U.S. patent application number 17/689207 was filed with the patent office on 2022-09-15 for hybrid pulsed/coherent lidar system.
The applicant listed for this patent is Luminar, LLC. Invention is credited to Jason M. Eichenholz, Joseph G. LaChapelle, Alex Michael Sincore.
Application Number | 20220291348 17/689207 |
Document ID | / |
Family ID | 1000006237767 |
Filed Date | 2022-09-15 |
United States Patent
Application |
20220291348 |
Kind Code |
A1 |
LaChapelle; Joseph G. ; et
al. |
September 15, 2022 |
HYBRID PULSED/COHERENT LIDAR SYSTEM
Abstract
In one embodiment, a lidar system includes a light source
configured to emit (i) local-oscillator light and (ii) pulses of
light, where each emitted pulse of light is coherent with a
corresponding temporal portion of the local-oscillator light. The
lidar system also includes a receiver configured to detect the
local-oscillator light and a received pulse of light, the received
pulse of light including a portion of one of the emitted pulses of
light scattered by a target located a distance from the lidar
system. The receiver includes a detector configured to produce a
photocurrent signal corresponding to the local-oscillator light and
the received pulse of light. The photocurrent signal includes a sum
of a first term, a second term, and a third term.
Inventors: |
LaChapelle; Joseph G.;
(Philomath, OR) ; Eichenholz; Jason M.; (Orlando,
FL) ; Sincore; Alex Michael; (Orlando, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Luminar, LLC |
Orlando |
FL |
US |
|
|
Family ID: |
1000006237767 |
Appl. No.: |
17/689207 |
Filed: |
March 8, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
63159095 |
Mar 10, 2021 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 7/4865 20130101;
G01S 17/931 20200101; G01S 7/4816 20130101; G01S 7/4868 20130101;
G01S 17/894 20200101 |
International
Class: |
G01S 7/481 20060101
G01S007/481; G01S 17/894 20060101 G01S017/894; G01S 7/4865 20060101
G01S007/4865; G01S 7/486 20060101 G01S007/486 |
Claims
1. A lidar system comprising: a light source configured to emit (i)
local-oscillator light and (ii) pulses of light, wherein each
emitted pulse of light is coherent with a corresponding temporal
portion of the local-oscillator light; a receiver configured to
detect the local-oscillator light and a received pulse of light,
the received pulse of light comprising a portion of one of the
emitted pulses of light scattered by a target located a distance
from the lidar system, wherein the receiver comprises: a detector
configured to produce a photocurrent signal corresponding to the
local-oscillator light and the received pulse of light, the
photocurrent signal comprising a sum of a first term, a second
term, and a third term, wherein (i) the first term corresponds to
an optical property of the received pulse of light, (ii) the second
term corresponds to a coherent mixing of the local-oscillator light
and the received pulse of light, and (iii) the third term
corresponds to an optical property of the local-oscillator light;
and a pulse-detection circuit configured to determine a
time-of-arrival for the received pulse of light based on the first
term and the second term; and a processor configured to determine
the distance from the lidar system to the target based on the
time-of-arrival for the received pulse of light.
2. The lidar system of claim 1, wherein the photocurrent signal is
proportional to |.epsilon..sub.Rx(t)+.epsilon..sub.LO(t)|.sup.2,
wherein: .epsilon..sub.Rx(t) represents an electric field of the
received pulse of light; and .epsilon..sub.LO(t) represents an
electric field of the local-oscillator light.
3. The lidar system of claim 2, wherein: the first term corresponds
to an optical power of the received pulse of light and is
represented by |.epsilon..sub.Rx(t)|.sup.2; the second term, which
corresponds to the coherent mixing of the local-oscillator light
and the received pulse of light, is represented by
2|.epsilon..sub.Rx(t)||.epsilon..sub.LO(t)|cos[.DELTA..omega.(t)t+.DELTA.-
.PHI.(t)], wherein: .DELTA..omega.o(t) represents a frequency
difference between the electric field of the received pulse of
light and the electric field of the local-oscillator light; and
.DELTA..PHI.(t) represents a phase difference between the electric
field of the received pulse of light and the electric field of the
local-oscillator light; and the third term corresponds to an
optical power of the local-oscillator light and is represented by
|.epsilon..sub.LO(t))|.sup.2.
4. The lidar system of claim 1, wherein the second term is a
coherent-mixing term that is proportional to a product of (i) an
amplitude of an electric field of the received pulse of light and
(ii) an amplitude of an electric field of the local-oscillator
light.
5. The lidar system of claim 4, wherein the coherent-mixing term of
the photocurrent signal is proportional to
E.sub.Rx(t)E.sub.LO(t)cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.Rx(t-
)-.PHI..sub.LO(t)], wherein: E.sub.Rx(t) represents the amplitude
of the electric field of the received pulse of light; E.sub.LO(t)
represents the amplitude of the electric field of the
local-oscillator light; .omega..sub.Rx represents a frequency of
the electric field of the received pulse of light; .omega..sub.LO
represents a frequency of the electric field of the
local-oscillator light; .PHI..sub.Rx(t) represents a phase of the
electric field of the received pulse of light; and .PHI..sub.LO(t)
represents a phase of the electric field of the local-oscillator
light.
6. The lidar system of claim 1, wherein, when the first term is
greater than the second term, the receiver is configured to act as
a pulsed-lidar receiver, wherein the pulse-detection circuit
determines the time-of-arrival for the received pulse of light
primarily based on the first term.
7. The lidar system of claim 6, wherein the first term being
greater than the second term is associated with the distance to the
target being less than a threshold distance or a reflectivity of
the target being greater than a threshold reflectivity.
8. The lidar system of claim 1, wherein, when the second term is
greater than the first term, the receiver is configured to act as a
coherent-lidar receiver, wherein the pulse-detection circuit
determines the time-of-arrival for the received pulse of light
primarily based on the second term.
9. The lidar system of claim 8, wherein the second term being
greater than the first term is associated with the distance to the
target being greater than a threshold distance or a reflectivity of
the target being less than a threshold reflectivity.
10. The lidar system of claim 1, wherein: the optical property of
the received pulse of light is an optical power, optical intensity,
optical energy, or electric field of the received pulse of light;
and the optical property of the local-oscillator light is an
optical power, optical intensity, optical energy, or electric field
of the local-oscillator light.
11. The lidar system of claim 1, wherein the local-oscillator light
and the received pulse of light are coherently mixed together at
the receiver to produce the photocurrent signal.
12. The lidar system of claim 1, further comprising an optical
combiner configured to: combine the local-oscillator light and the
received pulse of light to produce a combined beam comprising at
least a portion of the local-oscillator light and at least a
portion of the received pulse of light; and direct the combined
beam to the detector.
13. The lidar system of claim 1, wherein the detector comprises a
first input side and a second input side located opposite the first
input side, wherein the received pulse of light is incident on the
first input side of the detector, and the local-oscillator light is
incident on the second input side of the detector.
14. The lidar system of claim 1, further comprising an optical
polarization element configured to alter a polarization of the
emitted pulses of light, the local-oscillator light, or the
received pulse of light to allow the local-oscillator light and the
received pulse of light to be coherently mixed.
15. The lidar system of claim 14, wherein the optical polarization
element comprises (i) a quarter-wave plate configured to convert
the polarization of the local-oscillator light to circularly
polarized light or (ii) a depolarizer configured to depolarize the
polarization of the local-oscillator light.
16. The lidar system of claim 1, wherein: the light source
comprises: a seed laser diode configured to produce a seed optical
signal and the local-oscillator light; and a semiconductor optical
amplifier (SOA) configured to amplify temporal portions of the seed
optical signal to produce the emitted pulses of light, wherein each
amplified temporal portion of the seed optical signal corresponds
to one of the emitted pulses of light; and the lidar system further
comprises a photonic integrated circuit (PIC) comprising an optical
combiner and one or more optical waveguides, wherein: each of the
seed laser diode, the SOA, and the detector is attached to,
connected to, or integrated with the PIC; the optical waveguides
are configured to (i) convey the local-oscillator light to the
optical combiner, (ii) convey the received pulse of light to the
optical combiner, and (iii) convey a combined beam from the
combiner to the detector; and the optical combiner is configured to
combine the local-oscillator light and the received pulse of light
to produce the combined beam, the combined beam comprising at least
a portion of the local-oscillator light and at least a portion of
the received pulse of light.
17. The lidar system of claim 16, further comprising an input lens
attached to, connected to, or integrated with the PIC, wherein the
input lens is configured to focus the received pulse of light into
one of the optical waveguides of the PIC.
18. The lidar system of claim 1, wherein: the light source is
further configured to impart a spectral signature of one or more
different spectral signatures to each of the emitted pulses of
light; and the receiver further comprises a frequency-detection
circuit configured to determine, based on the second term of the
photocurrent signal, a spectral signature of the received pulse of
light.
19. The lidar system of claim 1, wherein the light source
comprises: a seed laser diode configured to produce a seed optical
signal and the local-oscillator light; and a semiconductor optical
amplifier (SOA) configured to amplify temporal portions of the seed
optical signal to produce the emitted pulses of light, wherein each
amplified temporal portion of the seed optical signal corresponds
to one of the emitted pulses of light.
20. The lidar system of claim 19, wherein each emitted pulse of
light being coherent with the corresponding temporal portion of the
local-oscillator light corresponds to the temporal portion of the
seed light that is amplified being coherent with the corresponding
temporal portion of the local-oscillator light.
21. The lidar system of claim 19, wherein the seed laser diode
comprises a front face from which the seed optical signal is
produced and a back face from which the local-oscillator light is
produced.
22. The lidar system of claim 19, wherein the light source further
comprises an optical splitter disposed between the seed laser diode
and the SOA, wherein the optical splitter is configured to split
off a portion of the seed optical signal to produce the
local-oscillator light.
23. The light source of claim 19, wherein the SOA comprises a
tapered optical waveguide extending from an input end of the SOA to
an output end of the SOA, wherein a width of the tapered optical
waveguide increases from the input end to the output end.
24. The lidar system of claim 19, wherein the light source further
comprises an electronic driver configured to: supply a
substantially constant electrical current to the seed laser diode
so that the seed optical signal comprises light having a
substantially constant optical power; and supply pulses of
electrical current to the SOA, wherein each pulse of current causes
the SOA to amplify one of the temporal portions of the seed optical
signal to produce one of the emitted pulses of light.
25. The lidar system of claim 19, wherein the light source is
configured as a three-terminal device, wherein (i) the light source
comprises a common anode, wherein an anode of the seed laser diode
is electrically connected to an anode of the SOA or (ii) the light
source comprises a common cathode, wherein a cathode of the seed
laser diode is electrically connected to a cathode of the SOA.
26. The lidar system of claim 19, wherein the light source is
configured as a four-terminal device, wherein: the seed laser diode
comprises a seed laser anode and a seed laser cathode; the SOA
comprises a SOA anode and a SOA cathode; the seed laser anode and
the SOA anode are electrically isolated from one another; and the
seed laser cathode and the SOA cathode are electrically isolated
from one another.
27. The lidar system of claim 1, wherein the light source
comprises: a seed laser diode configured to produce a seed optical
signal and the local-oscillator light; a semiconductor optical
amplifier (SOA) configured to amplify temporal portions of the seed
optical signal to produce initial pulses of light; and a
fiber-optic amplifier configured to receive the initial pulses of
light from the SOA and further amplify the initial pulses of light
to produce the emitted pulses of light, wherein each amplified
temporal portion of the seed optical signal corresponds to one of
the emitted pulses of light.
28. The lidar system of claim 1, wherein the emitted pulses of
light have optical characteristics comprising: a wavelength between
900 nanometers and 2000 nanometers; a pulse energy between 0.01
.mu.J and 100 .mu.J; a pulse repetition frequency between 80 kHz
and 10 MHz; and a pulse duration between 1 ns and 100 ns.
29. The lidar system of claim 1, wherein: the receiver further
comprises an electronic amplifier configured to amplify the
photocurrent signal to produce a voltage signal that corresponds to
the photocurrent signal; and the pulse-detection circuit comprises
one or more comparators coupled to one or more respective
time-to-digital converters (TDCs), wherein: each comparator is
configured to provide an electrical-edge signal to a corresponding
TDC when the voltage signal rises above or falls below a particular
threshold voltage; and the corresponding TDC is configured to
produce a time value corresponding to a time when the
electrical-edge signal was received, wherein the time-of-arrival
for the received pulse of light is determined based on one or more
time values produced by one or more of the TDCs.
30. The lidar system of claim 1, wherein: the time-of-arrival for
the received pulse of light corresponds to a round-trip time
(.DELTA.T) for the portion of the one of the emitted pulses of
light to travel to the target and back to the lidar system; and the
distance (D) to the target is determined from an expression
D=c.DELTA.T/2, wherein c is a speed of light.
31. The lidar system of claim 1, further comprising a scanner
configured to scan the emitted pulses of light across a field of
regard of the lidar system.
32. The lidar system of claim 31, wherein the scanner comprises: a
polygon mirror configured to scan the emitted pulses of light along
a first direction within the field of regard; and a scan mirror
configured to scan the emitted pulses of light along a second
direction within the field of regard, the second direction
different from the first direction.
33. The lidar system of claim 31, wherein scanning the emitted
pulses of light comprises scanning a field of view of the light
source and a field of view of the receiver across the field of
regard of the lidar system, wherein the light-source field of view
and the receiver field of view are scanned synchronously with
respect to one another, wherein a scanning speed of the
light-source field of view and a scanning speed of the receiver
field of view are approximately equal.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 63/159,095, filed 10 Mar. 2021, which is
incorporated by reference herein in its entirety.
TECHNICAL FIELD
[0002] This disclosure generally relates to lidar systems.
BACKGROUND
[0003] Light detection and ranging (lidar) is a technology that can
be used to measure distances to remote targets. Typically, a lidar
system includes a light source and an optical receiver. The light
source can include, for example, a laser which emits light having a
particular operating wavelength. The operating wavelength of a
lidar system may lie, for example, in the infrared, visible, or
ultraviolet portions of the electromagnetic spectrum. The light
source emits light toward a target which scatters the light, and
some of the scattered light is received back at the receiver. The
system determines the distance to the target based on one or more
characteristics associated with the received light. For example,
the lidar system may determine the distance to the target based on
the time of flight for a pulse of light emitted by the light source
to travel to the target and back to the lidar system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 illustrates an example light detection and ranging
(lidar) system.
[0005] FIG. 2 illustrates an example scan pattern produced by a
lidar system.
[0006] FIG. 3 illustrates an example lidar system with an example
rotating polygon mirror.
[0007] FIG. 4 illustrates an example light-source field of view
(FOV.sub.L) and receiver field of view (FOV.sub.R) for a lidar
system.
[0008] FIG. 5 illustrates an example unidirectional scan pattern
that includes multiple pixels and multiple scan lines.
[0009] FIG. 6 illustrates an example lidar system with a light
source that emits pulses of light and local-oscillator (LO)
light.
[0010] FIG. 7 illustrates an example receiver and an example
voltage signal corresponding to a received pulse of light.
[0011] FIG. 8 illustrates an example light source that includes a
seed laser diode and a semiconductor optical amplifier (SOA).
[0012] FIG. 9 illustrates an example light source that includes a
semiconductor optical amplifier (SOA) with a tapered optical
waveguide.
[0013] FIG. 10 illustrates an example light source with an optical
splitter that splits output light from a seed laser diode to
produce seed light and local-oscillator (LO) light.
[0014] FIG. 11 illustrates an example light source with a photonic
integrated circuit (PIC) that includes an optical-waveguide
splitter.
[0015] FIG. 12 illustrates an example light source that includes a
seed laser diode and a local-oscillator (LO) laser diode.
[0016] FIG. 13 illustrates an example light source that includes a
seed laser, a semiconductor optical amplifier (SOA), and a
fiber-optic amplifier.
[0017] FIG. 14 illustrates an example fiber-optic amplifier.
[0018] FIG. 15 illustrates example graphs of seed current
(I.sub.1), LO light, seed light, pulsed SOA current (I.sub.2), and
emitted optical pulses.
[0019] FIG. 16 illustrates example graphs of seed light, an emitted
optical pulse, a received optical pulse, LO light, and detector
photocurrent.
[0020] FIG. 17 illustrates an example voltage signal that results
from coherent mixing of LO light and a received pulse of light.
[0021] FIGS. 18-20 each illustrates an example receiver that
includes an optical combiner.
[0022] FIG. 21 illustrates an example receiver in which LO light
and an input beam are combined at a detector.
[0023] FIG. 22 illustrates an example receiver that includes a
two-sided detector.
[0024] FIG. 23 illustrates an example receiver that includes two
polarization beam-splitters.
[0025] FIGS. 24-27 each illustrates an example light source that
includes a seed laser, a semiconductor optical amplifier (SOA), and
one or more optical modulators.
[0026] FIG. 28 illustrates an example lidar system with a light
source that emits pulses of light and local-oscillator (LO)
light.
[0027] FIG. 29 illustrates an example light source and receiver
integrated into a photonic integrated circuit (PIC).
[0028] FIGS. 30-31 each illustrates an example photocurrent signal
that includes a pulse term, a coherent-mixing term, and a
local-oscillator (LO) term.
[0029] FIG. 32 illustrates an example graph with amplitudes of a
pulse term and a coherent-mixing term plotted versus distance to a
target.
[0030] FIG. 33 illustrates an example graph with amplitudes of a
pulse term and a coherent-mixing term plotted versus reflectivity
of a target.
[0031] FIG. 34 illustrates an example voltage signal that results
from the coherent mixing of LO light and a received pulse of light,
where the LO light and the received pulse of light have a frequency
difference of .DELTA.f.
[0032] FIG. 35 illustrates example graphs of seed current
(I.sub.1), seed light, an emitted optical pulse, a received optical
pulse, and LO light.
[0033] FIGS. 36-38 each illustrates example optical spectra of LO
light and a received pulse of light.
[0034] FIGS. 39-41 each illustrates an example photocurrent signal
plotted versus time.
[0035] FIGS. 42-43 each illustrates an example photocurrent signal
that includes a pulse term, a coherent-mixing term, and a
local-oscillator (LO) term.
[0036] FIG. 44 illustrates an example receiver that includes a
frequency-detection circuit with a derivative circuit and a
zero-crossing circuit.
[0037] FIG. 45 illustrates an example photocurrent signal and a
corresponding derivative signal.
[0038] FIG. 46 illustrates an example lidar system that emits n
pulses of light having n different respective spectral
signatures.
[0039] FIG. 47 illustrates an example lidar system configured to
determine a relative speed (Sr) of a target.
[0040] FIG. 48 illustrates an example computer system.
DESCRIPTION OF EXAMPLE EMBODIMENTS
[0041] FIG. 1 illustrates an example light detection and ranging
(lidar) system 100. In particular embodiments, a lidar system 100
may be referred to as a laser ranging system, a laser radar system,
a LIDAR system, a lidar sensor, or a laser detection and ranging
(LADAR or ladar) system. In particular embodiments, a lidar system
100 may include a light source 110, mirror 115, scanner 120,
receiver 140, or controller 150 (which may be referred to as a
processor). The light source 110 may include, for example, a laser
which emits light having a particular operating wavelength in the
infrared, visible, or ultraviolet portions of the electromagnetic
spectrum. As an example, light source 110 may include a laser with
one or more operating wavelengths between approximately 900
nanometers (nm) and 2000 nm. The light source 110 emits an output
beam of light 125 which may be continuous wave (CW), pulsed, or
modulated in any suitable manner for a given application. The
output beam of light 125 is directed downrange toward a remote
target 130. As an example, the remote target 130 may be located a
distance D of approximately 1 m to 1 km from the lidar system
100.
[0042] Once the output beam 125 reaches the downrange target 130,
the target may scatter or reflect at least a portion of light from
the output beam 125, and some of the scattered or reflected light
may return toward the lidar system 100. In the example of FIG. 1,
the scattered or reflected light is represented by input beam 135,
which passes through scanner 120 and is reflected by mirror 115 and
directed to receiver 140. In particular embodiments, a relatively
small fraction of the light from output beam 125 may return to the
lidar system 100 as input beam 135. As an example, the ratio of
input beam 135 average power, peak power, or pulse energy to output
beam 125 average power, peak power, or pulse energy may be
approximately 10.sup.-1, 10.sup.-2, 10.sup.-3, 10.sup.-4,
10.sup.-5, 10-6, 10.sup.-7, 10.sup.-8, 10.sup.-9, 10.sup.-10,
10.sup.-11, or 10.sup.-12. As another example, if a pulse of output
beam 125 has a pulse energy of 1 microjoule (.mu.J), then the pulse
energy of a corresponding pulse of input beam 135 may have a pulse
energy of approximately 10 nanojoules (nJ), 1 nJ, 100 picojoules
(pJ), 10 pJ, 1 pJ, 100 femtojoules (fJ), 10 fJ, 1 fJ, 100
attojoules (aJ), 10 aJ, 1 aJ, or 0.1 aJ.
[0043] In particular embodiments, output beam 125 may include or
may be referred to as an optical signal, output optical signal,
emitted optical signal, output light, emitted pulse of light, laser
beam, light beam, optical beam, emitted beam, emitted light, or
beam. In particular embodiments, input beam 135 may include or may
be referred to as a received optical signal, received pulse of
light, input pulse of light, input optical signal, return beam,
received beam, return light, received light, input light, scattered
light, or reflected light. As used herein, scattered light may
refer to light that is scattered or reflected by a target 130. As
an example, an input beam 135 may include: light from the output
beam 125 that is scattered by target 130; light from the output
beam 125 that is reflected by target 130; or a combination of
scattered and reflected light from target 130.
[0044] In particular embodiments, receiver 140 may receive or
detect photons from input beam 135 and produce one or more
representative signals. For example, the receiver 140 may produce
an output electrical signal 145 that is representative of the input
beam 135, and the electrical signal 145 may be sent to controller
150. In particular embodiments, receiver 140 or controller 150 may
include a processor, computing system (e.g., an ASIC or FPGA), or
other suitable circuitry. A controller 150 may be configured to
analyze one or more characteristics of the electrical signal 145
from the receiver 140 to determine one or more characteristics of
the target 130, such as its distance downrange from the lidar
system 100. This may be done, for example, by analyzing a time of
flight or a frequency or phase of a transmitted beam of light 125
or a received beam of light 135. If lidar system 100 measures a
time of flight of .DELTA.T (e.g., .DELTA.T represents a round-trip
time of flight for an emitted pulse of light to travel from the
lidar system 100 to the target 130 and back to the lidar system
100), then the distance D from the target 130 to the lidar system
100 may be expressed as D=c.DELTA.T/2, where c is the speed of
light (approximately 3.0.times.10.sup.8 m/s). As an example, if a
time of flight is measured to be .DELTA.T=300 ns, then the distance
from the target 130 to the lidar system 100 may be determined to be
approximately D=45.0 m. As another example, if a time of flight is
measured to be .DELTA.T=1.33 .mu.s, then the distance from the
target 130 to the lidar system 100 may be determined to be
approximately D=199.5 m. In particular embodiments, a distance D
from lidar system 100 to a target 130 may be referred to as a
distance, depth, or range of target 130. As used herein, the speed
of light c refers to the speed of light in any suitable medium,
such as for example in air, water, or vacuum. As an example, the
speed of light in vacuum is approximately 2.9979.times.10.sup.8
m/s, and the speed of light in air (which has a refractive index of
approximately 1.0003) is approximately 2.9970.times.10.sup.8
m/s.
[0045] In particular embodiments, light source 110 may include a
pulsed or CW laser. As an example, light source 110 may be a pulsed
laser configured to produce or emit pulses of light with a pulse
duration or pulse width of approximately 10 picoseconds (ps) to 100
nanoseconds (ns). The pulses may have a pulse duration
(.DELTA..tau.) of approximately 100 ps, 200 ps, 400 ps, 1 ns, 2 ns,
5 ns, 10 ns, 20 ns, 50 ns, 100 ns, or any other suitable pulse
duration. As another example, light source 110 may be a pulsed
laser that produces pulses with a pulse duration of approximately
1-5 ns. As another example, light source 110 may be a pulsed laser
that produces pulses at a pulse repetition frequency of
approximately 80 kHz to 10 MHz or a pulse period (e.g., a time
between consecutive pulses) of approximately 100 ns to 12.5 .mu.s.
In particular embodiments, light source 110 may have a
substantially constant pulse repetition frequency, or light source
110 may have a variable or adjustable pulse repetition frequency.
As an example, light source 110 may be a pulsed laser that produces
pulses at a substantially constant pulse repetition frequency of
approximately 640 kHz (e.g., 640,000 pulses per second),
corresponding to a pulse period of approximately 1.56 .mu.s. As
another example, light source 110 may have a pulse repetition
frequency (which may be referred to as a repetition rate) that can
be varied from approximately 200 kHz to 3 MHz. As used herein, a
pulse of light may be referred to as an optical pulse, a light
pulse, or a pulse.
[0046] In particular embodiments, light source 110 may include a
pulsed or CW laser that produces a free-space output beam 125
having any suitable average optical power. As an example, output
beam 125 may have an average power of approximately 1 milliwatt
(mW), 10 mW, 100 mW, 1 watt (W), 10 W, or any other suitable
average power. In particular embodiments, output beam 125 may
include optical pulses with any suitable pulse energy or peak
optical power. As an example, output beam 125 may include pulses
with a pulse energy of approximately 0.01 .mu.J, 0.1 .mu.J, 0.5
.mu.J, 1 .mu.J, 2 .mu.J, 10 .mu.J, 100 .mu.J, 1 mJ, or any other
suitable pulse energy. As another example, output beam 125 may
include pulses with a peak power of approximately 10 W, 100 W, 1
k.OMEGA., 5 k.OMEGA., 10 k.OMEGA., or any other suitable peak
power. The peak power (P.sub.peak) of a pulse of light can be
related to the pulse energy (E) by the expression
E=P.sub.peak.DELTA.t, where .DELTA.t is the duration of the pulse,
and the duration of a pulse may be defined as the full width at
half maximum duration of the pulse. For example, an optical pulse
with a duration of 1 ns and a pulse energy of 1 .mu.J has a peak
power of approximately 1 k.OMEGA.. The average power (P.sub.av) of
an output beam 125 can be related to the pulse repetition frequency
(PRF) and pulse energy by the expression P.sub.av=PRFE. For
example, if the pulse repetition frequency is 500 kHz, then the
average power of an output beam 125 with 1-.mu.J pulses is
approximately 0.5 W.
[0047] In particular embodiments, light source 110 may include a
laser diode, such as for example, a Fabry-Perot laser diode, a
quantum well laser, a distributed Bragg reflector (DBR) laser, a
distributed feedback (DFB) laser, a vertical-cavity
surface-emitting laser (VCSEL), a quantum dot laser diode, a
grating-coupled surface-emitting laser (GCSEL), a slab-coupled
optical waveguide laser (SCOWL), a single-transverse-mode laser
diode, a multi-mode broad area laser diode, a laser-diode bar, a
laser-diode stack, or a tapered-stripe laser diode. As an example,
light source 110 may include an aluminum-gallium-arsenide (AlGaAs)
laser diode, an indium-gallium-arsenide (InGaAs) laser diode, an
indium-gallium-arsenide-phosphide (InGaAsP) laser diode, or a laser
diode that includes any suitable combination of aluminum (Al),
indium (In), gallium (Ga), arsenic (As), phosphorous (P), or any
other suitable material. In particular embodiments, light source
110 may include a pulsed or CW laser diode with a peak emission
wavelength between 1200 nm and 1600 nm. As an example, light source
110 may include a current-modulated InGaAsP DFB laser diode that
produces optical pulses at a wavelength of approximately 1550 nm.
As another example, light source 110 may include a laser diode that
emits light at a wavelength between 1500 nm and 1510 nm.
[0048] In particular embodiments, light source 110 may include a
pulsed or CW laser diode followed by one or more
optical-amplification stages. For example, a seed laser diode may
produce a seed optical signal, and an optical amplifier may amplify
the seed optical signal to produce an amplified optical signal that
is emitted by the light source 110. In particular embodiments, an
optical amplifier may include a fiber-optic amplifier or a
semiconductor optical amplifier (SOA). For example, a pulsed laser
diode may produce relatively low-power optical seed pulses which
are amplified by a fiber-optic amplifier. As another example, a
light source 110 may include a fiber-laser module that includes a
current-modulated laser diode with an operating wavelength of
approximately 1550 nm followed by a single-stage or a multi-stage
erbium-doped fiber amplifier (EDFA) or erbium-ytterbium-doped fiber
amplifier (EYDFA) that amplifies the seed pulses from the laser
diode. As another example, light source 110 may include a
continuous-wave (CW) or quasi-CW laser diode followed by an
external optical modulator (e.g., an electro-optic amplitude
modulator). The optical modulator may modulate the CW light from
the laser diode to produce optical pulses which are sent to a
fiber-optic amplifier or SOA. As another example, light source 110
may include a pulsed or CW seed laser diode followed by a
semiconductor optical amplifier (SOA). The SOA may include an
active optical waveguide configured to receive light from the seed
laser diode and amplify the light as it propagates through the
waveguide. The optical gain of the SOA may be provided by pulsed or
direct-current (DC) electrical current supplied to the SOA. The SOA
may be integrated on the same chip as the seed laser diode, or the
SOA may be a separate device with an anti-reflection coating on its
input facet or output facet. As another example, light source 110
may include a seed laser diode followed by a SOA, which in turn is
followed by a fiber-optic amplifier. For example, the seed laser
diode may produce relatively low-power seed pulses which are
amplified by the SOA, and the fiber-optic amplifier may further
amplify the optical pulses.
[0049] In particular embodiments, light source 110 may include a
direct-emitter laser diode. A direct-emitter laser diode (which may
be referred to as a direct emitter) may include a laser diode which
produces light that is not subsequently amplified by an optical
amplifier. A light source 110 that includes a direct-emitter laser
diode may not include an optical amplifier, and the output light
produced by a direct emitter may not be amplified after it is
emitted by the laser diode. The light produced by a direct-emitter
laser diode (e.g., optical pulses, CW light, or frequency-modulated
light) may be emitted directly as a free-space output beam 125
without being amplified. A direct-emitter laser diode may be driven
by an electrical power source that supplies current pulses to the
laser diode, and each current pulse may result in the emission of
an output optical pulse.
[0050] In particular embodiments, light source 110 may include a
diode-pumped solid-state (DPSS) laser. A DPSS laser (which may be
referred to as a solid-state laser) may refer to a laser that
includes a solid-state, glass, ceramic, or crystal-based gain
medium that is pumped by one or more pump laser diodes. The gain
medium may include a host material that is doped with rare-earth
ions (e.g., neodymium, erbium, ytterbium, or praseodymium). For
example, a gain medium may include a yttrium aluminum garnet (YAG)
crystal that is doped with neodymium (Nd) ions, and the gain medium
may be referred to as a Nd:YAG crystal. A DPSS laser with a Nd:YAG
gain medium may produce light at a wavelength between approximately
1300 nm and approximately 1400 nm, and the Nd:YAG gain medium may
be pumped by one or more pump laser diodes with an operating
wavelength between approximately 730 nm and approximately 900 nm. A
DPSS laser may be a passively Q-switched laser that includes a
saturable absorber (e.g., a vanadium-doped crystal that acts as a
saturable absorber). Alternatively, a DPSS laser may be an actively
Q-switched laser that includes an active Q-switch (e.g., an
acousto-optic modulator or an electro-optic modulator). A passively
or actively Q-switched DPSS laser may produce output optical pulses
that form an output beam 125 of a lidar system 100.
[0051] In particular embodiments, an output beam of light 125
emitted by light source 110 may be a collimated optical beam having
any suitable beam divergence, such as for example, a full-angle
beam divergence of approximately 0.5 to 10 milliradians (mrad). A
divergence of output beam 125 may refer to an angular measure of an
increase in beam size (e.g., a beam radius or beam diameter) as
output beam 125 travels away from light source 110 or lidar system
100. In particular embodiments, output beam 125 may have a
substantially circular cross section with a beam divergence
characterized by a single divergence value. As an example, an
output beam 125 with a circular cross section and a full-angle beam
divergence of 2 mrad may have a beam diameter or spot size of
approximately 20 cm at a distance of 100 m from lidar system 100.
In particular embodiments, output beam 125 may have a substantially
elliptical cross section characterized by two divergence values. As
an example, output beam 125 may have a fast axis and a slow axis,
where the fast-axis divergence is greater than the slow-axis
divergence. As another example, output beam 125 may be an
elliptical beam with a fast-axis divergence of 4 mrad and a
slow-axis divergence of 2 mrad.
[0052] In particular embodiments, an output beam of light 125
emitted by light source 110 may be unpolarized or randomly
polarized, may have no specific or fixed polarization (e.g., the
polarization may vary with time), or may have a particular
polarization (e.g., output beam 125 may be linearly polarized,
elliptically polarized, or circularly polarized). As an example,
light source 110 may produce light with no specific polarization or
may produce light that is linearly polarized.
[0053] In particular embodiments, lidar system 100 may include one
or more optical components configured to reflect, focus, filter,
shape, modify, steer, or direct light within the lidar system 100
or light produced or received by the lidar system 100 (e.g., output
beam 125 or input beam 135). As an example, lidar system 100 may
include one or more lenses, mirrors, filters (e.g., band-pass or
interference filters), beam splitters, polarizers, polarizing beam
splitters, wave plates (e.g., half-wave or quarter-wave plates),
diffractive elements, holographic elements, isolators, optical
splitters, couplers, detectors, beam combiners, or collimators. The
optical components in a lidar system 100 may be free-space optical
components, fiber-coupled optical components, or a combination of
free-space and fiber-coupled optical components.
[0054] In particular embodiments, lidar system 100 may include a
telescope, one or more lenses, or one or more mirrors configured to
expand, focus, or collimate the output beam 125 or the input beam
135 to a desired beam diameter or divergence. As an example, the
lidar system 100 may include one or more lenses to focus the input
beam 135 onto a photodetector of receiver 140. As another example,
the lidar system 100 may include one or more flat mirrors or curved
mirrors (e.g., concave, convex, or parabolic mirrors) to steer or
focus the output beam 125 or the input beam 135. For example, the
lidar system 100 may include an off-axis parabolic mirror to focus
the input beam 135 onto a photodetector of receiver 140. As
illustrated in FIG. 1, the lidar system 100 may include mirror 115
(which may be a metallic or dielectric mirror), and mirror 115 may
be configured so that light beam 125 passes through the mirror 115
or passes along an edge or side of the mirror 115 and input beam
135 is reflected toward the receiver 140. As an example, mirror 115
(which may be referred to as an overlap mirror, superposition
mirror, or beam-combiner mirror) may include a hole, slot, or
aperture which output light beam 125 passes through. As another
example, rather than passing through the mirror 115, the output
beam 125 may be directed to pass alongside the mirror 115 with a
gap (e.g., a gap of width approximately 0.1 mm, 0.5 mm, 1 mm, 2 mm,
5 mm, or 10 mm) between the output beam 125 and an edge of the
mirror 115.
[0055] In particular embodiments, mirror 115 may provide for output
beam 125 and input beam 135 to be substantially coaxial so that the
two beams travel along approximately the same optical path (albeit
in opposite directions). The input and output beams being
substantially coaxial may refer to the beams being at least
partially overlapped or sharing a common propagation axis so that
input beam 135 and output beam 125 travel along substantially the
same optical path (albeit in opposite directions). As an example,
output beam 125 and input beam 135 may be parallel to each other to
within less than 10 mrad, 5 mrad, 2 mrad, 1 mrad, 0.5 mrad, or 0.1
mrad. As output beam 125 is scanned across a field of regard, the
input beam 135 may follow along with the output beam 125 so that
the coaxial relationship between the two beams is maintained.
[0056] In particular embodiments, lidar system 100 may include a
scanner 120 configured to scan an output beam 125 across a field of
regard of the lidar system 100. As an example, scanner 120 may
include one or more scanning mirrors configured to pivot, rotate,
oscillate, or move in an angular manner about one or more rotation
axes. The output beam 125 may be reflected by a scanning mirror,
and as the scanning mirror pivots or rotates, the reflected output
beam 125 may be scanned in a corresponding angular manner. As an
example, a scanning mirror may be configured to periodically pivot
back and forth over a 30-degree range, which results in the output
beam 125 scanning back and forth across a 60-degree range (e.g., a
0-degree rotation by a scanning mirror results in a 20-degree
angular scan of output beam 125).
[0057] In particular embodiments, a scanning mirror (which may be
referred to as a scan mirror) may be attached to or mechanically
driven by a scanner actuator or mechanism which pivots or rotates
the mirror over a particular angular range (e.g., over a 5.degree.
angular range, 30.degree. angular range, 60.degree. angular range,
120.degree. angular range, 360.degree. angular range, or any other
suitable angular range). A scanner actuator or mechanism configured
to pivot or rotate a mirror may include a galvanometer scanner, a
resonant scanner, a piezoelectric actuator, a voice coil motor, an
electric motor (e.g., a DC motor, a brushless DC motor, a
synchronous electric motor, or a stepper motor), a
microelectromechanical systems (MEMS) device, or any other suitable
actuator or mechanism. As an example, a scanner 120 may include a
scanning mirror attached to a galvanometer scanner configured to
pivot back and forth over a 1.degree. to 30.degree. angular range.
As another example, a scanner 120 may include a scanning mirror
that is attached to or is part of a MEMS device configured to scan
over a 1.degree. to 30.degree. angular range. As another example, a
scanner 120 may include a polygon mirror configured to rotate
continuously in the same direction (e.g., rather than pivoting back
and forth, the polygon mirror continuously rotates 360 degrees in a
clockwise or counterclockwise direction). The polygon mirror may be
coupled or attached to a synchronous motor configured to rotate the
polygon mirror at a substantially fixed rotational frequency (e.g.,
a rotational frequency of approximately 1 Hz, 10 Hz, 50 Hz, 100 Hz,
500 Hz, or 1,000 Hz).
[0058] In particular embodiments, scanner 120 may be configured to
scan the output beam 125 (which may include at least a portion of
the light emitted by light source 110) across a field of regard of
the lidar system 100. A field of regard (FOR) of a lidar system 100
may refer to an area, region, or angular range over which the lidar
system 100 may be configured to scan or capture distance
information. As an example, a lidar system 100 with an output beam
125 with a 30-degree scanning range may be referred to as having a
30-degree angular field of regard. As another example, a lidar
system 100 with a scanning mirror that rotates over a 30-degree
range may produce an output beam 125 that scans across a 60-degree
range (e.g., a 60-degree FOR). In particular embodiments, lidar
system 100 may have a FOR of approximately 10.degree., 20.degree.,
40.degree., 60.degree., 120.degree., 360.degree., or any other
suitable FOR.
[0059] In particular embodiments, scanner 120 may be configured to
scan the output beam 125 horizontally and vertically, and lidar
system 100 may have a particular FOR along the horizontal direction
and another particular FOR along the vertical direction. As an
example, lidar system 100 may have a horizontal FOR of 100 to 1200
and a vertical FOR of 2.degree. to 45.degree.. In particular
embodiments, scanner 120 may include a first scan mirror and a
second scan mirror, where the first scan mirror directs the output
beam 125 toward the second scan mirror, and the second scan mirror
directs the output beam 125 downrange from the lidar system 100. As
an example, the first scan mirror may scan the output beam 125
along a first direction, and the second scan mirror may scan the
output beam 125 along a second direction that is different from the
first direction (e.g., the first and second directions may be
approximately orthogonal to one another, or the second direction
may be oriented at any suitable non-zero angle with respect to the
first direction). As another example, the first scan mirror may
scan the output beam 125 along a substantially horizontal
direction, and the second scan mirror may scan the output beam 125
along a substantially vertical direction (or vice versa). As
another example, the first and second scan mirrors may each be
driven by galvanometer scanners. As another example, the first or
second scan mirror may include a polygon mirror driven by an
electric motor. In particular embodiments, scanner 120 may be
referred to as a beam scanner, optical scanner, or laser
scanner.
[0060] In particular embodiments, one or more scanning mirrors may
be communicatively coupled to controller 150 which may control the
scanning mirror(s) so as to guide the output beam 125 in a desired
direction downrange or along a desired scan pattern. In particular
embodiments, a scan pattern may refer to a pattern or path along
which the output beam 125 is directed. As an example, scanner 120
may include two scanning mirrors configured to scan the output beam
125 across a 600 horizontal FOR and a 200 vertical FOR. The two
scanner mirrors may be controlled to follow a scan path that
substantially covers the 60.degree..times.20.degree. FOR. As an
example, the scan path may result in a point cloud with pixels that
substantially cover the 60.degree..times.20.degree. FOR. The pixels
may be approximately evenly distributed across the
600.times.20.degree. FOR. Alternatively, the pixels may have a
particular nonuniform distribution (e.g., the pixels may be
distributed across all or a portion of the
60.degree..times.20.degree. FOR, and the pixels may have a higher
density in one or more particular regions of the
60.degree..times.20.degree. FOR).
[0061] In particular embodiments, a lidar system 100 may include a
scanner 120 with a solid-state scanning device. A solid-state
scanning device may refer to a scanner 120 that scans an output
beam 125 without the use of moving parts (e.g., without the use of
a mechanical scanner, such as a mirror that rotates or pivots). For
example, a solid-state scanner 120 may include one or more of the
following: an optical phased array scanning device; a
liquid-crystal scanning device; or a liquid lens scanning device. A
solid-state scanner 120 may be an electrically addressable device
that scans an output beam 125 along one axis (e.g., horizontally)
or along two axes (e.g., horizontally and vertically). In
particular embodiments, a scanner 120 may include a solid-state
scanner and a mechanical scanner. For example, a scanner 120 may
include an optical phased array scanner configured to scan an
output beam 125 in one direction and a galvanometer scanner that
scans the output beam 125 in an orthogonal direction. The optical
phased array scanner may scan the output beam relatively rapidly in
a horizontal direction across the field of regard (e.g., at a scan
rate of 50 to 1,000 scan lines per second), and the galvanometer
may pivot a mirror at a rate of 1-30 Hz to scan the output beam 125
vertically.
[0062] In particular embodiments, a lidar system 100 may include a
light source 110 configured to emit pulses of light and a scanner
120 configured to scan at least a portion of the emitted pulses of
light across a field of regard of the lidar system 100. One or more
of the emitted pulses of light may be scattered by a target 130
located downrange from the lidar system 100, and a receiver 140 may
detect at least a portion of the pulses of light scattered by the
target 130. A receiver 140 may be referred to as a photoreceiver,
optical receiver, optical sensor, detector, photodetector, or
optical detector. In particular embodiments, lidar system 100 may
include a receiver 140 that receives or detects at least a portion
of input beam 135 and produces an electrical signal that
corresponds to input beam 135. As an example, if input beam 135
includes an optical pulse, then receiver 140 may produce an
electrical current or voltage pulse that corresponds to the optical
pulse detected by receiver 140. As another example, receiver 140
may include one or more avalanche photodiodes (APDs) or one or more
single-photon avalanche diodes (SPADs). As another example,
receiver 140 may include one or more PN photodiodes (e.g., a
photodiode structure formed by a p-type semiconductor and a n-type
semiconductor, where the PN acronym refers to the structure having
p-doped and n-doped regions) or one or more PIN photodiodes (e.g.,
a photodiode structure formed by an undoped intrinsic semiconductor
region located between p-type and n-type regions, where the PIN
acronym refers to the structure having p-doped, intrinsic, and
n-doped regions). An APD, SPAD, PN photodiode, or PIN photodiode
may each be referred to as a detector, photodetector, or
photodiode. A detector may have an active region or an
avalanche-multiplication region that includes silicon, germanium,
InGaAs, InAsSb (indium arsenide antimonide), AlAsSb (aluminum
arsenide antimonide), or AlInAsSb (aluminum indium arsenide
antimonide). The active region may refer to an area over which a
detector may receive or detect input light. An active region may
have any suitable size or diameter, such as for example, a diameter
of approximately 10 .mu.m, 25 .mu.m, 50 .mu.m, 80 .mu.m, 100 .mu.m,
200 .mu.m, 500 .mu.m, 1 mm, 2 mm, or 5 mm.
[0063] In particular embodiments, receiver 140 may include
electronic circuitry that performs signal amplification, sampling,
filtering, signal conditioning, analog-to-digital conversion,
time-to-digital conversion, pulse detection, threshold detection,
rising-edge detection, or falling-edge detection. As an example,
receiver 140 may include a transimpedance amplifier that converts a
received photocurrent (e.g., a current produced by an APD in
response to a received optical signal) into a voltage signal. The
voltage signal may be sent to pulse-detection circuitry that
produces an analog or digital output signal 145 that corresponds to
one or more optical characteristics (e.g., rising edge, falling
edge, amplitude, duration, or energy) of a received optical pulse.
As an example, the pulse-detection circuitry may perform a
time-to-digital conversion to produce a digital output signal 145.
The electrical output signal 145 may be sent to controller 150 for
processing or analysis (e.g., to determine a time-of-flight value
corresponding to a received optical pulse).
[0064] In particular embodiments, a controller 150 (which may
include or may be referred to as a processor, an FPGA, an ASIC, a
computer, or a computing system) may be located within a lidar
system 100 or outside of a lidar system 100. Alternatively, one or
more parts of a controller 150 may be located within a lidar system
100, and one or more other parts of a controller 150 may be located
outside a lidar system 100. In particular embodiments, one or more
parts of a controller 150 may be located within a receiver 140 of a
lidar system 100, and one or more other parts of a controller 150
may be located in other parts of the lidar system 100. For example,
a receiver 140 may include an FPGA or ASIC configured to process an
output electrical signal from the receiver 140, and the processed
signal may be sent to a computing system located elsewhere within
the lidar system 100 or outside the lidar system 100. In particular
embodiments, a controller 150 may include any suitable arrangement
or combination of logic circuitry, analog circuitry, or digital
circuitry.
[0065] In particular embodiments, controller 150 may be
electrically coupled or communicatively coupled to light source
110, scanner 120, or receiver 140. As an example, controller 150
may receive electrical trigger pulses or edges from light source
110, where each pulse or edge corresponds to the emission of an
optical pulse by light source 110. As another example, controller
150 may provide instructions, a control signal, or a trigger signal
to light source 110 indicating when light source 110 should produce
optical pulses. Controller 150 may send an electrical trigger
signal that includes electrical pulses, where each electrical pulse
results in the emission of an optical pulse by light source 110. In
particular embodiments, the frequency, period, duration, pulse
energy, peak power, average power, or wavelength of the optical
pulses produced by light source 110 may be adjusted based on
instructions, a control signal, or trigger pulses provided by
controller 150. In particular embodiments, controller 150 may be
coupled to light source 110 and receiver 140, and controller 150
may determine a time-of-flight value for an optical pulse based on
timing information associated with when the pulse was emitted by
light source 110 and when a portion of the pulse (e.g., input beam
135) was detected or received by receiver 140. In particular
embodiments, controller 150 may include circuitry that performs
signal amplification, sampling, filtering, signal conditioning,
analog-to-digital conversion, time-to-digital conversion, pulse
detection, threshold detection, rising-edge detection, or
falling-edge detection.
[0066] In particular embodiments, lidar system 100 may include one
or more processors (e.g., a controller 150) configured to determine
a distance D from the lidar system 100 to a target 130 based at
least in part on a round-trip time of flight for an emitted pulse
of light to travel from the lidar system 100 to the target 130 and
back to the lidar system 100. The target 130 may be at least
partially contained within a field of regard of the lidar system
100 and located a distance D from the lidar system 100 that is less
than or equal to an operating distance (D.sub.OP) of the lidar
system 100. In particular embodiments, an operating distance (which
may be referred to as an operating range) of a lidar system 100 may
refer to a distance over which the lidar system 100 is configured
to sense or identify targets 130 located within a field of regard
of the lidar system 100. The operating distance of lidar system 100
may be any suitable distance, such as for example, 25 m, 50 m, 100
m, 200 m, 250 m, 500 m, or 1 km. As an example, a lidar system 100
with a 200-m operating distance may be configured to sense or
identify various targets 130 located up to 200 m away from the
lidar system 100. The operating distance D.sub.OP of a lidar system
100 may be related to the timer between the emission of successive
optical signals by the expression D.sub.OP=c.tau./2. For a lidar
system 100 with a 200-m operating distance (D.sub.OP=200 m), the
time .tau. between successive pulses (which may be referred to as a
pulse period, a pulse repetition interval (PRI), or a time period
between pulses) is approximately 2D.sub.OP/c.apprxeq.1.33 .mu.s.
The pulse period .tau. may also correspond to the time of flight
for a pulse to travel to and from a target 130 located a distance
D.sub.OP from the lidar system 100. Additionally, the pulse period
.tau. may be related to the pulse repetition frequency (PRF) by the
expression .tau.=1/PRF. For example, a pulse period of 1.33 .mu.s
corresponds to a PRF of approximately 752 kHz.
[0067] In particular embodiments, a lidar system 100 may be used to
determine the distance to one or more downrange targets 130. By
scanning the lidar system 100 across a field of regard, the system
may be used to map the distance to a number of points within the
field of regard. Each of these depth-mapped points may be referred
to as a pixel or a voxel. A collection of pixels captured in
succession (which may be referred to as a depth map, a point cloud,
or a frame) may be rendered as an image or may be analyzed to
identify or detect objects or to determine a shape or distance of
objects within the FOR. As an example, a point cloud may cover a
field of regard that extends 60.degree. horizontally and 15.degree.
vertically, and the point cloud may include a frame of 100-2000
pixels in the horizontal direction by 4-400 pixels in the vertical
direction.
[0068] In particular embodiments, lidar system 100 may be
configured to repeatedly capture or generate point clouds of a
field of regard at any suitable frame rate between approximately
0.1 frames per second (FPS) and approximately 1,000 FPS. As an
example, lidar system 100 may generate point clouds at a frame rate
of approximately 0.1 FPS, 0.5 FPS, 1 FPS, 2 FPS, 5 FPS, 10 FPS, 20
FPS, 100 FPS, 500 FPS, or 1,000 FPS. As another example, lidar
system 100 may be configured to produce optical pulses at a rate of
5.times.10.sup.5 pulses/second (e.g., the system may determine
500,000 pixel distances per second) and scan a frame of
1000.times.50 pixels (e.g., 50,000 pixels/frame), which corresponds
to a point-cloud frame rate of 10 frames per second (e.g., 10 point
clouds per second). In particular embodiments, a point-cloud frame
rate may be substantially fixed, or a point-cloud frame rate may be
dynamically adjustable. As an example, a lidar system 100 may
capture one or more point clouds at a particular frame rate (e.g.,
1 Hz) and then switch to capture one or more point clouds at a
different frame rate (e.g., 10 Hz). A slower frame rate (e.g., 1
Hz) may be used to capture one or more high-resolution point
clouds, and a faster frame rate (e.g., 10 Hz) may be used to
rapidly capture multiple lower-resolution point clouds.
[0069] In particular embodiments, a lidar system 100 may be
configured to sense, identify, or determine distances to one or
more targets 130 within a field of regard. As an example, a lidar
system 100 may determine a distance to a target 130, where all or
part of the target 130 is contained within a field of regard of the
lidar system 100. All or part of a target 130 being contained
within a FOR of the lidar system 100 may refer to the FOR
overlapping, encompassing, or enclosing at least a portion of the
target 130. In particular embodiments, target 130 may include all
or part of an object that is moving or stationary relative to lidar
system 100. As an example, target 130 may include all or a portion
of a person, vehicle, motorcycle, truck, train, bicycle,
wheelchair, pedestrian, animal, road sign, traffic light, lane
marking, road-surface marking, parking space, pylon, guard rail,
traffic barrier, pothole, railroad crossing, obstacle in or near a
road, curb, stopped vehicle on or beside a road, utility pole,
house, building, trash can, mailbox, tree, any other suitable
object, or any suitable combination of all or part of two or more
objects. In particular embodiments, a target may be referred to as
an object.
[0070] In particular embodiments, light source 110, scanner 120,
and receiver 140 may be packaged together within a single housing,
where a housing may refer to a box, case, or enclosure that holds
or contains all or part of a lidar system 100. As an example, a
lidar-system enclosure may contain a light source 110, mirror 115,
scanner 120, and receiver 140 of a lidar system 100. Additionally,
the lidar-system enclosure may include a controller 150. The
lidar-system enclosure may also include one or more electrical
connections for conveying electrical power or electrical signals to
or from the enclosure. In particular embodiments, one or more
components of a lidar system 100 may be located remotely from a
lidar-system enclosure. As an example, all or part of light source
110 may be located remotely from a lidar-system enclosure, and
pulses of light produced by the light source 110 may be conveyed to
the enclosure via optical fiber. As another example, all or part of
a controller 150 may be located remotely from a lidar-system
enclosure.
[0071] In particular embodiments, light source 110 may include an
eye-safe laser, or lidar system 100 may be classified as an
eye-safe laser system or laser product. An eye-safe laser, laser
system, or laser product may refer to a system that includes a
laser with an emission wavelength, average power, peak power, peak
intensity, pulse energy, beam size, beam divergence, exposure time,
or scanned output beam such that emitted light from the system
presents little or no possibility of causing damage to a person's
eyes. As an example, light source 110 or lidar system 100 may be
classified as a Class 1 laser product (as specified by the
60825-1:2014 standard of the International Electrotechnical
Commission (IEC)) or a Class I laser product (as specified by Title
21, Section 1040.10 of the United States Code of Federal
Regulations (CFR)) that is safe under all conditions of normal use.
In particular embodiments, lidar system 100 may be an eye-safe
laser product (e.g., with a Class 1 or Class I classification)
configured to operate at any suitable wavelength between
approximately 900 nm and approximately 2100 nm. As an example,
lidar system 100 may include a laser with an operating wavelength
between approximately 1200 nm and approximately 1400 nm or between
approximately 1400 nm and approximately 1600 nm, and the laser or
the lidar system 100 may be operated in an eye-safe manner. As
another example, lidar system 100 may be an eye-safe laser product
that includes a scanned laser with an operating wavelength between
approximately 900 nm and approximately 1700 nm. As another example,
lidar system 100 may be a Class 1 or Class I laser product that
includes a laser diode, fiber laser, or solid-state laser with an
operating wavelength between approximately 1200 nm and
approximately 1600 nm. As another example, lidar system 100 may
have an operating wavelength between approximately 1500 nm and
approximately 1510 nm.
[0072] In particular embodiments, one or more lidar systems 100 may
be integrated into a vehicle. As an example, multiple lidar systems
100 may be integrated into a car to provide a complete 360-degree
horizontal FOR around the car. As another example, 2-10 lidar
systems 100, each system having a 45-degree to 180-degree
horizontal FOR, may be combined together to form a sensing system
that provides a point cloud covering a 360-degree horizontal FOR.
The lidar systems 100 may be oriented so that adjacent FORs have an
amount of spatial or angular overlap to allow data from the
multiple lidar systems 100 to be combined or stitched together to
form a single or continuous 360-degree point cloud. As an example,
the FOR of each lidar system 100 may have approximately 1-30
degrees of overlap with an adjacent FOR. In particular embodiments,
a vehicle may refer to a mobile machine configured to transport
people or cargo. For example, a vehicle may include, may take the
form of, or may be referred to as a car, automobile, motor vehicle,
truck, bus, van, trailer, off-road vehicle, farm vehicle, lawn
mower, construction equipment, forklift, robot, golf cart,
motorhome, taxi, motorcycle, scooter, bicycle, skateboard, train,
snowmobile, watercraft (e.g., a ship or boat), aircraft (e.g., a
fixed-wing aircraft, helicopter, or dirigible), unmanned aerial
vehicle (e.g., drone), or spacecraft. In particular embodiments, a
vehicle may include an internal combustion engine or an electric
motor that provides propulsion for the vehicle.
[0073] In particular embodiments, one or more lidar systems 100 may
be included in a vehicle as part of an advanced driver assistance
system (ADAS) to assist a driver of the vehicle in operating the
vehicle. For example, a lidar system 100 may be part of an ADAS
that provides information (e.g., about the surrounding environment)
or feedback to a driver (e.g., to alert the driver to potential
problems or hazards) or that automatically takes control of part of
a vehicle (e.g., a braking system or a steering system) to avoid
collisions or accidents. A lidar system 100 may be part of a
vehicle ADAS that provides adaptive cruise control, automated
braking, automated parking, collision avoidance, alerts the driver
to hazards or other vehicles, maintains the vehicle in the correct
lane, or provides a warning if an object or another vehicle is in a
blind spot.
[0074] In particular embodiments, one or more lidar systems 100 may
be integrated into a vehicle as part of an autonomous-vehicle
driving system. As an example, a lidar system 100 may provide
information about the surrounding environment to a driving system
of an autonomous vehicle. An autonomous-vehicle driving system may
be configured to guide the autonomous vehicle through an
environment surrounding the vehicle and toward a destination. An
autonomous-vehicle driving system may include one or more computing
systems that receive information from a lidar system 100 about the
surrounding environment, analyze the received information, and
provide control signals to the vehicle's driving systems (e.g.,
brakes, accelerator, steering mechanism, lights, or turn signals).
As an example, a lidar system 100 integrated into an autonomous
vehicle may provide an autonomous-vehicle driving system with a
point cloud every 0.1 seconds (e.g., the point cloud has a 10 Hz
update rate, representing 10 frames per second). The
autonomous-vehicle driving system may analyze the received point
clouds to sense or identify targets 130 and their respective
locations, distances, or speeds, and the autonomous-vehicle driving
system may update control signals based on this information. As an
example, if lidar system 100 detects a vehicle ahead that is
slowing down or stopping, the autonomous-vehicle driving system may
send instructions to release the accelerator and apply the
brakes.
[0075] In particular embodiments, an autonomous vehicle may be
referred to as an autonomous car, driverless car, self-driving car,
robotic car, or unmanned vehicle. In particular embodiments, an
autonomous vehicle may refer to a vehicle configured to sense its
environment and navigate or drive with little or no human input. As
an example, an autonomous vehicle may be configured to drive to any
suitable location and control or perform all safety-critical
functions (e.g., driving, steering, braking, parking) for the
entire trip, with the driver not expected to control the vehicle at
any time. As another example, an autonomous vehicle may allow a
driver to safely turn their attention away from driving tasks in
particular environments (e.g., on freeways), or an autonomous
vehicle may provide control of a vehicle in all but a few
environments, requiring little or no input or attention from the
driver.
[0076] In particular embodiments, an autonomous vehicle may be
configured to drive with a driver present in the vehicle, or an
autonomous vehicle may be configured to operate the vehicle with no
driver present. As an example, an autonomous vehicle may include a
driver's seat with associated controls (e.g., steering wheel,
accelerator pedal, and brake pedal), and the vehicle may be
configured to drive with no one seated in the driver's seat or with
little or no input from a person seated in the driver's seat. As
another example, an autonomous vehicle may not include any driver's
seat or associated driver's controls, and the vehicle may perform
substantially all driving functions (e.g., driving, steering,
braking, parking, and navigating) without human input. As another
example, an autonomous vehicle may be configured to operate without
a driver (e.g., the vehicle may be configured to transport human
passengers or cargo without a driver present in the vehicle). As
another example, an autonomous vehicle may be configured to operate
without any human passengers (e.g., the vehicle may be configured
for transportation of cargo without having any human passengers
onboard the vehicle).
[0077] In particular embodiments, an optical signal (which may be
referred to as a light signal, a light waveform, an optical
waveform, an output beam, an emitted optical signal, or emitted
light) may include pulses of light, CW light, amplitude-modulated
light, frequency-modulated (FM) light, or any suitable combination
thereof. Although this disclosure describes or illustrates example
embodiments of lidar systems 100 or light sources 110 that produce
optical signals that include pulses of light, the embodiments
described or illustrated herein may also be applied, where
appropriate, to other types of optical signals, including
continuous-wave (CW) light, amplitude-modulated optical signals, or
frequency-modulated optical signals. For example, a lidar system
100 as described or illustrated herein may be a pulsed lidar system
and may include a light source 110 configured to produce pulses of
light. Alternatively, a lidar system 100 may be configured to
operate as a frequency-modulated continuous-wave (FMCW) lidar
system and may include a light source 110 configured to produce CW
light or a frequency-modulated optical signal.
[0078] In particular embodiments, a lidar system 100 may be a FMCW
lidar system where the emitted light from the light source 110
(e.g., output beam 125 in FIG. 1 or FIG. 3) includes
frequency-modulated light. A pulsed lidar system is a type of lidar
system 100 in which the light source 110 emits pulses of light, and
the distance to a remote target 130 is determined based on the
round-trip time-of-flight for a pulse of light to travel to the
target 130 and back. Another type of lidar system 100 is a
frequency-modulated lidar system, which may be referred to as a
frequency-modulated continuous-wave (FMCW) lidar system. A FMCW
lidar system uses frequency-modulated light to determine the
distance to a remote target 130 based on a frequency of received
light (which includes emitted light scattered by the remote target)
relative to a frequency of local-oscillator (LO) light. A
round-trip time for the emitted light to travel to a target 130 and
back to the lidar system may correspond to a frequency difference
between the received scattered light and the LO light. A larger
frequency difference may correspond to a longer round-trip time and
a greater distance to the target 130. The frequency difference
between the received scattered light and the LO light may be
referred to as a beat frequency.
[0079] For example, for a linearly chirped light source (e.g., a
frequency modulation that produces a linear change in frequency
with time), the larger the frequency difference between the LO
light and the received light, the farther away the target 130 is
located. The frequency difference may be determined by mixing the
received light with the LO light (e.g., by coupling the two beams
onto a detector so that they are coherently mixed or combined
together, or by mixing analog electric signals corresponding to the
received light and the emitted light) to produce a beat signal and
determining the beat frequency of the beat signal. For example, an
electrical signal from an APD may be analyzed using a fast Fourier
transform (FFT) technique to determine the frequency difference
between the emitted light and the received light. If a linear
frequency modulation m (e.g., in units of Hz/s) is applied to a CW
laser, then the round-trip time .DELTA.T may be related to the
frequency difference between the received scattered light and the
emitted light .DELTA..PHI. by the expression
.DELTA.T=.DELTA..PHI./m. Additionally, the distance D from the
target 130 to the lidar system 100 may be expressed as
D=c.DELTA..PHI./(2m), where c is the speed of light. For example,
for a light source 110 with a linear frequency modulation of
10.sup.12 Hz/s (or, 1 MHz/.mu.s), if a frequency difference
(between the received scattered light and the emitted light) of 330
kHz is measured, then the distance to the target is approximately
50 meters (which corresponds to a round-trip time of approximately
330 ns). As another example, a frequency difference of 1.33 MHz
corresponds to a target located approximately 200 meters away.
[0080] Alight source 110 for a FMCW lidar system may include (i) a
direct-emitter laser diode, (ii) a seed laser diode followed by a
SOA, (iii) a seed laser diode followed by a fiber-optic amplifier,
or (iv) a seed laser diode followed by a SOA and then a fiber-optic
amplifier. A seed laser diode or a direct-emitter laser diode may
be operated in a CW manner (e.g., by driving the laser diode with a
substantially constant DC current), and a frequency modulation may
be provided by an external modulator (e.g., an electro-optic phase
modulator may apply a frequency modulation to seed-laser light).
Alternatively, a frequency modulation may be produced by applying a
current modulation to a seed laser diode or a direct-emitter laser
diode. The current modulation (which may be provided along with a
DC bias current) may produce a corresponding refractive-index
modulation in the laser diode, which results in a frequency
modulation of the light emitted by the laser diode. The
current-modulation component (and the corresponding frequency
modulation) may have any suitable frequency or shape (e.g.,
piecewise linear, sinusoidal, triangle-wave, or sawtooth). For
example, the current-modulation component (and the resulting
frequency modulation of the emitted light) may increase or decrease
monotonically over a particular time interval. As another example,
the current-modulation component may include a triangle or sawtooth
wave with an electrical current that increases or decreases
linearly over a particular time interval, and the light emitted by
the laser diode may include a corresponding frequency modulation in
which the optical frequency increases or decreases approximately
linearly over the particular time interval. For example, a light
source 110 that emits light with a linear frequency change of 200
MHz over a 2-.mu.s time interval may be referred to as having a
frequency modulation m of 10.sup.14 Hz/s (or, 100 MHz/.mu.s).
[0081] FIG. 2 illustrates an example scan pattern 200 produced by a
lidar system 100. A scanner 120 of the lidar system 100 may scan
the output beam 125 (which may include multiple emitted optical
signals) along a scan pattern 200 that is contained within a FOR of
the lidar system 100. A scan pattern 200 (which may be referred to
as an optical scan pattern, optical scan path, scan path, or scan)
may represent a path or course followed by output beam 125 as it is
scanned across all or part of a FOR. Each traversal of a scan
pattern 200 may correspond to the capture of a single frame or a
single point cloud. In particular embodiments, a lidar system 100
may be configured to scan output optical beam 125 along one or more
particular scan patterns 200. In particular embodiments, a scan
pattern 200 may scan across any suitable field of regard (FOR)
having any suitable horizontal FOR (FOR.sub.H) and any suitable
vertical FOR (FOR.sub.V). For example, a scan pattern 200 may have
a field of regard represented by angular dimensions (e.g.,
FOR.sub.H.times.FOR.sub.Y) 40.degree..times.30.degree.,
90.degree..times.40.degree., or 60.degree..times.15.degree.. As
another example, a scan pattern 200 may have a FOR.sub.H greater
than or equal to 10.degree., 25.degree., 30.degree., 40.degree.,
60.degree., 90.degree., or 120.degree.. As another example, a scan
pattern 200 may have a FOR.sub.V greater than or equal to
2.degree., 5.degree., 10.degree., 15.degree., 20.degree.,
30.degree., or 45.degree..
[0082] In the example of FIG. 2, reference line 220 represents a
center of the field of regard of scan pattern 200. In particular
embodiments, reference line 220 may have any suitable orientation,
such as for example, a horizontal angle of 0.degree. (e.g.,
reference line 220 may be oriented straight ahead) and a vertical
angle of 0.degree. (e.g., reference line 220 may have an
inclination of 0.degree.), or reference line 220 may have a
non-zero horizontal angle or a non-zero inclination (e.g., a
vertical angle of +10.degree. or -10.degree.). In FIG. 2, if the
scan pattern 200 has a 60.degree..times.15.degree. field of regard,
then scan pattern 200 covers a .+-.30.degree. horizontal range with
respect to reference line 220 and a .+-.7.5.degree. vertical range
with respect to reference line 220. Additionally, optical beam 125
in FIG. 2 has an orientation of approximately -15.degree.
horizontal and +3.degree. vertical with respect to reference line
220. Optical beam 125 may be referred to as having an azimuth of
-15.degree. and an altitude of +3.degree. relative to reference
line 220. In particular embodiments, an azimuth (which may be
referred to as an azimuth angle) may represent a horizontal angle
with respect to reference line 220, and an altitude (which may be
referred to as an altitude angle, elevation, or elevation angle)
may represent a vertical angle with respect to reference line
220.
[0083] In particular embodiments, a scan pattern 200 may include
multiple pixels 210, and each pixel 210 may be associated with one
or more laser pulses or one or more distance measurements.
Additionally, a scan pattern 200 may include multiple scan lines
230, where each scan line represents one scan across at least part
of a field of regard, and each scan line 230 may include multiple
pixels 210. In FIG. 2, scan line 230 includes five pixels 210 and
corresponds to an approximately horizontal scan across the FOR from
right to left, as viewed from the lidar system 100. In particular
embodiments, a cycle of scan pattern 200 may include a total of
P.sub.x.times.P.sub.y pixels 210 (e.g., a two-dimensional
distribution of P.sub.x by P.sub.y pixels). As an example, scan
pattern 200 may include a distribution with dimensions of
approximately 100-2,000 pixels 210 along a horizontal direction and
approximately 4-400 pixels 210 along a vertical direction. As
another example, scan pattern 200 may include a distribution of
1,000 pixels 210 along the horizontal direction by 64 pixels 210
along the vertical direction (e.g., the frame size is 1000.times.64
pixels) for a total of 64,000 pixels per cycle of scan pattern 200.
In particular embodiments, the number of pixels 210 along a
horizontal direction may be referred to as a horizontal resolution
of scan pattern 200, and the number of pixels 210 along a vertical
direction may be referred to as a vertical resolution. As an
example, scan pattern 200 may have a horizontal resolution of
greater than or equal to 100 pixels 210 and a vertical resolution
of greater than or equal to 4 pixels 210. As another example, scan
pattern 200 may have a horizontal resolution of 100-2,000 pixels
210 and a vertical resolution of 4-400 pixels 210.
[0084] In particular embodiments, a pixel 210 may refer to a data
element that includes (i) distance information (e.g., a distance
from a lidar system 100 to a target 130 from which an associated
pulse of light was scattered) or (ii) an elevation angle and an
azimuth angle associated with the pixel (e.g., the elevation and
azimuth angles along which the associated pulse of light was
emitted). Each pixel 210 may be associated with a distance (e.g., a
distance to a portion of a target 130 from which an associated
laser pulse was scattered) or one or more angular values. As an
example, a pixel 210 may be associated with a distance value and
two angular values (e.g., an azimuth and altitude) that represent
the angular location of the pixel 210 with respect to the lidar
system 100. A distance to a portion of target 130 may be determined
based at least in part on a time-of-flight measurement for a
corresponding pulse. An angular value (e.g., an azimuth or
altitude) may correspond to an angle (e.g., relative to reference
line 220) of output beam 125 (e.g., when a corresponding pulse is
emitted from lidar system 100) or an angle of input beam 135 (e.g.,
when an input signal is received by lidar system 100). In
particular embodiments, an angular value may be determined based at
least in part on a position of a component of scanner 120. As an
example, an azimuth or altitude value associated with a pixel 210
may be determined from an angular position of one or more
corresponding scanning mirrors of scanner 120.
[0085] FIG. 3 illustrates an example lidar system 100 with an
example rotating polygon mirror 301. In particular embodiments, a
scanner 120 may include a polygon mirror 301 configured to scan
output beam 125 along a first direction and a scanning mirror 302
configured to scan output beam 125 along a second direction
different from the first direction (e.g., the first and second
directions may be approximately orthogonal to one another, or the
second direction may be oriented at any suitable non-zero angle
with respect to the first direction). In the example of FIG. 3,
scanner 120 includes two scanning mirrors: (1) a polygon mirror 301
that rotates along the .THETA..sub.x direction and (2) a scanning
mirror 302 that oscillates back and forth along the .THETA..sub.y
direction. The output beam 125 from light source 110, which passes
alongside mirror 115, is reflected by reflecting surface 320 of
scan mirror 302 and is then reflected by a reflecting surface
(e.g., surface 320A, 320B, 320C, or 320D) of polygon mirror 301.
Scattered light from a target 130 returns to the lidar system 100
as input beam 135. The input beam 135 reflects from polygon mirror
301, scan mirror 302, and mirror 115, which directs input beam 135
through focusing lens 330 and to the detector 340 of receiver 140.
The detector 340 may be a PN photodiode, a PIN photodiode, an APD,
a SPAD, or any other suitable detector. A reflecting surface 320
(which may be referred to as a reflective surface) may include a
reflective metallic coating (e.g., gold, silver, or aluminum) or a
reflective dielectric coating, and the reflecting surface 320 may
have any suitable reflectivity R at an operating wavelength of the
light source 110 (e.g., R greater than or equal to 70%, 80%, 90%,
95%, 98%, or 99%).
[0086] In particular embodiments, a polygon mirror 301 may be
configured to rotate along a .THETA..sub.x or .THETA..sub.y
direction and scan output beam 125 along a substantially horizontal
or vertical direction, respectively. A rotation along a
.THETA..sub.x direction may refer to a rotational motion of mirror
301 that results in output beam 125 scanning along a substantially
horizontal direction. Similarly, a rotation along a .THETA..sub.y
direction may refer to a rotational motion that results in output
beam 125 scanning along a substantially vertical direction. In FIG.
3, mirror 301 is a polygon mirror that rotates along the
.THETA..sub.x direction and scans output beam 125 along a
substantially horizontal direction, and mirror 302 pivots along the
.THETA..sub.y direction and scans output beam 125 along a
substantially vertical direction. In particular embodiments, a
polygon mirror 301 may be configured to scan output beam 125 along
any suitable direction. As an example, a polygon mirror 301 may
scan output beam 125 at any suitable angle with respect to a
horizontal or vertical direction, such as for example, at an angle
of approximately 0.degree., 10.degree., 20.degree., 30.degree.,
45.degree., 60.degree., 70.degree., 80.degree., or 90.degree. with
respect to a horizontal or vertical direction.
[0087] In particular embodiments, a polygon mirror 301 may refer to
a multi-sided object having reflective surfaces 320 on two or more
of its sides or faces. As an example, a polygon mirror may include
any suitable number of reflective faces (e.g., 2, 3, 4, 5, 6, 7, 8,
or 10 faces), where each face includes a reflective surface 320. A
polygon mirror 301 may have a cross-sectional shape of any suitable
polygon, such as for example, a triangle (with three reflecting
surfaces 320), square (with four reflecting surfaces 320), pentagon
(with five reflecting surfaces 320), hexagon (with six reflecting
surfaces 320), heptagon (with seven reflecting surfaces 320), or
octagon (with eight reflecting surfaces 320). In FIG. 3, the
polygon mirror 301 has a substantially square cross-sectional shape
and four reflecting surfaces (320A, 320B, 320C, and 320D). The
polygon mirror 301 in FIG. 3 may be referred to as a square mirror,
a cube mirror, or a four-sided polygon mirror. In FIG. 3, the
polygon mirror 301 may have a shape similar to a cube, cuboid, or
rectangular prism. Additionally, the polygon mirror 301 may have a
total of six sides, where four of the sides include faces with
reflective surfaces (320A, 320B, 320C, and 320D).
[0088] In particular embodiments, a polygon mirror 301 may be
continuously rotated in a clockwise or counter-clockwise rotation
direction about a rotation axis of the polygon mirror 301. The
rotation axis may correspond to a line that is perpendicular to the
plane of rotation of the polygon mirror 301 and that passes through
the center of mass of the polygon mirror 301. In FIG. 3, the
polygon mirror 301 rotates in the plane of the drawing, and the
rotation axis of the polygon mirror 301 is perpendicular to the
plane of the drawing. An electric motor may be configured to rotate
a polygon mirror 301 at a substantially fixed frequency (e.g., a
rotational frequency of approximately 1 Hz (or 1 revolution per
second), 10 Hz, 50 Hz, 100 Hz, 500 Hz, or 1,000 Hz). As an example,
a polygon mirror 301 may be mechanically coupled to an electric
motor (e.g., a synchronous electric motor) which is configured to
spin the polygon mirror 301 at a rotational speed of approximately
160 Hz (or, 9600 revolutions per minute (RPM)).
[0089] In particular embodiments, output beam 125 may be reflected
sequentially from the reflective surfaces 320A, 320B, 320C, and
320D as the polygon mirror 301 is rotated. This results in the
output beam 125 being scanned along a particular scan axis (e.g., a
horizontal or vertical scan axis) to produce a sequence of scan
lines, where each scan line corresponds to a reflection of the
output beam 125 from one of the reflective surfaces of the polygon
mirror 301. In FIG. 3, the output beam 125 reflects off of
reflective surface 320A to produce one scan line. Then, as the
polygon mirror 301 rotates, the output beam 125 reflects off of
reflective surfaces 320B, 320C, and 320D to produce a second,
third, and fourth respective scan line. In particular embodiments,
a lidar system 100 may be configured so that the output beam 125 is
first reflected from polygon mirror 301 and then from scan mirror
302 (or vice versa). As an example, an output beam 125 from light
source 110 may first be directed to polygon mirror 301, where it is
reflected by a reflective surface of the polygon mirror 301, and
then the output beam 125 may be directed to scan mirror 302, where
it is reflected by reflective surface 320 of the scan mirror 302.
In the example of FIG. 3, the output beam 125 is reflected from the
polygon mirror 301 and the scan mirror 302 in the reverse order. In
FIG. 3, the output beam 125 from light source 110 is first directed
to the scan mirror 302, where it is reflected by reflective surface
320, and then the output beam 125 is directed to the polygon mirror
301, where it is reflected by reflective surface 320A.
[0090] FIG. 4 illustrates an example light-source field of view
(FOV.sub.L) and receiver field of view (FOV.sub.R) for a lidar
system 100. A light source 110 of lidar system 100 may emit pulses
of light as the FOV.sub.L and FOV.sub.R are scanned by scanner 120
across a field of regard (FOR). In particular embodiments, a
light-source field of view may refer to an angular cone illuminated
by the light source 110 at a particular instant of time. Similarly,
a receiver field of view may refer to an angular cone over which
the receiver 140 may receive or detect light at a particular
instant of time, and any light outside the receiver field of view
may not be received or detected. As an example, as the light-source
field of view is scanned across a field of regard, a portion of a
pulse of light emitted by the light source 110 may be sent
downrange from lidar system 100, and the pulse of light may be sent
in the direction that the FOV.sub.L is pointing at the time the
pulse is emitted. The pulse of light may scatter off a target 130,
and the receiver 140 may receive and detect a portion of the
scattered light that is directed along or contained within the
FOV.sub.R.
[0091] In particular embodiments, scanner 120 may be configured to
scan both a light-source field of view and a receiver field of view
across a field of regard of the lidar system 100. Multiple pulses
of light may be emitted and detected as the scanner 120 scans the
FOV.sub.L and FOV.sub.R across the field of regard of the lidar
system 100 while tracing out a scan pattern 200. In particular
embodiments, the light-source field of view and the receiver field
of view may be scanned synchronously with respect to one another,
so that as the FOV.sub.L is scanned across a scan pattern 200, the
FOV.sub.R follows substantially the same path at the same scanning
speed. Additionally, the FOV.sub.L and FOV.sub.R may maintain the
same relative position to one another as they are scanned across
the field of regard. As an example, the FOV.sub.L may be
substantially overlapped with or centered inside the FOV.sub.R (as
illustrated in FIG. 4), and this relative positioning between
FOV.sub.L and FOV.sub.R may be maintained throughout a scan. As
another example, the FOV.sub.R may lag behind the FOV.sub.L by a
particular, fixed amount throughout a scan (e.g., the FOV.sub.R may
be offset from the FOV.sub.L in a direction opposite the scan
direction).
[0092] In particular embodiments, the FOV.sub.L may have an angular
size or extent .THETA..sub.L that is substantially the same as or
that corresponds to the divergence of the output beam 125, and the
FOV.sub.R may have an angular size or extent .THETA..sub.R that
corresponds to an angle over which the receiver 140 may receive and
detect light. In particular embodiments, the receiver field of view
may be any suitable size relative to the light-source field of
view. As an example, the receiver field of view may be smaller
than, substantially the same size as, or larger than the angular
extent of the light-source field of view. In particular
embodiments, the light-source field of view may have an angular
extent of less than or equal to 50 milliradians, and the receiver
field of view may have an angular extent of less than or equal to
50 milliradians. The FOV.sub.L may have any suitable angular extent
.THETA..sub.L, such as for example, approximately 0.1 mrad, 0.2
mrad, 0.5 mrad, 1 mrad, 1.5 mrad, 2 mrad, 3 mrad, 5 mrad, 10 mrad,
20 mrad, 40 mrad, or 50 mrad. Similarly, the FOV.sub.R may have any
suitable angular extent .THETA..sub.R, such as for example,
approximately 0.1 mrad, 0.2 mrad, 0.5 mrad, 1 mrad, 1.5 mrad, 2
mrad, 3 mrad, 5 mrad, 10 mrad, 20 mrad, 40 mrad, or 50 mrad. In
particular embodiments, the light-source field of view and the
receiver field of view may have approximately equal angular
extents. As an example, .THETA..sub.L and .THETA..sub.R may both be
approximately equal to 1 mrad, 2 mrad, or 4 mrad. In particular
embodiments, the receiver field of view may be larger than the
light-source field of view, or the light-source field of view may
be larger than the receiver field of view. As an example,
.THETA..sub.L may be approximately equal to 3 mrad, and
.THETA..sub.R may be approximately equal to 4 mrad. As another
example, .THETA..sub.R may be approximately L times larger than
.THETA..sub.L, where L is any suitable factor, such as for example,
1.1, 1.2, 1.5, 2, 3, 5, or 10.
[0093] In particular embodiments, a pixel 210 may represent or may
correspond to a light-source field of view or a receiver field of
view. As the output beam 125 propagates from the light source 110,
the diameter of the output beam 125 (as well as the size of the
corresponding pixel 210) may increase according to the beam
divergence .THETA..sub.L. As an example, if the output beam 125 has
a .THETA..sub.L of 2 mrad, then at a distance of 100 m from the
lidar system 100, the output beam 125 may have a size or diameter
of approximately 20 cm, and a corresponding pixel 210 may also have
a corresponding size or diameter of approximately 20 cm. At a
distance of 200 m from the lidar system 100, the output beam 125
and the corresponding pixel 210 may each have a diameter of
approximately 40 cm.
[0094] FIG. 5 illustrates an example unidirectional scan pattern
200 that includes multiple pixels 210 and multiple scan lines 230.
In particular embodiments, scan pattern 200 may include any
suitable number of scan lines 230 (e.g., approximately 1, 2, 5, 10,
20, 50, 100, 500, or 1,000 scan lines), and each scan line 230 of a
scan pattern 200 may include any suitable number of pixels 210
(e.g., 1, 2, 5, 10, 20, 50, 100, 200, 500, 1,000, 2,000, or 5,000
pixels). The scan pattern 200 illustrated in FIG. 5 includes eight
scan lines 230, and each scan line 230 includes approximately 16
pixels 210. In particular embodiments, a scan pattern 200 where the
scan lines 230 are scanned in two directions (e.g., alternately
scanning from right to left and then from left to right) may be
referred to as a bidirectional scan pattern 200, and a scan pattern
200 where the scan lines 230 are scanned in the same direction may
be referred to as a unidirectional scan pattern 200. The scan
pattern 200 in FIG. 2 may be referred to as a bidirectional scan
pattern, and the scan pattern 200 in FIG. 5 may be referred to as a
unidirectional scan pattern 200 where each scan line 230 travels
across the FOR in substantially the same direction (e.g.,
approximately from left to right as viewed from the lidar system
100). In particular embodiments, scan lines 230 of a unidirectional
scan pattern 200 may be directed across a FOR in any suitable
direction, such as for example, from left to right, from right to
left, from top to bottom, from bottom to top, or at any suitable
angle (e.g., at a 0.degree., 5.degree.. 10.degree., 30.degree., or
45.degree. angle) with respect to a horizontal or vertical axis. In
particular embodiments, each scan line 230 in a unidirectional scan
pattern 200 may be a separate line that is not directly connected
to a previous or subsequent scan line 230.
[0095] In particular embodiments, a unidirectional scan pattern 200
may be produced by a scanner 120 that includes a polygon mirror
(e.g., polygon mirror 301 of FIG. 3), where each scan line 230 is
associated with a particular reflective surface 320 of the polygon
mirror. As an example, reflective surface 320A of polygon mirror
301 in FIG. 3 may produce scan line 230A in FIG. 5. Similarly, as
the polygon mirror 301 rotates, reflective surfaces 320B, 320C, and
320D may successively produce scan lines 230B, 230C, and 230D,
respectively. Additionally, for a subsequent revolution of the
polygon mirror 301, the scan lines 230A', 230B', 230C', and 230D'
may be successively produced by reflections of the output beam 125
from reflective surfaces 320A, 320B, 320C, and 320D, respectively.
In particular embodiments, N successive scan lines 230 of a
unidirectional scan pattern 200 may correspond to one full
revolution of a N-sided polygon mirror. As an example, the four
scan lines 230A, 230B, 230C, and 230D in FIG. 5 may correspond to
one full revolution of the four-sided polygon mirror 301 in FIG. 3.
Additionally, a subsequent revolution of the polygon mirror 301 may
produce the next four scan lines 230A', 230B', 230C', and 230D' in
FIG. 5.
[0096] FIG. 6 illustrates an example lidar system 100 with a light
source 110 that emits pulses of light 400 and local-oscillator (LO)
light 430. The lidar system 100 in FIG. 6 includes a light source
110, a scanner 120, a receiver 140, and a controller 150 (which may
be referred to as a processor). The receiver 140 includes a
detector 340, an amplifier 350, a pulse-detection circuit 365, and
a frequency-detection circuit 600. The lidar system 100 illustrated
in FIG. 6 may be referred to as a hybrid pulsed/coherent lidar
system in which the light source 110 emits LO light 430 and pulses
of light 400, where each emitted pulse of light 400 is coherent
with a corresponding portion of the LO light 430. Additionally, the
receiver 140 in a hybrid pulsed/coherent lidar system may be
configured to detect the LO light 430 and a received pulse of light
410, where the LO light 430 and the received pulse of light 410
(which includes scattered light from one of the emitted pulses of
light 400) are coherently mixed together at the receiver 140. The
LO light 430 may be referred to as a local-oscillator optical
signal or a LO optical signal.
[0097] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may include a light source 110 configured to emit pulses
of light 400 and LO light 430. The emitted pulses of light 400 may
be part of an output beam 125 that is scanned by a scanner 120
across a field of regard of the lidar system 100, and the LO light
430 may be sent to a receiver 140 of the lidar system 100. The
light source 110 may include a seed laser that produces seed light
440 and the LO light 430. Additionally, the light source 110 may
include an optical amplifier that amplifies the seed light to
produce the emitted pulses of light 400. For example, the optical
amplifier may be a pulsed optical amplifier that amplifies temporal
portions of the seed light to produce the emitted pulses of light
400, where each amplified temporal portion of the seed light
corresponds to one of the emitted pulses of light 400. The pulses
of light 400 emitted by the light source 110 may have one or more
of the following optical characteristics: a wavelength between 900
nm and 2000 nm; a pulse energy between 0.01 .mu.J and 100 .mu.J; a
pulse repetition frequency between 80 kHz and 10 MHz; and a pulse
duration between 0.1 ns and 100 ns. For example, the light source
110 may emit pulses of light 400 with a wavelength of approximately
1550 nm, a pulse energy of approximately 0.5 pJ per pulse, a pulse
repetition frequency of approximately 750 kHz, and a pulse duration
of approximately 3 ns. As another example, the light source 110 may
emit pulses of light with a wavelength from approximately 1500 nm
to approximately 1510 nm.
[0098] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may include a scanner 120 configured to scan an output
beam 125 (which includes emitted pulses of light 400) across a
field of regard of the lidar system 100. The scanner 120 may
receive the output beam 125 from a light source 110, and the
scanner 120 may include one or more scanning mirrors configured to
scan the output beam 125. In addition to scanning the output beam
125, the scanner may also scan a FOV of the detector 340 across the
field of regard so that the output beam 125 (which corresponds to
the light-source FOV) and the detector FOV are scanned
synchronously, where the scanning speeds of the light-source FOV
and the detector FOV are equal. Additionally, the light-source FOV
and the detector FOV may have the same relative position to one
another as they are scanned across the field of regard (e.g., the
light-source FOV and the detector FOV may be fully or partially
overlapped, and the amount of overlap may remain approximately
fixed as they are scanned). Alternatively, the lidar system 100 may
be configured so that only the output beam 125 is scanned, and the
detector has a static FOV that is not scanned. In this case, the
input beam 135 (which includes received pulses of light 410) may
bypass the scanner 120 and be directed to the receiver 140 without
passing through the scanner 120.
[0099] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may include an optical combiner 420 configured to
optically combine LO light 430 with a received pulse of light 410.
The optical combiner 420 in FIG. 6 may be a free-space optical beam
combiner that reflects at least part of the LO light 430 and
transmits at least part of the input beam 135 so that the LO light
430 and the input beam 135 are spatially overlapped and propagate
substantially coaxially along the same path to the detector 340. As
another example, the combiner 420 in FIG. 6 may be a mirror that
reflects the LO light 430 and directs it to the detector 340, where
it is combined with the input beam 135. As another example, a
combiner 420 may include an integrated-optic component or a
fiber-optic component that spatially overlaps the LO light 430 and
the input beam 135 so that the LO light 430 and the input beam 135
propagate together in a waveguide or in a core of an optical
fiber.
[0100] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may include a receiver 140 that detects LO light 430 and
received pulses of light 410. A received pulse of light 410 may
include light from one of the emitted pulses of light 400 that is
scattered by a target 130 located a distance from the lidar system
100. The receiver 140 may include one or more detectors 340, and
the LO light 430 and a received pulse of light 410 may be
coherently mixed together at one or more of the detectors 340. One
or more of the detectors 340 may produce photocurrent signals that
correspond to the coherent mixing of the LO light 430 and the
received pulse of light 410. The lidar system 100 in FIG. 6
includes a receiver 140 with one detector 340 that receives the LO
light 430 and the pulse of light 410, which are coherently mixed
together at the detector 340. In response to the coherent mixing of
the received LO light 430 and pulse of light 410, the detector 340
produces a photocurrent signal i that is amplified by an electronic
amplifier 350.
[0101] In particular embodiments, a receiver 140 may include a
pulse-detection circuit 365 that determines a time-of-arrival for a
received pulse of light 410. The time-of-arrival for a received
pulse of light 410 may correspond to a time associated with a
rising edge, falling edge, peak, or temporal center of the received
pulse of light 410. The time-of-arrival may be determined based at
least in part on a photocurrent signal i produced by a detector 340
of the receiver 140. For example, a photocurrent signal i may
include a pulse of current corresponding to the received pulse of
light 410, and the electronic amplifier 350 may produce a voltage
signal 360 with a voltage pulse that corresponds to the pulse of
current. The pulse-detection circuit 365 (or a controller 150
coupled to the pulse-detection circuit) may determine the
time-of-arrival for the received pulse of light 410 based on a
characteristic of the voltage pulse (e.g., based on a time
associated with a rising edge, falling edge, peak, or temporal
center of the voltage pulse). For example, the pulse-detection
circuit 365 may receive an electronic trigger signal (e.g., from
the light source 110 or the controller 150) when a pulse of light
400 is emitted, and the pulse-detection circuit 365 may determine
the time-of-arrival for the received pulse of light 410 based on a
time associated with an edge, peak, or temporal center of the
voltage signal 360. The time-of-arrival may be determined based on
a difference between a time when the pulse 400 is emitted and a
time when the received pulse 410 is detected.
[0102] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may include a processor (e.g., controller 150) that
determines the distance to a target 130 based at least in part on a
time-of-arrival for a received pulse of light 410. The
time-of-arrival for the received pulse of light 410 may correspond
to a round-trip time (.DELTA.T) for at least a portion of an
emitted pulse of light 400 to travel to the target 130 and back to
the lidar system 100, where the portion of the emitted pulse of
light 400 that travels back to the target 130 corresponds to the
received pulse of light 410. The distance D to the target 130 may
be determined from the expression D=c.DELTA.T/2. For example, if
the pulse-detection circuit 365 determines that the time .DELTA.T
between emission of optical pulse 400 and receipt of optical pulse
410 is 1 .mu.s, then the controller 150 may determine that the
distance to the target 130 is approximately 150 m. In particular
embodiments, a round-trip time may be determined by a receiver 140,
by a controller 150, or by a receiver 140 and controller 150
together. For example, a receiver 140 may determine a round-trip
time by subtracting a time when a pulse 400 is emitted from a time
when a received pulse 410 is detected. As another example, a
receiver 140 may determine a time when a pulse 400 is emitted and a
time when a received pulse 410 is detected. These values may be
sent to a controller 150, and the controller 150 may determine a
round-trip time by subtracting the time when the pulse 400 is
emitted from the time when the received pulse 410 is detected.
[0103] In particular embodiments, a controller 150 of a lidar
system 100 may be coupled to one or more components of the lidar
system 100 via one or more data links 425. Each link 425 in FIG. 6
represents a data link that couples the controller 150 to another
component of the lidar system 100 (e.g., light source 110, scanner
120, receiver 140, pulse-detection circuit 365, or
frequency-detection circuit 600). Each data link 425 may include
one or more electrical links, one or more wireless links, or one or
more optical links, and the data links 425 may be used to send
data, signals, or commands to or from the controller 150. For
example, the controller 150 may send a command via a link 425 to
the light source 110 instructing the light source 110 to emit a
pulse of light 400. As another example, the pulse-detection circuit
365 may send a signal via a link 425 to the controller with
information about a received pulse of light 410 (e.g., a
time-of-arrival for the received pulse of light 410). Additionally,
the controller 150 may be coupled via a link (not illustrated in
FIG. 6) to a processor of an autonomous-vehicle driving system. The
autonomous-vehicle processor may receive point-cloud data from the
controller 150 and may make driving decisions based on the received
point-cloud data.
[0104] FIG. 7 illustrates an example receiver 140 and an example
voltage signal 360 corresponding to a received pulse of light 410.
A light source 110 of a lidar system 100 may emit a pulse of light
400, and a receiver 140 may be configured to detect a combined beam
422. The combined beam 422 in FIG. 7 includes LO light 430 and
input light 135, where the input light 135 includes one or more
received pulses of light 410. In particular embodiments, a receiver
140 of a lidar system 100 may include one or more detectors 340,
one or more amplifiers 350, one or more pulse-detection circuits
365, or one or more frequency-detection circuits 600. A
pulse-detection circuit 365 may include one or more comparators 370
or one or more time-to-digital converters (TDCs) 380. A
frequency-detection circuit 600 may include one or more electronic
filters 610 or one or more electronic amplitude detectors 620.
[0105] The receiver 140 illustrated in FIG. 7 includes a detector
340 configured to receive a combined beam 422 and produce a
photocurrent i that corresponds to the coherent mixing of the LO
light 430 and a received pulse of light 410 (which is part of the
input light 135). The photocurrent i produced by the detector 340
may be referred to as a photocurrent signal or an
electrical-current signal. The detector 340 may include an APD, PN
photodiode, or PIN photodiode. For example, the detector 340 may
include a silicon APD or PIN photodiode configured to detect light
at an 800-1100 nm operating wavelength of a lidar system 100, or
the detector 340 may include an InGaAs APD or PIN photodiode
configured to detect light at a 1200-1600 nm operating wavelength.
In FIG. 7, the detector 340 is coupled to an electronic amplifier
350 configured to receive the photocurrent i and produce a voltage
signal 360 that corresponds to the received photocurrent. For
example, the detector 340 may be an APD that produces a pulse of
photocurrent in response to coherent mixing of LO light 430 and a
received pulse of light 410, and the voltage signal 360 may be an
analog voltage pulse that corresponds to the pulse of photocurrent.
The amplifier 350 may include a transimpedance amplifier configured
to receive the photocurrent i and amplify the photocurrent to
produce a voltage signal that corresponds to the photocurrent
signal. Additionally, the amplifier 350 may include a voltage
amplifier that amplifies the voltage signal or an electronic filter
(e.g., a low-pass or high-pass filter) that filters the
photocurrent or the voltage signal.
[0106] In FIG. 7, the voltage signal 360 produced by the amplifier
350 is coupled to a pulse-detection circuit 365 and a
frequency-detection circuit 600. The pulse-detection circuit
includes N comparators (comparators 370-1, 370-2, . . . , 370-N),
and each comparator is supplied with a particular threshold or
reference voltage (V.sub.T1, V.sub.T2, . . . , V.sub.TN). For
example, the pulse-detection circuit 365 may include N=10
comparators, and the threshold voltages may be set to 10 values
between 0 volts and 1 volt (e.g., V.sub.T1=0.1 V, V.sub.T2=0.2 V,
and V.sub.T10=1.0 V). A comparator may produce an electrical-edge
signal (e.g., a rising or falling electrical edge) when the voltage
signal 360 rises above or falls below a particular threshold
voltage. For example, comparator 370-2 may produce a rising edge
when the voltage signal 360 rises above the threshold voltage
V.sub.T2. Additionally or alternatively, comparator 370-2 may
produce a falling edge when the voltage signal 360 falls below the
threshold voltage V.sub.T2.
[0107] The pulse-detection circuit 365 in FIG. 7 includes N
time-to-digital converters (TDCs 380-1, 380-2, . . . , 380-N), and
each comparator is coupled to one of the TDCs. Each comparator-TDC
pair in FIG. 7 (e.g., comparator 370-1 and TDC 380-1) may be
referred to as a threshold detector. A comparator may provide an
electrical-edge signal to a corresponding TDC, and the TDC may act
as a timer that produces an electrical output signal (e.g., a
digital signal, a digital word, or a digital value) that represents
a time when the edge signal is received from the comparator. For
example, if the voltage signal 360 rises above the threshold
voltage VTi, then the comparator 370-1 may produce a rising-edge
signal that is supplied to the input of TDC 380-1, and the TDC
380-1 may produce a digital time value corresponding to a time when
the edge signal was received by TDC 380-1. The digital time value
may be referenced to the time when a pulse of light is emitted, and
the digital time value may correspond to or may be used to
determine a round-trip time for the pulse of light to travel to a
target 130 and back to the lidar system 100. Additionally, if the
voltage signal 360 subsequently falls below the threshold voltage
VTi, then the comparator 370-1 may produce a falling-edge signal
that is supplied to the input of TDC 380-1, and the TDC 380-1 may
produce a digital time value corresponding to a time when the edge
signal was received by TDC 380-1.
[0108] In particular embodiments, a pulse-detection output signal
may be an electrical signal that corresponds to a received pulse of
light 410. For example, the pulse-detection output signal in FIG. 7
may be a digital signal that corresponds to the analog voltage
signal 360, which in turn corresponds to the photocurrent signal i,
which in turn corresponds to a received pulse of light 410. If an
input light signal 135 includes a received pulse of light 410, the
pulse-detection circuit 365 may receive a voltage signal 360
(corresponding to the photocurrent i) and produce a pulse-detection
output signal that corresponds to the received pulse of light 410.
The pulse-detection output signal may include one or more digital
time values from each of the TDCs 380 that received one or more
edge signals from a comparator 370, and the digital time values may
represent the analog voltage signal 360. The pulse-detection output
signal may be sent to a controller 150, and a time-of-arrival for
the received pulse of light 410 may be determined based at least in
part on the one or more time values produced by the TDCs. For
example, the time-of-arrival may be determined from a time
associated with the peak (e.g., Vpea) of the voltage signal 360 or
from a temporal center of the voltage signal 360. Alternatively,
the time-of-arrival may be determined from a time associated with a
rising edge of the voltage signal 360. The pulse-detection output
signal in FIG. 7 may correspond to the electrical output signal 145
in FIG. 1.
[0109] In particular embodiments, a pulse-detection output signal
may include one or more digital values that correspond to a time
interval between (1) a time when a pulse of light 400 is emitted
and (2) a time when a received pulse of light 410 is detected by a
receiver 140. The pulse-detection output signal in FIG. 7 may
include digital values from each of the TDCs that receive an edge
signal from a comparator, and each digital value may represent a
time interval between the emission of an optical pulse 400 by a
light source 110 and the receipt of an edge signal from a
comparator. For example, a light source 110 may emit a pulse of
light 400 that is scattered by a target 130, and a receiver 140 may
receive a portion of the scattered pulse of light as an input pulse
of light 410. When the light source emits the pulse of light 400, a
count value of the TDCs may be reset to zero counts. Alternatively,
the TDCs in receiver 140 may accumulate counts continuously over
two or more pulse periods (e.g., for 10, 100, 1,000, 10,000, or
100,000 pulse periods), and when a pulse of light 400 is emitted,
the current TDC count may be stored in memory. After the pulse of
light 400 is emitted, the TDCs may accumulate counts that
correspond to elapsed time (e.g., the TDCs may count in terms of
clock cycles or some fraction of clock cycles).
[0110] In FIG. 7, when TDC 380-1 receives an edge signal from
comparator 370-1, the TDC 380-1 may produce a digital signal that
represents the time interval between emission of the pulse of light
400 and receipt of the edge signal. For example, the digital signal
may include a digital value that corresponds to the number of clock
cycles that elapsed between emission of the pulse of light 400 and
receipt of the edge signal. Alternatively, if the TDC 380-1
accumulates counts over multiple pulse periods, then the digital
signal may include a digital value that corresponds to the TDC
count at the time of receipt of the edge signal. The
pulse-detection output signal may include digital values
corresponding to one or more times when a pulse of light 400 was
emitted and one or more times when a TDC received an edge signal. A
pulse-detection output signal from a pulse-detection circuit 365
may correspond to a received pulse of light 410 and may include
digital values from each of the TDCs that receive an edge signal
from a comparator. The pulse-detection output signal may be sent to
a controller 150, and the controller may determine the distance to
the target 130 based at least in part on the pulse-detection output
signal. Additionally or alternatively, the controller 150 may
determine an optical characteristic of a received pulse of light
410 based at least in part on the pulse-detection output signal
received from the TDCs of a pulse-detection circuit 365.
[0111] In particular embodiments, a receiver 140 of a lidar system
100 may include one or more analog-to-digital converters (ADCs). As
an example, instead of including multiple comparators and TDCs, a
receiver 140 may include an ADC that receives a voltage signal 360
from amplifier 350 and produces a digital representation of the
voltage signal 360. Although this disclosure describes or
illustrates example receivers 140 that include one or more
comparators 370 and one or more TDCs 380, a receiver 140 may
additionally or alternatively include one or more ADCs. As an
example, in FIG. 7, instead of the N comparators 370 and N TDCs
380, the receiver 140 may include an ADC configured to receive the
voltage signal 360 and produce a digital output signal that
includes digitized values that correspond to the voltage signal
360.
[0112] The example voltage signal 360 illustrated in FIG. 7
corresponds to a received pulse of light 410. The voltage signal
360 may be an analog signal produced by an electronic amplifier 350
and may correspond to a pulse of light detected by the receiver 140
in FIG. 7. The voltage levels on the y-axis correspond to the
threshold voltages V.sub.T1, V.sub.T2, . . . , V.sub.TN of the
respective comparators 370-1, 370-2, . . . , 370-N. The time values
t.sub.1, t.sub.2, t.sub.3, . . . , t.sub.N-1 correspond to times
when the voltage signal 360 exceeds the corresponding threshold
voltages, and the time values t'.sub.1, t'.sub.2, t'.sub.3, . . . ,
t'.sub.N-1 correspond to times when the voltage signal 360 falls
below the corresponding threshold voltages. For example, at time
t.sub.1 when the voltage signal 360 exceeds the threshold voltage
V.sub.T1, comparator 370-1 may produce an edge signal, and TDC
380-1 may output a digital value corresponding to the time
t'.sub.1. Additionally, the TDC 380-1 may output a digital value
corresponding to the time t'.sub.1 when the voltage signal 360
falls below the threshold voltage V.sub.T1. Alternatively, the
receiver 140 may include an additional TDC (not illustrated in FIG.
7) configured to produce a digital value corresponding to time
t'.sub.1 when the voltage signal 360 falls below the threshold
voltage V.sub.T1. The pulse-detection output signal from
pulse-detection circuit 365 may include one or more digital values
that correspond to one or more of the time values t.sub.1, t.sub.2,
t.sub.3, . . . , t.sub.N-1 and t'.sub.1, t'.sub.2, t'.sub.3, . . .
, t'.sub.N-1. Additionally, the pulse-detection output signal may
also include one or more values corresponding to the threshold
voltages associated with the time values. Since the voltage signal
360 in FIG. 7 does not exceed the threshold voltage V.sub.TN, the
corresponding comparator 370-N may not produce an edge signal. As a
result, TDC 380-N may not produce a time value, or TDC 380-N may
produce a signal indicating that no edge signal was received.
[0113] In particular embodiments, a pulse-detection output signal
produced by a pulse-detection circuit 365 of a receiver 140 may
correspond to or may be used to determine an optical characteristic
of a received pulse of light 410 detected by the receiver 140. An
optical characteristic of a received pulse of light 410 may
correspond to a peak optical intensity, a peak optical power, an
average optical power, an optical energy, a shape or amplitude, a
temporal duration, or a temporal center of the received pulse of
light 410. For example, a pulse of light 410 detected by receiver
140 may have one or more of the following optical characteristics:
a peak optical power between 1 nanowatt and 10 watts; a pulse
energy between 1 attojoule and 10 nanojoules; and a pulse duration
between 0.1 ns and 50 ns. In particular embodiments, an optical
characteristic of a received pulse of light 410 may be determined
from a pulse-detection output signal provided by one or more TDCs
380 of a pulse-detection circuit 365 (e.g., as illustrated in FIG.
7), or an optical characteristic may be determined from a
pulse-detection output signal provided by one or more ADCs of a
pulse-detection circuit 365.
[0114] In particular embodiments, a peak optical power or peak
optical intensity of a received pulse of light 410 may be
determined from one or more values of a pulse-detection output
signal provided by a receiver 140. As an example, a controller 150
may determine the peak optical power of a received pulse of light
410 based on a peak voltage (V.sub.peak) of the voltage signal 360.
The controller 150 may use a formula or lookup table that
correlates a peak voltage of the voltage signal 360 with a value
for the peak optical power. In the example of FIG. 7, the peak
optical power of a pulse of light 410 may be determined from the
threshold voltage V.sub.T(N-1), which is approximately equal to the
peak voltage V.sub.peak of the voltage signal 360 (e.g., the
threshold voltage V.sub.T(N-1) may be associated with a pulse of
light 410 having a peak optical power of 10 mW). As another
example, a controller 150 may apply a curve-fit or interpolation
operation to the values of a pulse-detection output signal to
determine the peak voltage of the voltage signal 360, and this peak
voltage may be used to determine the corresponding peak optical
power of a received pulse of light 410.
[0115] In particular embodiments, an energy of a received pulse of
light 410 may be determined from one or more values of a
pulse-detection output signal. For example, a controller 150 may
perform a summation of digital values that correspond to a voltage
signal 360 to determine an area under the voltage-signal curve, and
the area under the voltage-signal curve may be correlated with a
pulse energy of a received pulse of light 410. As an example, the
approximate area under the voltage-signal curve in FIG. 7 may be
determined by subdividing the curve into M subsections (where M is
approximately the number of time values included in the
pulse-detection output signal) and adding up the areas of each of
the subsections (e.g., using a numerical integration technique such
as a Riemann sum, trapezoidal rule, or Simpson's rule). For
example, the approximate area A under the voltage-signal curve 360
in FIG. 7 may be determined from a Riemann sum using the expression
.DELTA.=.SIGMA..sub.k=1.sup.MV.sub.Tk.times..DELTA.t.sub.k, where
V.sub.Tk is a threshold voltage associated with the time value
t.sub.k, and .DELTA.t.sub.k is a width of the subsection associated
with time value t.sub.k. In the example of FIG. 7, the voltage
signal 360 may correspond to a received pulse of light 410 with a
pulse energy of 1 picojoule.
[0116] In particular embodiments, a duration of a received pulse of
light 410 may be determined from a duration or width of a
corresponding voltage signal 360. For example, the difference
between two time values of a pulse-detection output signal may be
used to determine a duration of a received pulse of light 410. In
the example of FIG. 7, the duration of the pulse of light 410
corresponding to voltage signal 360 may be determined from the
difference (t'.sub.3-t.sub.3), which may correspond to a received
pulse of light 410 with a pulse duration of 4 nanoseconds. As
another example, a controller 150 may apply a curve-fit or
interpolation operation to the values of the pulse-detection output
signal, and the duration of the pulse of light 410 may be
determined based on the curve-fit or interpolation. One or more of
the approaches for determining an optical characteristic of a
received pulse of light 410 as described herein may be implemented
using a receiver 140 that includes multiple comparators 370 and
TDCs 380 (as illustrated in FIG. 7) or using a receiver 140 that
includes one or more ADCs.
[0117] In FIG. 7, the voltage signal 360 produced by amplifier 350
is coupled to a frequency-detection circuit 600 as well as a
pulse-detection circuit 365. The pulse-detection circuit 365 may
provide a pulse-detection output signal that is used to determine
time-domain information for a received pulse of light 410 (e.g., a
time-of-arrival, duration, or energy of the received pulse of light
410), and the frequency-detection circuit 600 may provide
frequency-domain information for the received pulse of light 410.
For example, the frequency-detection output signal of the
frequency-detection circuit 600 may include amplitude information
for particular frequency components of the received pulse of light
410. The frequency-detection output signal may include the
amplitude of one or more frequency components of a spectral
signature of a received pulse of light 410, and this amplitude
information may be sent to a controller 150 for further processing.
For example, the controller 150 may determine, based at least in
part on the frequency-component information, whether a received
pulse of light is a valid received pulse of light 410 or an
interfering pulse of light.
[0118] In particular embodiments, a frequency-detection circuit 600
may include multiple parallel frequency-measurement channels, and
each frequency-measurement channel may include a filter 610 and a
corresponding amplitude detector 620. In FIG. 7, the
frequency-detection circuit 600 includes M electronic filters
(filters 610-1, 610-2, . . . , 610-M), where each filter is
associated with a particular frequency component (frequencies
f.sub.a, f.sub.b, . . . , f.sub.M). Each filter 610 in FIG. 7 may
include an electronic band-pass filter having a particular
pass-band center frequency and width. For example, filter 610-2 may
be a band-pass filter with a center frequency f.sub.b of 1 GHz and
a pass-band width of 20 MHz. Each filter 610 may include a passive
filter implemented with one or more passive electronic components
(e.g., one or more resistors, inductors, or capacitors).
Alternatively, each filter 610 may include an active filter that
includes one or more active electronic components (e.g., one or
more transistors or op-amps) along with one or more passive
components.
[0119] In addition to the M electronic filters 610, the
frequency-detection circuit 600 in FIG. 7 also includes M
electronic amplitude detectors (amplitude detectors 620-1, 620-2, .
. . , 620-M). An amplitude detector 620 may be configured to
provide an output signal that corresponds to an amplitude (e.g., a
peak value, a size, or an energy) of an electrical signal received
from a filter 610. For example, filter 610-M may receive voltage
signal 360 and provide to amplitude detector 620-M the portion of
the voltage signal 360 having a frequency component at or near the
frequency f.sub.M. The amplitude detector 620-M may produce a
digital or analog output signal that corresponds to the amplitude,
peak value, size, or energy of the signal associated with the
frequency component f.sub.M. Each amplitude detector 620 may
include a sample-and-hold circuit, a peak-detector circuit, an
integrator circuit, or an ADC. For example, amplitude detector
620-M may include a sample-and-hold circuit and an ADC. The
sample-and-hold circuit may produce an analog voltage corresponding
to the amplitude of a signal received from filter 610-M, and the
ADC may produce a digital signal that represents the analog
voltage.
[0120] A frequency-detection circuit 600 may include 1, 2, 4, 8,
10, 20, 50, or any other suitable number of filters 610 and
amplitude detectors 620, and each filter may have a center
frequency between approximately 10 MHz and approximately 50 GHz.
Additionally, each filter 610 may include a band-pass filter having
a pass-band with a frequency width of approximately 1 MHz, 10 MHz,
20 MHz, 50 MHz, 100 MHz, 200 MHz, or any other suitable frequency
width. For example, a frequency-detection circuit 600 may include
16 band-pass filters 610, each with a different center frequency
between 100 MHz and 1 GHz. As another example, a
frequency-detection circuit 600 may include four band-pass filters
610 with center frequencies of approximately 200 MHz, 400 MHz, 600
MHz, and 800 MHz, and each filter may have a pass-band with a
frequency width of approximately 20 MHz. A 400-MHz filter with a
20-MHz pass-band may pass or transmit frequency components from
approximately 390 MHz to approximately 410 MHz and may attenuate
frequency components outside of that frequency range.
[0121] FIG. 8 illustrates an example light source 110 that includes
a seed laser diode 450 and a semiconductor optical amplifier (SOA)
460. In particular embodiments, a light source 110 of a lidar
system 100 may include (i) a seed laser 450 that produces seed
light 440 and LO light 430 and (ii) a pulsed optical amplifier 460
that amplifies temporal portions of the seed light 440 to produce
emitted pulses of light 400. In the example of FIG. 8, the seed
laser is a seed laser diode 450 that produces seed light 440 and LO
light 430. The seed laser diode 450 may include a Fabry-Perot laser
diode, a quantum well laser, a DBR laser, a DFB laser, a VCSEL, a
quantum dot laser diode, or any other suitable type of laser diode.
In FIG. 8, the pulsed optical amplifier is a semiconductor optical
amplifier (SOA) 460 that emits a pulse of light 400 that is part of
the output beam 125. A SOA 460 may include a semiconductor optical
waveguide that receives the seed light 440 from the seed laser
diode 450 and amplifies a temporal portion of the seed light 440 as
it propagates through the waveguide to produce an emitted pulse of
light 400. A SOA 460 may have an optical power gain of 20 decibels
(dB), 25 dB, 30 dB, 35 dB, 40 dB, 45 dB, or any other suitable
optical power gain. For example, a SOA 460 may have a gain of 40
dB, and a temporal portion of seed light 440 with an energy of 20
.mu.J may be amplified by the SOA 460 to produce a pulse of light
400 with an energy of approximately 0.2 pJ. A light source 110 that
includes a seed laser diode 450 that supplies seed light 440 that
is amplified by a SOA 460 may be referred to as a master-oscillator
power-amplifier laser (MOPA laser) or a MOPA light source. The seed
laser diode 450 may be referred to as a master oscillator, and the
SOA 460 may be referred to as a power amplifier.
[0122] In particular embodiments, a light source 110 may include an
electronic driver 480 that (i) supplies electrical current to a
seed laser 450 and (ii) supplies electrical current to a SOA 460.
In FIG. 8, the electronic driver 480 supplies seed current I.sub.1
to the seed laser diode 450 to produce the seed light 440 and the
LO light 430. The seed current I.sub.1 supplied to the seed laser
diode 450 may be a substantially constant DC electrical current so
that the seed light 440 and the LO light 430 each include
continuous-wave (CW) light or light having a substantially constant
optical power. For example, the seed current I.sub.1 may include a
DC current of approximately 1 mA, 10 mA, 100 mA, 200 mA, 500 mA, or
any other suitable DC electrical current. Additionally or
alternatively, the seed current I.sub.1 may include a pulse of
electrical current so that the seed light 440 includes seed pulses
of light that are amplified by the SOA 460. The seed laser 450 may
be pulsed with a pulse of current having a duration that is long
enough so that the wavelength of the seed-laser light emitted by
the seed laser 450 (e.g., seed light 440 and LO light 430)
stabilizes or reaches a substantially constant value at some time
during the pulse. For example, the duration of the current pulse
may be between 50 ns and 2 .mu.s, and the SOA 460 may be configured
to amplify a 5-ns temporal portion of the seed light 440 to produce
the emitted pulse of light 400. The temporal portion of the seed
light 440 that is selected for amplification may be located in time
near the middle or end of the electrical current pulse to allow
sufficient time for the wavelength of the seed-laser light to
stabilize.
[0123] In FIG. 8, the electronic driver 480 supplies SOA current
I.sub.2 to the SOA 460, and the SOA current I.sub.2 provides
optical gain to temporal portions of the seed light 440 that
propagate through the waveguide of the SOA 460. The SOA current
I.sub.2 may include pulses of electrical current, where each pulse
of current causes the SOA 460 to amplify one temporal portion of
the seed light 440 to produce an emitted pulse of light 400. The
SOA current I.sub.2 may have a duration of approximately 0.5 ns, 1
ns, 2 ns, 5 ns, 10 ns, 20 ns, 50 ns, 100 ns, or any other suitable
duration. The SOA current I.sub.2 may have a peak amplitude of
approximately 1 A, 2 A, 5 A, 10 A, 20 A, 50 A, 100 A, 200 A, 500 A,
or any other suitable peak current. For example, the SOA current
I.sub.2 supplied to the SOA 460 may include a series of current
pulses having a duration of approximately 5-10 ns and a peak
current of approximately 100 A. The series of current pulses may
result in the emission of a corresponding series of pulses of light
400, and each emitted pulse of light 400 may have a duration that
is less than or equal to the duration of the corresponding
electrical current pulse. For example, an electronic driver 480 may
supply 5-ns duration current pulses to the SOA 460 at a repetition
frequency of 700 kHz. This may result in emitted pulses of light
400 that have a duration of approximately 4 ns and a pulse
repetition frequency of 700 kHz.
[0124] A pulsed optical amplifier may refer to an optical amplifier
that is operated in a pulsed mode so that the output beam 125
emitted by the optical amplifier includes pulses of light 400. For
example, a pulsed optical amplifier may include a SOA 460 that is
operated in a pulsed mode by supplying the SOA 460 with pulses of
current. The seed light 440 may include CW light or light having a
substantially constant optical power, and each pulse of current
supplied to the SOA 460 may amplify a temporal portion of seed
light to produce an emitted pulse of light 400. As another example,
a pulsed optical amplifier may include an optical amplifier along
with an optical modulator. The optical modulator may be an
acousto-optic modulator (AOM) or an electro-optic modulator (EOM)
operated in a pulsed mode so that the modulator selectively
transmits pulses of light. The SOA 460 may also be operated in a
pulsed mode in synch with the optical modulator to amplify the
temporal portions of the seed light, or the SOA 460 may be supplied
with substantially DC current to operate as a CW optical amplifier.
The optical modulator may be located between the seed laser diode
450 and the SOA 460, and the optical modulator may be operated in a
pulsed mode to transmit temporal portions of the seed light 440
which are then amplified by the SOA 460. Alternatively, the optical
modulator may be located after the SOA 460, and the optical
modulator may be operated in a pulsed mode to transmit the emitted
pulses of light 400.
[0125] The seed laser diode 450 illustrated in FIG. 8 includes a
front face 452 and aback face 451. The seed light 440 is emitted
from the front face 452 and directed to the input end 461 of the
SOA 460. The LO light 430 is emitted from the back face 451 and
directed to the receiver 140 of the lidar system 100. The seed
light 440 or the LO light 430 may be emitted as a free-space beam,
and a light source 110 may include one or more lenses (not
illustrated in FIG. 10) that (i) collimate the LO light 430 emitted
from the back face 451, (ii) collimate the seed light 440 emitted
from the front face 452, or (iii) focus the seed light 440 into the
SOA 460.
[0126] In particular embodiments a front face 452 or a back face
451 may include a discrete facet formed by a semiconductor-air
interface (e.g., a surface formed by cleaving or polishing a
semiconductor structure to form the seed laser diode 450).
Additionally, the front face 452 or the back face 451 may include a
dielectric coating that provides a reflectivity (at the seed-laser
operating wavelength) of between approximately 50% and
approximately 99.9%. For example, the back face 451 may have a
reflectivity of 90% to 99.9% at a wavelength of the LO light 430.
The average power of the LO light 430 emitted from the back face
451 may depend at least in part on the reflectivity of the back
face 451, and a value for the reflectivity of the back face 451 may
be selected to provide a particular average power of the LO light
430. For example, the back face 451 may be configured to have a
reflectivity between 90% and 99%, and the seed laser diode 450 may
emit LO light 430 having an average optical power of 10 .mu.W to 1
mW. In some conventional laser diodes, the reflectivity of the back
face may be designed to be relatively high or as close to 100% as
possible in order to minimize the amount of light produced from the
back face or to maximize the amount of light produced from the
front face. In the seed laser diode 450 of FIG. 8, the reflectivity
of the back face 451 may be reduced to a lower value compared to a
conventional laser diode so that a particular power of LO light 430
is emitted from the back face 451. As an example, a conventional
laser diode may have a back face with a reflectivity of greater
than 98%, and a seed laser diode 450 may have a back face with a
reflectivity between 90% and 98%.
[0127] In particular embodiments, the wavelength of the seed light
440 and the wavelength of the LO light 430 may be approximately
equal. For example, a seed laser diode 450 may have a seed-laser
operating wavelength of approximately 1508 nm, and the seed light
440 and the LO light 430 may each have the same wavelength of
approximately 1508 nm. As another example, the wavelength of the
seed light 440 and the wavelength of the LO light 430 may be equal
to within some percentage (e.g., to within approximately 0.1%,
0.01%, or 0.001%) or to within some wavelength range (e.g., to
within approximately 0.1 nm, 0.01 nm, or 0.001 nm). If the
wavelengths are within 0.01% of 1508 nm, then the wavelengths of
the seed light 440 and the LO light 430 may each be in the range
from 1507.85 nm to 1508.15 nm).
[0128] FIG. 9 illustrates an example light source 110 that includes
a semiconductor optical amplifier (SOA) 460 with a tapered optical
waveguide 463. In particular embodiments, a SOA 460 may include an
input end 461, an output end 462, and an optical waveguide 463
extending from the input end 461 to the output end 462. The input
end 461 may receive the seed light 440 from the seed laser diode
450. The waveguide 463 may amplify a temporal portion of the seed
light 440 as the temporal portion propagates along the waveguide
463 from the input end 461 to the output end 462. The amplified
temporal portion may be emitted from the output end 462 as an
emitted pulse of light 400. The emitted pulse of light 400 may be
part of the output beam 125, and the light source 110 may include a
lens 490 configured to collect and collimate emitted pulses of
light 400 from the output end 462 to produce a collimated output
beam 125. The seed laser diode 450 in FIG. 9 may have a diode
length of approximately 100 .mu.m, 200 .mu.m, 500 .mu.m, 1 mm, or
any other suitable length. The SOA 460 may have an amplifier length
of approximately 1 mm, 2 mm, 3 mm, 5 mm, 10 mm, 20 mm, or any other
suitable length. For example, the seed laser diode 450 may have a
diode length of approximately 300 .mu.m, and the SOA 460 may have
an amplifier length of approximately 4 mm.
[0129] In particular embodiments, a waveguide 463 may include a
semiconductor optical waveguide formed at least in part by the
semiconductor material of the SOA 460, and the waveguide 463 may
confine light along transverse directions while the light
propagates through the SOA 460. In particular embodiments, a
waveguide 463 may have a substantially fixed width or a waveguide
463 may have a tapered width. For example, a waveguide 463 may have
a substantially fixed width of approximately 5 .mu.m, 10 .mu.m, 20
.mu.m, 50 .mu.m, 100 .mu.m, 200 .mu.m, 500 .mu.m, or any other
suitable width. In FIG. 9, the SOA 460 has a tapered waveguide 463
with a width that increases from the input end 461 to the output
end 462. For example, the width of the tapered waveguide 463 at the
input end 461 may be approximately equal to the width of the
waveguide of the seed laser diode 450 (e.g., the input end 461 may
have a width of approximately 1 .mu.m, 2 .mu.m, 5 .mu.m, 10 .mu.m,
or 50 .mu.m). At the output end 462 of the SOA 460, the tapered
waveguide 463 may have a width of approximately 50 .mu.m, 100
.mu.m, 200 .mu.m, 500 .mu.m, 1 mm, or any other suitable width. As
another example, the width of the tapered waveguide 463 may
increase linearly from a width of approximately 20 .mu.m at the
input end 461 to a width of approximately 250 .mu.m at the output
end 462.
[0130] In particular embodiments, the input end 461 or the output
end 462 of a SOA 460 may be a discrete facet formed by a
semiconductor-air interface. Additionally, the input end 461 or the
output end 462 may include a dielectric coating (e.g., an
anti-reflection coating to reduce the reflectivity of the input end
461 or the output end 462). An anti-reflection (AR) coating may
have a reflectivity at the seed-laser operating wavelength of less
than 5%, 2%, 0.5%, 0.1%, or any other suitable reflectivity value.
In FIG. 8, the input end 461 may have an AR coating that reduces
the amount of seed light 440 reflected by the input end 461. In
FIG. 8 or FIG. 9, the output end 462 may have an AR coating that
reduces the amount of amplified seed light reflected by the output
end 462. An AR coating applied to the input end 461 or the output
end 462 may also prevent the SOA 460 from acting as a laser by
emitting coherent light when no seed light 440 is present.
[0131] In particular embodiments, a light source 110 may include a
seed laser diode 450 and a SOA 460 that are integrated together and
disposed on or in a single chip or substrate. For example, a seed
laser diode 450 and a SOA 460 may each be fabricated separately and
then attached to the same substrate (e.g., using epoxy, adhesive,
or solder). The substrate may be electrically or thermally
conductive, and the substrate may have a coefficient of thermal
expansion (CTE) that is approximately equal to the CTE of the seed
laser 450 and the SOA 460. As another example, the seed laser diode
450 and the SOA 460 may be fabricated together on the same
substrate (e.g., using semiconductor-fabrication processes, such as
for example, lithography, deposition, and etching). The seed laser
diode 450 and the SOA 460 may each include InGaAs or InGaAsP
semiconductor structures, and the substrate may include indium
phosphide (InP). The InP substrate may be n-doped or p-doped so
that it is electrically conductive, and a portion of the InP
substrate may act as an anode or cathode for both the seed laser
diode 450 and the SOA 460. The substrate may be thermally coupled
to (i) a heat sink that dissipates heat produced by the seed laser
diode 450 or the SOA 460 or (ii) a temperature-control device
(e.g., a thermoelectric cooler) that stabilizes the temperature of
the seed laser diode 450 or the SOA 460 to a particular temperature
setpoint or to within a particular temperature range. In the
example of FIG. 8, the seed laser 450 and the SOA 460 may be
separate devices that are not disposed on a single substrate, and
the seed light 440 may be a free-space beam. Alternatively, in the
example of FIG. 8, the seed laser 450 and the SOA 460 may be
separate devices that are disposed together on a single substrate.
In the example of FIG. 9, the seed laser 450 and the SOA 460 may be
integrated together and disposed on or in a single chip or
substrate.
[0132] In FIG. 9, rather than having a discrete facet formed by a
semiconductor-air interface, the front face 452 of the seed laser
diode 450 and the input end 461 of the SOA 460 may be coupled
together without a semiconductor-air interface. For example, the
seed laser diode 450 may be directly connected to the SOA 460 so
that the seed light 440 is directly coupled from the seed laser
diode 450 into the waveguide 463 of the SOA 460. The front face 452
may be butt-coupled or affixed (e.g., using an optically
transparent adhesive) to the input end 461, or the seed laser diode
450 and the SOA 460 may be fabricated together so that there is no
separate front face 452 or input end 461 (e.g., the front face 452
and the input end 461 may be merged together to form a single
interface between the seed laser diode 450 and the SOA 460).
Alternatively, the seed laser diode 450 may be coupled to the SOA
460 via a passive optical waveguide that transmits the seed light
440 from the front face 452 of the seed laser diode 450 to the
input end 461 of the SOA 460.
[0133] In particular embodiments, during a period of time between
two successive temporal portions of seed light 440, a SOA 460 may
be configured to optically absorb most of the seed light 440
propagating in the SOA 460. The seed light 440 from the seed laser
diode 450 may be coupled into the waveguide 463 of the SOA 460.
Depending on the amount of SOA current 2 supplied to the SOA 460,
the seed light 440 may be optically amplified or optically absorbed
while propagating along the waveguide 463. If the SOA current
I.sub.2 exceeds a threshold gain value (e.g., 100 mA) that
overcomes the optical loss of the SOA 460, then the seed light 440
may be optically amplified by stimulated emission of photons.
Otherwise, if the SOA current I.sub.2 is less than the threshold
gain value, then the seed light 440 may be optically absorbed. The
process of optical absorption of the seed light 440 may include
photons of the seed light 440 being absorbed by electrons located
in the semiconductor structure of the SOA 460.
[0134] In particular embodiments, the SOA current I.sub.2 may
include pulses of current separated by a period of time that
corresponds to the pulse period .tau. of the light source 110, and
each pulse of current may result in the emission of a pulse of
light 400. For example, if the SOA current I.sub.2 includes 20-A
current pulses with a 10-ns duration, then for each current pulse,
a corresponding 10-ns temporal portion of the seed light 440 may be
amplified, resulting in the emission of a pulse of light 400.
During the time periods r between successive pulses of current, the
SOA current I.sub.2 may be set to approximately zero or to some
other value below the threshold gain value, and the seed light 440
present in the SOA 460 during those time periods may be optically
absorbed. The optical absorption of the SOA 460 when the SOA
current I.sub.2 is zero may be greater than or equal to
approximately 10 decibels (dB), 15 dB, 20 dB, 25 dB, or 30 dB. For
example, if the optical absorption is greater than or equal to 20
dB, then less than or equal to 1% of the seed light 440 that is
coupled into the input end 461 of the waveguide 463 may be emitted
from the output end 462 as unwanted leakage light. Having most of
the seed light 440 absorbed in the SOA 460 may prevent unwanted
seed light 440 (e.g., seed light 440 located between successive
pulses of light 400) from leaking out of the SOA 460 and
propagating through the rest of the lidar system 100. Additionally,
optically absorbing the unwanted seed light 440 may allow the seed
laser 450 to be operated with a substantially constant current
I.sub.1 or a substantially constant output power so that the
wavelengths of the seed light 440 and LO light 430 are stable and
substantially constant.
[0135] In particular embodiments, a SOA 460 may be electrically
configured as a diode with a p-doped region and a n-doped region
that form a p-n junction. The SOA may include an anode and a
cathode that convey SOA current I.sub.2 from an electronic driver
480 into or out of the p-n junction of the SOA 460. The anode may
correspond to the p-doped side of the semiconductor p-n junction,
and the cathode may correspond to the n-doped side. For example,
the anode of the SOA 460 may include or may be electrically coupled
to the p-doped region of the SOA 460, and the p-doped region may be
electrically coupled to an electrically conductive electrode
material (e.g., gold) deposited onto a surface of the SOA 460. The
cathode may include or may be electrically coupled to a n-doped
substrate located on the opposite side of the SOA 460.
Alternatively, the anode of the SOA 460 may include or may be
electrically coupled to a p-doped substrate of the SOA 460, and the
cathode may include or may be electrically coupled to an electrode
and a n-doped region of the SOA 460. The anode and cathode may be
electrically coupled to the electronic driver 480, and the driver
480 may supply a positive SOA current I.sub.2 that flows from the
driver 480 into the anode, through the SOA 460, out of the cathode,
and back to the driver 480. A positive SOA current I.sub.2 flowing
through the SOA 460 may correspond to the p-n junction of the SOA
being in a forward-biased state which allows the current to flow.
When considering the electrical current as being made up of a flow
of electrons, then for a positive SOA current, the electrons may be
viewed as flowing in the opposite direction (e.g., from the driver
480 into the cathode, through the SOA 460, and out of the anode and
back to the driver 480).
[0136] In particular embodiments, an electronic driver 480 may
electrically couple the SOA anode to the SOA cathode during a
period of time between two successive pulses of current. For
example, for most or all of the time period .tau. between two
successive pulses of current, the electronic driver 480 may
electrically couple the anode and cathode of the SOA 460.
Electrically coupling the anode and cathode may include
electrically shorting the anode directly to the cathode or coupling
the anode and cathode through a particular electrical resistance
(e.g., approximately 1.OMEGA., 10.OMEGA., or 100.OMEGA.).
Alternatively, electrically coupling the anode and the cathode may
include applying a reverse-bias voltage (e.g., approximately -1 V,
-5 V, or -10 V) to the anode and cathode, where the reverse-bias
voltage has a polarity that is opposite the forward-bias polarity
associated with the applied pulses of current. By electrically
coupling the anode to the cathode, the optical absorption of the
SOA may be increased. For example, the optical absorption of the
SOA 460 when the anode and cathode are electrically coupled may be
increased (compared to the anode and cathode not being electrically
coupled) by approximately 3 dB, 5 dB, 10 dB, 15 dB, or 20 dB. The
optical absorption of the SOA 460 when the anode and cathode are
electrically coupled may be greater than or equal to approximately
20 dB, 25 dB, 30 dB, 35 dB, or 40 dB. For example, the optical
absorption of a SOA 460 when the SOA current I.sub.2 is zero and
the anode and cathode are not electrically coupled may be 20 dB.
When the anode and cathode are electrically shorted together, the
optical absorption may increase by 10 dB to an optical absorption
of 30 dB. If the optical absorption of the SOA 460 is greater than
or equal to 30 dB, then less than or equal to 0.1% of the seed
light 440 that is coupled into the input end 461 of the waveguide
463 may be emitted from the output end 462 as unwanted leakage
light.
[0137] In particular embodiments, a light source 110 that includes
a seed laser diode 450 and a SOA 460 may be configured as a
three-terminal device. A three-terminal light source may include
(i) a common cathode and separate, electrically isolated anodes or
(ii) a common anode and separate, electrically isolated cathodes. A
seed laser diode 450 may be electrically configured as a diode with
a p-doped region (coupled to a seed laser anode) and a n-doped
region (coupled to a seed laser cathode), where the p-doped and
n-doped regions form a p-n junction. Similarly, a SOA 460 may be
electrically configured as a diode with a p-doped region (coupled
to a SOA anode) and a n-doped region (coupled to a SOA cathode),
where the p-doped and n-doped regions form a p-n junction. A seed
laser diode 450 and a SOA 460 may each have a cathode and an anode,
and a common-cathode configuration may refer to the cathodes of the
seed laser diode 450 and the SOA 460 being electrically connected
together into a single electrical terminal or contact that is
connected to an electronic driver 480. A light source 110
configured as a three-terminal common-cathode device may include a
seed laser anode, a SOA anode, and a common cathode. The seed laser
anode and the SOA anode may be electrically isolated from one
another, and the seed laser cathode and the SOA cathode may be
electrically connected together to form the common cathode.
Alternatively, a light source 110 may be configured as a
three-terminal common-anode device that includes a seed laser
cathode, a SOA cathode, and a common anode. The seed laser cathode
and the SOA cathode may be electrically isolated from one another,
and the seed laser anode and the SOA anode may be electrically
connected together to form the common anode.
[0138] Two terminals (e.g., two anodes or two cathodes) being
electrically isolated from one another may refer to the two
terminals having greater than a particular value of electrical
resistance between them (e.g., the resistance between two
electrically isolated anodes may be greater than 1 k.OMEGA., 10
k.OMEGA., 100 k.OMEGA., or 1 M.OMEGA.). Two terminals (e.g., two
anodes or two cathodes) being electrically connected may refer to
the two terminals having less than a particular value of electrical
resistance between them (e.g., the resistance between two
electrically connected cathodes may be less than 1 k.OMEGA., 100
.OMEGA., 10.OMEGA., or 1.OMEGA.). A common-anode or common-cathode
configuration may be provided by combining or electrically
connecting the respective anodes or cathodes through an
electrically conductive substrate. For example, a seed laser diode
450 and a SOA 460 may be fabricated separately and then affixed to
an electrically conductive substrate so that their anodes or
cathodes are electrically connected. As another example, a
substrate may include an electrically conductive semiconductor
material on which a seed laser diode 450 and SOA 460 are grown. The
seed laser diode 450 and the SOA 460 may each include an InGaAs or
InGaAsP semiconductor structure grown on an InP substrate. The InP
substrate may be n-doped so that it is electrically conductive, and
the cathodes of the seed laser diode 450 and the SOA 460 may each
be electrically connected to the InP substrate so that the InP
substrate acts as a common cathode. Alternatively, the InP
substrate may be p-doped, and the anodes of the seed laser diode
450 and the SOA 460 may each be electrically connected to the InP
substrate, which acts as a common anode.
[0139] One or more of the light sources 110 illustrated in FIGS.
8-11 and 28-29 and described herein may be configured as a
three-terminal device (with a common cathode or a common anode).
For example, the light source 110 in FIG. 9 may be configured as a
three-terminal common-cathode device having separate electrical
connections between the electronic driver 480 and each of these
three electrical terminals or contacts: (i) seed laser anode, (ii)
SOA anode, and (iii) common cathode. In a three-terminal
common-cathode device, the seed laser anode and the SOA anode may
be electrically isolated from one another, and an electronic driver
480 may drive the seed laser diode 450 and the SOA 460 by supplying
separate electrical signals to the seed laser anode and the SOA
anode. The common cathode may act as a common return path for
currents from the seed laser diode 450 and the SOA 460 to combine
and return to the electronic driver 480.
[0140] In particular embodiments, a light source 110 that includes
a seed laser diode 450 and a SOA 460 may be configured as a
four-terminal device. In a four-terminal light source 110, the seed
laser anode and the SOA anode may be electrically isolated from one
another, and instead of having a common cathode, the seed laser
cathode and the SOA cathode may also be electrically isolated from
one another. One or more of the light sources 110 described herein
may be configured as a four-terminal device. For example, the light
source 110 in each of FIGS. 8 and 9 may be configured as a
four-terminal device with two electrically isolated anodes (seed
laser anode and SOA anode) and two electrically isolated cathodes
(seed laser cathode and SOA cathode). A four-terminal light source
110 may have separate electrical connections between an electronic
driver 480 and each of these four electrical terminals or contacts:
(i) seed laser anode, (ii) seed laser cathode, (iii) SOA anode, and
(iv) SOA cathode. An electronic driver 480 may drive the anode and
cathode of the seed laser diode 450 separately or independently
from the anode and cathode of the SOA 460. As compared to a
three-terminal light source 110, a light source configured as a
four-terminal device may provide improved electrical isolation
between the seed laser diode 450 and the SOA 460. For example, in a
four-terminal light source 110, applying a pulse of current to the
SOA 460 may result in a reduced amount of unwanted cross-talk
current that is coupled to the seed laser diode 450.
[0141] FIG. 10 illustrates an example light source 110 with an
optical splitter 470 that splits output light 472 from a seed laser
diode 450 to produce seed light 440 and local-oscillator (LO) light
430. In particular embodiments, a light source 110 may include (i)
a seed laser diode 450 with a front face 452 from which seed-laser
output light 472 is emitted and (ii) an optical splitter 470 that
splits off a portion of the output light 472 to produce seed light
440 and LO light 430. The optical splitter 470 may be located
between the seed laser diode 450 and the SOA 460. In FIG. 10, the
output light 472 emitted by the seed laser diode 450 is a
free-space optical beam, and the optical splitter 470 is a
free-space optical beam-splitter that produces the free-space
beams: seed light 440 and LO light 430. In the examples of FIGS. 8
and 9, light emitted from the back face 451 of the seed laser diode
450 is used to produce the LO light 430. In contrast, in the
example of FIG. 10, both the seed light 440 and the LO light 430
are produced from the output light 472 emitted from the front face
452 of the seed laser diode 450. The seed light 440 is transmitted
through the splitter 470 and directed to the SOA 460, and the LO
light 430 is reflected by the splitter 470 and directed to the
receiver 140 of the lidar system 100. A light source 110 may
include one or more lenses (not illustrated in FIG. 10) that
collimate the seed-laser output light 472 or focus the seed light
440 into the waveguide 463 of the SOA 460.
[0142] The optical splitter 470 in FIG. 10 is a free-space optical
splitter that receives the seed-laser output light 472 as a
free-space optical beam and produces two free-space beams: seed
light 440 and LO light 430. In FIG. 10, the free-space optical
beam-splitter 470 reflects a first portion of the incident
seed-laser output light 472 to produce the LO light 430 and
transmits a second portion of the output light 472 to produce the
seed light 440. Alternatively, the beam-splitter 470 may be
arranged to reflect a portion of the output light 472 to produce
the seed light 440 and transmit a portion of the output light 472
to produce the LO light 430. The free-space beam-splitter 470 in
FIG. 10 may have a reflectivity of less than or equal to 1%, 2%,
5%, 10%, 20%, 50%, or any other suitable reflectivity value. For
example, the splitter 470 may reflect 10% or less of the incident
seed-laser output light 472 to produce the LO light 430, and the
remaining 90% or more of the output light 472 may be transmitted
through the splitter 470 to produce the seed light 440. As another
example, if the output light 472 has an average power of 25 mW and
the splitter 470 reflects approximately 4% of the output light 472,
then the LO light 430 may have an average power of approximately 1
mW, and the seed light 440 may have an average power of
approximately 24 mW. As used herein, a splitter 470 may refer to a
free-space optical splitter, a fiber-optic splitter, or an
optical-waveguide splitter. Additionally, an optical-waveguide
splitter may be referred to as an integrated-optic splitter.
[0143] In particular embodiments, a light source 110 may include a
fiber-optic splitter 470 that splits the seed-laser output light
472 to produce seed light 440 and LO light 430. Instead of using a
free-space optical splitter 470 (as illustrated in FIG. 10), a
light source 110 may use a fiber-optic splitter 470. The
fiber-optic splitter 470 may include one input optical fiber and
two or more output optical fibers, and light that is coupled into
the input optical fiber may be split between the output optical
fibers. The output light 472 may be coupled from the front face 452
of the seed laser diode 450 into the input optical fiber of the
fiber-optic splitter 470, and the fiber-optic splitter 470 may
split the output light 472 into the seed light 440 and the LO light
430. The output light 472 may be coupled into the input optical
fiber using one or more lenses, or the output light 472 may be
directly coupled into the input optical fiber (e.g., the input
optical fiber may be butt-coupled to the front face 452 of the seed
laser diode 450). The seed light 440 may be directed to the SOA 460
by a first output fiber, and the LO light 430 may be directed to a
receiver 140 by a second output fiber. The seed light 440 may be
coupled from the first output fiber into the waveguide 463 of the
SOA 460 by one or more lenses, or the seed light 440 may be
directly coupled into waveguide 463 (e.g., the first output fiber
may be butt-coupled to the input end 461 of the SOA 460). A
fiber-optic splitter 470 may split off less than or equal to 1%,
2%, 5%, 10%, 20%, 50%, or any other suitable amount of the output
light 472 to produce the LO light 430, and the remaining light may
form the seed light 440. For example, a fiber-optic splitter 470
may split off 10% or less of the output light 472 to produce the LO
light 430, which is directed to one output fiber. The remaining 90%
or more of the output light 472 may be directed to the other output
fiber as the seed light 440.
[0144] FIG. 11 illustrates an example light source 110 with a
photonic integrated circuit (PIC) 455 that includes an
optical-waveguide splitter 470. In particular embodiments, a light
source 110 may include an optical splitter 470 and a PIC 455, where
the optical splitter 470 is an optical-waveguide splitter of the
PIC. A PIC 455 (which may be referred to as a planar lightwave
circuit (PLC), an integrated-optic device, an integrated
optoelectronic device, or a silicon optical bench) may include one
or more optical waveguides or one or more optical-waveguide devices
(e.g., optical-waveguide splitter 470) integrated together into a
single device. A PIC 455 may include or may be fabricated from a
substrate that includes silicon, InP, glass (e.g., silica), a
polymer, an electro-optic material (e.g., lithium niobate
(LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3)), or any suitable
combination thereof. One or more optical waveguides may be formed
on or in a PIC substrate using micro-fabrication techniques, such
as for example, lithography, deposition, or etching. For example,
an optical waveguide may be formed on a glass or silicon substrate
by depositing and selectively etching material to form a ridge or
channel waveguide on the substrate. As another example, an optical
waveguide may be formed by implanting or diffusing a material into
a substrate (e.g., by diffusing titanium into a LiNbO.sub.3
substrate) to form a region in the substrate having a higher
refractive index than the surrounding substrate material.
[0145] In particular embodiments, an optical-waveguide splitter 470
may include an input port and two or more output ports. In FIG. 11,
the seed-laser output light 472 from the seed laser diode 450 is
coupled into the input optical waveguide (input port) of the
waveguide splitter 470, and the waveguide splitter 470 splits the
output light 472 between two output waveguides, output port 1 and
output port 2. The seed-laser output light 472 may be coupled from
the front face 452 of the seed laser diode 450 to the input port of
the splitter 470 using one or more lenses, or the seed laser diode
450 may be butt-coupled to the input port so that the output light
472 is directly coupled into the input port. The seed light 440 is
formed by the portion of output light 472 that is sent by the
splitter 470 to output port 1, and the LO light 430 is formed by
the portion of output light 472 that is sent by the splitter 470 to
output port 2. The waveguide splitter 470 directs the seed light
440 to output port 1, which is coupled to waveguide 463 of the SOA
460. Additionally, the waveguide splitter 470 directs the LO light
430 to output port 2, which sends the LO light 430 to a receiver
140. An optical-waveguide splitter 470 may split off less than or
equal to 1%, 2%, 5%, 10%, 20%, 50%, or any other suitable amount of
the output light 472 to produce the LO light 430, and the remaining
light may form the seed light 440. For example, the
optical-waveguide splitter 470 may send 10% or less of the output
light 472 to output port 2 to produce the LO light 430, and the
remaining 90% or more of the output light 472 may be sent to output
port 1 to produce the seed light 440.
[0146] In particular embodiments, a light source 110 may include
one or more discrete optical devices combined with a PIC 455. The
discrete optical devices (which may include a seed laser diode 450,
a SOA 460, one or more lenses, or one or more optical fibers) may
be configured to couple light into the PIC 455 or to receive light
emitted from the PIC 455. In the example of FIG. 11, the light
source 110 includes a PIC 455, a seed laser diode 450, and a SOA
460. The seed laser diode 450 and the SOA 460 may each be attached
or bonded to the PIC 455, or the seed laser diode 450, the SOA 460,
and the PIC 455 may be attached to a common substrate. For example,
the front face 452 of the seed laser diode 450 may be bonded to the
input port of the PIC 455 so that the output light 472 is directly
coupled into the input port. As another example, the input end 461
of the SOA 460 may be bonded to the output port 1 of the PIC 455 so
that the seed light 440 is directly coupled into the waveguide 463
of the SOA 460. As another example, the light source 110 may
include a lens (not illustrated in FIG. 11) attached to or
positioned near output port 2, and the lens may collect and
collimate the LO light 430. As another example, the light source
110 may include an optical fiber (not illustrated in FIG. 11)
attached to or positioned near output port 2, and the LO light 430
may be coupled into the optical fiber, which directs the LO light
430 to a receiver 140.
[0147] FIG. 12 illustrates an example light source 110 that
includes a seed laser diode 450a and a local-oscillator (LO) laser
diode 450b. In particular embodiments, a seed laser of a light
source 110 may include a seed laser diode 450a that produces seed
light 440 and a LO laser diode 450b that produces LO light 430.
Instead of having one laser diode that produces both the seed light
440 and the LO light 430 (e.g., as illustrated in FIGS. 8-11), a
light source 110 may include two laser diodes, one to produce the
seed light 440 and the other to produce the LO light 430. A light
source 110 with two laser diodes may not include an optical
splitter 470. Rather, the seed light 440 emitted by the seed laser
diode 450a may be coupled to a SOA 460, and the LO light 430
emitted by the LO laser diode 450b may be sent to a receiver 140.
For example, the seed laser diode 450a may be butt-coupled to the
input end 461 of the SOA 460, and the LO light 430 from the LO
laser diode 450b may be coupled into an optical fiber, which may
direct the LO light 430 to a receiver 140.
[0148] In particular embodiments, a seed laser diode 450a and a LO
laser diode 450b may be operated so that the seed light 440 and the
LO light 430 have a particular frequency offset. For example, the
seed light 440 and the LO light 430 may have an optical frequency
offset of approximately 0 Hz, 1 kHz, 1 MHz, 100 MHz, 1 GHz, 2 GHz,
5 GHz, 10 GHz, 20 GHz, or any other suitable frequency offset. An
optical frequency f (which may be referred to as a frequency or a
carrier frequency) and a wavelength .lamda. may be related by the
expression .lamda.f=c. For example, seed light 440 with a
wavelength of 1550 nm corresponds to seed light 440 with an optical
frequency of approximately 193.4 terahertz (THz). In some cases
herein, the terms wavelength and frequency may be used
interchangeably when referring to an optical property of light. For
example, LO light 430 having a substantially constant optical
frequency may be equivalent to the LO light 430 having a
substantially constant wavelength. As another example, LO light 430
having approximately the same wavelength as seed light 440 may also
be referred to as the LO light 430 having approximately the same
frequency as the seed light 440. As another example, LO light 430
having a particular wavelength offset from seed light 440 may also
be referred to as the LO light 430 having a particular frequency
offset from the seed light 440. An optical frequency offset
(.DELTA.f) and a wavelength offset (AX) may be related by the
expression .DELTA.f/f=-.DELTA..lamda./.lamda.. For example, for
seed light 440 with a 1550-nm wavelength, LO light 430 that has a
+10-GHz frequency offset from the seed light 440 corresponds to LO
light 430 with a wavelength offset of approximately -0.08-nm from
the 1550-nm wavelength of the seed light 440 (e.g., a wavelength
for the LO light 430 of approximately 1549.92 nm).
[0149] In particular embodiments, a seed laser diode 450a or a LO
laser diode 450b may be frequency locked so that they emit light
having a substantially fixed wavelength or so that there is a
substantially fixed frequency offset between the seed light 440 and
the LO light 430. Frequency locking a laser diode may include
locking the wavelength of the light emitted by the laser diode to a
stable frequency reference using, for example, an external optical
cavity, an atomic optical absorption line, or light injected into
the laser diode. For example, the seed laser diode 450a may be
frequency locked (e.g., using an external optical cavity), and some
of the light from the seed laser diode 450a may be injected into
the LO laser diode 450b to frequency lock the LO laser diode 450 to
approximately the same wavelength as the seed laser diode 450a. As
another example, the seed laser diode 450a and the LO laser diode
450b may each be separately frequency locked so that the two laser
diodes have a particular frequency offset (e.g., a frequency offset
of approximately 2 GHz).
[0150] FIG. 13 illustrates an example light source 110 that
includes a seed laser 450, a semiconductor optical amplifier (SOA)
460, and a fiber-optic amplifier 500. In particular embodiments, in
addition to a seed laser 450 and a pulsed optical amplifier 460, a
light source 110 may also include a fiber-optic amplifier 500 that
amplifies pulses of light 400a produced by the pulsed optical
amplifier 460. In FIG. 13, the SOA 460 may amplify temporal
portions of seed light 440 from the seed laser 450 to produce
pulses of light 400a, and the fiber-optic amplifier 500 may amplify
the pulses of light 400a from the SOA 460 to produce amplified
pulses of light 400b. The amplified pulses of light 400b may be
part of a free-space output beam 125 that is sent to a scanner 120
and scanned across a field of regard of a lidar system 100.
[0151] A SOA 460 and a fiber-optic amplifier 500 may each have an
optical power gain of 10 dB, 15 dB, 20 dB, 25 dB, 30 dB, 35 dB, 40
dB, or any other suitable optical power gain. In the example of
FIG. 13, the SOA 460 may have a gain of 30 dB, and the fiber-optic
amplifier 500 may have a gain of 20 dB, which corresponds to an
overall gain of 50 dB. A temporal portion of seed light 440 with an
energy of 5 pJ may be amplified by the SOA 460 (with a gain of 30
dB) to produce a pulse of light 400a with an energy of
approximately 5 nJ. The fiber-optic amplifier 500 may amplify the
5-nJ pulse of light 400a by 20 dB to produce an output pulse of
light 400b with an energy of approximately 0.5 pJ. The seed laser
450 in FIG. 13 produces seed light 440 and LO light 430. The seed
light 440 may be emitted from a front face 452 of a seed laser
diode 450, and the LO light 430 may be emitted from a back face 451
of the seed laser diode 450. Alternatively, the light source 110
may include a splitter 470 that splits seed-laser output light 472
to produce the seed light 440 and the LO light 430.
[0152] FIG. 14 illustrates an example fiber-optic amplifier 500. In
particular embodiments, a light source 110 of a lidar system 100
may include a fiber-optic amplifier 500 that amplifies pulses of
light 400a produced by a SOA 460 to produce an output beam 125 with
amplified pulses of light 400b. A fiber-optic amplifier 500 may be
terminated by a lens (e.g., output collimator 570) that produces a
collimated free-space output beam 125 which may be directed to a
scanner 120. In particular embodiments, a fiber-optic amplifier 500
may include one or more pump lasers 510, one or more pump WDMs 520,
one or more optical gain fibers 501, one or more optical isolators
530, one or more optical splitters 470, one or more detectors 550,
one or more optical filters 560, or one or more output collimators
570.
[0153] A fiber-optic amplifier 500 may include an optical gain
fiber 501 that is optically pumped (e.g., provided with energy) by
one or more pump lasers 510. The optically pumped gain fiber 501
may provide optical gain to each input pulse of light 400a while
propagating through the gain fiber 501. The pump-laser light may
travel through the gain fiber 501 in the same direction
(co-propagating) as the pulse of light 400a or in the opposite
direction (counter-propagating). The fiber-optic amplifier 500 in
FIG. 14 includes one co-propagating pump laser 510 on the input
side of the amplifier 500 and one counter-propagating pump laser
510 on the output side. A pump laser 510 may produce light at any
suitable wavelength to provide optical excitation to the gain
material of gain fiber 501 (e.g., a wavelength of approximately 808
nm, 810 nm, 915 m, 940 nm, 960 nm, 976 nm, or 980 nm). A pump laser
510 may be operated as a CW light source and may produce any
suitable amount of average optical pump power, such as for example,
approximately 1 W, 2 W, 5 W, 10 W, or 20 W of pump power. The
pump-laser light from a pump laser 510 may be coupled into gain
fiber 501 via a pump wavelength-division multiplexer (WDM) 520. A
pump WDM 520 may be used to combine or separate pump light and the
pulses of light 400a that are amplified by the gain fiber 501.
[0154] The fiber-optic core of a gain fiber 501 may be doped with a
gain material that absorbs pump-laser light and provides optical
gain to pulses of light 400a as they propagate along the gain fiber
501. The gain material may include rare-earth ions, such as for
example, erbium (Er.sup.3+), ytterbium (Yb.sup.3+), neodymium
(Nd.sup.3+), praseodymium (Pr.sup.3+), holmium (Ho.sup.3+), thulium
(Tm.sup.3+), dysprosium (Dy.sup.3+), or any other suitable
rare-earth element, or any suitable combination thereof. For
example, the gain fiber 501 may include a core doped with erbium
ions or with a combination of erbium and ytterbium ions. The
rare-earth dopants absorb pump-laser light and are "pumped" or
promoted into excited states that provide amplification to the
pulses of light 400a through stimulated emission of photons. The
rare-earth ions in excited states may also emit photons through
spontaneous emission, resulting in the production of amplified
spontaneous emission (ASE) light by the gain fiber 501.
[0155] Again fiber 501 may include a single-clad or multi-clad
optical fiber with a core diameter of approximately 6 .mu.m, 7
.mu.m, 8 .mu.m, 9 .mu.m, 10 .mu.m, 12 .mu.m, 20 .mu.m, 25 .mu.m, or
any other suitable core diameter. A single-clad gain fiber 501 may
include a core surrounded by a cladding material, and the pump
light and the pulses of light 400a may both propagate substantially
within the core of the gain fiber 501. A multi-clad gain fiber 501
may include a core, an inner cladding surrounding the core, and one
or more additional cladding layers surrounding the inner cladding.
The pulses of light 400a may propagate substantially within the
core, while the pump light may propagate substantially within the
inner cladding and the core. The length of gain fiber 501 in an
amplifier 500 may be approximately 0.5 m, 1 m, 2 m, 4 m, 6 m, 10 m,
20 m, or any other suitable gain-fiber length.
[0156] A fiber-optic amplifier 500 may include one or more optical
filters 560 located at the input or output side of the amplifier
500. An optical filter 560 (which may include an absorptive filter,
dichroic filter, long-pass filter, short-pass filter, band-pass
filter, notch filter, Bragg grating, or fiber Bragg grating) may
transmit light over a particular optical pass-band and
substantially block light outside of the pass-band. The optical
filter 560 in FIG. 14 is located at the output side of the
amplifier 500 and may reduce the amount of ASE from the gain fiber
501 that accompanies the output pulses of light 400b. For example,
the filter 560 may transmit light at the wavelength of the pulses
of light 400a (e.g., 1550 nm) and may attenuate light at
wavelengths away from a 5-nm pass-band centered at 1550 nm.
[0157] A fiber-optic amplifier 500 may include one or more optical
isolators 530. An isolator 530 may reduce or attenuate
backward-propagating light, which may destabilize or cause damage
to a seed laser diode 450, SOA 460, pump laser 510, or gain fiber
501. The isolators 530 in FIG. 14 may allow light to pass in the
direction of the arrow drawn in the isolator and block light
propagating in the reverse direction. Backward-propagating light
may arise from ASE light from gain fiber 501, counter-propagating
pump light from a pump laser 510, or optical reflections from one
or more optical interfaces of a fiber-optic amplifier 500. An
optical isolator 530 may prevent the destabilization or damage
associated with backward-propagating light by blocking most of the
backward-propagating light (e.g., by attenuating
backward-propagating light by greater than or equal to 5 dB, 10 dB,
20 dB, 30 dB, 40 dB, 50 dB, or any other suitable attenuation
value).
[0158] A fiber-optic amplifier 500 may include one or more optical
splitters 470 and one or more detectors 550. A splitter 470 may
split off a portion of light (e.g., approximately 0.1%, 0.5%, 1%,
2%, or 5% of light received by the splitter 470) and direct the
split off portion to a detector 550. In FIG. 14, each splitter 470
may split off and send approximately 1% of each pulse of light
(400a or 400b) to a detector 550. One or more detectors 550 may be
used to monitor the performance or health of a fiber-optic
amplifier 500. If an electrical signal from a detector 550 drops
below a particular threshold level, then a controller 150 may
determine that there is a problem with the amplifier 500 (e.g.,
there may be insufficient optical power in the input pulses of
light 400a or a pump laser 510 may be failing). In response to
determining that there is a problem with the amplifier 500, the
controller 150 may shut down or disable the amplifier 500, shut
down or disable the lidar system 100, or send a notification that
the lidar system 100 is in need of service or repair.
[0159] In particular embodiments, a fiber-optic amplifier 500 may
include an input optical fiber configured to receive input pulses
of light 400a from a SOA 460. The input optical fiber may be part
of or may be coupled or spliced to one of the components of the
fiber-optic amplifier 500. For example, pulses of light 400a may be
coupled into an optical fiber which is spliced to an input optical
fiber of the isolator 530 located at the input to the amplifier
500. As another example, the pulses of light 400a from a SOA 460
may be part of a free-space beam that is coupled into an input
optical fiber of fiber-optical amplifier 500 using one or more
lenses. As another example, an input optical fiber of fiber-optic
amplifier 500 may be positioned at or near the output end 462 of a
SOA 460 so that the pulses of light 400a are directly coupled from
the SOA 460 into the input optical fiber.
[0160] In particular embodiments, the optical components of a
fiber-optic amplifier 500 may be free-space components,
fiber-coupled components, or a combination of free-space and
fiber-coupled components. As an example, each optical component in
FIG. 14 may be a free-space optical component or a fiber-coupled
optical component. As another example, the input pulses of light
400a may be part of a free-space optical beam, and the isolator
530, splitter 470, and pump WDM 520 located on the input side of
the amplifier 500 may each be free-space optical components.
Additionally, the light from the pump laser 510 on the input side
may be a free-space beam that is combined with the input pulses of
light 400a by the pump WDM 520 on the input side, and the combined
pump-seed light may form a free-space beam that is coupled into the
gain fiber 501 via one or more lenses.
[0161] FIG. 15 illustrates example graphs of seed current
(I.sub.1), LO light 430, seed light 440, pulsed SOA current
(I.sub.2), and emitted optical pulses 400. Each of the parameters
(I.sub.1, LO light 430, seed light 440, I.sub.2, and emitted
optical pulses 400) in FIG. 15 is plotted versus time. The graph of
seed current I.sub.1 corresponds to a substantially constant DC
electrical current that is supplied to a seed laser diode 450.
Based on the DC electrical current I.sub.1, the LO light 430 and
seed light 440 produced by the seed laser diode 450 may each
include CW light or light having a substantially constant optical
power, as represented by the graphs of LO light 430 and seed light
440 in FIG. 15. For example, the LO light 430 may have a
substantially constant average optical power of approximately 1
.mu.W, 10 .mu.W, 100 .mu.W, 1 mW, 10 mW, 20 mW, 50 mW, or any other
suitable average optical power. As another example, the seed light
440 may have a substantially constant average optical power of
approximately 1 mW, 10 mW, 20 mW, 50 mW, 100 mW, 200 mW or any
other suitable average optical power. As another example, the LO
light 430 may have a substantially constant optical power of
approximately 10 .mu.W, and the seed light 440 may have a
substantially constant optical power of approximately 100 mW. The
LO light 430 or the seed light 440 having a substantially constant
optical power may correspond to the optical power being
substantially constant over particular time interval (e.g., a time
interval greater than or equal to the pulse period .tau., the
coherence time T.sub.c, or the time interval t.sub.b-t.sub.a). For
example, the power of the LO light 430 may vary by less than
.+-.1%, .+-.2%, or .+-.5% over a time interval greater than or
equal to the pulse period .tau..
[0162] In particular embodiments, CW light may refer to light
having a substantially fixed or stable optical frequency or
wavelength over a particular time interval (e.g., over pulse period
.tau., over coherence time T.sub.c, or over the time interval
t.sub.b-t.sub.a). Light with a substantially fixed or stable
optical frequency may refer to light having a variation in optical
frequency over a particular time interval of less than or equal to
.+-.0.1%, .+-.0.01%, .+-.0.001%, .+-.0.0001%, .+-.0.00001%,
.+-.0.000001%, or any other suitable variation. For example, if LO
light 430 with a 1550-nm wavelength (which corresponds to an
optical frequency of approximately 193.4 THz) has a frequency
variation of less than or equal to .+-.0.000001% over a particular
time interval, then the frequency of the LO light 430 may vary by
less than or equal to approximately .+-.1.94 MHz over the time
interval.
[0163] In particular embodiments, the average optical power for LO
light 430 may be set to a particular value based at least in part
on a saturation value of a receiver 140. For example, a seed laser
450 may be configured to emit LO light 430 having an average
optical power that is less than a saturation value of a receiver
140 (e.g., less than a saturation value of a detector 340 or an
amplifier 350 of the receiver 140). If a receiver 140 receives an
input optical signal (e.g., combined beam 422) that exceeds an
optical-power saturation value of the detector 340, then the
detector 340 may saturate or produce a photocurrent i that is
different from or distorted with respect to the input optical
signal. A detector 340 may saturate with an input optical power of
approximately 0.1 mW, 0.5 mW, 1 mW, 5 mW, 10 mW, 20 mW, or 100 mW.
If an amplifier 350 of a receiver 140 receives an input
photocurrent i that exceeds an electrical-current saturation value,
then the amplifier 350 may saturate or produce a voltage signal 360
that is different from or distorted with respect to the
photocurrent signal i. To prevent saturation of the detector 340 or
amplifier 350, the optical power of the input beam 135 or of the LO
light 430 may be selected to be below a saturation power of the
receiver 140. For example, a detector 340 may saturate with an
input optical power of 10 mW, and to prevent the detector 340 from
saturating, the optical power of a combined beam 422 may be limited
to less than 10 mW. In particular embodiments, a limit may be
applied to the average power of the LO light 430 to prevent
saturation. For example, a detector 340 may saturate with an
average optical power of 1 mW, and to prevent the detector 340 from
saturating, the average optical power of LO light 430 that is sent
to the detector 340 may be configured to be less than 1 mW. As
another example, the average optical power of the LO light 430 may
be set to a value between 1 .mu.W and 100 .mu.W to prevent
saturation effects in a detector 340.
[0164] In particular embodiments, the average optical power of LO
light 430 may be configured by adjusting or setting (i) an amount
of seed current I.sub.1 supplied to a seed laser diode 450, (ii) a
reflectivity of the back face 451 of the seed laser diode 450,
(iii) a reflectivity of a free-space splitter 470, or (iv) an
amount of light split off by a fiber-optic or optical-waveguide
splitter 470. In the example of FIG. 8 or FIG. 9, the seed current
I.sub.1 and the reflectivity of the back face 451 of the seed laser
diode 450 may be configured so that the average optical power of
the LO light 430 is set to a particular value (e.g., a value
between 10 .mu.W and 100 .mu.W). In the example of FIG. 10, the
seed current I.sub.1 and the reflectivity of the splitter 470 may
be configured so that the average optical power of the LO light 430
is set to a particular value (e.g., a value below 10 mW). In the
example of FIG. 11, the seed current supplied to the seed laser
diode 450 and the amount of light split off to output port 2 by the
optical-waveguide splitter 470 may be configured so that the
average optical power of the LO light 430 is set to a particular
value (e.g., a value below 1 mW).
[0165] In FIG. 15, the hatched regions 441 of the seed light 440
correspond to temporal portions of the seed light 440 that are
amplified by a SOA 460. The SOA current I.sub.2 includes pulses of
electrical current, and each pulse of current may cause the SOA 460
to amplify a corresponding temporal portion 441 of the seed light
440 to produce an emitted pulse of light 400. A temporal portion
441 of seed light 440 may refer to a portion of the seed light 440
located in a particular interval of time over which a pulse of
current I.sub.2 is applied to a SOA 460. For example, the portion
of seed light 440 located in the time interval between times ta and
tb in FIG. 15 corresponds to one temporal portion 441 of the seed
light 440. The corresponding pulse of SOA current between the times
t.sub.a and t.sub.b results in the amplification of the temporal
portion 441 and the emission of a pulse of light 400. The duration
of a temporal portion 441 (e.g., as represented by t.sub.b-t.sub.a)
or the duration of a SOA current pulse may be approximately 0.5 ns,
1 ns, 2 ns, 5 ns, 10 ns, 20 ns, 50 ns, 100 ns, or any other
suitable duration.
[0166] Each emitted pulse of light 400 in FIG. 15 may include a
temporal portion 441 of seed light 440 that is amplified by a SOA
460, and during the time period between successive pulses of SOA
current I.sub.2, the seed light 440 may be substantially absorbed
by the SOA 460. The emitted pulses of light 400 are part of an
output beam 125 and have a pulse duration of .DELTA..tau. and a
pulse period of .tau.. For example, the emitted pulses of light 400
may have a pulse period of approximately 100 ns, 200 ns, 500 ns, 1
.mu.s, 2 .mu.s, 5 .mu.s, 10 .mu.s, or any other suitable pulse
period. As another example, the emitted pulses of light 400 may
have a pulse duration of 1-10 ns and a pulse period of 0.5-2.0
.mu.s. In particular embodiments, when a current pulse is applied
to a SOA 460, there may be a time delay until the optical gain of
the SOA 460 builds up to exceed the optical loss of the SOA 460. As
a result, the pulse duration .DELTA..tau. of an emitted pulse of
light 400 may be less than or equal to the duration of a
corresponding pulse of SOA current I.sub.2. For example, a SOA
current pulse with a duration of 8 ns may produce an emitted pulse
of light 400 with a duration of 6 ns. In the example of FIG. 15,
the emitted pulses of light 400 may have a duration of
approximately 5 ns, and the SOA current pulses may have a duration
(e.g., as represented by t.sub.b-t.sub.a) of approximately 5 ns to
10 ns.
[0167] FIG. 16 illustrates example graphs of seed light 440, an
emitted optical pulse 400, a received optical pulse 410, LO light
430, and detector photocurrent i. Each of the parameters (seed
light 440, emitted optical pulse 400, received optical pulse 410,
LO light 430, and photocurrent i) in FIG. 15 is plotted versus
time. The seed light 440 may include CW light or light having a
substantially constant optical power, and the temporal portion 441
of the seed light 440 may be amplified by a SOA 460 to produce the
emitted pulse of light 400. The emitted pulse of light 400 is part
of output beam 125, and the received pulse of light 410 is part of
input beam 135. The received pulse of light 410, which is received
a time interval .DELTA.T after the pulse of light 400 is emitted,
may include light from the emitted optical pulse 400 that is
scattered by a target 130. The distance D from the lidar system 100
to the target 130 may be determined from the expression
D=c.DELTA.T/2.
[0168] In particular embodiments, a received pulse of light 410 and
LO light 430 may be combined and coherently mixed together at one
or more detectors 340 of a receiver 140. Each detector 340 may
produce a photocurrent signal i that corresponds to coherent mixing
of the received pulse of light 410 and the LO light 430. In FIG.
16, the received pulse of light 410 is coherently mixed with a
temporal portion 431 of the LO light 430 to produce a corresponding
pulse of detector photocurrent i. A temporal portion 431 of LO
light 430 may refer to a portion of the LO light 430 that is
coincident with a received pulse of light 410. In FIG. 16, temporal
portion 431 and the received pulse of light 410 are each located in
the time interval between times t.sub.c and t.sub.d. The coherent
mixing of the pulse of light 410 and the temporal portion 431 may
occur at a detector 340 of the receiver 140, and the detector 340
may produce a pulse of detector photocurrent i in response to the
coherent mixing. Coherent mixing of two optical signals (e.g., a
received pulse of light 410 and LO light 430) may be referred to as
optical mixing, mixing, optical interfering, coherent combining,
coherent detection, homodyne detection, or heterodyne
detection.
[0169] In particular embodiments, coherent mixing may occur when
two optical signals that are coherent with one another are
optically combined and then detected by a detector 340. If two
optical signals can be coherently mixed together, the two optical
signals may be referred to as being coherent with one another. Two
optical signals being coherent with one another may include two
optical signals (i) that have approximately the same optical
frequency, (ii) that have a particular optical frequency offset
(.DELTA.f), or (iii) that each have a substantially fixed or stable
optical frequency over a particular period of time. For example,
seed light 440 and LO light 430 in FIG. 16 may be coherent with one
another since they may have approximately the same optical
frequency or each of their frequencies may be substantially fixed
over a time period approximately equal to coherence time T.sub.c.
As another example, the emitted pulse of light 400 and the temporal
portion 431 of LO light 430 in FIG. 16 may be coherent with one
another. And since the received pulse of light 410 may include a
portion of the emitted pulse of light 400, the received pulse of
light 410 and the temporal portion 431 may also be coherent with
one another.
[0170] In particular embodiments, if two optical signals each have
a stable frequency over a particular period of time, then the two
optical signals may be (i) optically combined together and (ii)
coherently mixed at a detector 340. Optically combining two optical
signals (e.g., an input beam 135 and LO light 430) may refer to
combining two optical signals so that their respective electric
fields are summed together. Optically combining two optical signals
may include overlapping the two optical signals (e.g., with an
optical combiner 420) so that they are substantially coaxial and
travel together in the same direction and along approximately the
same optical path. Alternatively, optically combining two optical
signals may include directing the two optical signals to a detector
340 (e.g., without using an optical combiner) so that the two
optical signals overlap at or within the detector 340.
Additionally, optically combining two optical signals may include
overlapping the two optical signals so that at least a portion of
their respective polarizations have the same orientation. Once the
two optical signals are optically combined, they may be coherently
mixed at a detector 340, and the detector 340 may produce a
photocurrent signal i corresponding to the summed electrical fields
of the two optical signals.
[0171] In particular embodiments, a portion of seed light 440 may
be coherent with a portion of LO light 430. For example, LO light
430 and seed light 440 may be coherent with one another over a time
period approximately equal to the coherence time T.sub.c. In each
of FIGS. 8-11, the LO light 430 and the seed light 440 may be
coherent with one another since the two optical signals are derived
from the same seed laser diode 450. In FIG. 12, the LO light 430
and the seed light 440 may be coherent with one another since the
two optical signals may have a particular frequency offset. In FIG.
16, the temporal portion 441 of the seed light 440 may be coherent
with the temporal portion 431 of the LO light 430. Additionally,
the temporal portion 441 may be coherent with any portion of the LO
light 430 extending over at least the time interval .DELTA.T or
T.sub.c (e.g., from approximately time t.sub.a to at least time
t.sub.d). The coherence time T.sub.c may correspond to a time over
which light emitted by a seed laser diode 450 is coherent (e.g.,
the emitted light may have a substantially fixed or stable
frequency over a time interval of T.sub.c). The coherence length
L.sub.c is the distance over which the light from a seed laser
diode 450 is coherent, and the coherence time and coherence length
may be related by the expression L.sub.c=cT.sub.c. For example, a
seed laser diode 450 may have a coherence length of approximately
500 m, which corresponds to a coherence time of approximately 1.67
.mu.s. The seed light 440 and LO light 430 emitted by a seed laser
diode 450 may have a coherence length of approximately 1 m, 10 m,
50 m, 100 m, 300 m, 500 m, 1 km, or any other suitable coherence
length. Similarly, the seed light 440 and LO light 430 may have a
coherence time of approximately 3 ns, 30 ns, 150 ns, 300 ns, 1
.mu.s, 1.5 .mu.s, 3 .mu.s, or any other suitable coherence
time.
[0172] In particular embodiments, each emitted pulse of light 400
may be coherent with a corresponding temporal portion of LO light
430. In FIG. 16, the corresponding portion of the LO light 430 may
include any portion of the LO light 430 (including temporal portion
431) extending from approximately time t.sub.a to at least time
t.sub.d, and the emitted pulse of light 400 may be coherent with
any portion of the LO light 430 from time t.sub.a to time t.sub.d.
In FIG. 15, each emitted pulse of light 400 may be coherent with
the LO light 430 over a time period from when the pulse of light
400 is emitted until at least a time .tau. (the pulse period) after
the pulse is emitted. Similarly, in each of FIGS. 8-11, the emitted
pulse of light 400 may be coherent with the LO light 430 for at
least a time .tau. after the pulse 400 is emitted. In FIG. 13, the
fiber-optic amplifier 500 may preserve the coherence of the pulse
of light 400a, and the emitted pulse of light 400b may be coherent
with the LO light 430 for at least a time .tau. after the pulse
400b is emitted.
[0173] In particular embodiments, each emitted pulse of light 400
may include a temporal portion 441 of the seed light 440 that is
amplified by a SOA 460, and the amplification process may be a
coherent amplification process that preserves the coherence of the
temporal portion 441. Since the temporal portion 441 may be
coherent with a corresponding portion of the LO light 430, the
emitted pulse of light 400 may also be coherent with the same
portion of the LO light 430. An emitted pulse of light 400 being
coherent with a corresponding portion of LO light 430 may
correspond to temporal portion 441 being coherent with the
corresponding portion of the LO light 430. In the example of FIG.
16, the temporal portion 441 may be coherent with the LO light 430
over at least the time interval .DELTA.T or T.sub.c (e.g., from
approximately time t.sub.a to at least time t.sub.d). Since the
emitted pulse of light 400 may be coherent with the temporal
portion 441, the emitted pulse of light 400 may also be coherent
with any portion of the LO light 430 (including the temporal
portion 431) from approximately time t.sub.a until at least time
t.sub.d. An emitted pulse of light 400 being coherent with any
portion of LO light 430 in the time period from time t.sub.a until
at least time t.sub.d indicates that the emitted pulse of light 400
may be coherently mixed with any portion of the LO light 430
(including the temporal portion 431) over this same time period.
The received pulse of light 410 includes light from the emitted
pulse of light 400 (e.g., light from the emitted pulse of light 400
that is scattered by a target 130), and so the received pulse of
light 410 may be coherent with the emitted pulse of light 400.
Based on this, the received pulse of light 410 may also be
coherently mixed with any portion of the LO light 430 over the
t.sub.a to t.sub.d time period.
[0174] In particular embodiments, an emitted pulse of light 400
being coherent with a corresponding portion of LO light 430 may
correspond to the LO light 430 having a coherence length greater
than or equal to 2.times.D.sub.OP, where D.sub.OP is an operating
distance of the lidar system 100. The coherence length L.sub.c
being greater than or equal to 2.times.D.sub.OP corresponds to the
coherence time T.sub.c being greater than or equal to
2.times.D.sub.OP/c. Since the quantity 2.times.D.sub.op/c may be
approximately equal to the pulse period .tau., the coherence length
L.sub.c being greater than or equal to 2.times.D.sub.OP may
correspond to the coherence time T.sub.c being greater than or
equal the pulse period .tau.. The LO light 430 and the seed light
440 may be coherent with one another over the coherence time
T.sub.c, which corresponds to the temporal portion 441 in FIG. 16
being coherent with the LO light 430 over the coherence time
T.sub.c. Similarly, the emitted pulse of light 400, which includes
the temporal portion 441 amplified by the SOA 460, may be coherent
with the LO light 430 over the coherence time T.sub.c. If the
coherence length of the LO light 430 is greater than or equal to
2.times.D.sub.OP (or, if T.sub.c is greater than or equal to
.tau.), then an emitted pulse of light 400 may be coherent with any
portion of the LO light 430 (including the temporal portion 431)
from a time when the pulse of light 400 is emitted until at least a
time .tau. after the pulse is emitted. This indicates that a
received pulse of light 410 (which includes light from the emitted
pulse of light 400 scattered from a target 130) may be coherently
mixed with the LO light 430 as long as the distance D to the target
130 is within the operating distance of the lidar system 100 (e.g.,
D.ltoreq.D.sub.OP).
[0175] In particular embodiments, each emitted pulse of light 400
may be coherent with a corresponding portion of LO light 430, and
the corresponding portion of the LO light 430 may include temporal
portion 431 of the LO light 430. The temporal portion 431
represents the portion of the LO light 430 that is detected by a
receiver 140 at the time when the received pulse of light 410 is
detected by the receiver 140. In FIG. 16, the temporal portion 431
is coincident with the received pulse of light 410, and both
optical signals are located between times t.sub.c and t.sub.d.
Since the received pulse of light 410 includes scattered light from
the emitted pulse of light 400, the received pulse of light 410 may
be coherent with the temporal portion 431 of the LO light 430. The
received pulse of light 410 and the temporal portion 431 may be
coherently mixed together at a detector 340 of the receiver, and
the coherent mixing may result in a pulse of detector photocurrent
i, as illustrated in FIG. 16.
[0176] In particular embodiments, a received pulse of light 410 may
be coherent with a temporal portion 431 of LO light 430. In FIG.
16, the received pulse of light 410 and the temporal portion 431,
which are coherently mixed together, are coherent with one another.
In particular embodiments, the coherent mixing of a received pulse
of light 410 and a temporal portion 431 may not require that the
coherence time T.sub.c associated with seed light 440 or LO light
430 be greater than or equal to the pulse period .tau.. For
example, the received pulse of light 410 and the temporal portion
431 may be coherently mixed even if the coherence time is less than
.DELTA.T or less than the pulse period .tau.. Coherent mixing may
occur if the coherence time T.sub.c associated with the seed light
440 or the LO light 430 is greater than or equal to the duration of
the received pulse of light 410 or the duration of the temporal
portion 431. If a received pulse of light 410 and a temporal
portion 431 each has a substantially fixed or stable frequency over
at least the duration of the temporal portion 431, then the
received pulse of light 410 and the temporal portion 431 may be
coherently mixed together. As long as the received pulse of light
410 and the temporal portion 431 each has an optical frequency that
is substantially fixed or stable over the duration of the pulse of
light 410 or over the duration of the temporal portion 431, then
the two optical signals may be coherently mixed together. In the
example of FIG. 16, the received pulse of light 410 and the
temporal portion 431 may be coherent over the duration of the
temporal portion 431 (e.g., the coherence time T.sub.c may be
greater than or equal to t.sub.d-t.sub.c), and their electric
fields may be coherently combined (e.g., summed together) and
coherently mixed together.
[0177] FIG. 17 illustrates an example voltage signal 360 that
results from coherent mixing of LO light 430 and a received pulse
of light 410. The optical spectrum of the LO light 430 is
represented by a frequency-domain graph that illustrates relative
optical power versus optical frequency. The LO light 430 has a
center optical frequency of f.sub.0 and a relatively narrow
spectral linewidth of .DELTA.v.sub.1. For example, the optical
frequency f may be approximately 199.2 THz (corresponding to a
wavelength of approximately 1505 nm), and the spectral linewidth
.DELTA.v.sub.1 may be approximately 2 MHz. The upper
frequency-domain graph of the received pulse of light 410 indicates
that the received pulse of light 410 has an optical spectrum with
approximately the same center frequency f.sub.0 and a broader
spectral linewidth of .DELTA.v.sub.2. The lower time-domain graph
indicates that the received pulse of light 410 has a duration of
.DELTA.T. The coherent mixing of the LO light 430 and the received
pulse of light 410 at a detector 340 results in a pulse of
photocurrent with a duration of .DELTA..tau..sub.p. The
photocurrent signal i may be amplified by an amplifier 350 that
produces a corresponding voltage signal 360. The upper
voltage-signal graph illustrates the voltage signal 360 in the time
domain and includes a pulse of voltage with a duration of
.DELTA.T'. The lower voltage-signal graph in FIG. 17 is a
frequency-domain graph of the voltage signal 360 that indicates
that the voltage signal 360 has an electrical bandwidth of
.DELTA.v.
[0178] A pulse duration (.DELTA..tau.) and spectral linewidth
(.DELTA.v.sub.2) of a pulse of light may have an inverse
relationship where the product .DELTA..tau..DELTA.v.sub.2 (which
may be referred to as a time-bandwidth product) is equal to a
constant value. For example, a pulse of light with a Gaussian
temporal shape may have a time-bandwidth product equal to a
constant value that is greater than or equal to 0.441. If a
Gaussian pulse has a time-bandwidth product that is approximately
equal to 0.441, then the pulse may be referred to as a
transform-limited pulse. For a transform-limited Gaussian pulse,
the pulse duration (.DELTA..tau.) and spectral linewidth
(.DELTA.v.sub.2) may be related by the expression
.DELTA..tau..DELTA.v.sub.2=0.441. The inverse relationship between
pulse duration and spectral linewidth indicates that a
shorter-duration pulse has a larger spectral linewidth (and vice
versa). For example, the received pulse of light 410 in FIG. 17 may
be a transform-limited Gaussian pulse with (i) a pulse duration
.DELTA..tau. of 2 ns and a spectral linewidth .DELTA.v.sub.2 of
approximately 220 MHz or (ii) a pulse duration .DELTA..tau. of 4 ns
and a spectral linewidth .DELTA.v.sub.2 of approximately 110 MHz.
This inverse relationship between pulse duration and spectral
linewidth results from the Fourier-transform relationship between
time-domain and frequency-domain representations of a pulse. If a
Gaussian pulse of light has a time-bandwidth product that is
greater than 0.441, then the pulse of light may be referred to as a
non-transform-limited pulse of light. For example, the received
pulse of light 410 in FIG. 17 may be a non-transform-limited pulse
of light with a time-bandwidth product of 1, and the received pulse
of light 410 may have (i) a pulse duration .DELTA..tau. of 2 ns and
a spectral linewidth .DELTA.v.sub.2 of approximately 500 MHz or
(ii) a pulse duration .DELTA..tau. of 4 ns and a spectral linewidth
.DELTA.v.sub.2 of approximately 250 MHz.
[0179] The duration .DELTA..tau..sub.p of a pulse of photocurrent
may be greater than or equal to the duration .DELTA..tau. of the
corresponding received pulse of light 410. For example, due at
least in part to the finite temporal response of a detector 340,
the pulse of photocurrent may have a somewhat longer rise time,
fall time, or duration (e.g., a 0% to 20% longer rise time, fall
time, or duration) than the received pulse of light 410. Similarly,
the duration .DELTA..tau.' of a voltage pulse may be greater than
or equal to the duration .DELTA..tau..sub.p of the corresponding
pulse of photocurrent. For example, due at least in part to
electrical-bandwidth limitations of an electronic amplifier 350,
the pulse of voltage may have a somewhat longer rise time, fall
time, or duration (e.g., a 0% to 20% longer rise time, fall time,
or duration) than the pulse of photocurrent. In FIG. 17, the
received pulse of light may have a duration .DELTA..tau. of
approximately 5 ns, the pulse of photocurrent may have a duration
.DELTA..tau..sub.p of approximately 5.5 ns, and the pulse of
voltage may have a duration .DELTA..tau.' of approximately 6
ns.
[0180] In FIG. 17, the LO light 430 has a spectral linewidth of
.DELTA.v.sub.1, and the received pulse of light has a spectral
linewidth of .DELTA.v.sub.2. The voltage signal 360 has an
electrical bandwidth of .DELTA.v. A spectral linewidth of an
optical signal (e.g., seed light 440, LO light 430, or pulse of
light 410) may be referred to as a linewidth, optical linewidth,
bandwidth, or optical bandwidth. A spectral linewidth or an
electrical bandwidth may refer to an approximate width of a
spectrum as measured at the half-power points of the spectrum
(which may be referred to as the 3-dB points). A spectral linewidth
or an electrical bandwidth may be specified over a particular time
period, such as for example, over a period of time approximately
equal to a pulse duration (e.g., .DELTA..tau. or t.sub.b-t.sub.a),
a temporal-portion duration (e.g., t.sub.d-t.sub.c), a pulse period
.tau., a coherence time T.sub.c, or any other suitable period of
time. A spectral linewidth or an electrical bandwidth may be
specified over a time period of approximately 1 .mu.s, 10 .mu.s,
100 .mu.s, 1 ms, 10 ms, 100 ms, 1 s, 10 s, 100 s, or any other
suitable time period. For example, the LO light 430 in FIG. 17 may
have a spectral linewidth .DELTA.v.sub.1 of 4 MHz when measured
over a 100-ms time interval. A spectral linewidth for an optical
signal may be related to a variation in optical frequency of the
optical signal. For example, LO light 430 having a spectral
linewidth .DELTA.v.sub.1 of 4 MHz may correspond to LO light 430
having an optical-frequency variation of approximately .+-.2 MHz
over a 100-ms time interval.
[0181] In particular embodiments, seed light 440 or LO light 430
may have a spectral linewidth .DELTA.v.sub.1 of less than
approximately 50 MHz, 10 MHz, 5 MHz, 3 MHz, 1 MHz, 0.5 MHz, 100
kHz, or any other suitable spectral-linewidth value. In the example
of FIG. 17, the LO light 430 in FIG. 17 may have a spectral
linewidth .DELTA.v.sub.1 of approximately 3 MHz, and the
corresponding seed light (not illustrated in FIG. 17) may have
approximately the same spectral linewidth. When a temporal portion
441 of the seed light 440 is amplified to produce an emitted pulse
of light 400, the spectral linewidth of the emitted pulse of light
400 may have a broadened linewidth .DELTA.v.sub.2 that is greater
than .DELTA.v.sub.1. For example, an emitted pulse of light 400 and
a corresponding received pulse of light 410 may each have spectral
linewidth .DELTA.v.sub.2 of approximately 10 MHz, 50 MHz, 100 MHz,
200 MHz, 300 MHz, 500 MHz, 1 GHz, 10 GHz, 50 GHz, or any other
suitable linewidth. As another example, the LO light 430 in FIG. 17
may have a spectral linewidth .DELTA.v.sub.1 of 5 MHz, and the
received pulse of light 410 in FIG. 17 may have a spectral
linewidth .DELTA.v.sub.2 of 100 MHz. As another example, the
received pulse of light 410 in FIG. 17 may have a duration
.DELTA..tau. of approximately 3-6 ns and a spectral linewidth
.DELTA.v.sub.2 of approximately 75-150 MHz.
[0182] In particular embodiments, an electrical bandwidth .DELTA.v
of a voltage signal 360 may be approximately equal to a numeric
combination of the linewidths of the corresponding LO light 430 and
received pulse of light 410. The electrical bandwidth .DELTA.v may
be greater than both of the linewidths .DELTA.v.sub.1 and
.DELTA.v.sub.2. For example, the electrical bandwidth .DELTA.v may
be approximately equal to the sum of the linewidths of the LO light
430 and the received pulse of light 410 (e.g.,
.DELTA.v.apprxeq..DELTA.v.sub.1+.DELTA.v.sub.2). As another
example, the electrical bandwidth .DELTA.v may be approximately
equal to {square root over
(.DELTA.v.sub.1.sup.2+.DELTA.v.sub.2.sup.2)}. In FIG. 17, the LO
light 430 may have a spectral linewidth .DELTA.v.sub.1 of
approximately 3 MHz, and the received pulse of light 410 may have a
spectral linewidth .DELTA.v.sub.2 of approximately 150 MHz. The
electrical bandwidth .DELTA.v of the voltage signal 360 may be
approximately equal to the sum of the two linewidths, or 153
MHz.
[0183] In particular embodiments, a photocurrent signal i produced
by a detector 340 in response to coherent mixing of LO light 430
and a received pulse of light 410 may be expressed as
i(t)=k|.epsilon..sub.Rx(t)+.epsilon..sub.LO(t)|.sup.2, where the
variable t represents time. The parameter k is a constant, and k
may account for the responsivity of the detector 340 as well as
other constant parameters or conversion factors. For clarity, the
constant k or other constants (e.g., conversion constants or
factors of 2 or 4) may be excluded from expressions herein related
to a photocurrent signal i. For example, the photocurrent signal i,
which is proportional to
|.epsilon..sub.Rx(t)+.epsilon..sub.LO(t)|.sup.2, may be written as
i(t)=|.epsilon..sub.Rx(t)+.epsilon..sub.LO(t)|.sup.2, with the
proportionality constant k removed from the expression for clarity.
In the expression for i(t), .sub.RX(t) represents the electric
field of the received pulse of light 410, and .epsilon..sub.LO(t)
represents the electric field of the LO light 430. The electric
field of the received pulse of light 410 may be expressed as
E.sub.Rx cos[.omega..sub.Rxt+.PHI..sub.Rx(t)], where E.sub.Rx is
the amplitude of the electric field of the received pulse of light
410. The amplitude of the electric field of the received pulse of
light 410, which may be expressed as |.epsilon..sub.Rx(t)|,
E.sub.Rx(t), or E.sub.Rx, may vary with time (e.g., the
electric-field amplitude may have a time dependence corresponding
to the temporal shape of the received pulse of light). Similarly,
the electric field of the LO light 430 may be expressed as E.sub.LO
cos[.omega..sub.LOt+.PHI..sub.LO(t)], where E.sub.LO is the
amplitude of the electric field of the LO light 430. The amplitude
of the electric field of the LO light 430, which may be expressed
as |.epsilon..sub.LO(t)|, E.sub.LO(t), or E.sub.LO, may vary with
time or may be substantially constant (e.g., corresponding to the
substantially constant optical power of the LO light). The
frequency .omega..sub.Rx represents the optical frequency of the
electric field of the received pulse of light 410, and
.omega..sub.LO represents the optical frequency of the electric
field of the LO light 430. A frequency represented by .omega. is a
radial frequency (with units radians/s) and is related to the
frequency f (with units cycles/s) by the expression .omega.=2.pi.f.
Each of the frequencies .omega..sub.Rx and .omega..sub.LO, which
may be expressed as .omega..sub.Rx(t) or .omega..sub.LO(t), may
vary with time or may be substantially constant with time.
Similarly, a frequency difference (e.g., between a received pulse
of light 410 and LO light 430) may be expressed in cycles/s as
.DELTA.f or in radians/s as .DELTA..omega., where
.DELTA..omega.=2.pi..DELTA.f. The parameter .PHI..sub.Rx(t)
represents the phase of the electric field of the received pulse of
light 410, and .PHI..sub.LO(t) represents the phase of the electric
field of the LO light 430. Each of the phases .PHI..sub.Rx(t) and
.PHI..sub.LO(t), which may be expressed as .PHI..sub.Rx and
.PHI..sub.LO, may vary with time or may be substantially constant
with time.
[0184] In the expression for the photocurrent signal
i(t)=|.epsilon..sub.Rx(t)+.epsilon..sub.LO(t)|.sup.2, summing the
two electric fields and then taking the square of the magnitude of
that sum may correspond to coherent mixing of the LO light 430 and
the received pulse of light 410. The first step of summing the two
electric fields corresponds to optically combining or overlapping
the LO light 430 with the input beam 135 (which includes the
received pulse of light 410) so that their electric fields add
together. This may include either combining the two beams (LO light
430 and input beam 135) using an optical combiner 420 or combining
the two beams at a detector 340 without using an optical combiner
420. Additionally, summing the two electric fields may include
using an optical polarization element 465 so that at least portions
of the two electric fields are oriented along the same direction.
The second step of taking the square of the magnitude of the summed
electric fields occurs at the detector 340 and may correspond to
the detection by the detector 340 of light corresponding to the
summed electric fields (where the summed electric fields correspond
to the combined LO light 430 and received pulse of light 410). A
detector 340 may produce a photocurrent signal that is proportional
to the optical power or intensity of a received optical signal,
which in turn is proportional to the square of the electric field
of the received optical signal. This indicates that the
photocurrent signal i produced by the detector 340 is proportional
to the square of the electric field at the detector, and the
electric field at the detector includes the sum of the electric
fields of the LO light 430 and the received pulse of light 410. The
coherent mixing of LO light 430 and a received pulse of light 410
may occur at a receiver 140 of a lidar system 100. If a lidar
system 100 includes an optical combiner 420 that combines LO light
430 and input beam 135, the combiner 420 may be considered to be
part of the receiver 140. Similarly, if a lidar system 100 includes
a polarization element 465 that alters a polarization of the LO
light 430 or the input beam 135, the polarization element 465 may
be considered to be part of the receiver 140.
[0185] The expression for the photocurrent signal
i(t)=|.epsilon..sub.Rx(t)+.epsilon..sub.LO(t)|.sup.2, may be
expanded and written as
i(t)=|.epsilon..sub.Rx(t)|.sup.2+2|.epsilon..sub.Rx(t)||E.sub.LO(t)|cos[.-
DELTA..omega.(t)t+.DELTA..PHI.(t)]+|.epsilon..sub.LO(t))|.sup.2.
The frequency-difference term .DELTA..omega.(t) (which may be
expressed as .omega..sub.Rx(t)-.omega..sub.LO(t),
.omega..sub.Rx-.omega..sub.LO, or .DELTA..omega.) represents the
frequency difference between the electric field of the received
pulse of light 410 and the electric field of the LO light 430. The
frequency-difference term .DELTA..omega.(t) may have a value of
approximately zero (e.g., if the optical frequencies of the
received pulse of light 410 and the LO light 430 are approximately
the same), may have an approximately constant non-zero value (e.g.,
if the optical frequencies have an approximately constant frequency
difference), or may vary with time. Similarly, the phase-difference
term .DELTA..PHI.(t) (which may be expressed as
.PHI..sub.Rx(t)-.PHI..sub.LO(t), .PHI..sub.Rx-.PHI..sub.LO, or
.DELTA..PHI.) represents the phase difference between the electric
field of the received pulse of light 410 and the electric field of
the LO light 430. The phase-difference term may have a value of
approximately zero, may have an approximately constant non-zero
value, or may vary with time.
[0186] In the above expanded expression for the photocurrent signal
i, the terms |E.sub.Rx(t)| and | .sub.LO(t)| may be written as
E.sub.Rx and E.sub.LO, respectively, and the expanded expression
for the photocurrent signal i may then be written as
i(t)=E.sub.Rx.sup.2+2E.sub.RxE.sub.LO
cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.Rx(t)-.PHI..sub.LO(t)]+E.s-
ub.LO.sup.2. In this expanded expression for the photocurrent
signal i(t), the first term E.sub.Rx.sup.2 corresponds to the
optical power of the received pulse of light 410, and the first
term may be referred to as the pulse term. The third term
E.sub.LO.sup.2 corresponds to the optical power of the LO light 430
and may be referred to as the LO term. If the received pulse of
light 410 is a Gaussian pulse with a pulse width of .DELTA..tau.,
the first term may be expressed as E.sub.Rx.sup.2(t)=P.sub.Rx
exp[-(2 {square root over (ln 2)}t/.DELTA..tau.).sup.2], where
P.sub.Rx is the peak power of the received pulse of light 410. If
the LO light 430 has a substantially constant optical power, the
third term may be expressed as E.sub.LO.sup.2=P.sub.LO, where
P.sub.LO is the average power of the LO light 430. In particular
embodiments, a photocurrent signal i corresponding to the coherent
mixing of LO light 430 and a received pulse of light 410 may
include a coherent-mixing term. The second term in the above
expression, 2E.sub.RxE.sub.LO
cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.RX(t)-.PHI..sub.LO(t)],
corresponds to the coherent mixing of LO light 430 and the received
pulse of light 410, and the second term may be referred to as a
coherent-mixing term or a coherent-mixing cross-product term. If
the received pulse of light 410 and the LO light 430 have
approximately the same optical frequency, then (ax is approximately
equal to .omega..sub.LO, and the coherent-mixing term may be
expressed as 2E.sub.RxE.sub.LO
cos[.PHI..sub.Rx(t)-.PHI..sub.LO(t)]. The coherent-mixing term
represents coherent mixing between the electric fields of the
received pulse of light 410 and the LO light 430. The
coherent-mixing term is proportional to E.sub.RxE.sub.LO
cos[(.omega..sub.RX-.omega..sub.LO)t+.PHI..sub.Rx(t)-.PHI..sub.LO(t)].
Additionally, the coherent-mixing term is proportional to the
product of (i) E.sub.Rx, the amplitude of the electric field of the
received pulse of light 410 and (ii) E.sub.LO, the amplitude of the
electric field of the LO light 430. The amplitude of the electric
field of the received pulse of light 410 may be time dependent
(e.g., corresponding to a Gaussian or other pulse shape), and the
E.sub.LO term may be substantially constant, corresponding to an
optical power of LO light 430 that is substantially constant.
[0187] A hybrid pulsed/coherent lidar system 100 as described
herein may operate as a combination or "hybrid" of a
direct-detection pulsed lidar system and a coherent pulsed lidar
system. A hybrid pulsed/coherent lidar system 100 may have two
operating regimes depending on the distance or reflectivity of a
target 130. For a target 130 located relatively close to the lidar
system 100 or having a relatively high reflectivity, the lidar
system may act primarily as a direct-detection pulsed lidar system
and may detect a received pulse of light 410 primarily based on the
first term above (E.sub.Rx.sup.2), which corresponds to the power
of the received pulse of light 410. For a target 130 located
relatively far from the lidar system 100 or having a relatively low
reflectivity, the lidar system may act primarily as a coherent
pulsed lidar system and may detect a received pulse of light 410
primarily based on the coherent-mixing term 2E.sub.RxE.sub.LO
cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.Rx(t)-.PHI..sub.LO(t)],
which corresponds to the coherent mixing of LO light 430 and the
received pulse of light 410.
[0188] A hybrid pulsed/coherent lidar system 100 as described
herein may provide a higher sensitivity than a conventional
direct-detection pulsed lidar system (which may be referred to as a
non-coherent pulsed lidar system). For example, compared to a
conventional direct-detection pulsed lidar system, a hybrid lidar
system may be able to detect targets 130 that are farther away or
that have lower reflectivity. In a conventional direct-detection
pulsed lidar system, a received pulse of light may be directly
detected by a detector, without LO light and without coherent
mixing. The photocurrent signal produced in a conventional
direct-detection pulsed lidar system may correspond to the
E.sub.Rx.sup.2 term discussed above, which represents the power of
a received pulse of light. The size of the E.sub.Rx.sup.2 term may
be determined primarily by the distance to the target 130 and the
reflectivity of the target 130, and aside from boosting the energy
of the emitted pulses of light 400, increasing the size of the
E.sub.Rx.sup.2 term may not be practical or feasible. In a hybrid
lidar system 100 as discussed herein, the detected signal includes
the E.sub.Rx.sup.2 term as well as the coherent-mixing term, which
is proportional to the product of E.sub.Rx and E.sub.LO, and the
improved sensitivity of a hybrid lidar system 100 may result from
the addition of the coherent-mixing term. While it may not be
practical or feasible to increase the amplitude of E.sub.Rx for
far-away or low-reflectivity targets 130, the amplitude of the
E.sub.LO term may be increased by increasing the power of the LO
light 430. The power of the LO light 430 can be set to a level that
results in an effective boosting of the size of the coherent-mixing
term, which results in an increased sensitivity of the lidar system
100. In the case of a conventional direct-detection pulsed lidar
system, the signal of interest depends on E.sub.Rx.sup.2, the power
of the received pulse of light. In a hybrid pulsed/coherent lidar
system 100, the signal of interest, which depends on E.sub.Rx.sup.2
as well as the product of E.sub.Rx and E.sub.LO, may be increased
by increasing the power of the LO light 430. The LO light 430 acts
to effectively boost the coherent-mixing term, which may result in
an improved sensitivity of the lidar system 100.
[0189] FIGS. 18-20 each illustrates an example receiver 140 that
includes an optical combiner 420. In particular embodiments, a
receiver 140 of a hybrid pulsed/coherent lidar system 100 may
include an optical combiner 420 that (i) combines LO light 430 with
a received pulse of light 410 (which is part of an input beam 135)
to produce a combined beam 422 and (ii) directs the combined beam
422 to a detector 340. Optically combining LO light 430 with an
input beam 135 may include spatially overlapping the LO light 430
with the input beam 135 to produce a combined beam 422. A combined
beam 422 may include at least a portion of the LO light 430 and at
least a portion of the received pulse of light 410, and the optical
combiner 420 may direct the combined beam 422 to a detector 340.
For example, an optical combiner 420 may produce one combined beam
422 and direct the combined beam 422 to one detector 340 (e.g., as
illustrated in FIG. 18). As another example, an optical combiner
420 may produce one combined beam 422 and direct the combined beam
422 to two or more detectors 340 located in close proximity to one
another. As another example, an optical combiner 420 may produce
two or more combined beams 422 (each combined beam including a
portion of LO light 430 and a portion of a received pulse of light
410) and direct each of the combined beams 422 to one or more
detectors 340. In FIGS. 19 and 20, the optical combiner 420
produces two combined beams and directs each of the combined beams
to one detector. In an embodiment where each of the combined beams
422 is directed to two or more detectors 340, each of the two of
more detectors may be located in close proximity to one
another.
[0190] An optical combiner 420 (which may be referred to as a
combiner, a beam combiner, or an optical beam combiner) may include
an integrated-optic component, a fiber-optic component, or a
free-space optical component. Each of the optical combiners 420 in
FIGS. 18-19 may be an integrated-optic combiner 420 and may be part
of a PIC. An integrated-optic combiner 420, which may be referred
to as an optical-waveguide combiner, may include optical waveguides
that direct, combine, or split light. Alternatively, an optical
combiner 420 may be a fiber-optic combiner that includes two input
optical fibers (that receive LO light 430 and input beam 135) and
one or more output optical fibers to direct one or more combined
beams 422 to one or more respective detectors 340. The optical
combiner 420 in FIG. 20 is a free-space 2.times.2 optical combiner
that receives two free-space input beams (LO light 430 and input
beam 135) and combines the input beams to produce two combined
free-space output beams 422a and 422b.
[0191] In particular embodiments, a receiver 140 may include a
2.times.1 optical combiner 420 and one or more detectors 340. The
2.times.1 optical combiner 420 may include two input ports (that
receive the LO light 430 and the input beam 135) and one output
port that directs a combined beam 422 to the one or more detectors
340. For example, an optical combiner 420 may be a fiber-optic
component that includes two input optical fibers (that receive LO
light 430 and input beam 135) and one output optical fiber that
directs a combined beam 422 to one or more detectors 340. The
receiver 140 in FIG. 18 includes an integrated-optic 2.times.1
combiner 420 (with two input waveguides and one output waveguide)
and one detector 340. The 2.times.1 optical combiner 420 receives
two input beams (LO light 430 and input beam 135) and combines the
input beams to produce one combined output beam 422 that is
directed to the detector 340. The optical combiner 420 in FIG. 6
may be a free-space 2.times.1 optical combiner that receives two
free-space input beams (LO light 430 and input beam 135) and
combines the beams to produce one combined free-space output beam
422.
[0192] In particular embodiments, a receiver 140 may include a
2.times.p optical combiner 420 and p detectors 340, where p is an
integer greater than or equal to 2 and represents the number of
output ports and detectors. A 2.times.p optical combiner 420 may
have two input ports (that receive LO light 430 and input beam 135)
and p output ports. The combiner 420 may produce p combined beams
422, and each output port may direct one of the combined beams to
one of the p detectors 340. In FIGS. 19-20, the parameter p is 2,
and each of the combiners 420 is a 2.times.2 optical combiner 420.
The receiver 140 in FIG. 19 includes an integrated-optic 2.times.2
combiner 420 (with two input waveguides and two output waveguides)
and two detectors (340a, 340b). The receiver 140 in FIG. 20
includes a free-space 2.times.2 optical combiner 420 and two
detectors. In each of FIGS. 19-20, the 2.times.2 optical combiner
420 receives two input beams (LO light 430 and input beam 135) and
combines the input beams to produce two combined output beams 422a
and 422b that are directed to the respective detectors 340a and
340b. Each of the combined beams 422a and 422b may include a
portion of the LO light 430 and a portion of the received pulse of
light 410. For example, the combiner 420 may distribute the LO
light 430 and the input beam 135 approximately equally so that each
of the combined beams 422a and 422b includes approximately 50% of
the LO light 430 and approximately 50% of the input beam 135.
[0193] In particular embodiments, a receiver 140 may include a
2.times.p optical combiner 420 and p.times.m detectors 340, where p
is an integer greater than or equal to 2. The parameter m is an
integer greater than or equal to 1 and represents the number of
detectors located at each of the p output ports of the combiner. In
FIGS. 19-20, the parameter p is 2 and the parameter m is 1 so that
there is one detector located at each of the two output ports. In
other embodiments, the parameter m may be greater than or equal to
2, and there may be m detectors located at each output port, where
each group of m detectors may be located in close proximity to one
another. For example, adjacent detectors in a group of m detectors
may be separated by less than a size of the detector active region
or by less than a beam diameter of a combined beam 422 at the
detector plane.
[0194] In particular embodiments, a receiver 140 of a lidar system
100 may include one or more detectors 340 configured to produce one
or more respective photocurrent signals i corresponding to coherent
mixing of LO light 430 and a received pulse of light 410. The
receiver 140 in FIG. 18 includes one detector 340 that detects the
combined beam 422. The detector 340 may produce a photocurrent
signal i that corresponds to the coherent mixing of the LO light
430 and the received pulse of light 410. As described above, the
photocurrent signal may be expressed as
i(t)=E.sub.Rx.sup.2+2E.sub.RxE.sub.LO
cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.Rx(t)-.PHI..sub.LO(t)]+E.s-
ub.LO.sup.2. The voltage signal 360 produced by the amplifier 350
may correspond to the photocurrent signal i. For example, the
voltage signal 360 may be approximately proportional to the current
signal i(t), and the voltage signal 360 may be expressed as
v(t)=Gi(t), where G is the electronic gain of the amplifier 350 and
has units of volts/ampere.
[0195] The receiver 140 in each of FIGS. 19 and 20 includes the two
detectors 340a and 340b. The combiner 420 directs a first portion
422a of the combined light to detector 340a and directs a second
portion 422b of the combined light to detector 340b. In FIGS. 19
and 20, each of the detectors 340a and 340b produces a respective
photocurrent signal i.sub.a and i.sub.b. The portions of LO light
430 and received pulse of light 410 that make up the combined beam
422a may be coherently mixed at detector 340a to produce the
photocurrent signal i.sub.a. Similarly, the portions of LO light
430 and received pulse of light 410 that make up the combined beam
422b may be coherently mixed at detector 340b to produce the
photocurrent signal i.sub.b. In FIG. 19, each of the photocurrent
signals i.sub.a and i.sub.b is sent to one of the electronic
amplifiers 350a and 350b. The amplifiers 350a and 350b produce the
respective voltage signals 360a and 360b, which may be combined
together (e.g., voltage signals 360a and 360b may be added
together) to produce a combined voltage signal. The combined
voltage signal may correspond to the sum of the photocurrent
signals, i.sub.a+i.sub.b. For example, the sum of voltage signals
360a and 360b may be approximately proportional to i.sub.a+i.sub.b
and may be expressed as v(t)=G[i.sub.a(t)+i.sub.b(t)], where G is
the electronic gain of each of the amplifiers 350a and 350b.
Alternatively, the receiver 140 in FIG. 19 may be configured so
that the voltage signals 360a and 360b are not combined together
(e.g., each voltage signal may be sent separately to a
pulse-detection circuit). In FIG. 20, the two photocurrent signals
i.sub.a and i.sub.b are first added together, and the combined
photocurrent signal i.sub.a+i.sub.b is then sent to one electronic
amplifier 350. The combined photocurrent signal may be expressed as
i.sub.a(t)+i.sub.b(t)=E.sub.Rx.sup.2+2E.sub.RxE.sub.LO
cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.Rx(t)-.PHI..sub.LO(t)]+E.s-
ub.LO.sup.2. The voltage signal 360 may correspond to the combined
photocurrent signal and may be expressed as
v(t)=G[i.sub.a(t)+i.sub.b(t)], where G is the electronic gain of
the amplifier 350. In other embodiments, a receiver 140 may be
configured so that the photocurrent signals i.sub.a and i.sub.b are
subtracted (e.g., to produce a combined photocurrent signal
i.sub.a-i.sub.b) or so that the voltage signals 360a and 360b are
subtracted.
[0196] In particular embodiments, a receiver 140 may include one or
more lenses. For example, the receiver 140 in FIG. 18 may include
one lens (not illustrated in FIG. 18) that focuses the combined
beam 422 onto the detector 340. As another example, the receiver
140 in FIG. 18 may include a lens (not illustrated in FIG. 18) that
focuses the LO light 430 into an input optical waveguide of the
combiner 420. The receiver 140 may also include another lens (not
illustrated in FIG. 18) that focuses the input beam 135 into the
other input optical waveguide of the combiner 420. As another
example, the receiver 140 in FIG. 19 may include one or more lenses
(not illustrated in FIG. 19) that focus the combined beam 422a as a
free-space optical beam onto the detector 340a or that focus the
combined beam 422b as a free-space optical beam onto the detector
340b. Alternatively, each of the detectors 340a and 340b in FIG. 19
may be butt-coupled or affixed to an output port of the combiner
420 without an intervening lens. For example, detectors 340a and
340b may each be positioned close to an output port of the combiner
420 to directly receive the respective combined beams 422a and
422b. In FIG. 19, rather than being free-space optical beams, the
combined beams 422a and 422b may primarily be confined beams that
propagate through a waveguide of the combiner 420 and are directly
coupled, with a minimum of free-space propagation (e.g., less than
1 mm of free-space propagation), onto the detectors 340a and
340b.
[0197] FIG. 21 illustrates an example receiver 140 in which LO
light 430 and an input beam 135 are combined at a detector 340. In
particular embodiments, a receiver 140 may not include a discrete
or separate optical combiner. For example, the LO light 430 and the
input beam 135 may be directed to a detector 340 (e.g., by one or
more mirrors or lenses) as separate beams without first being
combined into a combined beam. Additionally, the LO light 430 and
the input beam 135 may be directed to the detector 340 so that the
two beams are non-collinear, non-coaxial, or incident on the
detector at a non-zero angle with respect to one another. The
receiver 140 in FIG. 21 does not include an optical combiner, and
the LO light 430 and the input beam 135 are combined at the
detector 340 (e.g., at or near an input surface of the detector 340
or within the detector 340). The focusing lens 330 receives the LO
light 430 and the input beam 135 as non-coaxial beams and focuses
each of the two beams onto the detector 340. Additionally, the
focusing lens 330 directs the LO light 430 and the input beam 135
to the detector 340 at an angle to one another.
[0198] FIG. 22 illustrates an example receiver 140 that includes a
two-sided detector 340. In particular embodiments, a receiver 140
may include a detector 340 that receives an input beam 135 directed
to one side of the detector and LO light 430 directed to an
opposite side of the detector. A two-sided detector 340 may be
referred to as a detector with two-sided illumination, a detector
that is illuminated from two sides, a dual-sided detector, a
double-sided detector, or a dual-entry detector. In FIG. 22, the
detector 340 has two optical-input sides (input side 1 and input
side 2), where input side 1 is located opposite input side 2. The
input beam 135 with the received pulse of light 410 is incident on
input side 1 of the detector, and the LO light 430 is incident on
input side 2 of the detector. The receiver 140 in FIG. 22 does not
include an optical combiner. The LO light 430 and the input beam
135 are directed to the detector 340 via opposite sides, and the LO
light 430 and the input beam 135 are combined within the detector
340. Input sides 1 and 2 may each include a dielectric coating
(e.g., an anti-reflection coating, a partially reflective coating,
or a high-reflectivity coating). For example, input side 1 may
include an anti-reflection coating with a reflectivity of less than
5% at a wavelength of the input beam 135.
[0199] FIG. 23 illustrates an example receiver 140 that includes
two polarization beam-splitters 650. In particular embodiments, a
receiver 140 may include a LO-light polarization splitter 650 that
splits LO light 430 into two orthogonal polarization components
(e.g., horizontal and vertical). Additionally, the receiver 140 may
include an input-beam polarization splitter 650 that splits an
input beam 135 (which includes a received pulse of light 410) into
the same two orthogonal polarization components. In FIG. 23, the
LO-light polarization beam-splitter (PBS) 650 splits the LO light
430 into a horizontally polarized LO-light beam 430H and a
vertically polarized LO-light beam 430V. Similarly, the input-beam
PBS 650 splits the input beam 135 into a horizontally polarized
input beam 135H and a vertically polarized input beam 135V. The
horizontally polarized beams are directed to a
horizontal-polarization receiver, and the vertically polarized
beams are directed to a vertical-polarization receiver. The
receiver 140 illustrated in FIG. 23 may be referred to as a
polarization-insensitive receiver since the receiver 140 may be
configured to detect received pulses of light 410 regardless of the
polarization of the received pulses of light 410.
[0200] In particular embodiments, a polarization-insensitive
receiver 140 as illustrated in FIG. 23 may be implemented with
free-space components, fiber-optic components, integrated-optic
components, or any suitable combination thereof. For example, the
two PBSs 650 may be free-space polarization beam-splitting cubes,
and the input beam 135 and the LO light 430 may be free-space
optical beams. As another example, the two PBSs 650 may be
fiber-optic components, and the input beam 135 and the LO light 430
may be conveyed to the PBSs 650 via optical fiber (e.g.,
single-mode optical fiber or polarization-maintaining optical
fiber). Additionally, the horizontally and vertically polarized
beams may be conveyed to the respective H-polarization and
V-polarization receivers via polarization-maintaining optical
fiber.
[0201] In particular embodiments, a receiver 140 may include a
horizontal-polarization receiver and a vertical-polarization
receiver. The H-polarization receiver may combine a horizontally
polarized LO-light beam 430H and a horizontally polarized input
beam 135H and produce one or more photocurrent signals
corresponding to coherent mixing of the two horizontally polarized
beams. Similarly, the V-polarization receiver may combine the
vertically polarized LO-light beam 430V and the vertically
polarized input beam 135V and produce one or more photocurrent
signals corresponding to coherent mixing of the two vertically
polarized beams. Each of the H-polarization and V-polarization
receivers may be similar to one of the receivers 140 illustrated in
FIGS. 18-22. The H-polarization and V-polarization receivers may
each preserve the polarization of the respective horizontally and
vertically polarized beams. For example, the H-polarization and
V-polarization receivers may each include polarization-maintaining
optical fiber that maintains the polarization of the beams.
Additionally or alternatively, the H-polarization and
V-polarization receivers may each include a PIC with optical
waveguides configured to maintain the polarization of the
beams.
[0202] The polarization of an input beam 135 may vary with time or
may not be controllable by a lidar system 100. For example, the
polarization of received pulses of light 410 may vary depending at
least in part on (i) the optical properties of a target 130 from
which pulses of light 400 are scattered or (ii) atmospheric
conditions encountered by pulses of light 400 while propagating to
the target 130 and back to the lidar system 100. However, since the
LO light 430 is produced and contained within the lidar system 100,
the polarization of the LO light 430 may be set to a particular
polarization state. For example, the polarization of the LO light
430 sent to the LO-light PBS 650 may be configured so that the
LO-light beams 430H and 430V produced by the PBS 650 have
approximately the same power. The LO light 430 produced by a seed
laser 450 may be linearly polarized, and a half-wave plate may be
used to rotate the polarization of the LO light 430 so that it is
oriented at approximately 45 degrees with respect to the LO-light
PBS 650. The LO-light PBS 650 may split the 45-degree polarized LO
light 430 into horizontal and vertical components having
approximately the same power. By providing a portion of the LO
light 430 to both the H-polarization receiver and the
V-polarization receiver, the receiver 140 in FIG. 23 may produce a
valid, non-zero output electrical signal regardless of the
polarization of the received pulse of light 410.
[0203] Coherent mixing of LO light 430 and a received pulse of
light 410 may require that the electric fields of the LO light 430
and the received pulse of light 410 are oriented in approximately
the same direction. For example, if LO light 430 and input beam 135
are both vertically polarized, then the two beams may be optically
combined together and coherently mixed at a detector 340. However,
if the two beams are orthogonally polarized (e.g., LO light 430 is
vertically polarized and input beam 135 is horizontally polarized),
then the two beams may not be coherently mixed, since their
electric fields are not oriented in the same direction.
Orthogonally polarized beams that are incident on a detector 340
may not be coherently mixed, resulting in little to no output
signal from a receiver 140. To mitigate problems with
polarization-related signal variation, a lidar system 100 may
include a polarization-insensitive receiver 140 (e.g., as
illustrated in FIG. 23). Additionally or alternatively, a lidar
system 100 may include an optical polarization element 465 to
ensure that at least a portion of the LO light 430 and input beam
135 have the same polarization.
[0204] A polarization-insensitive receiver 140 as illustrated in
FIG. 23 may ensure that the receiver 140 produces a valid, non-zero
output electrical signal in response to a received pulse of light
410, regardless of the polarization of the received pulse of light
410. For example, the output electrical signals from the
H-polarization and V-polarization receivers may be added together,
resulting in a combined output signal that is insensitive to the
polarization of the received pulse of light 410. If a received
pulse of light 410 is horizontally polarized, then the
H-polarization receiver may generate a non-zero output signal and
the V-polarization receiver may generate little to no output
signal. Similarly, if a received pulse of light 410 is vertically
polarized, then the H-polarization receiver may generate little to
no output signal and the V-polarization receiver may generate a
non-zero output signal. If a received pulse of light 410 has a
polarization that includes a vertical component and a horizontal
component, then each of the H-polarization and V-polarization
receivers may generate a non-zero output signal corresponding to
the respective polarization component. By adding together the
signals from the H-polarization and V-polarization receivers, a
valid, non-zero output electrical signal may be produced by the
receiver 140 regardless of the polarization of the received pulse
of light 410.
[0205] FIGS. 24-27 each illustrates an example light source 110
that includes a seed laser 450, a semiconductor optical amplifier
(SOA) 460, and one or more optical modulators 495. In particular
embodiments, a light source 110 may include a phase or amplitude
modulator 495 configured to change a frequency, phase, or amplitude
of seed light 440, LO light 430, or emitted pulse of light 400. An
optical phase or amplitude modulator 495 may include an
electro-optic modulator (EOM), an acousto-optic modulator (AOM), an
electro-absorption modulator, a liquid-crystal modulator, or any
other suitable type of optical phase or amplitude modulator. For
example, an optical modulator 495 may include an electro-optic
phase modulator or an AOM that changes the frequency or phase of
seed light 440 or LO light 430. As another example, an optical
modulator 495 may include an electro-optic amplitude modulator, an
electro-absorption modulator, or a liquid-crystal modulator that
changes the amplitude of the seed light 440 or LO light 430. An
optical modulator 495 may be a free-space modulator, a fiber-optic
modulator (e.g., with fiber-optic input or output ports), or an
integrated-optic modulator (e.g., a waveguide-based modulator
integrated into a PIC).
[0206] In particular embodiments, an optical modulator 495 may be
included in a seed laser diode 450 or a SOA 460. For example, a
seed laser diode 450 may include a waveguide section to which an
external electrical current or electric field may be applied to
change the carrier density or refractive index of the waveguide
section, resulting in a change in the frequency or phase of seed
light 440 or LO light 430. As another example, the frequency,
phase, or amplitude of seed light 440 or LO light 430 may be
changed by changing or modulating the seed current I.sub.1 or the
SOA current I.sub.2. In this case, the seed laser diode 450 or SOA
460 may not include a separate or discrete modulator, but rather, a
modulation function may be distributed within the seed laser diode
450 or SOA 460. For example, the optical frequency of the seed
light 440 or LO light 430 may be changed by changing the seed
current I.sub.1. Changing the seed current I.sub.1 may cause a
refractive-index change in the seed laser diode 450, which may
result in a change in the optical frequency of light produced by
the seed laser diode 450.
[0207] In FIG. 24, the light source 110 includes a modulator 495
located between the seed laser 450 and the optical splitter 470.
The seed-laser output light 472 passes through the modulator 495
and is then split by the splitter 470 to produce the seed light 440
and LO light 430. The modulator 495 in FIG. 24 may be configured to
change a frequency, phase, or amplitude of the seed-laser output
light 472. For example, the modulator 495 may be a phase modulator
that applies a time-varying phase shift to the seed-laser output
light 472, which may result in a frequency change of the seed-laser
output light 472. The modulator 495 may be driven in synch with the
emitted pulses of light 400 so that the emitted pulses of light 400
and the LO light 430 each have a different frequency change
imparted by the modulator 495.
[0208] In FIG. 25, the light source 110 includes a modulator 495
located between the seed laser 450 and the SOA 460. The modulator
495 in FIG. 25 may be configured to change a frequency, phase, or
amplitude of the seed light 440. For example, since the LO light
430 does not pass through the modulator 495, the modulator 495 may
change the optical frequency of the seed light 440 so that it is
different from the optical frequency of the LO light 430. In FIG.
26, the light source 110 includes a modulator 495 located in the
path of the LO light 430. The modulator 495 in FIG. 26 may be
configured to change a frequency, phase, or amplitude of the LO
light 430. For example, since the seed light 440 does not pass
through the modulator 495, the modulator 495 may change the optical
frequency of the LO light 430 so that it is different from the
optical frequency of the seed light 440. In FIG. 25 or 26, the seed
light 440 and LO light 430 may be produced by an optical splitter
470 that splits seed-laser output light 472 to produce the seed
light 440 and the LO light 430. Alternatively, in FIG. 25 or 26,
the seed light 440 may be emitted from a front face 452 of a seed
laser diode, and the LO light 430 may be emitted from the back face
451 of the seed laser diode.
[0209] In FIG. 27, the light source 110 includes three optical
modulators 495a, 495b, and 495c. In particular embodiments, a light
source 110 may include one, two, three, or any other suitable
number of modulators 495. Each of the modulators 495a, 495b, and
495c may be configured to change a frequency, phase, or amplitude
of the seed-laser output light 472, seed light 440, or LO light
430. For example, modulator 495b may be an amplitude modulator that
modulates the amplitude of the seed light 440 before passing
through the SOA 460. As another example, modulator 495b may be a
phase modulator that changes the frequency of the seed light 440.
As another example, modulator 495c may be a phase modulator that
changes the frequency of the LO light 430.
[0210] FIG. 28 illustrates an example lidar system 100 with alight
source 110 that emits pulses of light 400 and local-oscillator (LO)
light 430. The light source 110 includes a seed laser diode 450
that emits seed light 440 and LO light 430. The SOA 460, which has
a tapered waveguide 463 (e.g., a width of the SOA waveguide 463
increases from the input end 461 to the output end 462), amplifies
the seed light 440 to produce an output beam 125. For example, the
SOA 460 may amplify temporal portions of the seed light 440 to
produce an output beam 125 that includes emitted pulses of light
400, where each amplified temporal portion of the seed light 440
corresponds to one of the emitted pulses of light 400. The light
source 110 may include an electronic driver 480 (not illustrated in
FIG. 28) that (i) supplies a modulated or substantially constant
electrical current I.sub.1 to the seed laser diode 450 and (ii)
supplies pulses of current I.sub.2 to the SOA 460. Each pulse of
SOA current I.sub.2 may cause the SOA 460 to amplify a temporal
portion of the seed light 440 to produce an emitted pulse of light
400. Additionally, the electronic driver 480 may impart frequency
changes to seed light 440, emitted pulses of light 400, or LO light
430 based on the seed current I.sub.1 supplied to the seed laser
diode 450 or based on the SOA current I.sub.2 supplied to the SOA
460. For example, in addition to amplifying a temporal portion of
seed 460, each pulse of SOA current I.sub.2 supplied to the SOA 460
may also cause the SOA 460 to impart a spectral signature to the
corresponding emitted pulse of light 400. A light source 110 may
also include a fiber-optic amplifier 500 (not illustrated in FIG.
28) which may be similar to that illustrated in FIGS. 13-14 and
described herein. The fiber-optic amplifier 500 may receive pulses
of light from the SOA 460 and further amplify the pulses of light
to produce the output beam 125.
[0211] In the example of FIG. 28, the light source 110 emits an
output beam 125 that includes a pulse of light 400, and the scanner
120 scans the output beam 125 across a field of regard of the lidar
system. The receiver 140 detects a combined beam 422 that includes
LO light 430 and an input beam 135. The lidar system 100 in FIG. 28
may be referred to as a hybrid pulsed/coherent lidar system in
which the light source 110 emits LO light 430 and pulses of light
400, where each emitted pulse of light is coherent with a
corresponding temporal portion of the LO light 430. Additionally,
the receiver 140 in a hybrid pulsed/coherent lidar system may
detect the LO light 430 and a received pulse of light 410, where
the received pulse of light 410 includes scattered light from one
of the emitted pulses of light 400. The receiver 140 in FIG. 28
includes a detector 340, and the LO light 430 and the received
pulse of light 410 are combined by the optical combiner 420 and
coherently mixed together at the detector 340 to produce a
photocurrent signal i. Herein, a hybrid pulsed/coherent lidar
system 100 may be referred to as a hybrid pulsed-coherent lidar
system, a hybrid lidar system, or a lidar system. One or more of
the lidar systems 100 described or illustrated herein may be
configured to operate as a hybrid pulsed/coherent lidar system
100.
[0212] A hybrid pulsed/coherent lidar system 100 may include 1, 2,
4, or any other suitable number of detectors 340, and one or more
of the detectors may detect at least a portion of LO light 430 and
a received pulse of light 410 to produce a corresponding
photocurrent signal. The photocurrent signal i produced by a
detector 340 of a hybrid pulsed/coherent lidar system 100 may
include the sum of three terms: (i) the first term corresponds to
an optical property of the received pulse of light 410, (ii) the
second term is a coherent-mixing term corresponding to the coherent
mixing of the LO light 430 and the received pulse of light 410, and
(iii) the third term corresponds to an optical property of the LO
light 430. The optical property of the received pulse of light may
be the optical power, optical intensity, optical energy, or
electric field of the received pulse of light. Similarly, the
optical property of the LO light 430 may be the optical power,
optical intensity, optical energy, or electric field of the LO
light. For example, the first term may correspond to the optical
power of the received pulse of light 410 and may be expressed as
|.epsilon..sub.Rx(t)|.sup.2, E.sub.Rx.sup.2(t), or E.sub.Rx.sup.2.
Similarly, the third term may correspond to the optical power of
the LO light 430 and may be expressed as
|.epsilon..sub.LO(t)|.sup.2, E.sub.LO.sup.2(t), or E.sub.LO.sup.2.
The coherent-mixing term may be expressed as
2.epsilon..sub.Rx(t)||.epsilon..sub.LO(t)|cos[.DELTA..omega.(t)t+.DELTA..-
PHI.(t)], as
2E.sub.Rx(t)E.sub.LO(t)cos[(.omega..sub.Rx-.omega..sub.LO)t+.PHI..sub.Rx--
.PHI..sub.LO], or as 2E.sub.RxE.sub.LO
cos[.DELTA..omega.t+.DELTA..PHI.]. The photocurrent signal i may
also include additional terms which are not included here. For
example, the photocurrent signal may include one or more additional
terms associated with solar background light, other light sources
(e.g., car headlights), interference light from other lidar
systems, or electrical noise that induces a current.
[0213] The receiver 140 in FIG. 28 includes a pulse-detection
circuit 365 that may determine the time-of-arrival for the received
pulse of light 410 based on the first term and the second term of
the photocurrent signal i. The receiver 140 includes an electronic
amplifier 350 that produces a voltage signal 360 corresponding to
the photocurrent signal i, and the time-of-arrival for the received
pulse of light 410 may be determined from the voltage signal 360.
The voltage signal 360, which may be similar to or may be a
representation of the photocurrent signal i, may correspond to the
three terms of the photocurrent signal i. For example, the first
term of the photocurrent signal i may include a pulse of current
corresponding to the optical power of the received pulse of light
410, and the voltage signal 360 may include voltage pulse that
corresponds to the first term. Similarly, the second term may
include a pulse of current that corresponds to coherent mixing of
the LO light 430 and the received pulse of light 410, and the
voltage signal 360 may include a voltage pulse that corresponds to
the second term. Determining the time-of-arrival for the received
pulse of light 410 based on the first term and the second term of
the photocurrent signal i may include determining the
time-of-arrival for the receive pulse of light 410 based on the
voltage signal 360, since the voltage signal 360 may be similar to
or may be a representation of the photocurrent signal i.
[0214] In a hybrid pulsed/coherent lidar system 100, determining
the time-of-arrival for a received pulse of light 410 based on the
first term and the second term of the photocurrent signal i may
include determining the time-of-arrival based on: (i) primarily the
first term, (ii) primarily the second term, or (iii) a combination
of the first and second terms. For example, the time-of-arrival for
a received pulse of light 410 scattered from a nearby target (e.g.,
D<50 m) or a high-reflectivity target (e.g., R>80%) may be
determined primarily based on the first term (e.g., the optical
power of the received pulse of light 410). The time-of-arrival for
a received pulse of light 410 scattered from a relatively distant
target (e.g., D>150 m) or a low-reflectivity target (e.g.,
R<20%) may be determined primarily based on the second term, the
coherent-mixing term. The time-of-arrival for a received pulse of
light 410 scattered from a target located at an intermediate
distance (e.g., D.apprxeq.150 m) or having an intermediate
reflectivity (e.g., R.apprxeq.50%) may be determined based on both
the first and second terms (e.g., based on the sum of the first and
second terms).
[0215] The hybrid pulsed/coherent lidar system 100 in FIG. 28
includes an optical combiner 420 that combines LO light 430 with
the input beam 135 and directs the combined beam 422 to the
detector 340. The optical combiner 420 in FIG. 28 may be similar to
one of the optical combiners 420 described herein or illustrated in
FIG. 18, 19, or 20. Alternatively, a hybrid pulsed/coherent lidar
system 100 may not include an optical combiner, and the LO light
430 and the input beam 135 may be combined at the detector 340
(e.g., using a technique described herein or illustrated in FIG. 21
or 22).
[0216] In particular embodiments, a lidar system 100 may include an
optical polarization element 465 that alters the polarization of an
emitted pulse of light 400, LO light 430, or a received pulse of
light 410. An optical polarization element 465, which may be
referred to as a polarization element, may allow the LO light 430
and the received pulse of light 410 to be coherently mixed. For
example, an optical polarization element may alter the polarization
of the LO light 430 so that, regardless of the polarization of a
received pulse of light 410, the LO light 430 and the received
pulse of light 410 may be coherently mixed together. The optical
polarization element may ensure that at least a portion of the
received pulse of light 410 and the LO light 430 have polarizations
that are oriented in the same direction. An optical polarization
element may include one or more quarter-wave plates, one or more
half-wave plates, one or more optical polarizers, one or more
optical depolarizers, polarization-maintaining optical fiber,
highly birefringent optical fiber, or any suitable combination
thereof. For example, an optical polarization element may include a
quarter-wave plate that converts the polarization of the LO light
430, the output beam 125, or the input beam 135 to a substantially
circular or elliptical polarization. An optical polarization
element may include a free-space optical component, a fiber-optic
component, an integrated-optic component, or any suitable
combination thereof.
[0217] An optical polarization element may be included in a
receiver 140 as an alternative to configuring a receiver to be a
polarization-insensitive receiver. For example, rather than
producing horizontally polarized beams and vertically polarized
beams and having two receiver channels (e.g., H-polarization
receiver and V-polarization receiver) as illustrated in FIG. 23, a
receiver 140 may include an optical polarization element that
ensures that at least a portion of the LO light 430 and the
received pulse of light 410 may be coherently mixed together. An
optical polarization element may be included in each of the
receivers 140 illustrated in FIGS. 18-22 to allow the receiver to
coherently mix the LO light 430 and a received pulse of light 410
regardless of the polarization of the received pulse of light
410.
[0218] In particular embodiments, an optical polarization element
(e.g., a quarter-wave plate) may convert the polarization of the LO
light 430 into circularly or elliptically polarized light. For
example, the LO light 430 produced by a seed laser 450 may be
linearly polarized, and a quarter-wave plate may convert the
linearly polarized LO light 430 into circularly polarized light.
The circularly polarized LO light 430 may include both vertical and
horizontal polarization components. So, regardless of the
polarization of a received pulse of light 410, at least a portion
of the circularly polarized LO light 430 may be coherently mixed
with the received pulse of light 410. In the example of FIG. 28,
the polarization element 465 may include a quarter-wave plate that
converts the LO light 430 into circularly polarized light prior to
the LO light 430 being combined with the input beam 135.
[0219] In particular embodiments, an optical polarization element
may include an optical depolarizer that depolarizes the
polarization of the LO light 430. For example, the LO light 430
produced by a seed laser 450 may be linearly polarized, and an
optical depolarizer may convert the linearly polarized LO light 430
into depolarized light having a polarization that is substantially
random or scrambled. The depolarized LO light 430 may include two
or more different polarizations so that, regardless of the
polarization of a received pulse of light 410, at least a portion
of the depolarized LO light 430 may be coherently mixed with the
received pulse of light 410. An optical depolarizer may include a
Cornu depolarizer, a Lyot depolarizer, a wedge depolarizer, or any
other suitable depolarizer element.
[0220] FIG. 29 illustrates an example light source 110 and receiver
140 integrated into a photonic integrated circuit (PIC) 455. The
PIC 455 in FIG. 29 may be part of a hybrid pulsed/coherent lidar
system 100. In particular embodiments, a hybrid pulsed/coherent
lidar system 100 may include a light source 110, a receiver 140,
and a processor or controller 150, and at least part of the light
source 110 or at least part of the receiver 140 may be disposed on
or in a PIC 455. In the example of FIG. 29, both the light source
110 and the receiver 140 are disposed on or in the PIC 455. As
another example, the receiver 140 may be disposed on or in the PIC
455, and the light source 110 may be packaged separately from the
PIC 455. All or part of a processor or controller 150 may be
attached to, electrically coupled to, or located near the PIC
455.
[0221] In particular embodiments, a PIC 455 that is part of a lidar
system 100 may include one or more seed laser diodes 450, one or
more optical waveguides 479, one or more optical isolators 530, one
or more splitters 470, one or more SOAs 460, one or more lenses
490, one or more polarization elements 465, one or more combiners
420, or one or more detectors 340. The PIC 455 in FIG. 29 includes
the following optical components: seed laser diode 450, optical
isolator 530, splitter 470, SOA 460, output lens 490a, polarization
element 465, input lens 490b, combiner 420, detector 340, and
amplifier 350. Additionally, the PIC 455 includes optical
waveguides 479 that convey light from one optical component to
another. The waveguides 479 may be passive optical waveguides
formed in a PIC substrate material that includes silicon, InP,
glass, polymer, or lithium niobate. The amplifier 350 may be
attached to, electrically coupled to, or located near the PIC 455.
One or more optical components of the light source 110 or receiver
140 may be fabricated separately and then integrated with the PIC
455. For example, the waveguides 479, splitter 470, and combiner
420 may be fabricated as part of the PIC 455, and the seed laser
diode 450, isolator 530, SOA 460, lenses 490a and 490b, or detector
340 may be fabricated separately and then integrated into the PIC
455. An optical component may be integrated into the PIC 455 by
attaching or connecting the optical component to the PIC 455 or to
a substrate to which the PIC 455 is also attached. For example, the
seed laser diode 450 and the SOA may be attached to the PIC 445 or
to a substrate using epoxy, adhesive, or solder.
[0222] In particular embodiments, a PIC 455 may include one or more
optical waveguides 479, one or more optical splitters 470, or one
or more optical combiners 420. The one or more waveguides 479,
splitters 470, or combiners 420 may be configured to convey, split,
or combine the seed-laser output light 472, seed light 440, LO
light 430, emitted pulses of light 400, or received pulses of light
410. In FIG. 29, the optical splitter 470 is an optical-waveguide
splitter 470 that splits the seed-laser output light 472 to produce
the seed light 440 and the LO light 430. The integrated-optic
2.times.1 combiner 420 in FIG. 29 (which may be similar to the
combiner 420 illustrated in FIG. 18) combines the input beam 135,
which includes the received pulse of light 410, with the LO light
430 to produce a combined beam. The LO light 430 and the input beam
135 are each conveyed to the combiner 420 by one or more optical
waveguides 479, and the combined beam 422 is conveyed to the
detector by a waveguide 479. In other embodiments, a PIC 455 may
include an integrated-optic 2.times.2 combiner 420 (e.g., as
illustrated in FIG. 19). The optical waveguides 479 in FIG. 29 may
be referred to as passive optical waveguides (e.g., to distinguish
them from the waveguide 463 of a SOA 460, which may be referred to
as an active optical waveguide).
[0223] In particular embodiments, a PIC 455 may include one or more
optical waveguides 479 that direct seed light 440 to a SOA 460 or
that direct LO light 430 and input light 135 to a receiver 140. For
example, a light source 110 may include a PIC 455 with an optical
waveguide 479 that receives seed light 440 from a seed laser diode
450 and directs the seed light 440 to a SOA 460. As another
example, an optical waveguide 479 may receive seed-laser output
light 472 from a seed laser diode 450 and direct a portion of the
seed-laser output light 472 (which corresponds to the seed light
440) to a SOA 460. In FIG. 29, an optical waveguide 479 of the PIC
455 receives the seed-laser output light 472 from the front face
452 of the seed laser diode 450 and directs the output light 472
through the isolator 530 and then to the input port of the splitter
470. The splitter 470 splits the seed-laser output light 472 to
produce the seed light 440 and the LO light 430. One optical
waveguide 479 directs the seed light 440 from output port 1 of the
splitter 470 to the SOA 460, and another optical waveguide 479
directs the LO light 430 from output port 2 of the splitter 470 to
the combiner 420 of the receiver 140. Alternatively, a PIC 455 may
include a light source similar to that illustrated in FIG. 8 or 9,
and the PIC may not include a splitter 470. The LO light 430 may be
coupled from the back face 451 of the seed laser diode 450 into a
waveguide 479 of the PIC 455, and the seed light 440 may be coupled
to the SOA 460 either directly (e.g., from the seed laser diode
directly to the SOA) or via an optical waveguide 479.
[0224] In particular embodiments, a PIC 455 may include one or more
lenses 490 configured to collimate light emitted from the PIC 455
or focus light into the PIC 455. A lens 490 may be attached to,
connected to, or integrated with the PIC 455. For example, a lens
490 may be fabricated separately and then attached to the PIC 455
(or to a substrate to which the PIC is attached) using epoxy,
adhesive, or solder. The output lens 490a in FIG. 29 may collimate
the emitted pulses of light 400 from the SOA 460 to produce a
collimated free-space output beam 125. The output beam 125 may be
scanned across a field of regard by a scanner 120 (not illustrated
in FIG. 29). Light from an emitted pulse of light 400 may be
scattered by a target 130, and a portion of the scattered light may
be directed to the receiver 140 as a received pulse of light 410.
The input lens 490b in FIG. 29 may focus the received pulse of
light 410 into a waveguide 479 of the PIC 455, which directs the
received pulse of light 410 to the combiner 420. The combiner 420
combines the received pulse of light 410 with the LO light 430 and
directs the combined beam 422 to the detector 340 via a waveguide
479. The detector 340 may be butt-coupled or affixed (e.g., with
epoxy, adhesive, or solder) to the output waveguide 479 of the
combiner 420 so that the combined beam 422 is directly coupled to
the detector 340. The LO light 430 and the received pulse of light
410 are coherently mixed together at the detector 340, and the
detector 340 produces a photocurrent signal i, which is directed to
the amplifier 350.
[0225] In particular embodiments, a lidar system 100 may include
alight source 110 with an optical isolator 530. In FIG. 29, the
light source 110 includes a seed laser diode 450, an optical
isolator 530, and a SOA 460, where the optical isolator 530 is
located between the seed laser diode 450 and the SOA 460. The
optical isolator 530 may be an integrated-optic isolator, a
fiber-optic isolator, or a free-space isolator. The isolator 530 in
FIG. 29 may include a Faraday-type isolator or a filter-type
isolator and may be configured to (i) transmit seed light 440 to
the SOA 460 and (ii) reduce an amount of light that propagates from
the SOA 460 toward the seed laser diode 450.
[0226] The optical polarization element 465 in FIG. 29 may alter
the polarization of the LO light 430 so that the LO light 430 and
the received pulse of light 410 may be coherently mixed. The
polarization element 465 may ensure that at least a portion of the
received pulse of light 410 and at least a portion of the LO light
430 have polarizations that are oriented in the same direction. The
polarization element 465 may include one or more quarter-wave
plates, one or more half-wave plates, one or more optical
polarizers, one or more optical depolarizers, or any suitable
combination thereof. For example, the polarization element 465 may
include a quarter-wave plate that converts linearly polarized LO
light 430 produced by the seed laser diode 450 into circular or
elliptically polarized light. In the example of FIG. 29, the
polarization element 465 may be an integrated-optic component that
is fabricated as part of the PIC, or the polarization element 465
may be fabricated separately and then attached to the PIC 455 (or
to a substrate to which the PIC is attached) using epoxy, adhesive,
or solder.
[0227] FIGS. 30-31 each illustrates an example photocurrent signal
i that includes a pulse term, a coherent-mixing term, and a
local-oscillator (LO) term. The pulse term, coherent-mixing term,
and the LO term may be referred to as the first term, second term,
and third term, respectively. The three graphs on the left in each
of FIGS. 30-31 illustrate example temporal behavior of each of the
three terms separately: (1) the pulse term,
|.epsilon..sub.Rx(t)|.sup.2, (2) the coherent-mixing term,
2|.epsilon..sub.Rx(t)||.epsilon..sub.LO(t)|cos[.DELTA..PHI.(t)t+.DELTA..P-
HI.(t)], and (3) the LO term, |.epsilon..sub.LO(t)|.sup.2. The
graph on the right in each of FIGS. 30-31, which corresponds to the
photocurrent signal i produced by a detector 340, illustrates the
sum of the three terms shown separately on the left. The
photocurrent signal i may represent the electrical current produced
by a detector 340 in response to detecting a received pulse of
light 410. The pulse term and the coherent-mixing term each has a
temporal shape of a pulse (e.g., a Gaussian pulse), which may
correspond to the temporal shape of the received pulse of light
410. The LO term is substantially constant, which corresponds to
the optical power of the LO light 430 being approximately
constant.
[0228] A voltage signal 360 produced by an electronic amplifier 350
from a photocurrent signal i may have a shape or temporal behavior
that is similar to the photocurrent signal. For example, a voltage
signal 360 produced from the photocurrent signal i in FIG. 30 may
include a relatively large voltage pulse (corresponding to the sum
of the first and second terms) and a relatively small offset
voltage (corresponding to the third term). One or more of the
characteristics of the voltage pulse (e.g., duration, rise time,
fall time, or shape) may be somewhat different from the
corresponding characteristics of the current pulse in the
photocurrent signal i. For example, due to electrical-bandwidth
limitations of an amplifier 350, the duration, rise time, or fall
time of the voltage pulse may be somewhat longer (e.g., between 0%
and 20% longer) than the corresponding characteristic of the
current pulse.
[0229] The amplitudes shown in FIGS. 30-31 represent a peak height
or a difference between maximum and minimum values over a time
interval associated with a received pulse of light 410. The
amplitude A.sub.1 represents the peak height or the difference
between maximum and minimum values of the pulse term. Similarly,
the amplitude A.sub.2 represents the peak height or the difference
between maximum and minimum values of the coherent-mixing term. The
sum of the two amplitudes A.sub.1+A.sub.2 represents the amplitude
or the difference between maximum and minimum values of the
photocurrent signal i.
[0230] In FIG. 30, the amplitude of the pulse term (A.sub.1) is
significantly larger than the amplitude of the coherent-mixing term
(A.sub.2). The pulse term being significantly larger than the
coherent-mixing term indicates that the photocurrent signal i may
correspond to a received pulse of light 410 that is scattered from
a nearby target 130 or a high-reflectivity target 130. For example,
the photocurrent signal i may be produced by a received pulse of
light 410 scattered from a target 130 that is located a distance D
of less than 50 meters away and that has a reflectivity greater
than 70%. The pulse term being significantly larger than the
coherent-mixing term may indicate that the hybrid pulsed/coherent
lidar system 100 is operating primarily as a direct-detection
pulsed lidar system in which the time-of-arrival for the received
pulse of light 410 is determined primarily based on the pulse term
(the first term).
[0231] In FIG. 31, the amplitude of the coherent-mixing term
(A.sub.2) is significantly larger than the amplitude of the pulse
term (A.sub.1). The coherent-mixing term being significantly larger
than the pulse term indicates that the photocurrent signal i may
correspond to a received pulse of light 410 scattered from a
relatively distant target 130 or a low-reflectivity target 130. For
example, the photocurrent signal i may be produced by a received
pulse of light 410 scattered from a target 130 that is located
greater than 150 meters away and that has a reflectivity of less
than 20%. The coherent-mixing term being significantly larger than
the pulse term may indicate that the hybrid pulsed/coherent lidar
system 100 is operating primarily as a coherent pulsed lidar system
in which the time-of-arrival for the received pulse of light 410 is
determined primarily based on the coherent-mixing term (the second
term).
[0232] FIG. 32 illustrates an example graph with amplitudes of a
pulse term and a coherent-mixing term plotted versus distance (D)
to a target 130. The pulse term, which is the first term in the
expression for photocurrent signal i, may be expressed as
E.sub.Rx.sup.2, and the amplitude A.sub.1 of the pulse term
corresponds to the peak of the pulse term for a received pulse of
light 410. The coherent-mixing term, which is the second term in
the expression for photocurrent signal i, may be expressed as
2E.sub.RxE.sub.LO cos[.DELTA..omega.t+.DELTA..PHI.], and the
amplitude A.sub.2 of the coherent-mixing term corresponds to the
peak of the coherent-mixing term for a received pulse of light 410.
The two amplitude curves in FIG. 32 are plotted versus distance,
and the values of each curve at a particular distance D correspond
to the amplitudes of the pulse and coherent-mixing terms for a
received pulse of light 410 scattered from a target 130 located at
the distance D. The reflectivity of the target 130 considered in
FIG. 32 may be taken as a fixed value (e.g., the target may have a
fixed reflectivity of 50%). Both curves in FIG. 32 decrease
monotonically with increasing distance to the target 130,
indicating that the amplitudes of both the pulse term and the
coherent-mixing term decrease as the target gets farther from the
lidar system 100.
[0233] The sum of the amplitudes of the two curves in FIG. 32
(which corresponds to A.sub.1+A.sub.2) represents the amplitude or
maximum change of the photocurrent signal i for a received pulse of
light 410. The detection and determination of a time-of-arrival for
the received pulse of light 410 may be based on the sum of the
pulse term and the coherent-mixing term, and the amplitude of this
sum corresponds to A.sub.1+A.sub.2. For target distances between
zero and D.sub.1, the pulse term is greater than the
coherent-mixing term (so that A.sub.1>A.sub.2). In this case (of
which FIG. 30 is an example), the time-of-arrival for a received
pulse of light 410 may be determined primarily based on the pulse
term. While both the pulse term and the coherent-mixing term
contribute to the photocurrent signal i and to the detection of a
received pulse of light 410, for distances between zero and
D.sub.1, a hybrid pulsed/coherent lidar system 100 may be referred
to as operating primarily as a direct-detection pulsed lidar
system, since the pulse term provides a greater contribution to the
photocurrent signal i. For target distances between D.sub.1 and
D.sub.3, the coherent-mixing term is greater than the pulse term
(so that A.sub.2>A.sub.1). In this case (of which FIG. 31 is an
example), the time-of-arrival for a received pulse of light 410 may
be determined primarily based on the coherent-mixing term. While
both the pulse term and the coherent-mixing term contribute to the
photocurrent signal i and to the detection of a received pulse of
light 410, for distances between D.sub.1 and D.sub.3, a hybrid
pulsed/coherent lidar system 100 may be referred to as operating
primarily as a coherent pulsed lidar system, since the
coherent-mixing term provides a greater contribution to the
photocurrent signal i.
[0234] For distances close to D.sub.1, the amplitudes of the pulse
term and coherent-mixing term are approximately equal, and the
time-of-arrival for a received pulse of light 410 may be determined
based on both the pulse term and the coherent-mixing term. In this
case, a hybrid pulsed/coherent lidar system 100 may be referred to
as operating as both a direct-detection pulsed lidar system and as
a coherent pulsed lidar system, since the pulse term and the
coherent-mixing term provide approximately equal contributions to
the photocurrent signal i. The distance D.sub.1 may correspond to a
cross-over distance where a hybrid pulsed/coherent lidar system 100
switches from acting primarily as a direct-detection pulsed lidar
system (for distances less than D.sub.1) to acting primarily as a
coherent pulsed lidar system (for distances greater than D.sub.1).
The cross-over distance D.sub.1 may have any suitable value and may
depend on the reflectivity of the target 130. For example, D.sub.1
may be 30 meters for a target with a reflectivity of 5%, 100 meters
for a target with a reflectivity of 50%, and 200 meters for a
target with a reflectivity of 80%.
[0235] For distances greater than D.sub.3, the amplitude of the sum
of the pulse term and the coherent-mixing term is less than the
noise level of the receiver 140, and the lidar system 100 may not
be able to detect a received pulse of light 410 scattered from a
target 130 located farther than D.sub.3. The detector 340 or
electronic amplifier 350 may add electronic noise (e.g., shot noise
or thermal noise) to the photocurrent signal i or to the voltage
signal 360, and when the level of electronic noise from the
receiver 140 exceeds an amplitude that corresponds to
A.sub.1+A.sub.2, a lidar system 100 may not be able to detect a
received pulse of light 410. The distance D.sub.3 may be referred
to as the operating distance (D.sub.OP) of the hybrid
pulsed/coherent lidar system 100, and D.sub.3 may depend on the
target reflectivity R. For example, D.sub.3 may be 100 meters for a
target with a reflectivity of 5%, 250 meters for a target with a
reflectivity of 50%, and 350 meters for a target with a
reflectivity of 80%. In FIG. 32, without the coherent-mixing term,
the lidar system may operate as a direct-detection pulsed lidar
system, and the operating distance may be approximately equal to
D.sub.2, where the amplitude of the pulse term A.sub.1 is equal to
the receiver noise level. Operating as a hybrid pulsed/coherent
lidar system 100 allows the operating distance to extend beyond
D.sub.2 to the distance D.sub.3. For example, for a target with 80%
reflectivity, the distance D.sub.2 may be approximately 250 meters,
and the distance D.sub.3 may be approximately 350 meters, which
corresponds to a 100-m increase in operating distance for a hybrid
pulsed/coherent lidar system 100 as compared to a direct-detection
pulsed lidar system.
[0236] FIG. 33 illustrates an example graph with amplitudes of a
pulse term and a coherent-mixing term plotted versus reflectivity
(R) of a target 130. The two amplitude curves in FIG. 33 are
plotted versus target reflectivity, and the values of each curve at
a particular reflectivity R correspond to the amplitudes of the
pulse and coherent-mixing terms for a received pulse of light 410
scattered from a target 130 having a reflectivity of R. The
distance to the target 130 considered in FIG. 33 may be taken as a
fixed value (e.g., the target may be located at a fixed distance
from the lidar system, such as for example, a distance of 100
meters). Both curves in FIG. 33 decrease monotonically as the
target reflectivity decreases, indicating that the amplitudes of
both the pulse term and the coherent-mixing term decrease as the
reflectivity of the target decreases.
[0237] The sum of the amplitudes of the two curves in FIG. 33
(which corresponds to A.sub.1+A.sub.2) represents the amplitude or
maximum change of the photocurrent signal i for a received pulse of
light 410. The detection and determination of a time-of-arrival for
the received pulse of light 410 may be based on the sum of the
pulse term and the coherent-mixing term, and the amplitude of this
sum corresponds to A.sub.1+A.sub.2. For target reflectivities
between 100% and R.sub.1, the pulse term is greater than the
coherent-mixing term (so that A.sub.1>A.sub.2). In this case (of
which FIG. 30 is an example), the time-of-arrival fora received
pulse of light 410 may be determined primarily based on the pulse
term. For reflectivities between 100% and R.sub.1, a hybrid
pulsed/coherent lidar system 100 may be referred to as operating
primarily as a direct-detection pulsed lidar system, since the
pulse term provides a greater contribution to the photocurrent
signal i than the coherent-mixing term. For reflectivities between
R.sub.1 and R.sub.3, the coherent-mixing term is greater than the
pulse term (so that A.sub.2>A.sub.1). In this case (of which
FIG. 31 is an example), the time-of-arrival for a received pulse of
light 410 may be determined primarily based on the coherent-mixing
term. For reflectivities between R.sub.1 and R.sub.3, a hybrid
pulsed/coherent lidar system 100 may be referred to as operating
primarily as a coherent pulsed lidar system, since the
coherent-mixing term provides a greater contribution to the
photocurrent signal i than the pulse term.
[0238] For reflectivities close to R.sub.1, the amplitudes of the
pulse term and coherent-mixing term are approximately equal, and
the time-of-arrival for a received pulse of light 410 may be
determined based on both the pulse term and the coherent-mixing
term. In this case, a hybrid pulsed/coherent lidar system 100 may
be referred to as operating as both a direct-detection pulsed lidar
system and as a coherent pulsed lidar system, since the pulse term
and the coherent-mixing term provide approximately equal
contributions to the photocurrent signal i. The reflectivity
R.sub.1 may correspond to a cross-over reflectivity where a hybrid
pulsed/coherent lidar system 100 switches from acting primarily as
a direct-detection pulsed lidar system (for reflectivities greater
than R.sub.1) to acting primarily as a coherent pulsed lidar system
(for reflectivities less than R.sub.1). The cross-over reflectivity
R.sub.1 may have any suitable value and may depend on the distance
of the target 130. For example, R.sub.1 may be 80% for a target
located at 200 meters from the lidar system, 50% for a target
located at 100 meters, and 5% for a target located at 30
meters.
[0239] For reflectivities less than R.sub.3, the amplitude of the
sum of the pulse term and the coherent-mixing term is less than the
noise level of the receiver 140, and the lidar system 100 may not
be able to detect a received pulse of light 410 scattered from a
target 130 with a reflectivity less than R.sub.3. The reflectivity
R.sub.3 may be referred to as the operating reflectivity (R.sub.OP)
of the hybrid pulsed/coherent lidar system 100 and may depend on
the target distance D. For example, R.sub.3 may be 20% for a target
located 200 meters from the lidar system, 10% for a target located
at 100 meters, and 2% for a target located at 30 meters. In FIG.
33, without the coherent-mixing term, the lidar system may operate
as a direct-detection pulsed lidar system, and the operating
reflectivity may be approximately equal to R.sub.2, where the
amplitude of the pulse term A.sub.1 is equal to the receiver noise
level. Operating as a hybrid pulsed/coherent lidar system 100
allows the operating reflectivity to extend to reflectivities less
than R.sub.2 to the lower reflectivity value R.sub.3. For example,
for a target located 200 meters from the lidar system, the
reflectivity R.sub.2 may be approximately 60%, and the reflectivity
R.sub.3 may be approximately 20%, which corresponds to an
improvement in the sensitivity of the lidar system for
low-reflectivity targets.
[0240] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may be referred to as operating primarily as a
direct-detection pulsed lidar system when a received pulse of light
410 produces a photocurrent signal i where the pulse term (the
first term of the photocurrent signal) is greater than the
coherent-mixing term (the second term), which corresponds to
A.sub.1>A.sub.2. While both the pulse term and the
coherent-mixing term contribute to the photocurrent signal i and to
the detection of a received pulse of light 410, a hybrid
pulsed/coherent lidar system 100 may be referred to as operating
primarily as a direct-detection pulsed lidar system when
A.sub.1>A.sub.2, since the pulse term provides a greater
contribution to the photocurrent signal i. Additionally, the
receiver 140 of the lidar system may be referred to as acting as a
pulsed-lidar receiver, and the pulse-detection circuit 365 of the
receiver (or a controller 150 coupled to the pulse-detection
circuit) may be referred to as determining the time-of-arrival for
the received pulse of light 410 primarily based on the pulse term.
The pulse term being greater than the coherent-mixing term may be
associated with the target 130 from which the received pulse of
light 410 was scattered (i) being located less than a threshold
distance from the lidar system or (ii) having a reflectivity
greater than a threshold reflectivity. For example, a hybrid
pulsed/coherent lidar system 100 may operate primarily as a
direct-detection pulsed lidar system for targets located less than
150 meters from the lidar system and having a reflectivity of
greater than 50%. As another example, for a target 130 with a
reflectivity of 50%, the lidar system may operate primarily as a
direct-detection pulsed lidar system when the target is located
less than 150 meters from the lidar system. As another example, for
a target 130 located 150 meters from the lidar system, the lidar
system may operate primarily as a direct-detection pulsed lidar
system when the target reflectivity is greater than 50%.
[0241] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may be referred to as operating primarily as a coherent
pulsed lidar system when a received pulse of light 410 produces a
photocurrent signal i where the coherent-mixing term is greater
than the pulse term, which corresponds to A.sub.2>A.sub.1. While
both the pulse term and the coherent-mixing term contribute to the
photocurrent signal i and to the detection of a received pulse of
light 410, a hybrid pulsed/coherent lidar system 100 may be
referred to as operating primarily as a coherent pulsed lidar
system when A.sub.2>A.sub.1, since the coherent-mixing term
provides a greater contribution to the photocurrent signal i.
Additionally, the receiver 140 of the lidar system may be referred
to as acting as a coherent-lidar receiver, and the pulse-detection
circuit 365 of the receiver (or a controller 150 coupled to the
pulse-detection circuit) may be referred to as determining the
time-of-arrival for the received pulse of light 410 primarily based
on the coherent-mixing term. The coherent-mixing term being greater
than the pulse term may be associated with the target 130 from
which the received pulse of light 410 was scattered (i) being
located greater than a threshold distance from the lidar system or
(ii) having a reflectivity less than a threshold reflectivity. For
example, a hybrid pulsed/coherent lidar system 100 may operate
primarily as a coherent pulsed lidar system for targets located
more than 150 meters from the lidar system and having a
reflectivity of less than 50%. As another example, for a target 130
with a reflectivity of 50%, the lidar system may operate primarily
as a coherent pulsed lidar system when the target is located more
than 150 meters from the lidar system. As another example, for a
target 130 located 150 meters from the lidar system, the lidar
system may operate primarily as a coherent pulsed lidar system when
the target reflectivity is less than 50%.
[0242] FIG. 34 illustrates an example voltage signal 360 that
results from the coherent mixing of LO light 430 and a received
pulse of light 410, where the LO light and the received pulse of
light have a frequency difference of .DELTA.f. The optical spectrum
of the LO light 430 indicates that the LO light 430 has a center
optical frequency of f.sub.0 and a relatively narrow spectral
linewidth of .DELTA.v.sub.1. The received pulse of light 410 has a
duration of .DELTA..tau. and an optical spectrum with a center
optical frequency of f.sub.1 and a broader spectral linewidth of
.DELTA.v.sub.2. The optical frequency of the pulse of light 410 is
shifted by .DELTA.f with respect to the frequency of the LO light
430 so that f.sub.1=f.sub.0+.DELTA.f. The coherent mixing of the LO
light 430 and the received pulse of light 410 at a detector 340
results in a pulse of photocurrent with a duration of
.DELTA..tau..sub.p. The photocurrent signal i may be amplified by
an amplifier 350 that produces a corresponding voltage signal 360.
The upper voltage-signal graph illustrates the voltage signal 360
in the time domain and includes a pulse of voltage with a duration
of .DELTA..tau.'.
[0243] In FIG. 34, the photocurrent signal i and the corresponding
voltage pulse each includes temporal pulsations (which may be
referred to as pulsations or as amplitude modulation). Each
pulsation is separated by a time interval 1/.DELTA.f, which
corresponds to the temporal pulsations occurring at a frequency of
.DELTA.f. The lower voltage-signal graph is a frequency-domain
graph of the voltage signal 360 that indicates that the voltage
signal 360 is centered at a frequency of .DELTA.f and has an
electrical bandwidth of .DELTA.v. The voltage signal 360 being
centered at the frequency .DELTA.f indicates that the voltage
signal 360 has a frequency component at approximately .DELTA.f,
which corresponds to the periodic time-domain pulsations with time
interval 1/.DELTA.f. The frequency component .DELTA.f in the
voltage signal 360 arises from the frequency offset of .DELTA.f
between the received pulse of light 410 and the LO light 430. The
coherent mixing of LO light 430 and the received pulse of light 410
may result in a photocurrent signal i with a coherent-mixing term
that may be expressed as E.sub.RxE.sub.LO
cos[2.pi..DELTA.ft+.PHI..sub.Rx-.PHI..sub.LO] or as
E.sub.RxE.sub.LO cos[.DELTA..omega.t+.PHI..sub.Rx-.PHI..sub.LO],
where .DELTA..omega.=2.pi..DELTA.f. Here, since the optical
frequencies of the LO light 430 and the received pulse of light 410
are offset by .DELTA.f, the coherent-mixing term varies
periodically with a frequency of .DELTA.f. This temporal variation
in the coherent-mixing term corresponds to the periodic temporal
pulsations and the frequency component of .DELTA.f in the
photocurrent signal i and the voltage signal 360 in FIG. 34. The
graphs in FIG. 34 are similar to those in FIG. 17, with the
difference being that in FIG. 34, the LO light 430 and the received
pulse of light 410 have a frequency difference of .DELTA.f (which
gives rise to the temporal pulsations in the photocurrent signal i
and the voltage signal 360).
[0244] A frequency difference .DELTA.f between LO light 430 and a
pulse of light (e.g., an emitted pulse of light 400 or a received
pulse of light 410) may be referred to as a frequency offset, a
frequency shift, a frequency change, or a spectral shift. A
frequency difference .DELTA.f may have any suitable value between
approximately 10 MHz and approximately 50 GHz, such as for example
a value of approximately 10 MHz, 100 MHz, 200 MHz, 500 MHz, 1 GHz,
2 GHz, 10 GHz, or 50 GHz. The frequency difference .DELTA.f may be
configured to be greater than 1/.DELTA..tau. (where .DELTA..tau. is
the duration of the emitted pulse of light 400 or the received
pulse of light 410) or greater than 1/.DELTA..tau.' (where
.DELTA..tau.' is the duration of a voltage pulse corresponding to a
received pulse of light 410). For example, the frequency difference
.DELTA.f may be approximately equal to 2/.DELTA..tau.,
4/.DELTA..tau., 10/.DELTA..tau., 20/.DELTA..tau., or any other
suitable factor of 1/.DELTA..tau.. As another example, an emitted
pulse of light 400 with a duration .DELTA..tau. of 5 ns may have a
frequency difference .DELTA.f of greater than 200 MHz. As another
example, a light source 110 that emits 5-ns pulses of light 400 may
be configured so that the emitted pulses of light have a 1-GHz
frequency offset with respect to the LO light 430. Having .DELTA.f
greater than 1/.DELTA..tau. may ensure that voltage signal 360
includes a sufficient number of pulsations that are distinct from
the overall pulse envelope of the voltage signal 360. In FIG. 34,
.DELTA.f is approximately equal to 3/.DELTA..tau., and the voltage
signal 360 includes approximately seven pulsations superimposed on
the pulse envelope. For example, the received pulse of light 410
may have a duration .DELTA..tau. of 4 ns, and the frequency
difference .DELTA.f may be approximately 750 MHz. This 3.times.
difference between .DELTA.f and 1/.DELTA..tau. may allow the
frequency component .DELTA.f in the voltage signal 360 to be
determined (e.g., by a frequency-detection circuit 600) distinctly
from a frequency component associated with the overall pulse
envelope of the voltage signal 360. A frequency difference .DELTA.f
may be selected to be less than a maximum electrical bandwidth of a
receiver 140 so that the receiver is able to detect the temporal
pulsations associated with the frequency difference. For example,
the receiver 140 may have an electrical bandwidth from
approximately 100 MHz to approximately 1 GHz, and a light source
110 may be configured so that the frequency difference .DELTA.f is
within the 100-MHz to 1-GHz range.
[0245] FIG. 35 illustrates example graphs of seed current
(I.sub.1), seed light 440, an emitted optical pulse 400, a received
optical pulse 410, and LO light 430. The graphs in FIG. 35 each
illustrates a particular quantity plotted versus time, including
the temporal behavior of both the optical power and the optical
frequency of the seed light 440 and the LO light 430. In particular
embodiments, a light source 110 may change an optical frequency of
seed-laser output light 472, seed light 440, LO light 430, or
emitted pulses of light 400 by changing the seed current I.sub.1
supplied to a seed laser diode 450 or by changing the SOA current
I.sub.2 supplied to a SOA 460. Rather than incorporating a discrete
optical modulator 495 into a light source 110, a light source 110
may impart optical frequency changes based on the electrical
current supplied to the seed laser diode 450 or the SOA 460. For
example, the light source 110 illustrated in FIG. 6, 8, 9, 10, 11,
12, or 13 may not include a modulator 495 and may impart an optical
frequency change based on the electrical current supplied to the
seed laser diode 450 or the SOA 460. Changing the electrical
current supplied to a seed laser diode 450 or a SOA 460 may cause a
corresponding change in the optical frequency of the light emitted
by the seed laser diode 450 or the SOA 460 (e.g., the change in
optical frequency may result from a change in refractive index,
carrier density, or temperature associated with the change in
electrical current). For example, an electronic driver 480 may
supply a seed laser diode 450 with a time-varying seed current
I.sub.1 that results in a frequency offset of .DELTA.f between a
received pulse of light 410 and a corresponding temporal portion
431 of LO light 430.
[0246] In particular embodiments, a seed current I.sub.1 may be
alternated between K+1 different current values (where K equals 1,
2, 3, 4, or any other suitable positive integer) so that (i) each
temporal portion 441 (and each corresponding emitted pulse of light
400) has a particular optical frequency of K different frequencies
and (ii) each corresponding temporal portion 431 of the LO light
430 has one particular optical frequency that is different from
each of the other K frequencies. In the example of FIG. 35, the
parameter K is 1, and the seed current I.sub.1 supplied to a seed
laser diode 450 alternates between the two values i.sub.0 and
i.sub.1. The difference of .DELTA.i (where
.DELTA.i=i.sub.0-i.sub.1) between the two seed-current values may
be approximately 1 mA, 2 mA, 5 mA, 10 mA, 20 mA, or any other
suitable difference in seed current. For example, an electronic
driver 480 may supply seed currents of approximately i.sub.0=102 mA
and i.sub.1=100 mA, corresponding to a seed-current difference of 2
mA. The seed laser diode 450 produces seed light 440 and LO light
430, and the optical power of the seed light 440 and the LO light
430 may exhibit changes when the seed current I.sub.1 is changed.
For example, when the seed current I.sub.1 is reduced from i.sub.0
to i.sub.1, the optical power of the seed light 440 or the LO light
430 may be reduced by less than approximately 1 mW, 5 mW, or 10 mW.
Additionally, when the seed current I.sub.1 is changed between the
values i.sub.0 and i.sub.1, the optical frequency of the seed light
440 and the LO light 430 may change by .DELTA.f between the
respective values f.sub.0 and f.sub.1. The frequency change
.DELTA.f caused by a change in seed current I.sub.1 may be any
suitable frequency change between approximately 10 MHz and
approximately 50 GHz, such as for example, a frequency change of
100 MHz, 500 MHz, 1 GHz, 2 GHz, or 5 GHz.
[0247] In particular embodiments, an electronic driver 480 may (i)
supply electrical current i.sub.1 to a seed laser diode 450 during
a time interval when a pulse of light 400 is emitted by a light
source 110 and (ii) supply a different electrical current i.sub.0
to the seed laser diode 450 for a period of time after the pulse of
light 400 is emitted and prior to the emission of a subsequent
pulse of light 400. Switching the electrical current from i.sub.1
to i.sub.0 may result in a change of the frequency of the LO light
430 by .DELTA.f where the frequency change is with respect to: (i)
the frequency of the seed light 440 or LO light 430 during the time
interval when the pulse of light 400 is emitted and (ii) the
frequency of the emitted pulse of light 400. A photocurrent signal
produced by coherent mixing of a received pulse of light 410 with
the LO light 430 may include a frequency component at a frequency
of approximately .DELTA.f In FIG. 35, the seed current I.sub.1 is
alternated in time between two current values (i.sub.0 and i.sub.1)
so that (i) the temporal portion 441 of the seed light 440 has a
frequency f.sub.1 and (ii) the LO light 430 (including the temporal
portion 431) during a period of time after the pulse of light 400
is emitted has a frequency of f.sub.0, where
f.sub.1=f.sub.0+.DELTA.f. The emitted optical pulse 400 and the
received optical pulse 410 may each have optical frequencies of
approximately f.sub.1, corresponding to the frequency of the
temporal portion 441. The received optical pulse 410 may be
coherently mixed with the temporal portion 431 of the LO light 430
(which may have a frequency of f.sub.0) between the times t.sub.c
and t.sub.d to produce a photocurrent signal having temporal
pulsations with a frequency of approximately .DELTA.f. The
frequency component .DELTA.f of a corresponding voltage signal 360
may be detected or measured by a frequency-detection circuit 600 to
determine a spectral signature of the received optical pulse
410.
[0248] In particular embodiments, seed current I.sub.1 and SOA
current I.sub.2 maybe synched together so that (i) the seed current
I.sub.1 is set to a first value when a pulse of SOA current is
supplied to the SOA 460 and (ii) the seed current I.sub.1 is set to
a second value during the time periods between successive pulses of
SOA current. In FIG. 35, when a pulse of light 400 is emitted
(between times t.sub.a and t.sub.b), the seed current I.sub.1 is
set to the value i.sub.1, and during the time periods between
successive pulses of light 400, the seed current I.sub.1 is set to
the value i.sub.0. The seed current I.sub.1 may be set to the value
i.sub.0 for a period of time less than or equal to the pulse period
.tau., which corresponds to the time between successive pulses of
light 400. For example, the seed current I.sub.1 may be set to
i.sub.0 from time t.sub.b until at least time t.sub.d. At or before
a time when a subsequent pulse of light 400 (not illustrated in
FIG. 35) is emitted, the seed current I.sub.1 may be switched back
to the value i.sub.1, which changes the frequency of the seed light
440 and LO light 430 back to f.sub.1. After that subsequent pulse
of light 400 is emitted, the seed current I.sub.1 may again be set
to the value i.sub.0, which changes the frequency of the LO light
430 by .DELTA.f to f.sub.0.
[0249] In particular embodiments, an electronic driver 480 may
supply seed current I.sub.1 to a seed laser diode 450 where the
seed current I.sub.1 includes: (i) a substantially constant
electrical current (e.g., a DC current) and (ii) a modulated
electrical current. The modulated electrical current may include
any suitable waveform, such as for example, a sinusoidal, square,
pulsed, sawtooth, or triangle waveform. The constant-current
portion of the seed current I.sub.1 may include a DC current of
approximately 50 mA, 100 mA, 200 mA, 500 mA, or any other suitable
DC electrical current, and the modulated portion of the seed
current I.sub.1 may be smaller, with an amplitude of less than or
equal to 1 mA, 5 mA, 10 mA, or 20 mA. The modulated portion of the
electrical current may produce a corresponding frequency or
amplitude modulation in the seed light 440 or the LO light 430. For
example, the modulated electrical current may be applied to the
seed laser diode 450 when a pulse of light 400 is emitted so that
the emitted pulse of light 400 includes a corresponding frequency
or amplitude modulation. The modulated electrical current may not
be applied during the time period between successive pulses of
light 400, and so, during this time the LO light 430 may not
include a corresponding frequency or amplitude modulation. When a
received pulse of light 410 is coherently mixed with the LO light
430, the photocurrent signal may have a characteristic frequency
component corresponding to the frequency or amplitude modulation
applied to the emitted pulse of light 400. For example, the
characteristic frequency component may be detected or measured by a
frequency-detection circuit 600 to determine whether a received
pulse of light is a valid received pulse of light.
[0250] FIGS. 36-38 each illustrates example optical spectra of LO
light 430 and a received pulse of light 410. In each of FIGS.
36-38, the optical spectrum of the LO light 430 has a center
optical frequency of f.sub.0 and a relatively narrow spectral
linewidth, and the optical spectrum of the received pulse of light
410 has a broader spectral linewidth. In FIG. 36, the optical
spectrum of the received pulse of light 410 also has a center
optical frequency of f.sub.0, and in FIG. 37, the optical spectrum
of the received pulse of light 410 is shifted by .DELTA.f with
respect to the frequency of the LO light 430. The optical spectra
in FIG. 36 are similar to those illustrated in FIG. 17, and the
optical spectra in FIG. 37 are similar to those illustrated in FIG.
34. In FIG. 38, the optical spectrum of the received pulse of light
410 is nonuniform, having an overall shape that is asymmetric and
including some variation or ripples in the spectrum as a function
of frequency. The optical spectra in each of the FIGS. 36-38 may
correspond to LO light 430 and a received pulse of light 410 that
are coherently mixed together at a detector 340 to produce a
photocurrent signal i. In the time domain, the resulting
photocurrent signal may include a pulse of current as well as
temporal pulsations (e.g., as illustrated in FIGS. 39-41). The LO
light 430 and the received pulse of light 410 in FIG. 37 may
produce a photocurrent signal with temporal pulsations based on the
frequency difference .DELTA.f (e.g., similar to that illustrated in
FIG. 34). In FIGS. 36 and 38, the coherent mixing of the LO light
430 and the received pulse of light 410 may result in a
photocurrent signal that includes temporal pulsations, with the
temporal pulsations resulting from coherent mixing between (i)
frequency components of the LO light 430 located near the center
optical frequency and (ii) frequency components of the received
pulse of light 410 located away from the optical frequency f.sub.0.
For example, in FIG. 38, the LO light 430 may be coherently mixed
with one or more frequency components of the received pulse of
light 410 in the frequency range from f.sub.1 to f.sub.0 to produce
temporal pulsations with one or more frequencies in the range from
f.sub.1-f.sub.0 to f.sub.2-f.sub.0.
[0251] FIGS. 39-41 each illustrates an example photocurrent signal
i plotted versus time. In particular embodiments, a photocurrent
signal i produced by coherent mixing of LO light 430 and a received
pulse of light 410 may include a pulse of current and temporal
pulsations. In each of FIGS. 39-41, the photocurrent signal i
includes a pulse of current along with temporal pulsations in
current that are superimposed onto the pulse of current. The pulse
of current may correspond to a received pulse of light 410, and the
temporal pulsations may result from the coherent mixing of the
received pulse of light with LO light 430. The photocurrent signal
i in FIG. 39 (which is similar to the photocurrent signal in FIG.
34) includes temporal pulsations that are periodic with an
approximately fixed period of time between adjacent pulsations. The
photocurrent signal i in FIG. 40 includes periodic temporal
pulsations with a period of time between adjacent pulsations that
decreases with time (e.g., the frequency of the pulsations
increases with time). The photocurrent signal i in FIG. 41 includes
a pulse of current with pulsations of nonuniform amplitude and
period superimposed onto the current pulse. A photocurrent signal i
may include temporal pulsations having any suitable amplitude,
shape, frequency, or period. The photocurrent signal i in FIG. 39
may be referred to as having uniform periodic temporal pulsations
with a fixed frequency, and the photocurrent signal i in FIG. 40
may be referred to as having uniform periodic temporal pulsations
with a changing frequency. The photocurrent signal i in FIG. 41 may
be referred to as having nonuniform temporal pulsations of varying
amplitude and period.
[0252] In particular embodiments, a light source 110 of a hybrid
pulsed/coherent lidar system 100 may impart a spectral signature to
an emitted pulse of light 400. The light source 110 may emit LO
light 430 and pulses of light 400, where each emitted pulse of
light 400 includes a spectral signature of one or more different
spectral signatures. A spectral signature (which may be referred to
as a frequency signature, frequency tag, or frequency change) may
correspond to the presence or absence of particular frequency
components that are imparted to an emitted pulse of light 400. The
LO light 430 may have a relatively narrow spectral linewidth
centered at a particular optical frequency, and a spectral
signature imparted to an emitted pulse of light 400 may correspond
to a difference (e.g., a broadening or a shifting) in the optical
spectrum of the emitted pulse of light with respect to the LO
light. A spectral signature of an emitted pulse of light 400 may
include one or more of (i) a spectral linewidth that is broadened
with respect to the spectral linewidth of LO light 430 and (ii) a
spectral linewidth that is shifted with respect to the LO light
430. The spectral signature of a received pulse of light 410 that
includes scattered light from a corresponding emitted pulse of
light 400 may be substantially the same as or may be similar to the
spectral signature of the corresponding emitted pulse of light. The
optical spectrum of each of the received pulses of light 410 in
FIGS. 36-38 are broadened with respect to the LO light optical
spectrum. For example, LO light 430 may have a spectral linewidth
of <10 MHz, and a received pulse of light 410 may have a
spectral linewidth of >50 MHz. A broadened optical spectrum may
include an optical spectrum that is broadened substantially
uniformly or symmetrically (e.g., as illustrated in FIGS. 36-37) or
that is broadened nonuniformly or asymmetrically or that includes
variation or ripples in the optical spectrum (e.g., as illustrated
in FIG. 38). In addition to being broadened, the optical spectrum
of the received pulse of light 410 in FIG. 37 is shifted by
.DELTA.f with respect to the LO light optical spectrum. A spectral
shift may be achieved by (i) shifting the optical spectrum of LO
light 430, (ii) shifting the optical spectrum of an emitted pulse
of light 400, or (iii) shifting both optical spectra. For example,
in FIG. 37, a light source 110 may (i) shift the spectrum of the LO
light 430 by -.DELTA.f with respect to an emitted pulse of light
400 (which corresponds to the received pulse of light 410), (ii)
shift the spectrum of the emitted pulse of light 400 by +.DELTA.f
with respect to the LO light 430, or (iii) shift the spectrum of
the LO light by -.DELTA.f/2 and shift the spectrum of the emitted
pulse of light 400 by +.DELTA.f/2.
[0253] In particular embodiments, LO light 430 may act as a
reference optical signal, and coherent mixing of a received pulse
of light 410 with LO light may allow the spectral signature of the
received pulse of light to be determined. Coherent mixing of LO
light 430 and a received pulse of light 410 may be viewed as a
down-conversion or heterodyne process that shifts the spectral
signature of a received pulse of light from an optical frequency
range (e.g., 150-350 THz) down to an electronic frequency range
(e.g., 10 MHz-50 GHz). By shifting the spectral signature into an
electronic frequency range, the spectral signature may be
determined using an electronic measurement technique. A spectral
signature may include a broadening or shifting of the optical
spectrum of an emitted pulse of light 400 with respect to LO light
430, which corresponds to a signature that is primarily associated
with the frequency domain (rather than the time domain). That is,
an emitted pulse of light that includes a spectral signature may
have a time-domain pulse shape (e.g., a Gaussian pulse) that does
not include a significant amount of amplitude modulation
superimposed onto the temporal pulse shape. In a hybrid
pulsed/coherent lidar system with spectral signatures, the spectral
signatures may be encoded primarily onto the optical spectrum of an
emitted pulse of light rather than encoding an
amplitude-modulation-type signature onto the temporal shape of the
pulse of light. When a received pulse of light 410 that includes a
spectral signature is coherently mixed with LO light 430, the
resulting coherent-mixing term may include temporal pulsations that
arise from the spectral signature. In this way, an optical
frequency-domain signal (the spectral signature) may be converted
into an electronic time-domain signal (the temporal pulsations),
which may be measured using a time-domain-based
electronic-measurement technique. Thus, while a spectral signature
may be considered primarily to be part of the optical spectrum or
frequency domain of a pulse of light, in a hybrid pulsed/coherent
lidar system 100, the determination of the spectral signature of a
pulse of light may be based primarily on a time-domain measurement
of temporal pulsations resulting from coherent mixing.
[0254] In particular embodiments, a light source 110 may impart a
spectral signature to an emitted pulse of light 400 using one or
more of: (i) an optical modulator 495, (ii) based on seed current
I.sub.1 supplied to a seed laser diode 450, (iii) based on SOA
current I.sub.2 supplied to a SOA 460. For example, an optical
modulator 495 (e.g., as illustrated in FIGS. 24-27) or seed current
I.sub.1 (e.g., as illustrated in FIG. 35) may be used to shift the
optical frequency of an emitted pulse of light 400 with respect to
LO light 430. Additionally or alternatively, a pulse of SOA current
I.sub.2 supplied to a SOA 460 may be used to impart a spectral
signature to an emitted pulse of light 400.
[0255] In particular embodiments, a light source 110 may impart a
spectral signature to an emitted pulse of light 400 using an
optical modulator 495. For example, a light source 110 may include
an optical modulator 495 similar to that illustrated in FIG. 24,
25, 26, or 27. A phase modulator may impart a spectral signature to
an emitted pulse of light 400 by shifting the frequency of LO light
430 or seed light 440. For example, the modulator 495 in FIG. 25
may be an electro-optic phase modulator that applies a time-varying
phase shift to the seed light 440, which may result in a frequency
shift of the seed light 440 with respect to the LO light 430. The
resulting emitted pulse of light 400 may include approximately the
same frequency shift with respect to the LO light 430. As another
example, the modulator 495 in FIG. 26 may be a phase modulator that
shifts the frequency of the LO light 430 with respect to the seed
light 440.
[0256] In particular embodiments, a light source 110 may impart a
spectral signature to an emitted pulse of light 400 based on seed
current I.sub.1 supplied to a seed laser diode 450. For example, a
light source 110 may include (i) a seed laser diode 440 that
produces seed light 440 and LO light 430 and (ii) a SOA 460 that
amplifies temporal portions of the seed light 440 to produce
emitted pulses of light 440. A spectral signature imparted to an
emitted pulse of light 400 may include a shifted optical spectrum
in which the emitted pulse of light 400 and the LO light 430 are
offset by a frequency difference of .DELTA.f. An electronic driver
480 may supply seed current I.sub.1 to the seed laser diode 450,
and (as illustrated in FIG. 35) the optical frequency of the seed
light 440 may be changed by changing the seed current I.sub.1 by a
particular amount (A.sub.1) to cause a frequency difference of
.DELTA.f between the emitted pulse of light 400 and the LO light
430. In FIG. 35, the electrical current supplied to the seed laser
diode is ii when the seed laser diode produces temporal portion 441
that is amplified to produce the emitted pulse of light 400. After
the temporal portion 441 of the seed light 440 is produced, the
electrical current supplied to the seed laser diode is changed to
i.sub.0, where .DELTA.i=i.sub.0-i.sub.1. The SOA 460, which
amplifies the temporal portion 441 of the seed light 440 to produce
the emitted pulse of light 400, may substantially maintain the
optical frequency of the seed light 440. As a result, the emitted
pulse of light 400 or the corresponding received pulse of light 410
may also have approximately the same optical frequency offset of
.DELTA.f with respect to the LO light 430.
[0257] In particular embodiments, a light source 110 may impart a
spectral signature to an emitted pulse of light 400 based on SOA
current I.sub.2 supplied to a SOA 460. For example, in addition to
or instead of imparting a frequency change to an emitted pulse of
light 400 based on the seed current I.sub.1, a light source 110 may
impart a frequency change to an emitted pulse of light based on the
SOA current I.sub.2 supplied to a SOA 460. A light source 110 may
include (i) a seed laser diode 440 that produces seed light 440 and
LO light 430 and (ii) a SOA 460 that amplifies temporal portions of
the seed light 440 to produce emitted pulses of light 440. An
electronic driver 480 may supply (i) a substantially constant seed
current I.sub.1 to the seed laser diode and (ii) pulses of
electrical current I.sub.2 to the SOA 460, where each pulse of
current causes the SOA 460 to amplify a temporal portion 441 of the
seed light 440 to produce a corresponding emitted pulse of light
400. In addition to amplifying the temporal portion 441, the pulse
of current may also cause the SOA 460 to impart a spectral
signature to the amplified temporal portion so that the
corresponding emitted pulse of light 400 includes the spectral
signature. A spectral signature may be imparted to a temporal
portion 441 while propagating through and being amplified by the
SOA 460, resulting in an emitted pulse of light 400 that includes
the spectral signature.
[0258] Seed light 440 may have a relatively narrow spectral
linewidth that is approximately equal to the spectral linewidth of
the LO light 430 (e.g., .DELTA.v.sub.1 in FIG. 34), and amplifying
a temporal portion 441 of seed light 440 may result in the
linewidth being broadened according to the inverse relationship
between pulse duration and spectral linewidth. For example, in FIG.
34, the pulse duration (.DELTA..tau.) and the spectral linewidth
(.DELTA.v.sub.2) of the received pulse of light 410 may be related
by the expression .DELTA..tau..DELTA.v.sub.2.gtoreq.0.441. For
example, if the pulse duration .DELTA..tau. is 2 ns, then the
spectral linewidth .DELTA.v.sub.2 may be greater than approximately
220 MHz. At least part of a spectral signature imparted to an
emitted pulse of light may result from spectral broadening due to
the time-bandwidth relationship between pulse duration and spectral
linewidth. In addition to broadening the spectral linewidth of an
emitted pulse of light 400 based on the time-bandwidth
relationship, a light source 110 may also impart at least part of a
spectral signature to the emitted pulse of light 400 through one or
more nonlinear optical effects. For example, in a light source 110
that includes a seed laser diode 450 and a SOA 460, one or more of
the following effects occurring in the seed laser diode 450 or the
SOA 460 may impart a spectral signature to an emitted pulse of
light: four-wave mixing, Kerr nonlinear optical effect, self-phase
modulation, coupled-cavity effects between the seed laser diode and
the SOA, stimulated Raman scattering (SRS), stimulated Brillouin
scattering (SBS), and plasma dispersion effect. A spectral
signature associated with one or more nonlinear optical effects may
cause a broadening of the spectral linewidth of an emitted pulse of
light 400 or may cause a shift in the optical frequency of an
emitted pulse of light 400. A pulse of SOA current I.sub.2 may
include an amplitude modulation (e.g., a linear or sinusoidal
current variation added to the current pulse), and a spectral
signature may be imparted to an emitted pulse of light 400 based at
least in part on the amplitude modulation of the current pulse.
Alternatively, a pulse of SOA current I.sub.2 may not include any
additional modulation so that the pulse of SOA current increases
approximately monotonically, may be held approximately constant for
some time, and then decreases approximately monotonically (e.g., as
illustrated by the SOA current graph in FIG. 15), and a spectral
signature may be imparted to an emitted pulse of light 400 based on
(i) the time-bandwidth relationship between pulse duration and
spectral linewidth or (ii) one or more nonlinear optical
effects.
[0259] In particular embodiments, one or more characteristics of a
spectral signature imparted to an emitted pulse of light 400 may
depend on an amplitude, duration, rise time, fall time, or shape of
a corresponding pulse of electrical current supplied to the SOA
460. For example, a spectral signature may include a broadening of
the spectral linewidth of an emitted pulse of light 400 with
respect to the spectral linewidth of LO light 430, and the amount
of spectral broadening may depend at least in part on the
amplitude, duration, rise time, fall time, or shape of the
corresponding pulse of SOA current I.sub.2. A pulse of SOA current
with a shorter duration, a shorter rise time, or a shorter fall
time may be associated with a greater amount of spectral
broadening. As another example, a spectral signature may include a
shift in the optical frequency of an emitted pulse of light 400,
and the amount of spectral shift may depend at least in part on the
amplitude, duration, rise time, fall time, or shape of the
corresponding pulse of SOA current. A pulse of SOA current with a
shorter duration, a shorter rise time, or a shorter fall time may
be associated with a greater amount of spectral shift. In
particular embodiments, an electronic driver 480 may be configured
to supply pulses of current I.sub.2 to a SOA 460, where each pulse
of current imparts to each corresponding emitted pulse of light 400
a spectral signature of two or more different spectral signatures.
For example, an electronic driver 480 may supply electrical current
pulses having two or more different durations or rise times, and
each current-pulse duration or rise time may result in an emitted
pulse of light 400 having a particular pulse duration and a
corresponding particular spectral linewidth. Current pulses with
shorter durations or shorter rise times may result in emitted
pulses of light 400 having shorter pulse durations and broader
spectral linewidths.
[0260] FIGS. 42-43 each illustrates an example photocurrent signal
i that includes a pulse term, a coherent-mixing term, and a
local-oscillator (LO) term. The three graphs on the left in each of
FIGS. 42-43 illustrate example temporal behavior of each of the
three terms separately: (1) the pulse term,
|.epsilon..sub.Rx(t)|.sup.2, (2) the coherent-mixing term,
2|.epsilon..sub.Rx(t)||.epsilon..sub.LO(t)|cos[.DELTA..omega.(t)t+.DELTA.-
.PHI.(t)], and (3) the LO term, |.epsilon..sub.LO(t)|.sup.2. The
graph on the right in each of FIGS. 42-43, which corresponds to the
photocurrent signal i produced by a detector 340, illustrates the
sum of the three terms shown separately on the left. The
photocurrent signal i may represent the electrical current produced
by a detector 340 in response to detecting a received pulse of
light 410. The pulse term has a temporal shape of a pulse (e.g., a
Gaussian pulse), which may correspond to the temporal shape of the
received pulse of light. The LO term is substantially constant,
which corresponds to the optical power of the LO light 430 being
approximately constant. The coherent-mixing term in each of FIGS.
42-43 includes a time varying amplitude modulation superimposed
onto a temporal shape of a pulse. The amplitude modulation (which
may be referred to as temporal pulsations) of the coherent-mixing
term may correspond to a spectral signature of the received pulse
of light 410. The temporal pulsations of the coherent-mixing term
are also included in the corresponding photocurrent signal i, which
equals the sum of the pulse term, coherent-mixing term, and LO
term. The graphs in FIGS. 42-43 are similar to the graphs in FIGS.
30-31, except the coherent-mixing term and the photocurrent signal
in each of FIGS. 42-43 include temporal pulsations that may
correspond to a spectral signature. The photocurrent signal in FIG.
43 is similar to the photocurrent signal in FIG. 34.
[0261] The amplitude of the pulse term (A.sub.1) in FIG. 42 is
significantly larger than the amplitude of the coherent-mixing term
(A.sub.2), which may correspond to a received pulse of light 410
that is scattered from a nearby target 130 or a high-reflectivity
target 130. In this case, a hybrid pulsed/coherent lidar system 100
may be operating primarily as a direct-detection pulsed lidar
system. The amplitude of the coherent-mixing term (A.sub.2) in FIG.
43 is significantly larger than the amplitude of pulse term
(A.sub.1), which may correspond to a received pulse of light 410
that is scattered from a relatively distant target 130 or a
low-reflectivity target 130. In this case, a hybrid pulsed/coherent
lidar system 100 may be operating primarily as a coherent pulsed
lidar system.
[0262] A voltage signal 360 produced by an electronic amplifier 350
from the photocurrent signal in FIGS. 42-43 may have a shape or
temporal behavior that is similar to the photocurrent signal. For
example, a voltage signal 360 produced from the photocurrent signal
i in FIG. 42 may include a pulse shape with relatively small
temporal pulsations corresponding to the temporal pulsations of the
coherent-mixing term. As another example, a voltage signal 360
produced from the photocurrent signal i in FIG. 43 may include a
pulse shape with relatively large temporal pulsations corresponding
to the temporal pulsations of the coherent-mixing term.
[0263] In particular embodiments, a receiver 140 of a hybrid
pulsed/coherent lidar system 100 may include a pulse-detection
circuit 365 that determines a time-of-arrival for a received pulse
of light 410. The time-of-arrival may be determined based on the
first term (the pulse term) and the second term (the
coherent-mixing term) of a photocurrent signal i produced by a
detector 340. Additionally, a receiver 140 of a hybrid
pulsed/coherent lidar system 100 may include a frequency-detection
circuit 600 that determines a spectral signature of the received
pulse of light 410. The spectral signature of the received pulse of
light 410 may be determined based on the second term (the
coherent-mixing term) of the photocurrent signal i. The second term
of the photocurrent signal i (as well as the corresponding voltage
signal 360) may include temporal pulsations (which may be referred
to as amplitude modulation) that correspond to the spectral
signature of the received pulse of light 410. The
frequency-detection circuit 600 may determine the spectral
signature of the received pulse of light 410 based on the temporal
pulsations of the corresponding voltage signal 360. For example,
the frequency-detection circuit 600 may determine a frequency or
amplitude of one or more frequency components associated with the
temporal pulsations. One frequency component of the temporal
pulsations may be approximately equal to a frequency difference
.DELTA.f between the received pulse of light and the LO light 430,
and the frequency-detection circuit may determine the spectral
signature of the received pulse of light by determining the
frequency .DELTA.f of the temporal pulsations or by determining an
amplitude of the frequency component at the frequency .DELTA.f.
[0264] In particular embodiments, a frequency-detection circuit 600
may include multiple parallel frequency-measurement channels, where
each frequency-measurement channel includes an electronic band-pass
filter 610 and a corresponding amplitude detector 620 (e.g., as
illustrated in FIG. 7). A spectral signature of a received pulse of
light 410 may include one or more frequency components, and the
corresponding photocurrent signal i and voltage signal 360 may
include substantially the same frequency components. The
frequency-detection circuit 600 may determine a spectral signature
of a received pulse of light 410 by determining the frequency or
amplitude of one or more frequency components of a voltage signal
360, where the voltage signal corresponds to a photocurrent signal
i associated with the received pulse of light. For example, the
photocurrent signal i in FIG. 34 may have a frequency component
with a frequency of .DELTA.f and a frequency-detection circuit 600
may include a frequency-measurement channel with a band-pass filter
610 with a center frequency of approximately .DELTA.f. As another
example, the photocurrent signal i in FIG. 40 may have multiple
frequency components or a range of frequency components from
approximately 400 MHz to approximately 1 GHz, and a
frequency-detection circuit 600 may include multiple band-pass
filters 610 with center frequencies distributed in the 400-MHz to
1-GHz range. As another example, the photocurrent signal i in FIG.
41 may have two or more distinct frequency components, and a
frequency-detection circuit 600 may include two
frequency-measurement channels configured to determine an amplitude
of each of the frequency components.
[0265] The frequency-detection circuit 600 illustrated in FIG. 7
may be used to determine the spectral signature of a received pulse
of light 410 by determining the frequency or amplitude of one or
more frequency components of a photocurrent signal i associated
with the received pulse of light. For example, a
frequency-detection circuit 600 may determine whether a voltage
signal 360 includes one or more particular frequency components,
where each of the frequency components corresponds to one of the
frequency-measurement channels of the frequency-detection circuit
600. Additionally, the frequency-detection circuit 600 may
determine an amplitude of each of the one or more particular
frequency components. The spectral signature of the received pulse
of light 410 as determined by the frequency-detection circuit 600
may include a list of one or more frequency components in the
voltage signal 360. Additionally, the spectral-signature list may
include an amplitude of each of the frequency components in the
voltage signal 360. In the example of FIG. 7, the
frequency-detection circuit 600 includes M band-pass filters 610
and M amplitude detectors 620. Each band-pass filter 610 has a
center frequency corresponding to a particular frequency component
(from f.sub.a to f.sub.M), and each amplitude detector 620 may
produce a signal corresponding to the amplitude of the particular
frequency component. The frequency-detection output signal produced
by the frequency-detection circuit 600 may include M digital values
corresponding to the amplitudes of the M frequency components. For
example, a frequency-detection circuit 600 may include 10 band-pass
filters 610, each filter having a center frequency of 100 MHz, 200
MHz, . . . , 900 MHz, or 1 GHz, and the frequency-detection output
signal may include 10 digital values corresponding to the
amplitudes of each of the 10 frequency components.
[0266] In particular embodiments, a hybrid pulsed/coherent lidar
system 100 may include a controller 150 (which may be referred to
as a processor) that determines whether a spectral signature of a
received pulse of light 410 matches a spectral signature of an
emitted pulse of light 400. The processor may compare the spectral
signature of the received pulse of light 400 to the spectral
signature of the emitted pulse of light 400 to determine a
spectral-signature score that represents an amount of correlation
between the two spectral signatures. If the spectral-signature
score is greater than a particular threshold value, then the
processor may determine that the two spectral signatures match. The
two spectral signatures matching may indicate that the received
pulse of light 410 is associated with a particular emitted pulse of
light 400, which indicates that the received pulse of light
includes light from the emitted pulse of light that was scattered
from a target 130. The threshold value for determining that two
spectral signatures match may be any suitable value, such as for
example, 1.0 (indicating a 100% correlation between the two
spectral signatures), 0.9 (indicating a 90% correlation), or 0.8
(indicating an 80% correlation). For example, a frequency-detection
circuit 600 may include 10 frequency-measurement channels that
measure 10 different frequency components, and based on the
amplitudes of the 10 frequency components, a processor may
determine whether a received pulse of light 410 is associated with
a particular emitted pulse of light 400. If all 10 frequency
components match (e.g., 100% correlation between the two spectral
signatures), then the processor may determine that the received
pulse of light 410 is associated with the particular emitted pulse
of light 400. Alternatively, if 8 or more frequency components
match (e.g., .gtoreq.80% correlation), then the processor may
determine that the received pulse of light 410 is associated with
the particular emitted pulse of light 400.
[0267] In particular embodiments, a controller 150 may determine,
based on the amplitudes of one or more frequency components
associated with a received pulse of light 410, whether the received
pulse of light 410 is associated with a particular emitted pulse of
light 400. If one or more frequency components of a received pulse
of light 410 match a spectral signature of a particular emitted
pulse of light 400, then the controller 150 may determine that the
received pulse of light 410 is associated with the particular
emitted pulse of light 400. A received pulse of light 410 being
associated with an emitted pulse of light 400 may refer to the
received pulse of light including a portion light from the emitted
pulse of light (e.g., the received pulse of light includes light
from the emitted pulse of light that was scattered from a target
130). Otherwise, if the frequency components do not match, then the
controller 150 may determine that the received pulse of light 410
is not associated with the particular emitted pulse of light 400
(e.g., the received pulse of light 410 does not include scattered
light from the emitted pulse of light 400). For example, the
received pulse of light 410 may be associated with a different
pulse of light 400 emitted by the light source 110 of the lidar
system 100, or the received pulse of light 410 may be associated
with an interfering optical signal emitted by a different light
source external to the lidar system 100. As another example, a
particular pulse of light 400 emitted by the light source 110 may
include a spectral signature that produces a coherent-mixing term
with an amplitude modulation at one or more particular frequencies
(e.g., 600 MHz and 1 GHz), and a frequency-detection circuit 600
may include filters 610 and amplitude detectors 620 that determine
the amplitude of the frequency components for a received pulse of
light 410. If the amplitudes of the two frequency components are
each greater than a particular threshold value (or within a range
of two particular threshold values), then the controller 150 may
determine that the received pulse of light 410 is associated with
and includes light from the particular emitted pulse of light 400.
Otherwise, if the amplitude one or both frequency components are
less than the particular threshold value, then the controller 150
may determine that the received pulse of light 410 is not
associated with and does not include light from the particular
emitted pulse of light 400. Additionally or alternatively, if the
amplitude of a different frequency component (e.g., a 0.8-GHz
frequency component) that is not part of a particular spectral
signature is greater than a particular threshold value, then the
controller may determine that the received pulse of light 400 is
not associated with an emitted pulse of light 400 having that
particular spectral signature.
[0268] In particular embodiments, a spectral signature of a
received pulse of light 410 matching a spectral signature of an
emitted pulse of light 400 may correspond to the spectral signature
of the received pulse of light including at least a particular
minimum amount of frequency components associated with the spectral
signature of the emitted pulse of light. Determining whether two
spectral signatures match may require that some minimum number
(e.g., greater than 2, 4, or 8) or some minimum percentage (e.g.,
greater than 50%, 75%, or 90%) of the frequency components are
included in each of the spectral signatures. For example, an
emitted pulse of light 400 may include a spectral signature
associated with the four frequency components 400 MHz, 500 MHz, 600
MHz, and 700 MHz. A received pulse of light 410 with a spectral
signature that includes all of the four frequency components (e.g.,
100% of the frequency components) may be determined to match the
spectral signature of the emitted pulse of light 400.
Alternatively, a received pulse of light 410 with a spectral
signature that includes at least three of the four frequency
components (e.g., at least 75% of the frequency components) may be
determined to match the spectral signature of the emitted pulse of
light 400. Additionally, a spectral signature of a received pulse
of light 410 matching a spectral signature of an emitted pulse of
light 400 may further correspond to the spectral signature of the
received pulse of light including less than a particular maximum
amount of frequency components that are not associated with the
spectral signature of the emitted pulse of light. In addition to a
requirement that some minimum number or percentage of frequency
components are included in each of the spectral signatures,
determining whether two spectral signatures match may also require
the occurrence of less than some maximum number (e.g., less than 1,
2, or 3) or percentage (e.g., less than 5%, 10%, or 20%) of
non-matching frequency components. For the emitted pulse of light
400 that includes a spectral signature associated with the four
frequency components 400 MHz, 500 MHz, 600 MHz, and 700 MHz, if the
received pulse of light 410 also includes two non-matching
frequency components (e.g., at 200 MHz and 900 MHz), then the two
spectral signatures may be determined not to match (e.g., the
criteria may require no more than one non-matching frequency
component). Alternatively, the criteria may require zero
non-matching frequency components, and if the received pulse of
light 410 also includes one non-matching frequency component, then
the two spectral signatures may be determined not to match.
Determining whether two spectral signatures match based on the
presence of a particular number or percentage of frequency
components may be referred to as determining a spectral-signature
score, where the spectral signature score represents an amount of
correlation between the two spectral signatures.
[0269] In particular embodiments, a frequency-detection circuit 600
may include a matched filter. The matched filter may be used to
compare the spectral signature of an emitted pulse of light 400
with the spectral signature of a received pulse of light 410 to
determine a spectral-signature score representing an amount of
correlation between the two spectral signatures.
[0270] FIG. 44 illustrates an example receiver 140 that includes a
frequency-detection circuit 600 with a derivative circuit 630 and a
zero-crossing circuit 640. In particular embodiments, a hybrid
pulsed/coherent lidar system 100 may include a receiver 140 with a
frequency-detection circuit 600 that includes a derivative circuit
630 and a zero-crossing circuit 640. The derivative circuit 630 may
receive a voltage signal 360 and produce a derivative signal 631
corresponding to a first derivative with respect to time of the
voltage signal. The voltage signal 360 may correspond to the
photocurrent signal i, and the derivative signal 631 may correspond
to a first derivative of the photocurrent signal i. The derivative
circuit 630 may include an analog differentiator, such as for
example, an operational amplifier with a series capacitor located
at the inverting input terminal and a resistor located across the
operational amplifier to provide negative feedback. The derivative
signal 631 may be an analog voltage signal that is proportional to
the first derivative with respect to time of the voltage signal
360. The zero-crossing circuit 640 may determine two or more zero
crossings 641 of the derivative signal 631. Each zero crossing may
include a time value indicating a time at which the derivative
signal 631 crosses the x-axis, where the x-axis corresponds to a
value of zero volts for the derivative signal. The zero-crossing
circuit 640 may include a comparator 370 followed by a timer
circuit (e.g., a TDC 380), and the threshold voltage for the
comparator may be set to approximately zero volts. When the
derivative signal 631 crosses zero volts, the comparator 370 may
produce an electrical-edge signal, and the timer circuit may
produce a digital value that represents a time when the edge signal
is received from the comparator. The frequency-detection output
signal may include two or more digital time values, each time value
corresponding to one of the zero crossings 641.
[0271] FIG. 45 illustrates an example photocurrent signal i and a
corresponding derivative signal 631. The photocurrent signal i
includes temporal pulsations that may correspond to a spectral
signature, and the derivative signal 631 represents a first
derivative of the photocurrent signal. Each zero crossing 641 of
the derivative signal 631 corresponds to a peak or valley (e.g., a
point with zero slope) of the photocurrent signal i. The derivative
signal 631 in FIG. 45 includes seven zero crossings 641,
represented by the seven time values t.sub.1, t.sub.2, t.sub.3,
t.sub.4, t.sub.5, t.sub.6, and t.sub.7. The zero crossings 641 may
be referred to as a spectral-signature pattern and may represent
the spectral signature of a received pulse of light 410
corresponding to the photocurrent signal i.
[0272] The frequency-detection circuit 600 in FIG. 44 may send a
frequency-detection output signal that includes the time values of
the zero crossings 641 to a controller 150. Based on the zero
crossings 641, the controller 150 may determine whether the
spectral signature of a received pulse of light 410 matches a
spectral signature of an emitted pulse of light 400. Determining
whether the two spectral signatures match may include comparing or
correlating the zero crossings 641 associated with the received
pulse of light 410 with zero crossings associated with the emitted
pulse of light 400. The processor may compare the zero crossings
641 of the received pulse of light 410 to the zero crossings of the
emitted pulse of light to determine a spectral-signature score that
represents an amount of correlation between the two spectral
signatures. If the spectral-signature score is greater than a
particular threshold value, then the processor may determine that
the two spectral signatures match. Determining that two spectral
signatures match may require greater than 70% correlation, 80%
correlation, 90% correlation, or any other suitable amount of
correlation between zero-crossing values associated with the two
spectral signatures. For example, if 8 out of 10 zero-crossing
values match (indicating an 80% correlation), then the processor
may determine that the two spectral signatures match.
Alternatively, if 7 out of 10 zero-crossing values match
(indicating less than 80% correlation), then the processor may
determine that the two spectral signatures do not match. Comparing
two sets of zero crossings 641 may include a direct comparison of
time values (e.g., compare the time intervals t.sub.2-t.sub.1,
t.sub.3-t.sub.1, t.sub.4-t.sub.1, etc. of the received pulse of
light 410 with corresponding time intervals of the emitted pulse of
light 400). Alternatively, comparing two sets of zero crossings 641
may include comparing ratios of time intervals, which may allow for
scaling, distortion, or stretching of one set of zero crossings
with respect to another (e.g., due to a Doppler shift). For
example, the scaled time-interval values
(t.sub.3-t.sub.1)/(t.sub.2-t.sub.1),
(t.sub.4-t.sub.1)/(t.sub.2-t.sub.1),
(t.sub.5-t.sub.1)/(t.sub.2-t.sub.1), etc. of the received pulse of
light 410 may be compared with corresponding scaled time-interval
values of the emitted pulse of light 400.
[0273] In particular embodiments, a light source 110 may impart a
spectral signature to emitted pulses of light 400 in a
deterministic manner so that each emitted pulse of light includes a
predetermined spectral signature. The light source 110 may impart a
spectral signature to each emitted pulse of light 400 using an
optical modulator 495, seed current I.sub.1, or SOA current
I.sub.2, and each emitted pulse of light may include a
predetermined spectral signature of one or more different spectral
signatures. The light source 110 may impart substantially the same
spectral signature to each of the emitted pulses of light 400, or
the light source 110 may impart two or more different spectral
signatures so that each emitted pulse of light 400 includes one of
the different spectral signatures. For example, a light source 110
may impart a spectral signature to each emitted pulse of light 400
using an optical modulator 495, and the spectral signature imparted
to an emitted pulse of light 400 may depend on the electronic drive
signal (e.g., RF power or frequency) supplied to the modulator 495.
An optical modulator 495 may be operated with the same drive signal
for each emitted pulse of light 400, and each emitted pulse of
light 400 may have substantially the same spectral signature.
Alternatively, an optical modulator 495 may be operated with n
different drive signals (where n is an integer greater than or
equal to 2), and each emitted pulse of light 400 may have one of n
different corresponding spectral signatures. The spectral
signatures imparted by the optical modulator 495 may be
deterministic in that an imparted spectral signature may be
determined primarily based on the drive signal supplied to the
modulator. For example, two emitted pulses of light may have
substantially the same spectral signature if the same drive
parameters are supplied to the optical modulator 495.
[0274] In particular embodiments, a light source 110 may impart
spectral signatures to emitted pulses of light 400 in a
pseudo-random manner so that each emitted pulse of light includes a
non-predetermined spectral signature. Pseudo-random spectral
signatures (which may be referred to as non-deterministic spectral
signatures or random spectral signatures) may be produced through a
process that includes at least some random or non-deterministic
addition of frequency components to an emitted pulse of light 400.
For example, a light source 110 may impart a spectral signature to
each emitted pulse of light 400 based on the SOA current I.sub.2
supplied to a SOA 460. The particular spectral signature imparted
to an emitted pulse of light 400 may depend on the pulse
characteristics (e.g., amplitude, duration, rise time, fall time,
or shape) of a corresponding pulse of electrical current supplied
to the SOA 460. Frequency components may be added to an emitted
pulse of light 400 based on the inverse relationship between the
duration (.DELTA..tau.) and the spectral linewidth
(.DELTA.v.sub.2), and this process may be substantially
deterministic (e.g., based on the relationship
.DELTA..tau..DELTA.v.sub.2.gtoreq.0.441). Frequency components may
also be added to an emitted pulse of light based on one or more
nonlinear optical effects occurring within the seed laser diode 450
or the SOA 460, and these effects may be substantially
non-deterministic. That is, due at least in part to the
pseudo-random nature of the nonlinear optical effects, two emitted
pulses of light 400 produced by two pulses of current with
substantially the same pulse characteristics may have different
spectral signatures. For example, an electronic driver 480 may
supply pulses of current I.sub.2 to a SOA 460, where each pulse of
current has substantially the same amplitude, duration, rise time,
fall time, and shape, and the corresponding emitted pulses of light
400 may each have a different spectral signature. In case the
random variation of spectral signatures may not provide enough
variation to differentiate between different pulses of light, an
electronic driver 480 may change the pulse characteristics of the
pulses of current supplied to the SOA 460. For example, to produce
n emitted pulses of light 400 having n significantly different
spectral signatures, an electronic driver 480 may supplied pulses
of current having n different pulse characteristics (e.g., n
different rise times or durations). The n different spectral
signatures may be differentiated from one another since they may
include different frequency components based on (i) the different
pulse characteristics and (ii) the random behavior of nonlinear
optical effects.
[0275] In particular embodiments, a frequency-detection circuit 600
of a hybrid pulsed/coherent lidar system 100 may determine a
spectral signature of an emitted pulse of light 400. In addition to
determining the spectral signatures of received pulses of light
410, a frequency-detection circuit 600 may also determine the
spectral signatures of one or more of the emitted pulses of light
400. For example, if spectral signatures are imparted to emitted
pulses of light 400 in a pseudo-random manner, then the spectral
signature of each emitted pulse of light may be determined using a
frequency-detection circuit 600. A portion of each emitted pulse of
light 400 may be split off and sent to the receiver 140. For
example, prior to an emitted pulse of light 400 exiting the lidar
system 100, an optical splitter 470 may split off a relatively
small portion of the emitted pulse of light 400 (e.g., <10% of
the pulse energy may be split off). The receiver 140 may detect LO
light 430 and the split-off portion of the emitted pulse of light,
and a detector 340 may produce a photocurrent signal corresponding
to coherent mixing of the LO light 430 and the split-off portion of
the emitted pulse of light. The frequency-detection circuit 600 may
determine the spectral signature of the emitted pulse of light
based on the second term of the photocurrent signal resulting from
coherent mixing of the LO light 430 and the split-off portion of
the emitted pulse of light. For example, the frequency-detection
circuit 600 may produce a set of zero-crossing values 641 that
represent the spectral signature of the emitted pulse of light 400,
and a controller 150 may receive and store the set of zero-crossing
values. The zero-crossing values for the emitted pulse of light 400
may be compared with zero-crossing values associated with a
subsequently received pulse of light 410 to determine whether the
spectral signature of the received pulse of light 410 matches the
spectral signature of the emitted pulse of light 400. Additionally,
the controller 150 may store zero-crossing values associated with
the n most recently emitted pulses of light 400, where n is an
integer greater than or equal to 2 (e.g., n may have a value of 2,
4, 8, 16, or 50). When a pulse of light is received, the
zero-crossing values for the received pulse of light 410 may be
compared with each of the zero-crossing values of the n most
recently emitted pulses of light 400 to determine whether the
received pulse of light 410 is associated with one of the recently
emitted pulses of light.
[0276] FIG. 46 illustrates an example lidar system 100 that emits n
pulses of light 400 having n different respective spectral
signatures. In particular embodiments, a light source 110 may emit
n pulses of light, where each emitted pulse of light has one of n
different spectral signatures, and n is an integer greater than or
equal to 2 (e.g., n may have a value of 2, 4, 8, 16, or 50). In
FIG. 46, pulse 400-1 has a spectral signature that corresponds to
photocurrent signal i-1, and pulse 400-n has a spectral signature
that corresponds to photocurrent signal i-n. The n pulses of light
may be emitted in a sequence from pulse 400-1 to pulse 400-n, and
temporally adjacent pulses of light may be separated by a
particular time interval (e.g., a time interval between 20 ns and 5
.rho.s). The n pulses of light may represent the n pulses of light
most recently emitted from the lidar system 100.
[0277] A controller 150 may store spectral-signature information
associated with each of the n emitted pulses of light 400. For
example, a processor may store zero-crossing values associated with
each of the n emitted pulses of light 400. One of the n pulses of
light 400 may scatter from a target 130, and a portion of the
scattered light may return to the lidar system as a received pulse
of light 410. In FIG. 46, pulse 400-n may scatter from the target
130, and a portion of the scattered light may return to the lidar
system 100 as received pulse of light 410 with a spectral signature
that corresponds to photocurrent signal i-410. A
frequency-detection circuit 600 may determine the spectral
signature of the received pulse of light 410 (e.g., based on
determining zero-crossing values associated with the received pulse
of light), and the processor may compare the spectral-signature
information of the received pulse of light 410 to the stored
spectral-signature information for each of the n emitted pulses of
light. For example, the processor may determine n
spectral-signature scores, where each score represents an amount of
correlation between the spectral signature of the received pulse of
light 410 and the spectral signature of one of the n emitted pulses
of light 400. The processor may determine that the spectral
signature of the received pulse of light 410 matches the spectral
signature of a particular emitted pulse of light, based on the
particular emitted pulse of light having the highest
spectral-signature score (which indicates that the received pulse
of light 410 is associated with the particular emitted pulse of
light). If none of the n spectral-signature scores exceeds a
particular threshold value (e.g., a threshold value of 80%
correlation), then the processor may determine that the spectral
signature of the received pulse of light 410 does not match any of
the n different spectral signatures (e.g., the received pulse of
light 410 may be an interfering optical signal originating from a
source outside the lidar system). In FIG. 46, the spectral
signature of the received pulse of light 410 (which corresponds to
photocurrent signal i-410) may be determined to match the spectral
signature of emitted pulse of light 400-n.
[0278] In particular embodiments, a light source 110 of a hybrid
pulsed/coherent lidar system 100 may emit LO light 430 and pulses
of light 400, each emitted pulse of light having a spectral
signature of one or more different spectral signatures. The
spectral signatures may include one spectral signature (e.g., each
emitted pulse of light 400 may have the same spectral signature) or
multiple different spectral signatures, and the spectral signatures
may be imparted to the emitted pulses of light 400 in a
deterministic manner or in a pseudo-random manner. One emitted
pulse of light 400 with a particular spectral signature of the one
or more different spectral signatures may scatter from a target
130, and a portion of the scattered light may return to the lidar
system as a received pulse of light 410. A detector 340 may produce
a photocurrent signal corresponding to coherent mixing of the LO
light 430 and the received pulse of light 410, and the
coherent-mixing term of the photocurrent signal may include
temporal pulsations. A frequency-detection circuit 600 may
determine a spectral signature of the received pulse of light 410
based on the coherent-mixing term, and a processor (e.g.,
controller 150) may determine whether the spectral signature of the
received pulse of light 410 matches the particular spectral
signature of the emitted pulse of light. For example, the spectral
signature of the received pulse of light 410 may be substantially
the same as or may be similar to the particular spectral signature
of the emitted pulse of light. The two spectral signatures may be
determined to match based on a spectral-signature score being
greater than a particular threshold value, the spectral-signature
score representing an amount of correlation between the two
spectral signatures. In response to determining that the spectral
signature of the received pulse of light 410 matches the particular
spectral signature of the emitted pulse of light 400, the processor
may determine that the received pulse of light 410 is associated
with the emitted pulse of light 400, which indicates that the
received pulse of light includes a scattered portion of light from
the emitted pulse of light. Additionally or alternatively, in
response to determining that the spectral signature of the received
pulse of light 410 matches the particular spectral signature of the
emitted pulse of light 400, the processor may determine the
distance (D) from the lidar system 100 to the target 130 (e.g.,
based on the expression D=c.DELTA.T/2).
[0279] In particular embodiments, a light source 110 of a hybrid
pulsed/coherent lidar system 100 may emit a pulse of light 400
having a particular spectral signature. The emitted pulse of light
400 may scatter from a target 130, and a portion of the scattered
light may return to the lidar system as a first received pulse of
light 410. Additionally, a second received pulse of light 410 may
be detected by the receiver 140 of the lidar system 100, and the
second received pulse of light 410 may have a second spectral
signature that is different from the particular spectral signature
of the emitted pulse of light 400. A frequency-detection circuit
600 may determine the second spectral signature of the second
received pulse of light 410 based on a coherent-mixing term
resulting from coherent mixing of the LO light and the second
received pulse of light 410. A processor may determine that the
second received pulse of light 410 is not associated with the
emitted pulse of light 400 based on the second spectral signature
of the second received pulse of light not matching the particular
spectral signature of the emitted pulse of light. For example, the
two spectral signatures may be determined to not match based on a
spectral-signature score being less than a particular threshold
value, the spectral-signature score representing an amount of
correlation between the two spectral signatures.
[0280] In particular embodiments, a light source 110 of a hybrid
pulsed/coherent lidar system 100 may emit a first pulse of light
having a first spectral signature and a second pulse of light
having a second spectral signature different from the first
spectral signature. The two emitted pulses of light may scatter
from one or more targets 130, and a portion of scattered light may
return to the lidar system as a first received pulse of light and a
second received pulse of light. A frequency-detection circuit 600
may determine a spectral signature of the second received pulse of
light based on a coherent-mixing term resulting from coherent
mixing of the LO light and the second received pulse of light. A
processor may determine that the second received pulse of light 410
is not associated with the first emitted pulse of light based on
the spectral signature of the second received pulse of light not
matching the first spectral signature of the first emitted pulse of
light. Additionally or alternatively, the processor may determine
that the second received pulse of light 410 is associated with the
second emitted pulse of light based on the spectral signature of
the second received pulse of light matching the second spectral
signature of the second emitted pulse of light.
[0281] In particular embodiments, a receiver 140 or a controller
150 may determine whether a received pulse of light 410 (i) is a
valid received pulse of light that is associated with one of the
pulses of light 400 emitted by the lidar source 110, (ii) is a
valid received pulse of light that is associated with a particular
emitted pulse of light 400, or (iii) is an interfering optical
signal that is not associated with any of the emitted pulses of
light 400. A light source 110 may emit pulses of light 400 where
each emitted pulse of light 400 has a particular spectral signature
of one or more different spectral signatures. The spectral
signatures may be used to determine whether a received pulse of
light 410 is a valid received pulse of light that is associated
with an emitted pulse of light 400. A valid received pulse of light
410 may refer to a received pulse of light that includes scattered
light from a pulse of light 400 that was emitted by the light
source 110. For example, a light source 110 may emit pulses of
light 400 that each include the same spectral signature. If a
received pulse of light 410 matches that same spectral signature,
then the received pulse of light may be determined to be a valid
received pulse of light that is associated with an emitted pulse of
light 400. As another example, a light source 110 may emit pulses
of light 400 that each include one spectral signature of two or
more different spectral signatures. The light source 110 may emit
pulses of light 400 with spectral signatures that alternate (e.g.,
sequentially or in a pseudo-random manner) between two, three,
four, or any other suitable number of different spectral
signatures. If a received pulse of light 410 matches a particular
spectral signature of one of the emitted pulses of light, then the
received pulse of light may be determined to be a valid received
pulse of light 410 that is associated with that emitted pulse of
light 400. Emitting pulses of light 400 that have different
spectral signatures may allow a frequency-detection circuit 600 and
controller 150 to prevent problems with ambiguity as to which
emitted pulse of light a received pulse of light 410 is associated
with. A received pulse of light 410 may be unambiguously associated
with an emitted pulse of light 400 based on the spectral signature
of the received pulse of light 410 matching the spectral signature
of the emitted pulse of light 400.
[0282] If the spectral signature of a received pulse of light 410
does not match any of one or more different spectral signatures
imparted to emitted pulses of light 400, then a controller 150 may
determine that the received pulse of light is invalid or is not
associated with any of the emitted pulses of light. For example,
the received pulse of light may be an interfering optical signal
sent from a light source external to the lidar system 100. An
interfering optical signal may refer to an optical signal that is
sent by a light source external to the lidar system 100. For
example, another lidar system may emit a pulse of light that is
detected by the receiver 140, and the received pulse of light may
be determined to be an interfering optical signal if it does not
match any of the spectral signatures of the emitted pulses of light
400. A controller 150 may distinguish valid received pulses of
light from interfering pulses by comparing the spectral signature
of a received pulse of light with spectral signatures imparted to
emitted pulses of light 400. If a received pulse of light is
determined to be an interfering optical signal, the interfering
optical signal may be discarded or ignored since it is not
associated with any of the emitted pulses of light 400. A lidar
system 100 may refrain from determining a time-of-arrival or
determining a distance to a target 130 until a received pulse of
light 410 is determined to be valid. For example, a receiver 140 or
controller 150 may first verify that a received pulse of light 410
is valid before determining a time-of-arrival for the received
pulse of light or determining a distance to a target 130 associated
with the received pulse of light. If a received pulse of light 410
is determined to be an interfering optical signal, the receiver 140
may not perform further analysis to determine the time-of-arrival
or to determine a distance to a target.
[0283] FIG. 47 illustrates an example lidar system 100 configured
to determine a relative speed (S.sub.r) of a target 130. In
particular embodiments, a processor (e.g., controller 150) of a
hybrid pulsed/coherent lidar system 100 may determine a speed of a
target 130 with respect to the lidar system 100 based on a
frequency difference (.DELTA.F) between (i) a spectral signature of
an emitted pulse of light 400A and (ii) a spectral signature of a
received pulse of light 410A. The speed of the target 130 that is
determined may be a radial speed of the target relative to the
lidar system as measured along a line from the lidar system 100 to
the target, and the radial speed may not include a transverse speed
component that is directed orthogonal to the line. A radial speed
of a target 130 with respect to the lidar system 100 may refer to
the apparent speed of the target 130 from the perspective of the
lidar system 100. For example, the radial speed of the target 130
is 10 m/s in each of these scenarios: (i) the target 130 is moving
at 10 m/s towards the lidar system 100, and the lidar system 100 is
standing still, (ii) the target 130 is standing still, and the
lidar system 100 is moving at 10 m/s towards the target 130, (iii)
the target 130 and the lidar system 100 are each moving at 5 m/s
towards one another, and (iv) the target 130 is moving at 10 m/s
away from the lidar system 100, and the lidar system 100 is moving
towards the target 130 at 20 m/s. A positive radial speed of the
target 130 corresponds to a positive value for the frequency
difference .DELTA.F and corresponds to the target and lidar system
100 moving towards each other. A negative radial speed of the
target 130 corresponds to a negative value for .DELTA.F and
corresponds to the target and lidar system 100 moving away from
each other.
[0284] In FIG. 47, the target 130 is moving towards the lidar
system 100 with a speed S.sub.r, and an emitted pulse of light 400A
scatters from the moving target 130 and produces a scattered pulse
of light 410A that is received by the lidar system 100. The LO
light 430 has a center optical frequency of f.sub.0. The emitted
pulse of light 400A has a center optical frequency of f.sub.1, and
the received pulse of light 410A has a higher center optical
frequency of f.sub.2 (e.g., f.sub.2>f.sub.1). Because the target
130 is moving towards the lidar system 100, the frequency of the
received pulse of light 410A is upshifted with respect to the
frequency of the emitted pulse of light 400 due to the Doppler
effect, and the amount of upshift is proportional to the relative
speed of the target 130. If the target 130 were moving away from
the lidar system 100, then the frequency of the received pulse of
light 410A would be downshifted so that f.sub.2<f.sub.1. The
temporal pulsations of the photocurrent signal i-400 of the emitted
pulse of light 400A have a period of 1/(f.sub.1-f.sub.0), which
corresponds to a pulsation frequency of (f.sub.1-f.sub.0). The
temporal pulsations of the photocurrent signal i-410 of the
received pulse of light 410A have a period of 1/(f.sub.2-f.sub.0),
which corresponds to a pulsation frequency of (f.sub.2-f.sub.0).
The frequency difference .DELTA.F between the two photocurrent
signals (which may be referred to as a frequency difference between
the two spectral signatures) may be expressed as
.DELTA.F=(f.sub.2-f.sub.0)-(f.sub.1-f.sub.0), or
.DELTA.F=f.sub.2-f.sub.1. The speed of the target (S.sub.r)
relative to the lidar system 100 is a radial speed of the target
relative to the lidar system and may be determined from the
expression S.sub.r=.DELTA.F.lamda./2, where .lamda. is a wavelength
of the emitted pulse of light.
[0285] When an emitted pulse of light 400A is scattered from a
target 130 that is moving with respect to the lidar system 100, the
resulting scattered pulse of light 410A has its frequency shifted
due to the Doppler effect. An emitted pulse of light with a center
optical frequency of f that is scattered from a target 130 moving
with a speed S.sub.r (where S.sub.r is the radial speed of the
target 130 relative to the lidar system 100) has its frequency
shifted by
.DELTA. .times. F = 2 .times. S r c f , ##EQU00001##
where c is the speed of light. This expression may be rewritten as
.DELTA.F=2S.sub.r/.lamda., where .lamda. is the wavelength of the
pulse of light. The frequency difference .DELTA.F may be determined
from the frequencies (f.sub.2-f.sub.0) and (f.sub.1-f.sub.0) of the
respective photocurrent signals i-410 and i-400, and the relative
radial speed of the target 130 may then be determined from the
expression S.sub.r=.DELTA.F.lamda./2. For example, for an emitted
pulse of light 400A with a wavelength of 1550 nm, the relationship
may be written as S.sub.r=.DELTA.F.times.[0.775 (m/s)/MHz]. This
expression indicates that, for an operating wavelength of 1550 nm,
every 1 MHz of frequency shift corresponds to a 0.775-m/s relative
speed between the lidar system 100 and the target 130. A frequency
difference .DELTA.F of +32 MHz corresponds to a target 130 moving
towards the lidar system 100 with a relative speed of approximately
25 m/s (or, approximately 55 miles per hour). Similarly, a
frequency difference of -32 MHz corresponds to the target 130
moving away from the lidar system at a speed of 25 m/s.
[0286] FIG. 48 illustrates an example computer system 4800. In
particular embodiments, one or more computer systems 4800 may
perform one or more steps of one or more methods described or
illustrated herein. In particular embodiments, one or more computer
systems 4800 may provide functionality described or illustrated
herein. In particular embodiments, software running on one or more
computer systems 4800 may perform one or more steps of one or more
methods described or illustrated herein or may provide
functionality described or illustrated herein. Particular
embodiments may include one or more portions of one or more
computer systems 4800. In particular embodiments, a computer system
may be referred to as a processor, a controller, a computing
device, a computing system, a computer, a general-purpose computer,
or a data-processing apparatus. Herein, reference to a computer
system may encompass one or more computer systems, where
appropriate.
[0287] Computer system 4800 may take any suitable physical form. As
an example, computer system 4800 may be an embedded computer
system, a system-on-chip (SOC), a single-board computer system
(SBC), a desktop computer system, a laptop or notebook computer
system, a mainframe, a mesh of computer systems, a server, a tablet
computer system, or any suitable combination of two or more of
these. As another example, all or part of computer system 4800 may
be combined with, coupled to, or integrated into a variety of
devices, including, but not limited to, a camera, camcorder,
personal digital assistant (PDA), mobile telephone, smartphone,
electronic reading device (e.g., an e-reader), game console, smart
watch, clock, calculator, television monitor, flat-panel display,
computer monitor, vehicle display (e.g., odometer display or
dashboard display), vehicle navigation system, lidar system, ADAS,
autonomous vehicle, autonomous-vehicle driving system, cockpit
control, camera view display (e.g., display of a rear-view camera
in a vehicle), eyewear, or head-mounted display. Where appropriate,
computer system 4800 may include one or more computer systems 4800;
be unitary or distributed; span multiple locations; span multiple
machines; span multiple data centers; or reside in a cloud, which
may include one or more cloud components in one or more networks.
Where appropriate, one or more computer systems 4800 may perform
without substantial spatial or temporal limitation one or more
steps of one or more methods described or illustrated herein. As an
example, one or more computer systems 4800 may perform in real time
or in batch mode one or more steps of one or more methods described
or illustrated herein. One or more computer systems 4800 may
perform at different times or at different locations one or more
steps of one or more methods described or illustrated herein, where
appropriate.
[0288] As illustrated in the example of FIG. 48, computer system
4800 may include a processor 4810, memory 4820, storage 4830, an
input/output (I/O) interface 4840, a communication interface 4850,
or a bus 4860. Computer system 4800 may include any suitable number
of any suitable components in any suitable arrangement.
[0289] In particular embodiments, processor 4810 may include
hardware for executing instructions, such as those making up a
computer program. As an example, to execute instructions, processor
4810 may retrieve (or fetch) the instructions from an internal
register, an internal cache, memory 4820, or storage 4830; decode
and execute them; and then write one or more results to an internal
register, an internal cache, memory 4820, or storage 4830. In
particular embodiments, processor 4810 may include one or more
internal caches for data, instructions, or addresses. Processor
4810 may include any suitable number of any suitable internal
caches, where appropriate. As an example, processor 4810 may
include one or more instruction caches, one or more data caches, or
one or more translation lookaside buffers (TLBs). Instructions in
the instruction caches may be copies of instructions in memory 4820
or storage 4830, and the instruction caches may speed up retrieval
of those instructions by processor 4810. Data in the data caches
may be copies of data in memory 4820 or storage 4830 for
instructions executing at processor 4810 to operate on; the results
of previous instructions executed at processor 4810 for access by
subsequent instructions executing at processor 4810 or for writing
to memory 4820 or storage 4830; or other suitable data. The data
caches may speed up read or write operations by processor 4810. The
TLBs may speed up virtual-address translation for processor 4810.
In particular embodiments, processor 4810 may include one or more
internal registers for data, instructions, or addresses. Processor
4810 may include any suitable number of any suitable internal
registers, where appropriate. Where appropriate, processor 4810 may
include one or more arithmetic logic units (ALUs); may be a
multi-core processor; or may include one or more processors
4810.
[0290] In particular embodiments, memory 4820 may include main
memory for storing instructions for processor 4810 to execute or
data for processor 4810 to operate on. As an example, computer
system 4800 may load instructions from storage 4830 or another
source (such as, for example, another computer system 4800) to
memory 4820. Processor 4810 may then load the instructions from
memory 4820 to an internal register or internal cache. To execute
the instructions, processor 4810 may retrieve the instructions from
the internal register or internal cache and decode them. During or
after execution of the instructions, processor 4810 may write one
or more results (which may be intermediate or final results) to the
internal register or internal cache. Processor 4810 may then write
one or more of those results to memory 4820. One or more memory
buses (which may each include an address bus and a data bus) may
couple processor 4810 to memory 4820. Bus 4860 may include one or
more memory buses. In particular embodiments, one or more memory
management units (MMUs) may reside between processor 4810 and
memory 4820 and facilitate accesses to memory 4820 requested by
processor 4810. In particular embodiments, memory 4820 may include
random access memory (RAM). This RAM may be volatile memory, where
appropriate. Where appropriate, this RAM may be dynamic RAM (DRAM)
or static RAM (SRAM). Memory 4820 may include one or more memories
4820, where appropriate.
[0291] In particular embodiments, storage 4830 may include mass
storage for data or instructions. As an example, storage 4830 may
include a hard disk drive (HDD), a floppy disk drive, flash memory,
an optical disc, a magneto-optical disc, magnetic tape, or a
Universal Serial Bus (USB) drive or a combination of two or more of
these. Storage 4830 may include removable or non-removable (or
fixed) media, where appropriate. Storage 4830 may be internal or
external to computer system 4800, where appropriate. In particular
embodiments, storage 4830 may be non-volatile, solid-state memory.
In particular embodiments, storage 4830 may include read-only
memory (ROM). Where appropriate, this ROM may be mask ROM (MROM),
programmable ROM (PROM), erasable PROM (EPROM), electrically
erasable PROM (EEPROM), flash memory, or a combination of two or
more of these. Storage 4830 may include one or more storage control
units facilitating communication between processor 4810 and storage
4830, where appropriate. Where appropriate, storage 4830 may
include one or more storages 4830.
[0292] In particular embodiments, I/O interface 4840 may include
hardware, software, or both, providing one or more interfaces for
communication between computer system 4800 and one or more I/O
devices. Computer system 4800 may include one or more of these I/O
devices, where appropriate. One or more of these I/O devices may
enable communication between a person and computer system 4800. As
an example, an I/O device may include a keyboard, keypad,
microphone, monitor, mouse, printer, scanner, speaker, camera,
stylus, tablet, touch screen, trackball, another suitable I/O
device, or any suitable combination of two or more of these. An I/O
device may include one or more sensors. Where appropriate, I/O
interface 4840 may include one or more device or software drivers
enabling processor 4810 to drive one or more of these I/O devices.
I/O interface 4840 may include one or more I/O interfaces 4840,
where appropriate.
[0293] In particular embodiments, communication interface 4850 may
include hardware, software, or both providing one or more
interfaces for communication (such as, for example, packet-based
communication) between computer system 4800 and one or more other
computer systems 4800 or one or more networks. As an example,
communication interface 4850 may include a network interface
controller (NIC) or network adapter for communicating with an
Ethernet or other wire-based network or a wireless NIC (WNIC); a
wireless adapter for communicating with a wireless network, such as
a WI-FI network; or an optical transmitter (e.g., a laser or a
light-emitting diode) or an optical receiver (e.g., a
photodetector) for communicating using fiber-optic communication or
free-space optical communication. Computer system 4800 may
communicate with an ad hoc network, a personal area network (PAN),
an in-vehicle network (IVN), a local area network (LAN), a wide
area network (WAN), a metropolitan area network (MAN), or one or
more portions of the Internet or a combination of two or more of
these. One or more portions of one or more of these networks may be
wired or wireless. As an example, computer system 4800 may
communicate with a wireless PAN (WPAN) (such as, for example, a
BLUETOOTH WPAN), a WI-FI network, a Worldwide Interoperability for
Microwave Access (WiMAX) network, a cellular telephone network
(such as, for example, a Global System for Mobile Communications
(GSM) network), or other suitable wireless network or a combination
of two or more of these. As another example, computer system 4800
may communicate using fiber-optic communication based on 100
Gigabit Ethernet (100 GbE), 10 Gigabit Ethernet (10 GbE), or
Synchronous Optical Networking (SONET). Computer system 4800 may
include any suitable communication interface 4850 for any of these
networks, where appropriate. Communication interface 4850 may
include one or more communication interfaces 4850, where
appropriate.
[0294] In particular embodiments, bus 4860 may include hardware,
software, or both coupling components of computer system 4800 to
each other. As an example, bus 4860 may include an Accelerated
Graphics Port (AGP) or other graphics bus, a controller area
network (CAN) bus, an Enhanced Industry Standard Architecture
(EISA) bus, a front-side bus (FSB), a HYPERTRANSPORT (HT)
interconnect, an Industry Standard Architecture (ISA) bus, an
INFINIBAND interconnect, a low-pin-count (LPC) bus, a memory bus, a
Micro Channel Architecture (MCA) bus, a Peripheral Component
Interconnect (PCI) bus, a PCI-Express (PCIe) bus, a serial advanced
technology attachment (SATA) bus, a Video Electronics Standards
Association local bus (VLB), or another suitable bus or a
combination of two or more of these. Bus 4860 may include one or
more buses 4860, where appropriate.
[0295] In particular embodiments, various modules, circuits,
systems, methods, or algorithm steps described in connection with
the implementations disclosed herein may be implemented as
electronic hardware, computer software, or any suitable combination
of hardware and software. In particular embodiments, computer
software (which may be referred to as software, computer-executable
code, computer code, a computer program, computer instructions, or
instructions) may be used to perform various functions described or
illustrated herein, and computer software may be configured to be
executed by or to control the operation of computer system 4800. As
an example, computer software may include instructions configured
to be executed by processor 4810. In particular embodiments, owing
to the interchangeability of hardware and software, the various
illustrative logical blocks, modules, circuits, or algorithm steps
have been described generally in terms of functionality. Whether
such functionality is implemented in hardware, software, or a
combination of hardware and software may depend upon the particular
application or design constraints imposed on the overall
system.
[0296] In particular embodiments, a computing device may be used to
implement various modules, circuits, systems, methods, or algorithm
steps disclosed herein. As an example, all or part of a module,
circuit, system, method, or algorithm disclosed herein may be
implemented or performed by a general-purpose single- or multi-chip
processor, a digital signal processor (DSP), an ASIC, a FPGA, any
other suitable programmable-logic device, discrete gate or
transistor logic, discrete hardware components, or any suitable
combination thereof. A general-purpose processor may be a
microprocessor, or, any conventional processor, controller,
microcontroller, or state machine. A processor may also be
implemented as a combination of computing devices, e.g., a
combination of a DSP and a microprocessor, a plurality of
microprocessors, one or more microprocessors in conjunction with a
DSP core, or any other such configuration.
[0297] In particular embodiments, one or more implementations of
the subject matter described herein may be implemented as one or
more computer programs (e.g., one or more modules of
computer-program instructions encoded or stored on a
computer-readable non-transitory storage medium). As an example,
the steps of a method or algorithm disclosed herein may be
implemented in a processor-executable software module which may
reside on a computer-readable non-transitory storage medium. In
particular embodiments, a computer-readable non-transitory storage
medium may include any suitable storage medium that may be used to
store or transfer computer software and that may be accessed by a
computer system. Herein, a computer-readable non-transitory storage
medium or media may include one or more semiconductor-based or
other integrated circuits (ICs) (such, as for example,
field-programmable gate arrays (FPGAs) or application-specific ICs
(ASICs)), hard disk drives (HDDs), hybrid hard drives (HHDs),
optical discs (e.g., compact discs (CDs), CD-ROM, digital versatile
discs (DVDs), blu-ray discs, or laser discs), optical disc drives
(ODDs), magneto-optical discs, magneto-optical drives, floppy
diskettes, floppy disk drives (FDDs), magnetic tapes, flash
memories, solid-state drives (SSDs), RAM, RAM-drives, ROM, SECURE
DIGITAL cards or drives, any other suitable computer-readable
non-transitory storage media, or any suitable combination of two or
more of these, where appropriate. A computer-readable
non-transitory storage medium may be volatile, non-volatile, or a
combination of volatile and non-volatile, where appropriate.
[0298] In particular embodiments, certain features described herein
in the context of separate implementations may also be combined and
implemented in a single implementation. Conversely, various
features that are described in the context of a single
implementation may also be implemented in multiple implementations
separately or in any suitable sub-combination. Moreover, although
features may be described above as acting in certain combinations
and even initially claimed as such, one or more features from a
claimed combination may in some cases be excised from the
combination, and the claimed combination may be directed to a
sub-combination or variation of a sub-combination.
[0299] While operations may be depicted in the drawings as
occurring in a particular order, this should not be understood as
requiring that such operations be performed in the particular order
shown or in sequential order, or that all operations be performed.
Further, the drawings may schematically depict one more example
processes or methods in the form of a flow diagram or a sequence
diagram. However, other operations that are not depicted may be
incorporated in the example processes or methods that are
schematically illustrated. For example, one or more additional
operations may be performed before, after, simultaneously with, or
between any of the illustrated operations. Moreover, one or more
operations depicted in a diagram may be repeated, where
appropriate. Additionally, operations depicted in a diagram may be
performed in any suitable order. Furthermore, although particular
components, devices, or systems are described herein as carrying
out particular operations, any suitable combination of any suitable
components, devices, or systems may be used to carry out any
suitable operation or combination of operations. In certain
circumstances, multitasking or parallel processing operations may
be performed. Moreover, the separation of various system components
in the implementations described herein should not be understood as
requiring such separation in all implementations, and it should be
understood that the described program components and systems may be
integrated together in a single software product or packaged into
multiple software products.
[0300] Various embodiments have been described in connection with
the accompanying drawings. However, it should be understood that
the figures may not necessarily be drawn to scale. As an example,
distances or angles depicted in the figures are illustrative and
may not necessarily bear an exact relationship to actual dimensions
or layout of the devices illustrated.
[0301] The scope of this disclosure encompasses all changes,
substitutions, variations, alterations, and modifications to the
example embodiments described or illustrated herein that a person
having ordinary skill in the art would comprehend. The scope of
this disclosure is not limited to the example embodiments described
or illustrated herein. Moreover, although this disclosure describes
or illustrates respective embodiments herein as including
particular components, elements, functions, operations, or steps,
any of these embodiments may include any combination or permutation
of any of the components, elements, functions, operations, or steps
described or illustrated anywhere herein that a person having
ordinary skill in the art would comprehend.
[0302] The term "or" as used herein is to be interpreted as an
inclusive or meaning any one or any combination, unless expressly
indicated otherwise or indicated otherwise by context. Therefore,
herein, the expression "A or B" means "A, B, or both A and B." As
another example, herein, "A, B or C" means at least one of the
following: A; B; C; A and B; A and C; B and C; A, B and C. An
exception to this definition will occur if a combination of
elements, devices, steps, or operations is in some way inherently
mutually exclusive.
[0303] As used herein, words of approximation such as, without
limitation, "approximately, "substantially," or "about" refer to a
condition that when so modified is understood to not necessarily be
absolute or perfect but would be considered close enough to those
of ordinary skill in the art to warrant designating the condition
as being present. The extent to which the description may vary will
depend on how great a change can be instituted and still have one
of ordinary skill in the art recognize the modified feature as
having the required characteristics or capabilities of the
unmodified feature. In general, but subject to the preceding
discussion, a numerical value herein that is modified by a word of
approximation such as "approximately" may vary from the stated
value by .+-.0.5%, .+-.1%, .+-.2%, .+-.3%, .+-.4%, .+-.5%, .+-.10%,
.+-.12%, or .+-.15%. The term "substantially constant" refers to a
value that varies by less than a particular amount over any
suitable time interval. For example, a value that is substantially
constant may vary by less than or equal to 20%, 10%, 1%, 0.5%, or
0.1% over a time interval of approximately 10.sup.4 s, 10.sup.3 s,
10.sup.2 s, 10 s, 1 s, 100 ms, 10 ms, 1 ms, 100 .mu.s, 10 .mu.s, or
1 .mu.s. The term "substantially constant" may be applied to any
suitable value, such as for example, an optical power, a pulse
repetition frequency, an electrical current, a wavelength, an
optical or electrical frequency, or an optical or electrical
phase.
[0304] As used herein, the terms "first," "second," "third," etc.
may be used as labels for nouns that they precede, and these terms
may not necessarily imply a particular ordering (e.g., a particular
spatial, temporal, or logical ordering). As an example, a system
may be described as determining a "first result" and a "second
result," and the terms "first" and "second" may not necessarily
imply that the first result is determined before the second
result.
[0305] As used herein, the terms "based on" and "based at least in
part on" may be used to describe or present one or more factors
that affect a determination, and these terms may not exclude
additional factors that may affect a determination. A determination
may be based solely on those factors which are presented or may be
based at least in part on those factors. The phrase "determine A
based on B" indicates that B is a factor that affects the
determination of A. In some instances, other factors may also
contribute to the determination of A. In other instances, A may be
determined based solely on B.
* * * * *