U.S. patent application number 16/059638 was filed with the patent office on 2020-02-13 for dual-mode lidar system.
The applicant listed for this patent is LUMINAR TECHNOLOGIES, INC.. Invention is credited to Joseph G. LaChapelle.
Application Number | 20200049821 16/059638 |
Document ID | / |
Family ID | 69230235 |
Filed Date | 2020-02-13 |
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United States Patent
Application |
20200049821 |
Kind Code |
A1 |
LaChapelle; Joseph G. |
February 13, 2020 |
DUAL-MODE LIDAR SYSTEM
Abstract
A method in a lidar system comprises emitting a pulse of light,
detecting at least a portion of the emitted pulse of light
scattered by a target located a distance from the lidar system, and
determining the distance from the lidar system to the target based
at least in part on a round-trip time of flight for the emitted
pulse of light to travel from the lidar system to the target and
back to the lidar system. The method further comprises emitting a
series of pulses of light having particular pulse-frequency
characteristics, detecting at least a portion of the series of
emitted pulses of light scattered by the target, and comparing the
pulse-frequency characteristics of the series of emitted pulses of
light with corresponding pulse-frequency characteristics of the
detected series of scattered pulses of light to determine a
velocity of the target with respect to the lidar system.
Inventors: |
LaChapelle; Joseph G.;
(Philomath, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LUMINAR TECHNOLOGIES, INC. |
Orlando |
FL |
US |
|
|
Family ID: |
69230235 |
Appl. No.: |
16/059638 |
Filed: |
August 9, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 17/931 20200101;
G01N 21/47 20130101; G01S 17/42 20130101; G01S 7/4818 20130101;
G01S 17/58 20130101; G02B 26/121 20130101; G01S 7/484 20130101;
G01S 17/87 20130101; G01S 7/4868 20130101; G01S 17/10 20130101;
G01S 17/93 20130101 |
International
Class: |
G01S 17/10 20060101
G01S017/10; G01S 17/58 20060101 G01S017/58; G01S 17/93 20060101
G01S017/93; G02B 26/12 20060101 G02B026/12; G01N 21/47 20060101
G01N021/47 |
Claims
1. A method comprising: emitting, by a light source of a lidar
system, a pulse of light; detecting, by a receiver of the lidar
system, at least a portion of the emitted pulse of light scattered
by a target located a distance from the lidar system; determining,
by a processor of the lidar system, the distance from the lidar
system to the target based at least in part on a round-trip time of
flight for the emitted pulse of light to travel from the lidar
system to the target and back to the lidar system; and if (i) the
distance to the target is greater than a particular maximum
distance or (ii) at least one of a horizontal scan angle or a
vertical scan angle is outside a respective particular range within
a field of regard of the lidar system, then refraining from
emitting a series of pulses of light, otherwise: emitting, by the
light source, the series of pulses of light, the series of emitted
pulses of light having particular pulse-frequency characteristics;
detecting, by the receiver, at least a portion of the series of
emitted pulses of light scattered by the target; comparing, by a
comparison module, the pulse-frequency characteristics of the
series of emitted pulses of light with corresponding
pulse-frequency characteristics of the detected series of scattered
pulses of light to determine a velocity of the target with respect
to the lidar system.
2. The method of claim 1, wherein emitting the series of pulses is
in response to determining the distance from the lidar system to
the target.
3. The method of claim 1, wherein comparing the pulse-frequency
characteristics comprises: electrically mixing an electrical signal
corresponding to the series of emitted pulses of light with an
electrical signal corresponding to the detected series of scattered
pulses of light to produce a mixed output signal; determining one
or more intermediate frequencies of the mixed output signal; and
determining the velocity of the target with respect to the lidar
system based on the one or more intermediate frequencies.
4. The method of claim 1, wherein comparing the pulse-frequency
characteristics comprises: digitizing an electrical signal
corresponding to the detected series of scattered pulses of light;
determining a time or frequency characteristic of the digitized
electrical signal; and comparing the determined time or frequency
characteristic with a corresponding time or frequency
characteristic of the series of emitted pulses of light to
determine the velocity of the target with respect to the lidar
system.
5. The method of claim 1, wherein the series of emitted pulses of
light comprises greater than or equal to 10 pulses having a pulse
repetition frequency of greater than 10 MHz, each of the pulses
having a pulse duration of less than 10 ns.
6. The method of claim 1, wherein the pulse-frequency
characteristics of the series of emitted pulses of light comprise a
constant pulse repetition frequency.
7-10. (canceled)
11. The method of claim 1, including emitting the series of pulses
of light only if a vehicle in which the lidar system operates is
stopped.
12. The method of claim 1, further comprising: determining a type
of object to which the target corresponds; emitting the series of
pulses of light if the determined type corresponds to a dynamic
type; and refraining from emitting the series of pulses of light if
the determined type corresponds to a static type.
13. The method of claim 1, further comprising: receiving an
indication of a region of interest within the field of regard of
the lidar system; and refraining from emitting the series of pulses
of light if the target is not located within the region of
interest.
14. (canceled)
15. The method of claim 1, further comprising reducing a pulse
energy of the series of emitted pulses of light if the distance to
the target is less than a particular minimum distance.
16-17. (canceled)
18. The method of claim 1, further comprising: detecting another
series of pulses of light emitted by another lidar system;
determining that the pulse-frequency characteristics of the series
of emitted pulses of light do not match corresponding
pulse-frequency characteristics of the another series of pulses of
light; and disregarding the other series of pulses of light.
19. A lidar system comprising: a light source configured to emit a
pulse of light; a receiver configured to detect at least a portion
of the emitted pulse of light scattered by a target located a
distance from the lidar system; and a processor configured to
determine the distance from the lidar system to the target based at
least in part on a round-trip time of flight for the emitted pulse
of light to travel from the lidar system to the target and back to
the lidar system; wherein if (i) the distance to the target is
greater than a particular maximum distance or (ii) at least one of
a horizontal scan angle or a vertical scan angle is outside a
respective particular range within a field of regard of the lidar
system, then the light source is further configured to refrain from
emitting a series of pulses of light, otherwise: the light source
is further configured to emit a series of pulses of light having
particular pulse-frequency characteristics; the receiver is further
configured to detect at least a portion of the series of emitted
pulses of light scattered by the target; and the lidar system
further comprises a comparison module configured to compare the
pulse-frequency characteristics of the series of emitted pulses of
light with corresponding pulse-frequency characteristics of the
detected series of scattered pulses of light to determine a
velocity of the target with respect to the lidar system.
20. The lidar system of claim 19, wherein the light source
comprises: a pulsed laser diode configured to produce optical seed
pulses; and one or more optical amplifiers configured to amplify
the optical seed pulses to produce the emitted pulses of light.
21. The lidar system of claim 19, wherein the light source
comprises a direct-emitter laser diode configured to produce the
emitted pulses of light.
22. The lidar system of claim 19, further comprising a scanner
configured to scan the emitted pulses of light across the field of
regard of the lidar system.
23. The lidar system of claim 22, wherein the scanner comprises one
or more mirrors, wherein each mirror is mechanically driven by a
galvanometer scanner, a resonant scanner, a microelectromechanical
systems (MEMS) device, a voice coil motor, or a synchronous
electric motor.
24. The lidar system of claim 22, wherein the scanner comprises: a
first mirror driven by a first galvanometer scanner that scans the
emitted pulses of light along a first direction; and a second
mirror driven by a second galvanometer scanner that scans the
emitted pulses of light along a second direction substantially
orthogonal to the first direction.
25. The lidar system of claim 22, wherein the scanner comprises: a
first mirror configured to scan the emitted pulses of light along a
first direction; and a polygon mirror configured to scan the
emitted pulses of light along a second direction substantially
orthogonal to the first direction.
26. The lidar system of claim 19, wherein the receiver comprises an
avalanche photodiode (APD) configured to receive light scattered by
the target and produce an electrical current corresponding to the
received light.
27. A method comprising: emitting, by a light source of a lidar
system, a series of pulses of light having particular
pulse-frequency characteristics; detecting, by a receiver of the
lidar system, at least a portion of the series of emitted pulses of
light scattered by a first target located a first distance from the
lidar system; determining, by a processor of the lidar system, the
first distance from the lidar system to the first target based at
least in part on a round-trip time of flight for the series of
emitted pulses of light to travel from the lidar system to the
first target and back to the lidar system; and comparing, by a
comparison module, the pulse-frequency characteristics of the
series of emitted pulses of light with corresponding
pulse-frequency characteristics of the detected series of scattered
pulses of light to determine a velocity of the first target with
respect to the lidar system, wherein each of the emitting, the
detecting, the determining, and the comparing occurs in a first
instance to generate a value of a first pixel, the method further
comprising, in a second instance: emitting, by the light source, a
single pulse of light; detecting, by the receiver, at least a
portion of the emitted pulse of light scattered by a second target
located a second distance from the lidar system; and determining,
by the processor of the lidar system, the second distance from the
lidar system to the second target based at least in part on a
round-trip time of flight for the emitted pulse of light to travel
from the lidar system to the second target and back to the lidar
system to generate a value of a second pixel, wherein each of the
first pixel and the second pixel correspond to respective ranging
events of equal duration.
28. (canceled)
29. The method of claim 27, wherein comparing the pulse-frequency
characteristics includes: determining a time or frequency
characteristic of an electrical signal corresponding to the
detected series of scattered pulses of light; and comparing the
determined time or frequency characteristic with a corresponding
time or frequency characteristic of the series of emitted pulses of
light to determine the velocity of the first target with respect to
the lidar system.
30. The method of claim 27, further comprising not modulating a
frequency of the light.
31. A method comprising: emitting, by a light source of a lidar
system, a pulse of light; detecting, by a receiver of the lidar
system, at least a portion of the emitted pulse of light scattered
by a target located a distance from the lidar system; determining,
by a processor of the lidar system, the distance from the lidar
system to the target based at least in part on a round-trip time of
flight for the emitted pulse of light to travel from the lidar
system to the target and back to the lidar system; emitting, by the
light source, a series of pulses of light having particular
pulse-frequency characteristics, wherein the pulse-frequency
characteristics of the series of emitted pulses of light comprise
(i) an increasing pulse repetition frequency, (ii) a decreasing
pulse repetition frequency, or (iii) a pseudo-random sequence of
pulses of light having pseudo-random intervals of time between the
pulses; detecting, by the receiver, at least a portion of the
series of emitted pulses of light scattered by the target; and
comparing, by a comparison module, the pulse-frequency
characteristics of the series of emitted pulses of light with
corresponding pulse-frequency characteristics of the detected
series of scattered pulses of light to determine a velocity of the
target with respect to the lidar system.
32. A method comprising: emitting, by a light source of a lidar
system, a pulse of light; detecting, by a receiver of the lidar
system, at least a portion of the emitted pulse of light scattered
by a target located a distance from the lidar system; determining,
by a processor of the lidar system, the distance from the lidar
system to the target based at least in part on a round-trip time of
flight for the emitted pulse of light to travel from the lidar
system to the target and back to the lidar system; selecting, by
the processor and in view of the determined distance to the target,
one or more of: (i) a pulse energy of pulses in a series of pulses
of light, (ii) a width of pulses in the series of pulses of light,
and (iii) a number of pulses to be included in the series of pulses
of light; emitting, by the light source, the series of pulses of
light, the series of emitted pulses of light having particular
pulse-frequency characteristics; detecting, by the receiver, at
least a portion of the series of emitted pulses of light scattered
by the target; and comparing, by a comparison module, the
pulse-frequency characteristics of the series of emitted pulses of
light with corresponding pulse-frequency characteristics of the
detected series of scattered pulses of light to determine a
velocity of the target with respect to the lidar system.
Description
FIELD OF TECHNOLOGY
[0001] This disclosure generally relates to lidar systems and, more
particularly, detecting both the distance to a target and a
velocity of the target.
BACKGROUND
[0002] The background description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent it is described in
this background section, as well as aspects of the description that
may not otherwise qualify as prior art at the time of filing, are
neither expressly nor impliedly admitted as prior art against the
present disclosure.
[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 be, 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 then scatters the light.
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 returned light. For example,
the system may determine the distance to the target based on the
time of flight of a returned light pulse. However, a pulsed lidar
system generally cannot determine the velocity of a target based on
the returned light pulse.
SUMMARY
[0004] According to one implementation, a method comprises
emitting, by a light source of a lidar system, a pulse of light;
detecting, by a receiver of the lidar system, at least a portion of
the emitted pulse of light scattered by a target located a distance
from the lidar system; determining, by a processor of the lidar
system, the distance from the lidar system to the target based at
least in part on a round-trip time of flight for the emitted pulse
of light to travel from the lidar system to the target and back to
the lidar system; emitting, by the light source, a series of pulses
of light having particular pulse-frequency characteristics;
detecting, by the receiver, at least a portion of the series of
emitted pulses of light scattered by the target; and comparing, by
a comparison module, the pulse-frequency characteristics of the
series of emitted pulses of light with corresponding
pulse-frequency characteristics of the detected series of scattered
pulses of light to determine a velocity of the target with respect
to the lidar system.
[0005] Another embodiment of the techniques of this disclosure is
lidar system comprising a light source configured to emit a pulse
of light; a receiver configured to detect at least a portion of the
emitted pulse of light scattered by a target located a distance
from the lidar system; and a processor configured to determine the
distance from the lidar system to the target based at least in part
on a round-trip time of flight for the emitted pulse of light to
travel from the lidar system to the target and back to the lidar
system. The light source is further configured, in response to
determining the distance to the target, to emit a series of pulses
of light having particular pulse-frequency characteristics. The
receiver is further configured to detect at least a portion of the
series of emitted pulses of light scattered by the target. The
lidar system further comprises a comparator module configured to
compare the pulse-frequency characteristics of the series of
emitted pulses of light with corresponding pulse-frequency
characters of the detected series of scattered pulses of light to
determine a velocity of the target with respect to the lidar
system.
[0006] Yet another embodiment is a method comprising emitting, by a
light source, a series of pulses of light having particular
pulse-frequency characteristics; detecting, by a receiver of the
lidar system, at least a portion of the series of emitted pulses of
light scattered by scattered by a target located a distance from
the lidar system; determining, by a processor of the lidar system,
the distance from the lidar system to the target based at least in
part on a round-trip time of flight for the emitted series of
pulses of light to travel from the lidar system to the target and
back to the lidar system; and comparing, by a comparison module,
the pulse-frequency characteristics of the series of emitted pulses
of light with corresponding pulse-frequency characteristics of the
detected series of scattered pulses of light to determine a
velocity of the target with respect to the lidar system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a block diagram of an example dual-mode light
detection and ranging (lidar) system capable of detecting both
distances to targets and velocities of targets, implemented
according to one embodiment of the techniques of this
disclosure;
[0008] FIG. 2 illustrates in more detail several components that
can operate in the system of FIG. 1 to scan a field of regard of
the lidar system;
[0009] FIG. 3 is a block diagram of a lidar system in which the
scanner includes a polygon mirror and can operate in a two-eye
configuration with overlapping fields of regard;
[0010] FIG. 4 illustrates an example configuration in which the
components of FIG. 1 scan a 360-degree field of regard through a
window in a rotating housing;
[0011] FIG. 5 illustrates another configuration in which the
components of FIG. 1 scan a 360-degree field of regard through a
substantially transparent stationary housing;
[0012] FIG. 6 illustrates an example scan pattern which the lidar
system of FIG. 1 equipped with a scanner of FIG. 2 can produce when
scanning a field of regard;
[0013] FIG. 7 illustrates an example scan pattern which the lidar
system of FIG. 1 can produce when scanning a field of regard using
multiple beams;
[0014] FIG. 8 illustrates an example scan pattern which the lidar
system of FIG. 1 equipped with a polygon mirror can produce when
scanning a field of regard;
[0015] FIG. 9 schematically illustrates fields of view (FOVs) of a
light source and a detector that can operate in the lidar system of
FIG. 1;
[0016] FIG. 10 illustrates an example configuration of the lidar
system of FIG. 1 or another suitable lidar system, in which a laser
is disposed away from sensor components;
[0017] FIG. 11 illustrates an example vehicle in which the lidar
system of FIG. 1 can operate;
[0018] FIG. 12 illustrates an example InGaAs avalanche photodiode
which can operate in the lidar system of FIG. 1;
[0019] FIG. 13 illustrates an example photodiode coupled to a
pulse-detection circuit, which can operate in the lidar system of
FIG. 1;
[0020] FIG. 14 illustrates an example scenario in which a vehicle
equipped with a dual-mode lidar system of this disclosure detects
the distance to, and the velocity of, a target headed generally
toward the vehicle;
[0021] FIG. 15 illustrates an example scenario in which a vehicle
equipped with a dual-mode lidar system of this disclosure detects
the distance to, and the velocity of, a target headed generally
away from the vehicle;
[0022] FIG. 16 illustrates an example scenario in which a vehicle
equipped with a lidar system of this disclosure detects the
distance to, and the velocity of, a target moving along a line
generally perpendicular to the forward-facing direction of the
vehicle;
[0023] FIG. 17 is a timing diagram of an example ranging event
during which the dual-mode lidar system of FIG. 1 can emit pulses
to determine the distance to, and the velocity of, a target;
[0024] FIG. 18A illustrates an example pulse train with equal
intervals between successive pulses, which the lidar system of FIG.
1 can generate to determine the velocity of a target;
[0025] FIG. 18B illustrates an example "chirped" pulse train with
increasing intervals between successive pulses, which the lidar
system of FIG. 1 can generate to determine the velocity of a
target;
[0026] FIG. 18C illustrates an example pulse train with random
intervals between successive pulses, which the lidar system of FIG.
1 can generate to determine the velocity of a target;
[0027] FIG. 19 is a block diagram of an example module for
extracting a signal representative of a velocity of a target using
a heterodyne technique, which can be implemented in the lidar
system of FIG. 1;
[0028] FIGS. 20A and 20B illustrate a relationship between signals
corresponding to the emitted pulse train and the velocity of the
vehicle, in frequency domain;
[0029] FIG. 21 is a diagram of an example circuit that can be
implemented in the system of FIG. 1 to encode an outbound pulse
train with a certain pattern and determine the pattern encoded in a
return pulse train;
[0030] FIG. 22 schematically illustrates detecting rising and
falling edges of pulses making up a pulse train that can be
generated in the lidar system of FIG. 1;
[0031] FIG. 23 is a block diagram of an example circuit that uses
time-to-digital converters (TDCs) to determine the pattern of a
pulse train;
[0032] FIG. 24 is a flow diagram of an example method for
determining a distance to a target and the velocity of the target,
which can be implemented in the lidar system of this disclosure;
and
[0033] FIG. 25 is a flow diagram of an example method for
determining a distance to a target and the velocity of the target,
which can be implemented in the lidar system of this
disclosure.
DETAILED DESCRIPTION
[0034] A dual-mode lidar of system of this disclosure can generate
pulses of light to determine a distance to a target as well as the
velocity of the target relative to the lidar system. The dual-mode
lidar system has at least two operational modes: in the first mode,
the system emits an individual pulse of light, detects light
scattered by a remote target and corresponding to the individual
pulse of light (e.g., having the same wavelength), and determines
the distance to the target based on the time it took the scattered
light to reach the lidar system; in the second mode, the system
generates a burst (a series, a train) of pulses with certain
pulse-frequency characteristics, such as a constant separation
between successive pulses, and determines the velocity of the
target based on the change in this characteristic (e.g., the
frequency shift due to the Doppler effect). The dual-mode lidar
system in both cases operates as a pulsed lidar system.
[0035] Alternatively, to determine the velocity of a target, a
frequency-modulated continuous-wave (FM CW) lidar system can use
frequency-modulated coherent light (e.g., from a frequency-stable
CW laser). Such a system can determine the velocity of a target
based on the Doppler effect by determining by how much the
frequency of the light scattered by the object is shifted due to
the relative velocity of the object. However, when a lidar system
is pulsed rather than continuous-wave-based, the system generally
cannot determine velocity based on the Doppler effect because it
may not be practical (or possible) to perform coherent detection
with a pulsed light source (e.g., it is difficult to store a
portion of an emitted pulse to interfere with scattered light from
the emitted pulse).
[0036] As discussed in more detail below, the dual-mode lidar
system of this disclosure addresses these issues by using
particular characteristics of bursts of pulses. Because this system
determines velocity using a quasi-Doppler effect that does not
require optical coherence, this lidar system can be described as a
non-coherent pulsed Doppler system.
[0037] The dual-mode lidar system in some cases operates according
to the two modes during the same ranging event by first emitting a
pulse of light to determine a distance to a target and, immediately
upon receiving a return corresponding to the emitted pulse,
emitting a pulse train to determine the velocity of the target.
Both the distance measurement and the velocity measurement can be
provided as respective properties of the same pixel in a
point-cloud. The dual-mode lidar system thus can eliminate lag time
between distance and velocity measurements almost entirely.
Moreover, by conducting distance and velocity measurements using
the same optical path, the dual-model lidar system eliminates
optical misalignment between the two measurements (whereas using
two different instruments to determine distance and velocity
produces offsets between measurements).
[0038] In another implementation, the dual-mode lidar system can
operate in different modes during different ranging events, so as
to maximize the distance at which the system can determine the
velocity of a target. As a more specific example, the dual-mode
lidar system can measure the distance for a certain pixel and the
velocity for the adjacent pixel, with the entire duration of a
ranging event allocated to a single measurement of distance or
velocity. This approach may be useful when a controller of the
dual-mode lidar system or of the vehicle has previously determined
that the two pixels likely cover the same remote target.
[0039] Depending on the implementation, the dual-mode lidar system
can operate at the same wavelength in the two modes or at different
wavelengths. Further, the dual-mode lidar system in some cases
imparts a pattern to the pulse train so as to check whether the
return signal corresponds to the imparted pattern, transformed due
to the Doppler effect when the target is moving, as discussed
above. The dual-mode lidar system in this manner can reduce
cross-talk, or erroneous detections of light from other lidar
systems.
[0040] Still further, a dual-mode lidar system in some
implementations transitions to the velocity-detection mode only in
response to determining that one or more conditions are satisfied,
such as sufficient time remaining in the ranging event or the
target being disposed within a certain distance, prior results of
processing the field of regard indicating that the target is a of a
certain type, the scan angle being within a certain range, etc.
[0041] An example lidar system in which these techniques can be
implemented is considered next with reference to FIGS. 1-5,
followed by a discussion of the techniques which the lidar system
can implement to scan a field of regard and generate individual
pixels (FIGS. 6-9). An example implementation in a vehicle is then
discussed with reference to FIGS. 10 and 11, and example photo
detector and pulse-detection circuit are discussed with reference
to FIGS. 12 and 13. Example scenarios in which a lidar system of
this disclosure can determine velocities of targets are discussed
with reference to FIG. 14-16, and the techniques this lidar system
can implement to operate in the two modes are discussed with
reference to FIGS. 17-25.
System Overview
[0042] FIG. 1 illustrates an example light detection and ranging
(lidar) system 100. The 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.
The lidar system 100 may include a light source 110, a mirror 115,
a scanner 120, a receiver 140, and a controller 150. The light
source 110 may be, for example, a laser which emits light having a
particular operating wavelength in the infrared, visible, or
ultraviolet portions of the electromagnetic spectrum. As a more
specific example, the light source 110 may include a laser with an
operating wavelength between approximately 1.2 .mu.m and 1.7
.mu.m.
[0043] In operation, the light source 110 emits an output beam of
light 125 which may be continuous-wave, 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 located a
distance D from the lidar system 100 and at least partially
contained within a field of regard of the system 100. Depending on
the scenario and/or the implementation of the lidar system 100, D
can be between 1 m and 1 km, for example.
[0044] Once the output beam 125 reaches the downrange target 130,
the target 130 may scatter or, in some cases, 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 the scanner
120, which may be referred to as a beam scanner, optical scanner,
or laser scanner. The input beam 135 passes through the scanner 120
to the mirror 115, which may be referred to as an overlap mirror,
superposition mirror, or beam-combiner mirror. The mirror 115 in
turn directs the input beam 135 to the receiver 140. The input 135
may contain only a relatively small fraction of the light from the
output beam 125. For example, the ratio of average power, peak
power, or pulse energy of the input beam 135 to average power, peak
power, or pulse energy of the output beam 125 may be approximately
10.sup.-1, 10.sup.-2, 10.sup.-3, 10.sup.-4, 10.sup.-5, 10.sup.-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 the output beam 125
has a pulse energy of 1 microjoule (.mu.J), then the pulse energy
of a corresponding pulse of the 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, or 1 aJ.
[0045] The output beam 125 may be referred to as a laser beam,
light beam, optical beam, emitted beam, or just beam; and the input
beam 135 may be referred to as a 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 the target 130. The input beam 135 may
include light from the output beam 125 that is scattered by the
target 130, light from the output beam 125 that is reflected by the
target 130, or a combination of scattered and reflected light from
target 130.
[0046] The operating wavelength of a lidar system 100 may lie, for
example, in the infrared, visible, or ultraviolet portions of the
electromagnetic spectrum. The Sun also produces light in these
wavelength ranges, and thus sunlight can act as background noise
which can obscure signal light detected by the lidar system 100.
This solar background noise can result in false-positive detections
or can otherwise corrupt measurements of the lidar system 100,
especially when the receiver 140 includes SPAD detectors (which can
be highly sensitive).
[0047] Generally speaking, the light from the Sun that passes
through the Earth's atmosphere and reaches a terrestrial-based
lidar system such as the system 100 can establish an optical
background noise floor for this system. Thus, in order for a signal
from the lidar system 100 to be detectable, the signal must rise
above the background noise floor. It is generally possible to
increase the signal-to-noise (SNR) ratio of the lidar system 100 by
raising the power level of the output beam 125, but in some
situations it may be desirable to keep the power level of the
output beam 125 relatively low. For example, increasing transmit
power levels of the output beam 125 can result in the lidar system
100 not being eye-safe.
[0048] In some implementations, the lidar system 100 operates at
one or more wavelengths between approximately 1400 nm and
approximately 1600 nm. For example, the light source 110 may
produce light at approximately 1550 nm.
[0049] In some implementations, the lidar system 100 operates at
frequencies at which atmospheric absorption is relatively low. For
example, the lidar system 100 can operate at wavelengths in the
approximate ranges from 980 nm to 1110 nm or from 1165 nm to 1400
nm.
[0050] In other implementations, the lidar system 100 operates at
frequencies at which atmospheric absorption is high. For example,
the lidar system 100 can operate at wavelengths in the approximate
ranges from 930 nm to 980 nm, from 1100 nm to 1165 nm, or from 1400
nm to 1460 nm.
[0051] According to some implementations, the lidar system 100 can
include an eye-safe laser, or the lidar system 100 can 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 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. For example, the light source 110 or lidar system 100 may be
classified as a Class 1 laser product (as specified by the 60825-1
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 some implementations,
the lidar system 100 may be classified as an eye-safe laser product
(e.g., with a Class 1 or Class I classification) configured to
operate at any suitable wavelength between approximately 1400 nm
and approximately 2100 nm. In some implementations, the light
source 110 may include a laser with an operating wavelength between
approximately 1400 nm and approximately 1600 nm, and the lidar
system 100 may be operated in an eye-safe manner. In some
implementations, the light source 110 or the lidar system 100 may
be an eye-safe laser product that includes a scanned laser with an
operating wavelength between approximately 1530 nm and
approximately 1560 nm. In some implementations, the lidar system
100 may be a Class 1 or Class I laser product that includes a fiber
laser or solid-state laser with an operating wavelength between
approximately 1400 nm and approximately 1600 nm.
[0052] The receiver 140 may receive or detect photons from the
input beam 135 and generate one or more representative signals. For
example, the receiver 140 may generate an output electrical signal
145 that is representative of the input beam 135. The receiver may
send the electrical signal 145 to the controller 150. Depending on
the implementation, the controller 150 may include one or more
processors, an application-specific integrated circuit (ASIC), a
field-programmable gate array (FPGA), and/or other suitable
circuitry configured to analyze one or more characteristics of the
electrical signal 145 to determine one or more characteristics of
the target 130, such as its distance downrange from the lidar
system 100. More particularly, the controller 150 may analyze the
time of flight or phase modulation for the beam of light 125
transmitted by the light source 110. If the lidar system 100
measures a time of flight of T (e.g., 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=cT/2, where c is the speed of light
(approximately 3.0.times.10.sup.8 m/s).
[0053] As a more specific example, if the lidar system 100 measures
the time of flight to be T=300 ns, then the lidar system 100 can
determine the distance from the target 130 to the lidar system 100
to be approximately D=45.0 m. As another example, the lidar system
100 measures the time of flight to be T=1.33 .mu.s and accordingly
determines that the distance from the target 130 to the lidar
system 100 is approximately D=199.5 m. The distance D from lidar
system 100 to the target 130 may be referred to as a distance,
depth, or range of the 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. 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.
[0054] The target 130 may be located a distance D from the lidar
system 100 that is less than or equal to a maximum range R.sub.MAX
of the lidar system 100. The maximum range R.sub.MAX (which also
may be referred to as a maximum distance) of a lidar system 100 may
correspond to the maximum distance over which the lidar system 100
is configured to sense or identify targets that appear in a field
of regard of the lidar system 100. The maximum range of lidar
system 100 may be any suitable distance, such as for example, 25 m,
50 m, 100 m, 200 m, 500 m, or 1 km. As a specific example, a lidar
system with a 200-m maximum range may be configured to sense or
identify various targets located up to 200 m away. For a lidar
system with a 200-m maximum range (R.sub.MAX=200 m), the time of
flight corresponding to the maximum range is approximately
2R.sub.MAX/c.apprxeq.1.33 .mu.s.
[0055] In some implementations, the light source 110, the scanner
120, and the receiver 140 may be packaged together within a single
housing 155, which may be a box, case, or enclosure that holds or
contains all or part of a lidar system 100. The housing 155
includes a window 157 through which the beams 125 and 135 pass. In
one example implementation, the lidar-system housing 155 contains
the light source 110, the overlap mirror 115, the scanner 120, and
the receiver 140 of a lidar system 100. The controller 150 may
reside within the same housing 155 as the components 110, 120, and
140, or the controller 150 may reside remotely from the
housing.
[0056] Moreover, in some implementations, the housing 155 includes
multiple lidar sensors, each including a respective scanner and a
receiver. Depending on the particular implementation, each of the
multiple sensors can include a separate light source or a common
light source. The multiple sensors can be configured to cover
non-overlapping adjacent fields of regard or partially overlapping
fields of regard, depending on the implementation.
[0057] The housing 155 may be an airtight or watertight structure
that prevents water vapor, liquid water, dirt, dust, or other
contaminants from getting inside the housing 155. The housing 155
may be filled with a dry or inert gas, such as for example dry air,
nitrogen, or argon. The housing 155 may include one or more
electrical connections for conveying electrical power or electrical
signals to and/or from the housing.
[0058] The window 157 may be made from any suitable substrate
material, such as for example, glass or plastic (e.g.,
polycarbonate, acrylic, cyclic-olefin polymer, or cyclic-olefin
copolymer). The window 157 may include an interior surface (surface
A) and an exterior surface (surface B), and surface A or surface B
may include a dielectric coating having particular reflectivity
values at particular wavelengths. A dielectric coating (which may
be referred to as a thin-film coating, interference coating, or
coating) may include one or more thin-film layers of dielectric
materials (e.g., SiO.sub.2, TiO.sub.2, Al.sub.2O.sub.3,
Ta.sub.2O.sub.5, MgF.sub.2, LaF.sub.3, or AlF.sub.3) having
particular thicknesses (e.g., thickness less than 1 .mu.m) and
particular refractive indices. A dielectric coating may be
deposited onto surface A or surface B of the window 157 using any
suitable deposition technique, such as for example, sputtering or
electron-beam deposition.
[0059] The dielectric coating may have a high reflectivity at a
particular wavelength or a low reflectivity at a particular
wavelength. A high-reflectivity (HR) dielectric coating may have
any suitable reflectivity value (e.g., a reflectivity greater than
or equal to 80%, 90%, 95%, or 99%) at any suitable wavelength or
combination of wavelengths. A low-reflectivity dielectric coating
(which may be referred to as an anti-reflection (AR) coating) may
have any suitable reflectivity value (e.g., a reflectivity less
than or equal to 5%, 2%, 1%, 0.5%, or 0.2%) at any suitable
wavelength or combination of wavelengths. In particular
embodiments, a dielectric coating may be a dichroic coating with a
particular combination of high or low reflectivity values at
particular wavelengths. For example, a dichroic coating may have a
reflectivity of less than or equal to 0.5% at approximately
1550-1560 nm and a reflectivity of greater than or equal to 90% at
approximately 800-1500 nm.
[0060] In some implementations, surface A or surface B has a
dielectric coating that is anti-reflecting at an operating
wavelength of one or more light sources 110 contained within
enclosure 155. An AR coating on surface A and surface B may
increase the amount of light at an operating wavelength of light
source 110 that is transmitted through the window 157.
Additionally, an AR coating at an operating wavelength of the light
source 110 may reduce the amount of incident light from output beam
125 that is reflected by the window 157 back into the housing 155.
In an example implementation, each of surface A and surface B has
an AR coating with reflectivity less than 0.5% at an operating
wavelength of light source 110. As an example, if the light source
110 has an operating wavelength of approximately 1550 nm, then
surface A and surface B may each have an AR coating with a
reflectivity that is less than 0.5% from approximately 1547 nm to
approximately 1553 nm. In another implementation, each of surface A
and surface B has an AR coating with reflectivity less than 1% at
the operating wavelengths of the light source 110. For example, if
the housing 155 encloses two sensor heads with respective light
sources, the first light source emits pulses at a wavelength of
approximately 1535 nm and the second light source emits pulses at a
wavelength of approximately 1540 nm, then surface A and surface B
may each have an AR coating with reflectivity less than 1% from
approximately 1530 nm to approximately 1545 nm.
[0061] The window 157 may have an optical transmission that is
greater than any suitable value for one or more wavelengths of one
or more light sources 110 contained within the housing 155. As an
example, the window 157 may have an optical transmission of greater
than or equal to 70%, 80%, 90%, 95%, or 99% at a wavelength of
light source 110. In one example implementation, the window 157 can
transmit greater than or equal to 95% of light at an operating
wavelength of the light source 110. In another implementation, the
window 157 transmits greater than or equal to 90% of light at the
operating wavelengths of the light sources enclosed within the
housing 155.
[0062] Surface A or surface B may have a dichroic coating that is
anti-reflecting at one or more operating wavelengths of one or more
light sources 110 and high-reflecting at wavelengths away from the
one or more operating wavelengths. For example, surface A may have
an AR coating for an operating wavelength of the light source 110,
and surface B may have a dichroic coating that is AR at the
light-source operating wavelength and HR for wavelengths away from
the operating wavelength. A coating that is HR for wavelengths away
from a light-source operating wavelength may prevent most incoming
light at unwanted wavelengths from being transmitted through the
window 117. In one implementation, if light source 110 emits
optical pulses with a wavelength of approximately 1550 nm, then
surface A may have an AR coating with a reflectivity of less than
or equal to 0.5% from approximately 1546 nm to approximately 1554
nm. Additionally, surface B may have a dichroic coating that is AR
at approximately 1546-1554 nm and HR (e.g., reflectivity of greater
than or equal to 90%) at approximately 800-1500 nm and
approximately 1580-1700 nm.
[0063] Surface B of the window 157 may include a coating that is
oleophobic, hydrophobic, or hydrophilic. A coating that is
oleophobic (or, lipophobic) may repel oils (e.g., fingerprint oil
or other non-polar material) from the exterior surface (surface B)
of the window 157. A coating that is hydrophobic may repel water
from the exterior surface. For example, surface B may be coated
with a material that is both oleophobic and hydrophobic. A coating
that is hydrophilic attracts water so that water may tend to wet
and form a film on the hydrophilic surface (rather than forming
beads of water as may occur on a hydrophobic surface). If surface B
has a hydrophilic coating, then water (e.g., from rain) that lands
on surface B may form a film on the surface. The surface film of
water may result in less distortion, deflection, or occlusion of an
output beam 125 than a surface with a non-hydrophilic coating or a
hydrophobic coating.
[0064] With continued reference to FIG. 1, the light source 110 may
include a pulsed laser configured to produce or emit pulses of
light with a certain pulse duration. In an example implementation,
the pulse duration or pulse width of the pulsed laser is
approximately 10 picoseconds (ps) to 20 nanoseconds (ns). In
another implementation, the light source 110 is a pulsed laser that
produces pulses with a pulse duration of approximately 1-4 ns. In
yet another implementation, the light source 110 is a pulsed laser
that produces pulses at a pulse repetition frequency of
approximately 100 kHz to 5 MHz or a pulse period (e.g., a time
between successive pulses) of approximately 200 ns to 10 .mu.s. The
light source 110 may have a substantially constant or a variable
pulse repetition frequency, depending on the implementation. As an
example, the 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, the light source 110 may have a pulse repetition
frequency that can be varied from approximately 500 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, and a pulse repetition frequency
may be referred to as a pulse rate.
[0065] In general, the output beam 125 may have any suitable
average optical power, and the output beam 125 may include optical
pulses with any suitable pulse energy or peak optical power. Some
examples of the average power of the output beam 125 include the
approximate values of 1 mW, 10 mW, 100 mW, 1 W, and 10 W. Example
values of pulse energy of the output beam 125 include the
approximate values of 0.1 .mu.J, 1 .mu.J, 10 .mu.J, 100 .mu.J, and
1 mJ. Examples of peak power values of pulses included in the
output beam 125 are the approximate values of 10 W, 100 W, 1 kW, 5
kW, 10 kW. An example optical pulse with a duration of 1 ns and a
pulse energy of 1 .mu.J has a peak power of approximately 1 kW. If
the pulse repetition frequency is 500 kHz, then the average power
of the output beam 125 with 1-.mu.J pulses is approximately 0.5 W,
in this example.
[0066] The light source 110 may include a laser diode, such as a
Fabry-Perot laser diode, a quantum well laser, a distributed Bragg
reflector (DBR) laser, a distributed feedback (DFB) laser, or a
vertical-cavity surface-emitting laser (VCSEL). The laser diode
operating in the light source 110 may be an
aluminum-gallium-arsenide (AlGaAs) laser diode, an
indium-gallium-arsenide (InGaAs) laser diode, or an
indium-gallium-arsenide-phosphide (InGaAsP) laser diode, or any
other suitable diode. In some implementations, the light source 110
includes a pulsed laser diode with a peak emission wavelength of
approximately 1400-1600 nm. Further, the light source 110 may
include a laser diode that is current-modulated to produce optical
pulses.
[0067] In some implementations, the light source 110 includes a
pulsed laser diode followed by one or more optical-amplification
stages. For example, the light source 110 may be a fiber-laser
module that includes a current-modulated laser diode with a peak
wavelength of approximately 1550 nm, followed by a single-stage or
a multi-stage erbium-doped fiber amplifier (EDFA). As another
example, the 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 modulator), and the output of the modulator
may be fed into an optical amplifier. A light source 110 that
includes a laser diode followed by one or more
optical-amplification stages may be referred to as a master
oscillator power amplifier (MOPA) in which the laser diode acts as
a master oscillator. In yet other implementations, the light source
110 may include a pulsed solid-state laser or a pulsed fiber
laser.
[0068] In some implementations, the output beam of light 125
emitted by the light source 110 is a collimated optical beam with
any suitable beam divergence, such as a divergence of approximately
0.1 to 3.0 milliradian (mrad). Divergence of the output beam 125
may refer to an angular measure of an increase in beam size (e.g.,
a beam radius or beam diameter) as the output beam 125 travels away
from the light source 110 or the lidar system 100. The output beam
125 may have a substantially circular cross section with a beam
divergence characterized by a single divergence value. For example,
the output beam 125 with a circular cross section and a divergence
of 1 mrad may have a beam diameter or spot size of approximately 10
cm at a distance of 100 m from the lidar system 100. In some
implementations, the output beam 125 may be an astigmatic beam or
may have a substantially elliptical cross section and may be
characterized by two divergence values. As an example, the 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, the output beam 125 may be an astigmatic beam with a
fast-axis divergence of 2 mrad and a slow-axis divergence of 0.5
mrad.
[0069] The 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., the output beam 125 may be
linearly polarized, elliptically polarized, or circularly
polarized). As an example, the light source 110 may produce
linearly polarized light, and the lidar system 100 may include a
quarter-wave plate that converts this linearly polarized light into
circularly polarized light. The lidar system 100 may transmit the
circularly polarized light as the output beam 125, and receive the
input beam 135, which may be substantially or at least partially
circularly polarized in the same manner as the output beam 125
(e.g., if the output beam 125 is right-hand circularly polarized,
then the input beam 135 may also be right-hand circularly
polarized). The input beam 135 may pass through the same
quarter-wave plate (or a different quarter-wave plate), resulting
in the input beam 135 being converted to linearly polarized light
which is orthogonally polarized (e.g., polarized at a right angle)
with respect to the linearly polarized light produced by light
source 110. As another example, the lidar system 100 may employ
polarization-diversity detection where two polarization components
are detected separately. The output beam 125 may be linearly
polarized, and the lidar system 100 may split the input beam 135
into two polarization components (e.g., s-polarization and
p-polarization) which are detected separately by two photodiodes
(e.g., a balanced photoreceiver that includes two photodiodes).
[0070] With continued reference to FIG. 1, the output beam 125 and
input beam 135 may be substantially coaxial. In other words, the
output beam 125 and input beam 135 may at least partially overlap
or share a common propagation axis, so that the input beam 135 and
the output beam 125 travel along substantially the same optical
path (albeit in opposite directions). As the lidar system 100 scans
the output beam 125 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.
[0071] The lidar system 100 also may include one or more optical
components configured to condition, shape, filter, modify, steer,
or direct the output beam 125 and/or the input beam 135. For
example, lidar system 100 may include one or more lenses, mirrors,
filters (e.g., bandpass or interference filters), beam splitters,
polarizers, polarizing beam splitters, wave plates (e.g., half-wave
or quarter-wave plates), diffractive elements, or holographic
elements. In some implementations, lidar system 100 includes a
telescope, one or more lenses, or one or more mirrors to expand,
focus, or collimate the output beam 125 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 an active region of
the 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 an active region of receiver 140. As illustrated in FIG. 1,
the lidar system 100 may include the mirror 115, which may be a
metallic or dielectric mirror. The mirror 115 may be configured so
that the light beam 125 passes through the mirror 115. As an
example, mirror 115 may include a hole, slot, or aperture through
which the output light beam 125 passes. As another example, the
mirror 115 may be configured so that at least 80% of the output
beam 125 passes through the mirror 115 and at least 80% of the
input beam 135 is reflected by the mirror 115. In some
implementations, the mirror 115 may provide for the output beam 125
and the input beam 135 to be substantially coaxial, so that the
beams 125 and 135 travel along substantially the same optical path,
in opposite directions.
[0072] Generally speaking, the scanner 120 steers the output beam
125 in one or more directions downrange. The scanner 120 may
include one or more scanning mirrors and one or more actuators
driving the mirrors to rotate, tilt, pivot, or move the mirrors in
an angular manner about one or more axes, for example. For example,
the first mirror of the scanner may scan the output beam 125 along
a first direction, and the second mirror may scan the output beam
125 along a second direction that is substantially orthogonal to
the first direction. Example implementations of the scanner 120 are
discussed in more detail below with reference to FIG. 2.
[0073] The scanner 120 may be configured to scan the output beam
125 over a 5-degree angular range, 20-degree angular range,
30-degree angular range, 60-degree angular range, or any other
suitable angular range. For example, a scanning mirror may be
configured to periodically rotate over a 15-degree range, which
results in the output beam 125 scanning across a 30-degree range
(e.g., a .THETA.-degree rotation by a scanning mirror results in a
2.THETA.-degree angular scan of the output beam 125). A field of
regard (FOR) of the 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. When the lidar system 100
scans the output beam 125 within a 30-degree scanning range, the
lidar system 100 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 the
output beam 125 that scans across a 60-degree range (e.g., a
60-degree FOR). In various implementations, the lidar system 100
may have a FOR of approximately 10.degree., 20.degree., 40.degree.,
60.degree., 120.degree., or any other suitable FOR. The FOR also
may be referred to as a scan region.
[0074] The scanner 120 may be configured to scan the output beam
125 horizontally and vertically, and the lidar system 100 may have
a particular FOR along the horizontal direction and another
particular FOR along the vertical direction. For example, the lidar
system 100 may have a horizontal FOR of 10.degree. to 120.degree.
and a vertical FOR of 2.degree. to 45.degree..
[0075] The one or more scanning mirrors of the scanner 120 may be
communicatively coupled to the 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 general, a
scan pattern may refer to a pattern or path along which the output
beam 125 is directed, and also may be referred to as an optical
scan pattern, optical scan path, or scan path. As an example, the
scanner 120 may include two scanning mirrors configured to scan the
output beam 125 across a 60.degree. horizontal FOR and a 20.degree.
vertical FOR. The two scanner mirrors may be controlled to follow a
scan path that substantially covers the 60.degree..times.20.degree.
FOR. The lidar system 100 can use the scan path to generate a point
cloud with pixels that substantially cover the
60.degree..times.20.degree. FOR. The pixels may be approximately
evenly distributed across the 60.degree..times.20.degree. FOR.
Alternately, the pixels may have a particular non-uniform
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).
[0076] In operation, the light source 110 may emit pulses of light
which the scanner 120 scans across a FOR of lidar system 100. The
target 130 may scatter one or more of the emitted pulses, and the
receiver 140 may detect at least a portion of the pulses of light
scattered by the target 130.
[0077] The receiver 140 may be referred to as (or may include) a
photoreceiver, optical receiver, optical sensor, detector,
photodetector, or optical detector. The receiver 140 in some
implementations receives or detects at least a portion of the input
beam 135 and produces an electrical signal that corresponds to the
input beam 135. For example, if the input beam 135 includes an
optical pulse, then the receiver 140 may produce an electrical
current or voltage pulse that corresponds to the optical pulse
detected by the receiver 140. In an example implementation, the
receiver 140 includes one or more avalanche photodiodes (APDs) or
one or more single-photon avalanche diodes (SPADs). In another
implementation, the receiver 140 includes one or more PN
photodiodes (e.g., a photodiode structure formed by a p-type
semiconductor and a n-type semiconductor) 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).
[0078] The receiver 140 may have an active region or an
avalanche-multiplication region that includes silicon, germanium,
or InGaAs. The active region of receiver 140 may have any suitable
size, such as for example, a diameter or width of approximately
50-500 .mu.m. The receiver 140 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. For example, the 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 receiver 140 may direct
the voltage signal to pulse-detection circuitry that produces an
analog or digital output signal 145 that corresponds to one or more
characteristics (e.g., rising edge, falling edge, amplitude, or
duration) of a received optical pulse. For example, the
pulse-detection circuitry may perform a time-to-digital conversion
to produce a digital output signal 145. The receiver 140 may send
the electrical output signal 145 to the controller 150 for
processing or analysis, e.g., to determine a time-of-flight value
corresponding to a received optical pulse.
[0079] The controller 150 may be electrically coupled or otherwise
communicatively coupled to one or more of the light source 110, the
scanner 120, and the receiver 140. The controller 150 may receive
electrical trigger pulses or edges from the light source 110, where
each pulse or edge corresponds to the emission of an optical pulse
by the light source 110. The controller 150 may provide
instructions, a control signal, or a trigger signal to the light
source 110 indicating when the light source 110 should produce
optical pulses. For example, the controller 150 may send an
electrical trigger signal that includes electrical pulses, where
the light source 110 emits an optical pulse in response to each
electrical pulse. Further, the controller 150 may cause the light
source 110 to adjust one or more of the frequency, period,
duration, pulse energy, peak power, average power, or wavelength of
the optical pulses produced by light source 110.
[0080] The 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., the input beam 135) was detected or received by the
receiver 140. The 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.
[0081] As indicated above, the 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
can 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. For example, a depth map may cover a field
of regard that extends 60.degree. horizontally and 15.degree.
vertically, and the depth map may include a frame of 100-2000
pixels in the horizontal direction by 4-400 pixels in the vertical
direction.
[0082] The 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. For example, the 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. In an example implementation, the lidar system 100 is
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). The point-cloud frame rate may be substantially fixed or
dynamically adjustable, depending on the implementation. For
example, the 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).
In general, the lidar system can use a slower frame rate (e.g., 1
Hz) to capture one or more high-resolution point clouds, and use a
faster frame rate (e.g., 10 Hz) to rapidly capture multiple
lower-resolution point clouds.
[0083] The field of regard of the lidar system 100 can overlap,
encompass, or enclose at least a portion of the target 130, which
may include all or part of an object that is moving or stationary
relative to lidar system 100. For example, the 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.
[0084] The controller 150 can include a distance/velocity mode
selector 160 that determines whether the lidar system operates in
the distance-determination mode or the velocity-determination mode.
Depending on the implementation, the mode selector 160 can be
implemented as a software module, a hardware module, or a
combination thereof. The mode selector 160 can provide electrical
signals to the light source 110 to control when the light source
generates pulses and, in some cases, other parameters of the pulses
such as amplitude, wavelength, etc.
[0085] Now referring to FIG. 2, a scanner 162 and a receiver 164
can operate in the lidar system of FIG. 1 as the scanner 120 and
the receiver 140, respectively. More generally, the scanner 162 and
the receiver 164 can operate in any suitable lidar system.
[0086] The scanner 162 may include any suitable number of mirrors
driven by any suitable number of mechanical actuators. For example,
the scanner 162 may include a galvanometer scanner, a resonant
scanner, a piezoelectric actuator, a polygonal scanner, a
rotating-prism scanner, a voice coil motor, a DC motor, a brushless
DC motor, a stepper motor, or a microelectromechanical systems
(MEMS) device, or any other suitable actuator or mechanism.
[0087] A galvanometer scanner (which also may be referred to as a
galvanometer actuator) may include a galvanometer-based scanning
motor with a magnet and coil. When an electrical current is
supplied to the coil, a rotational force is applied to the magnet,
which causes a mirror attached to the galvanometer scanner to
rotate. The electrical current supplied to the coil may be
controlled to dynamically change the position of the galvanometer
mirror. A resonant scanner (which may be referred to as a resonant
actuator) may include a spring-like mechanism driven by an actuator
to produce a periodic oscillation at a substantially fixed
frequency (e.g., 1 kHz). A MEMS-based scanning device may include a
mirror with a diameter between approximately 1 and 10 mm, where the
mirror is rotated using electromagnetic or electrostatic actuation.
A voice coil motor (which may be referred to as a voice coil
actuator) may include a magnet and coil. When an electrical current
is supplied to the coil, a translational force is applied to the
magnet, which causes a mirror attached to the magnet to move or
rotate.
[0088] In an example implementation, the scanner 162 includes a
single mirror configured to scan an output beam 170 along a single
direction (e.g., the scanner 162 may be a one-dimensional scanner
that scans along a horizontal or vertical direction). The mirror
may be a flat scanning mirror attached to a scanner actuator or
mechanism which scans the mirror over a particular angular range.
The mirror may be driven by one actuator (e.g., a galvanometer) or
two actuators configured to drive the mirror in a push-pull
configuration. When two actuators drive the mirror in one direction
in a push-pull configuration, the actuators may be located at
opposite ends or sides of the mirror. The actuators may operate in
a cooperative manner so that when one actuator pushes on the
mirror, the other actuator pulls on the mirror, and vice versa. In
another example implementation, two voice coil actuators arranged
in a push-pull configuration drive a mirror along a horizontal or
vertical direction.
[0089] In some implementations, the scanner 162 may include one
mirror configured to be scanned along two axes, where two actuators
arranged in a push-pull configuration provide motion along each
axis. For example, two resonant actuators arranged in a horizontal
push-pull configuration may drive the mirror along a horizontal
direction, and another pair of resonant actuators arranged in a
vertical push-pull configuration may drive mirror along a vertical
direction. In another example implementation, two actuators scan
the output beam 170 along two directions (e.g., horizontal and
vertical), where each actuator provides rotational motion along a
particular direction or about a particular axis.
[0090] The scanner 162 also may include one mirror driven by two
actuators configured to scan the mirror along two substantially
orthogonal directions. For example, a resonant actuator or a
galvanometer actuator may drive one mirror along a substantially
horizontal direction, and a galvanometer actuator may drive the
mirror along a substantially vertical direction. As another
example, two resonant actuators may drive a mirror along two
substantially orthogonal directions.
[0091] In some implementations, the scanner 162 includes two
mirrors, where one mirror scans the output beam 170 along a
substantially horizontal direction and the other mirror scans the
output beam 170 along a substantially vertical direction. In the
example of FIG. 2, the scanner 162 includes two mirrors, a mirror
180-1 and a mirror 180-2. The mirror 180-1 may scan the output beam
170 along a substantially horizontal direction, and the mirror
180-2 may scan the output beam 170 along a substantially vertical
direction (or vice versa). Mirror 180-1 or mirror 180-2 may be a
flat mirror, a curved mirror, or a polygon mirror with two or more
reflective surfaces.
[0092] The scanner 162 in other implementations includes two
galvanometer scanners driving respective mirrors. For example, the
scanner 162 may include a galvanometer actuator that scans the
mirror 180-1 along a first direction (e.g., vertical), and the
scanner 162 may include another galvanometer actuator that scans
the mirror 180-2 along a second direction (e.g., horizontal). In
yet another implementation, the scanner 162 includes two mirrors,
where a galvanometer actuator drives one mirror, and a resonant
actuator drives the other mirror. For example, a galvanometer
actuator may scan the mirror 180-1 along a first direction, and a
resonant actuator may scan the mirror 180-2 along a second
direction. The first and second scanning directions may be
substantially orthogonal to one another, e.g., the first direction
may be substantially vertical, and the second direction may be
substantially horizontal. In yet another implementation, the
scanner 162 includes two mirrors, where one mirror is a polygon
mirror that is rotated in one direction (e.g., clockwise or
counter-clockwise) by an electric motor (e.g., a brushless DC
motor). For example, mirror 180-1 may be a polygon mirror that
scans the output beam 170 along a substantially horizontal
direction, and mirror 180-2 may scan the output beam 170 along a
substantially vertical direction. A polygon mirror may have two or
more reflective surfaces, and the polygon mirror may be
continuously rotated in one direction so that the output beam 170
is reflected sequentially from each of the reflective surfaces. A
polygon mirror may have a cross-sectional shape that corresponds to
a polygon, where each side of the polygon has a reflective surface.
For example, a polygon mirror with a square cross-sectional shape
may have four reflective surfaces, and a polygon mirror with a
pentagonal cross-sectional shape may have five reflective
surfaces.
[0093] To direct the output beam 170 along a particular scan
pattern, the scanner 162 may include two or more actuators driving
a single mirror synchronously. For example, the two or more
actuators can drive the mirror synchronously along two
substantially orthogonal directions to make the output beam 170
follow a scan pattern with substantially straight lines. In some
implementations, the scanner 162 may include two mirrors and
actuators driving the two mirrors synchronously to generate a scan
pattern that includes substantially straight lines. For example, a
galvanometer actuator may drive the mirror 180-2 with a
substantially linear back-and-forth motion (e.g., the galvanometer
may be driven with a substantially sinusoidal or triangle-shaped
waveform) that causes the output beam 170 to trace a substantially
horizontal back-and-forth pattern, and another galvanometer
actuator may scan the mirror 180-1 along a substantially vertical
direction. The two galvanometers may be synchronized so that for
every 64 horizontal traces, the output beam 170 makes a single
trace along a vertical direction. Whether one or two mirrors are
used, the substantially straight lines can be directed
substantially horizontally, vertically, or along any other suitable
direction.
[0094] The scanner 162 also may apply a dynamically adjusted
deflection along a vertical direction (e.g., with a galvanometer
actuator) as the output beam 170 is scanned along a substantially
horizontal direction (e.g., with a galvanometer or resonant
actuator) to achieve the straight lines. If a vertical deflection
is not applied, the output beam 170 may trace out a curved path as
it scans from side to side. In some implementations, the scanner
162 uses a vertical actuator to apply a dynamically adjusted
vertical deflection as the output beam 170 is scanned horizontally
as well as a discrete vertical offset between each horizontal scan
(e.g., to step the output beam 170 to a subsequent row of a scan
pattern).
[0095] With continued reference to FIG. 2, an overlap mirror 190 in
this example implementation is configured to overlap the input beam
172 and output beam 170, so that the beams 170 and 172 are
substantially coaxial. In FIG. 2, the overlap mirror 190 includes a
hole, slot, or aperture 192 through which the output beam 170
passes, and a reflecting surface 194 that reflects at least a
portion of the input beam 172 toward the receiver 164. The overlap
mirror 190 may be oriented so that input beam 172 and output beam
170 are at least partially overlapped.
[0096] In some implementations, the overlap mirror 190 may not
include a hole 192. For example, the output beam 170 may be
directed to pass by a side of mirror 190 rather than passing
through an aperture 192. The output beam 170 may pass alongside
mirror 190 and may be oriented at a slight angle with respect to
the orientation of the input beam 172. As another example, the
overlap mirror 190 may include a small reflective section
configured to reflect the output beam 170, and the rest of the
overlap mirror 190 may have an AR coating configured to transmit
the input beam 172.
[0097] The input beam 172 may pass through a lens 196 which focuses
the beam onto an active region 166 of the receiver 164. The active
region 166 may refer to an area over which receiver 164 may receive
or detect input light. The active region may have any suitable size
or diameter d, such as for example, a diameter of approximately 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. The overlap mirror 190 may have a reflecting surface
194 that is substantially flat or the reflecting surface 194 may be
curved (e.g., the mirror 190 may be an off-axis parabolic mirror
configured to focus the input beam 172 onto an active region of the
receiver 140).
[0098] The aperture 192 may have any suitable size or diameter
.PHI..sub.1, and the input beam 172 may have any suitable size or
diameter .PHI..sub.2, where .PHI..sub.2 is greater than
.PHI..sub.1. For example, the aperture 192 may have a diameter
.PHI..sub.1 of approximately 0.2 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 5
mm, or 10 mm, and the input beam 172 may have a diameter
.PHI..sub.2 of approximately 2 mm, 5 mm, 10 mm, 15 mm, 20 mm, 30
mm, 40 mm, or 50 mm. In some implementations, the reflective
surface 194 of the overlap mirror 190 may reflect 70% or more of
input beam 172 toward the receiver 164. For example, if the
reflective surface 194 has a reflectivity R at an operating
wavelength of the light source, then the fraction of input beam 172
directed toward the receiver 164 may be expressed as
R.times.[1-(.PHI..sub.1/.PHI..sub.2).sup.2]. As a more specific
example, if R is 95%, .PHI..sub.1 is 2 mm, and .PHI..sub.2 is 10
mm, then approximately 91% of the input beam 172 may be directed
toward the receiver 164 by the reflective surface 194.
[0099] FIG. 3 illustrates another example lidar system 200 that can
operate in a dual mode to determine the distance to a target as
well as the velocity of the target. The lidar system 200 includes a
polygon mirror 202 driven by a motor 204. The lidar system 200
operates in a two-eye configuration, with a first eye 206A and a
second eye 206B. The first eye 206A includes a collimator 210A, a
scan mirror 212A, and a receiver 214A, and the second eye 206B
includes a collimator 210B, a scan mirror 212B, and a receiver
214B. The polygon mirror 202 may be in the form of a rotatable
block with multiple reflective surfaces angularly offset from one
another along the polygon periphery of the rotatable block. In this
example implementation, the polygon mirror 202 has six reflective
surfaces 220A, 220B, . . . 220F; however, the polygon mirror 202 in
general can include any suitable number of surfaces, e.g., three,
four, five, eight, etc. The motor 204 imparts rotation to the
rotatable polygon mirror 202. The scan mirrors 212A and 212B are
configured to rotate, in an oscillatory manner within a certain
angular range, about the respective axis orthogonal to the axis of
rotation of the polygon mirror 202.
[0100] A light source 222 can be a fiber laser that includes a seed
laser diode. The output of the light source 222 can be provided to
the collimators 210A and 210B via fiber-optic cables 224A and 224B,
free-space coupling, or in any other suitable manner. While the
lidar system 200 uses collimators coupled to a shared light source,
in other implementations of this system each eye can include its
own direct-emitted laser diode. The light source 222 in this case
can be made of multiple direct-emitter laser diodes (e.g.,
high-power laser diodes) that directly emit the pulses without
requiring optical amplification. The laser diodes can be housed in
the respective sensor heads.
[0101] In operation, the collimators 210A and 210B direct output
beams 226A and 226B to the scan mirrors 212A and 212B,
respectively. The scan mirrors 212A and 212B then reflect these
beams toward non-adjacent reflective surfaces of the polygon mirror
202, which then directs the output beams 226A and 226B to
respective fields of regard. Input beams 228A and 228B are incident
on non-adjacent reflective surfaces of the polygon mirror 202 and
are reflected toward the scan mirrors 212A and 212B, respectively.
The input beams 228A and 228B then propagate toward the receivers
214A and 214B. In other implementations, input and output beams
from different eyes can be incident on adjacent surfaces of the
polygon mirror.
[0102] Referring back to FIG. 1, the scanner 120 in some
implementations includes a polygon mirror similar to the polygon
mirror 202 and one or two mirrors similar to the scan mirrors 212A
and 212B, depending on the number of eyes of the lidar system.
[0103] In the configuration of FIG. 3, the field of regard (FOR) of
the lidar system includes the FOR is the combination of the FOR of
the first eye 206A, which extends from the left boundary 216A-L to
the right boundary 216A-R, and the FOR of the second eye 206B,
which extends from the left boundary 216B-L to the right boundary
216B-R. The FORs of the two eyes define an overlap region 218. In
some implementations, the lidar system 200 generates a higher
density of information when scanning the overlap region 218. In
another implementation, the lidar system 200 operates the first eye
in distance-determination mode and the second eye in the
velocity-determination mode, only when scanning the overlap region
218. To this end, the light source 222 can include separately
operable lasers or a shared laser with separately operable optical
modulators responsible for transmitting different respective
optical signals via the links 224A and 224B. A controller similar
to the distance/velocity mode selector 160 can provide different
electrical signals to these separately operable lasers or optical
modulators.
[0104] FIG. 4 illustrates an example configuration in which several
components of the lidar system 100 or another suitable system may
operate to scan a 360-degree view of regard. Generally speaking,
the field of view of a light source in this configuration follows a
circular trajectory and accordingly defines a circular scan pattern
on a two-dimensional plane. All points on the trajectory remain at
the same elevation relative to the ground level, according to one
implementation. In this case, separate beams may follow the
circular trajectory with certain vertical offsets relative to each
other. In another implementation, the points of the trajectory may
define a spiral scan pattern in three-dimensional space. A single
beam can be sufficient to trace out the spiral scan pattern but, if
desired, multiple beams can be used.
[0105] In the example of FIG. 4, a rotating scan module 230
revolves around a central axis in one or both directions as
indicated. An electric motor may drive the rotating scan module 230
around the central axis at a constant speed, for example. The
rotating scan module 230 includes a scanner, a receiver, an overlap
mirror, etc. The components of the rotating module 230 may be
similar to the scanner 120, the receiver 140, and the overlap
mirror 115 discussed above. In some implementations, the rotating
scan module 230 also includes a light source and a controller. In
other implementations, the light source and/or the controller are
disposed apart from the rotating scan module 230 and/or exchange
optical and electrical signals with the components of the rotating
scan module 230 via corresponding links.
[0106] The rotating scan module 230 may include a housing 232 with
a window 234. Similar to the window 157 of FIG. 1, the window 234
may be made of glass, plastic, or any other suitable material. The
window 234 allows outbound beams as well as return signals to pass
through the housing 232. The arc length defined by the window 234
can correspond to any suitable percentage of the circumference of
the housing 232. For example, the arc length can correspond to 5%,
20%, 30%, 60%, or possibly even 100% of the circumference.
[0107] Now referring to FIG. 5, a rotating scan module 236 is
generally similar to the rotating scan module 230. In this
implementation, however, the components of the rotating scan module
236 are disposed on a platform 237 which rotates inside a
stationary circular housing 238. In this implementation, the
circular housing 238 is substantially transparent to light at the
lidar-system operating wavelength to pass inbound and outbound
light signals. The circular housing 238 in a sense defines a
circular window similar to the window 234 and may be made of
similar material.
Generating Pixels within a Field of Regard
[0108] FIG. 6 illustrates an example scan pattern 240 which the
lidar system 100 of FIG. 1 or the lidar system 200 of FIG. 3 can
produce, along with an example image 241 captured by the camera
101. The scan pattern 240 overlaps the FOR of the camera 101 at
least partially, and thus the lidar system 100 or 200 emits pulses
in the direction of objects depicted in the image 241. However,
some or all of the objects in the image 241 may be outside the
maximum range of the lidar system 100 or 200.
[0109] The lidar system 100 may be configured to scan output
optical beam 125 along one or more scan patterns 240. In some
implementations, the scan pattern 240 corresponds to a 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 certain scan pattern may have a field of regard
represented by angular dimensions (e.g., FOR.sub.H.times.FOR.sub.V)
40.degree..times.30.degree., 90.degree..times.40.degree., or
60.degree..times.15.degree.. As another example, a certain scan
pattern 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 yet another example, a certain scan pattern 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.. In
the example of FIG. 6, reference line 246 represents a center of
the field of regard of scan pattern 240. The reference line 246 may
have any suitable orientation, such as, a horizontal angle of
0.degree. (e.g., reference line 246 may be oriented straight ahead)
and a vertical angle of 0.degree. (e.g., reference line 246 may
have an inclination of 0.degree.), or the reference line 246 may
have a nonzero horizontal angle or a nonzero inclination (e.g., a
vertical angle of +10.degree. or -10.degree.. In FIG. 6, if the
scan pattern 240 has a 60.degree..times.15.degree. field of regard,
then the scan pattern 240 covers a .+-.30.degree. horizontal range
with respect to reference line 246 and a .+-.7.5.degree. vertical
range with respect to reference line 246. Additionally, the optical
beam 125 in FIG. 6 has an orientation of approximately -15.degree.
horizontal and +3.degree. vertical with respect to reference line
246. The beam 125 may be referred to as having an azimuth of
-15.degree. and an altitude of +3.degree. relative to the reference
line 246. An azimuth (which may be referred to as an azimuth angle)
may represent a horizontal angle with respect to the reference line
246, and an altitude (which may be referred to as an altitude
angle, elevation, or elevation angle) may represent a vertical
angle with respect to the reference line 246. The azimuth angle
also can be referred to herein as the horizontal scan angle, and
altitude angle can be referred to herein as the vertical scan
angle.
[0110] The scan pattern 240 may include multiple pixels 242, and
each pixel 242 may be associated with one or more laser pulses and
one or more corresponding distance measurements. A cycle of scan
pattern 240 may include a total of P.sub.x.times.P.sub.y pixels 242
(e.g., a two-dimensional distribution of P.sub.x by P.sub.y
pixels). For example, the scan pattern 240 may include a
distribution with dimensions of approximately 100-2,000 pixels 242
along a horizontal direction and approximately 4-400 pixels 242
along a vertical direction. As another example, the scan pattern
240 may include a distribution of 1,000 pixels 242 along the
horizontal direction by 64 pixels 242 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 240. The number of pixels
242 along a horizontal direction may be referred to as a horizontal
resolution of the scan pattern 240, and the number of pixels 242
along a vertical direction may be referred to as a vertical
resolution of the scan pattern 240. As an example, the scan pattern
240 may have a horizontal resolution of greater than or equal to
100 pixels 242 and a vertical resolution of greater than or equal
to 4 pixels 242. As another example, the scan pattern 240 may have
a horizontal resolution of 100-2,000 pixels 242 and a vertical
resolution of 4-400 pixels 242.
[0111] Each pixel 242 may be associated with a distance (e.g., a
distance to a portion of a target 130 from which the corresponding
laser pulse was scattered) or one or more angular values. As an
example, the pixel 242 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 242 with respect to the lidar
system 100. A distance to a portion of the 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 246) of the output beam 125 (e.g., when a corresponding pulse
is emitted from lidar system 100) or an angle of the input beam 135
(e.g., when an input signal is received by lidar system 100). In
some implementations, the lidar system 100 determines an angular
value based at least in part on a position of a component of the
scanner 120. For example, an azimuth or altitude value associated
with the pixel 242 may be determined from an angular position of
one or more corresponding scanning mirrors of the scanner 120.
[0112] In some implementations, the lidar system 100 concurrently
directs multiple beams across the field of regard. In the example
implementation of FIG. 7, the lidar system generates output beams
250A, 250B, 250C, . . . 250N etc., each of which follows a linear
scan pattern 254A, 254B, 254C, . . . 254N. The number of parallel
lines can be 2, 4, 12, 20, or any other suitable number. The lidar
system 100 may angularly separate the beams 250A, 250B, 250C, . . .
250N, so that, for example, the separation between beams 250A and
250B at a certain distance may be 30 cm, and the separation between
the same beams 250A and 250B at a longer distance may be 50 cm.
[0113] Similar to the scan pattern 240, each of the linear scan
patterns 254A-N includes pixels associated with one or more laser
pulses and distance measurements. FIG. 7 illustrates example pixels
252A, 252B and 252C along the scan patterns 254A, 254B and 254C,
respectively. The lidar system 100 in this example may generate the
values for the pixels 252A-252N at the same time, thus increasing
the rate at which values for pixels are determined.
[0114] Depending on the implementation, the lidar system 100 may
output the beams 250A-N at the same wavelength or different
wavelengths. The beam 250A for example may have the wavelength of
1540 nm, the beam 250B may have the wavelength of 1550 nm, the beam
250C may have the wavelength of 1560 nm, etc. The number of
different wavelengths the lidar system 100 uses need not match the
number of beams. Thus, the lidar system 100 in the example
implementation of FIG. 7 may use M wavelengths with N beams, where
1.ltoreq.M.ltoreq.N.
[0115] FIG. 8 illustrates an example scan pattern which a lidar
system that uses a rotating polygon mirror (e.g., the lidar system
200 of FIG. 3) can generate. The lidar in this case directs an
output beam 226A or 226B along a pattern 262 made up of slanted
lines (due to the oscillation of the scan mirror 212A or 212B along
the vertical dimension during rotation of the polygon mirror
202).
[0116] Next, FIG. 9 illustrates an example light-source field of
view (FOV.sub.L) and receiver field of view (FOV.sub.R) for the
lidar system 100. The light source 110 may emit pulses of light as
the FOV.sub.L and FOV.sub.R are scanned by the scanner 120 across a
field of regard (FOR). The 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. For
example, as the scanner 120 scans the light-source field of view
across a field of regard, the lidar system 100 may send the pulse
of light in the direction the FOV.sub.L is pointing at the time the
light source 110 emits the pulse. The pulse of light may scatter
off the 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.
[0117] An instantaneous FOV may refer to an angular cone being
illuminated by a pulse directed along the direction the
light-source FOV is pointing at the instant the pulse of light is
emitted. Thus, while the light-source FOV and the detector FOV are
scanned together in a synchronous manner (e.g., the scanner 120
scans both the light-source FOV and the detector FOV across the
field of regard along the same scan direction and at the same scan
speed, maintaining the same relative position to each other), the
instantaneous FOV remains "stationary," and the detector FOV
effectively moves relative to the instantaneous FOV. More
particularly, when a pulse of light is emitted, the scanner 120
directs the pulse along the direction in which the light-source FOV
currently is pointing. Each instantaneous FOV (IFOV) corresponds to
a pixel. Thus, each time a pulse is emitted, the lidar system 100
produces or defines an IFOV (or pixel) that is fixed in place and
corresponds to the light-source FOV at the time when the pulse is
emitted. During operation of the scanner 120, the detector FOV
moves relative to the light-source IFOV but does not move relative
to the light-source FOV.
[0118] In some implementations, the scanner 120 is 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. The lidar system
100 may emit and detect multiple pulses of light as the scanner 120
scans the FOV.sub.L and FOV.sub.R across the field of regard while
tracing out the scan pattern 240. The scanner 120 in some
implementations scans the light-source field of view and the
receiver field of view synchronously with respect to one another.
In this case, as the scanner 120 scans FOV.sub.L across a scan
pattern 240, 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 the
scanner 120 scans FOV.sub.L and FOV.sub.R across the field of
regard. For example, the FOV.sub.L may be substantially overlapped
with or centered inside the FOV.sub.R (as illustrated in FIG. 9),
and the scanner 120 may maintain this relative positioning between
FOV.sub.L and FOV.sub.R 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).
[0119] 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. The
receiver field of view may be any suitable size relative to the
light-source field of view. For 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 some
implementations, the light-source field of view has an angular
extent of less than or equal to 50 milliradians, and the receiver
field of view has 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. 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 3 mrad. In some implementations, the receiver field of
view is larger than the light-source field of view, or the
light-source field of view is larger than the receiver field of
view. For example, .THETA..sub.L may be approximately equal to 1.5
mrad, and .THETA..sub.R may be approximately equal to 3 mrad.
[0120] A pixel 242 may represent or correspond to a light-source
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 242) 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 242 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 242 may each have a diameter of
approximately 40 cm.
A Lidar System Operating in a Vehicle
[0121] As indicated above, one or more lidar systems 100 may be
integrated into a vehicle. In one example implementation, multiple
lidar systems 100 may be integrated into a car to provide a
complete 360-degree horizontal FOR around the car. As another
example, 4-10 lidar systems 100, each system having a 45-degree to
90-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
1-15 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, 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), or spacecraft. In particular
embodiments, a vehicle may include an internal combustion engine or
an electric motor that provides propulsion for the vehicle.
[0122] In some implementations, one or more lidar systems 100 are
included in a vehicle as part of an advanced driver assistance
system (ADAS) to assist a driver of the vehicle in the driving
process. For example, a lidar system 100 may be part of an ADAS
that provides information 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. The 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.
[0123] In some cases, one or more lidar systems 100 are integrated
into a vehicle as part of an autonomous-vehicle driving system. In
an example implementation, the lidar system 100 provides
information about the surrounding environment to a driving system
of an autonomous vehicle. An autonomous-vehicle driving system may
include one or more computing systems that receive information from
the lidar system 100 about the surrounding environment, analyze the
received information, and provide control signals to the vehicle's
driving systems (e.g., steering wheel, accelerator, brake, or turn
signal). For example, the 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 the 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.
[0124] An autonomous vehicle may be referred to as an autonomous
car, driverless car, self-driving car, robotic car, or unmanned
vehicle. An autonomous vehicle may be a vehicle configured to sense
its environment and navigate or drive with little or no human
input. For 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.
[0125] 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).
[0126] In some implementations, a light source of a lidar system is
located remotely from some of the other components of the lidar
system such as the scanner and the receiver. Moreover, a lidar
system implemented in a vehicle may include fewer light sources
than scanners and receivers.
[0127] FIG. 10 illustrates an example configuration in which a
laser-sensor link 320 includes an optical link 330 and an
electrical link 350 coupled between a laser 300 and a sensor 310.
The laser 300 may be configured to emit pulses of light and may be
referred to as a laser system, laser head, or light source. The
laser 300 may include, may be part of, may be similar to, or may be
substantially the same as the light source 110 illustrated in FIG.
1 and discussed above. Further, the scanner 302, the receiver 304,
the controller 306, and the mirror 308 may be similar to the
scanner 120, the receiver 140, the controller 150, and the mirror
115 discussed above. In the example of FIG. 10, the laser 300 is
coupled to the remotely located sensor 310 by a laser-sensor link
320 (which may be referred to as a link). The sensor 310 may be
referred to as a sensor head and may include the mirror 308, the
scanner 302, the receiver 304, and the controller 306. In an
example implementation, the laser 300 includes a pulsed laser diode
(e.g., a pulsed DFB laser) followed by an optical amplifier, and
light from the laser 300 is conveyed by an optical fiber of the
laser-sensor link 320 of a suitable length to the scanner 120 in a
remotely located sensor 310.
[0128] The laser-sensor link 320 may include any suitable number of
optical links 330 (e.g., 0, 1, 2, 3, 5, or 10) and any suitable
number of electrical links 350 (e.g., 0, 1, 2, 3, 5, or 10). In the
example configuration depicted in FIG. 10, the laser-sensor link
320 includes one optical link 330 from the laser 300 to an output
collimator 340 and one electrical link 350 that connects the laser
300 to the controller 150. The optical link 330 may include optical
fiber (which may be referred to as fiber-optic cable or fiber) that
conveys, carries, transports, or transmits light between the laser
300 and the sensor 310. The optical fiber may be, for example,
single-mode (SM) fiber, multi-mode (MM) fiber, large-mode-area
(LMA) fiber, polarization-maintaining (PM) fiber, photonic-crystal
or photonic-bandgap fiber, gain fiber (e.g., rare-earth-doped
optical fiber for use in an optical amplifier), or any suitable
combination thereof. The output collimator 340 receives optical
pulses conveyed from the laser 300 by the optical link 330 and
produces a free-space optical beam 312 that includes the optical
pulses. The output collimator 340 directs the free-space optical
beam 312 through the mirror 308 and to the scanner 302.
[0129] The electrical link 350 may include electrical wire or cable
(e.g., a coaxial cable or twisted-pair cable) that conveys or
transmits electrical power and/or one or more electrical signals
between the laser 300 and the sensor 310. For example, the laser
300 may include a power supply or a power conditioner that provides
electrical power to the laser 300, and additionally, the power
supply or power conditioner may provide power to one or more
components of the sensor 310 (e.g., the scanner 304, the receiver
304, and/or the controller 306) via the one or more electrical
links 350. The electrical link 350 in some implementations may
convey electrical signals that include data or information in
analog or digital format. Further, the electrical link 350 may
provide an interlock signal from the sensor 310 to the laser 300.
If the controller 306 detects a fault condition indicating a
problem with the sensor 310 or the overall lidar system, the
controller 306 may change a voltage on the interlock line (e.g.,
from 5 V to 0 V) indicating that the laser 300 should shut down,
stop emitting light, or reduce the power or energy of emitted
light. A fault condition may be triggered by a failure of the
scanner 302, a failure of the receiver 304, or by a person or
object coming within a threshold distance of the sensor 310 (e.g.,
within 0.1 m, 0.5 m, 1 m, 5 m, or any other suitable distance).
[0130] As discussed above, a lidar system can include one or more
processors to determine a distance D to a target. In the
implementation illustrated in FIG. 10, the controller 306 may be
located in the laser 300 or in the sensor 310, or parts of the
controller 150 may be distributed between the laser 300 and the
sensor 310. In an example implementation, each sensor head 310 of a
lidar system includes electronics (e.g., an electronic filter,
transimpedance amplifier, threshold detector, or time-to-digital
(TDC) converter) configured to receive or process a signal from the
receiver 304 or from an APD or SPAD of the receiver 304.
Additionally, the laser 300 may include processing electronics
configured to determine a time-of-flight value or a distance to the
target based on a signal received from the sensor head 310 via the
electrical link 350.
[0131] Next, FIG. 11 illustrates an example vehicle 354 with a
lidar system 351 that includes a laser 352 with multiple sensor
heads 360 coupled to the laser 352 via multiple laser-sensor links
370. The laser 352 and the sensor heads 360 may be similar to the
laser 300 and the sensor 310 discussed above, in some
implementations. For example, each of the laser-sensor links 370
may include one or more optical links and/or one or more electrical
links. The sensor heads 360 in FIG. 11 are positioned or oriented
to provide a greater than 30-degree view of an environment around
the vehicle. More generally, a lidar system with multiple sensor
heads may provide a horizontal field of regard around a vehicle of
approximately 30.degree., 45.degree., 60.degree., 90.degree.,
120.degree., 180.degree., 270.degree., or 360.degree.. Each of the
sensor heads may be attached to or incorporated into a bumper,
fender, grill, side panel, spoiler, roof, headlight assembly,
taillight assembly, rear-view mirror assembly, hood, trunk, window,
or any other suitable part of the vehicle.
[0132] In the example of FIG. 11, four sensor heads 360 are
positioned at or near the four corners of the vehicle (e.g., the
sensor heads may be incorporated into a light assembly, side panel,
bumper, or fender), and the laser 352 may be located within the
vehicle (e.g., in or near the trunk). The four sensor heads 360 may
each provide a 90.degree. to 120.degree. horizontal field of regard
(FOR), and the four sensor heads 360 may be oriented so that
together they provide a complete 360-degree view around the
vehicle. As another example, the lidar system 351 may include six
sensor heads 360 positioned on or around a vehicle, where each of
the sensor heads 360 provides a 60.degree. to 90.degree. horizontal
FOR. As another example, the lidar system 351 may include eight
sensor heads 360, and each of the sensor heads 360 may provide a
45.degree. to 60.degree. horizontal FOR. As yet another example,
the lidar system 351 may include six sensor heads 360, where each
of the sensor heads 360 provides a 70.degree. horizontal FOR with
an overlap between adjacent FORs of approximately 10.degree.. As
another example, the lidar system 351 may include two sensor heads
360 which together provide a forward-facing horizontal FOR of
greater than or equal to 30.degree..
[0133] Data from each of the sensor heads 360 may be combined or
stitched together to generate a point cloud that covers a greater
than or equal to 30-degree horizontal view around a vehicle. For
example, the laser 352 may include a controller or processor that
receives data from each of the sensor heads 360 (e.g., via a
corresponding electrical link 370) and processes the received data
to construct a point cloud covering a 360-degree horizontal view
around a vehicle or to determine distances to one or more targets.
The point cloud or information from the point cloud may be provided
to a vehicle controller 372 via a corresponding electrical,
optical, or radio link 370. In some implementations, the point
cloud is generated by combining data from each of the multiple
sensor heads 360 at a controller included within the laser 352 and
provided to the vehicle controller 372. In other implementations,
each of the sensor heads 360 includes a controller or process that
constructs a point cloud for a portion of the 360-degree horizontal
view around the vehicle and provides the respective point cloud to
the vehicle controller 372. The vehicle controller 372 then
combines or stitches together the points clouds from the respective
sensor heads 360 to construct a combined point cloud covering a
360-degree horizontal view. Still further, the vehicle controller
372 in some implementations communicates with a remote server to
process point cloud data.
[0134] In any event, the vehicle 354 may be an autonomous vehicle
where the vehicle controller 372 provides control signals to
various components 390 within the vehicle 354 to maneuver and
otherwise control operation of the vehicle 354. The components 390
are depicted in an expanded view in FIG. 11 for ease of
illustration only. The components 390 may include an accelerator
374, brakes 376, a vehicle engine 378, a steering mechanism 380,
lights 382 such as brake lights, head lights, reverse lights,
emergency lights, etc., a gear selector 384, and/or other suitable
components that effectuate and control movement of the vehicle 354.
The gear selector 384 may include the park, reverse, neutral, drive
gears, etc. Each of the components 390 may include an interface via
which the component receives commands from the vehicle controller
372 such as "increase speed," "decrease speed," "turn left 5
degrees," "activate left turn signal," etc. and, in some cases,
provides feedback to the vehicle controller 372.
[0135] In some implementations, the vehicle controller 372 receives
point cloud data from the laser 352 or sensor heads 360 via the
link 370 and analyzes the received point cloud data to sense or
identify targets 130 and their respective locations, distances,
speeds, shapes, sizes, type of target (e.g., vehicle, human, tree,
animal), etc. The vehicle controller 372 then provides control
signals via the link 370 to the components 390 to control operation
of the vehicle based on the analyzed information. For example, the
vehicle controller 372 may identify an intersection based on the
point cloud data and determine that the intersection is the
appropriate location at which to make a left turn. Accordingly, the
vehicle controller 372 may provide control signals to the steering
mechanism 380, the accelerator 374, and brakes 376 for making a
proper left turn. In another example, the vehicle controller 372
may identify a traffic light based on the point cloud data and
determine that the vehicle 354 needs to come to a stop. As a
result, the vehicle controller 372 may provide control signals to
release the accelerator 374 and apply the brakes 376.
Example Receiver Implementation
[0136] FIG. 12 illustrates an example InGaAs avalanche photodiode
(APD) 400. Referring back to FIG. 1, the receiver 140 may include
one or more APDs 400 configured to receive and detect light from
input light such as the beam 135. More generally, the APD 400 can
operate in any suitable receiver of input light. The APD 400 may be
configured to detect a portion of pulses of light which are
scattered by a target located downrange from the lidar system in
which the APD 400 operates. For example, the APD 400 may receive a
portion of a pulse of light scattered by the target 130 depicted in
FIG. 1, and generate an electrical-current signal corresponding to
the received pulse of light.
[0137] The APD 400 may include doped or undoped layers of any
suitable semiconductor material, such as for example, silicon,
germanium, InGaAs, InGaAsP, or indium phosphide (InP).
Additionally, the APD 400 may include an upper electrode 402 and a
lower electrode 406 for coupling the ADP 400 to an electrical
circuit. The APD 400 for example may be electrically coupled to a
voltage source that supplies a reverse-bias voltage V to the APD
400. Additionally, the APD 400 may be electrically coupled to a
transimpedance amplifier which receives electrical current
generated by the APD 400 and produces an output voltage signal that
corresponds to the received current. The upper electrode 402 or
lower electrode 406 may include any suitable electrically
conductive material, such as for example a metal (e.g., gold,
copper, silver, or aluminum), a transparent conductive oxide (e.g.,
indium tin oxide), a carbon-nanotube material, or polysilicon. In
some implementations, the upper electrode 402 is partially
transparent or has an opening to allow input light 410 to pass
through to the active region of the APD 400. In FIG. 12, the upper
electrode 402 may have a ring shape that at least partially
surrounds the active region of the APD 400, where the active region
refers to an area over which the APD 400 may receive and detect the
input light 410. The active region may have any suitable size or
diameter d, such as for example, a diameter of approximately 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.
[0138] The APD 400 may include any suitable combination of any
suitable semiconductor layers having any suitable doping (e.g.,
n-doped, p-doped, or intrinsic undoped material). In the example of
FIG. 12, the InGaAs APD 400 includes a p-doped InP layer 420, an
InP avalanche layer 422, an absorption layer 424 with n-doped
InGaAs or InGaAsP, and an n-doped InP substrate layer 426.
Depending on the implementation, the APD 400 may include separate
absorption and avalanche layers, or a single layer may act as both
an absorption and avalanche region. The APD 400 may operate
electrically as a PN diode or a PIN diode, and, during operation,
the APD 400 may be reverse-biased with a positive voltage V applied
to the lower electrode 406 with respect to the upper electrode 402.
The applied reverse-bias voltage V may have any suitable value,
such as for example approximately 5 V, 10 V, 20 V, 30 V, 50 V, 75
V, 100 V, or 200 V.
[0139] In FIG. 12, photons of the input light 410 may be absorbed
primarily in the absorption layer 424, resulting in the generation
of electron-hole pairs (which may be referred to as photo-generated
carriers). For example, the absorption layer 424 may be configured
to absorb photons corresponding to the operating wavelength of the
lidar system 100 (e.g., any suitable wavelength between
approximately 1400 nm and approximately 1600 nm). In the avalanche
layer 422, an avalanche-multiplication process occurs where
carriers (e.g., electrons or holes) generated in the absorption
layer 424 collide with the semiconductor lattice of the absorption
layer 424, and produce additional carriers through impact
ionization. This avalanche process can repeat numerous times so
that one photo-generated carrier may result in the generation of
multiple carriers. As an example, a single photon absorbed in the
absorption layer 424 may lead to the generation of approximately
10, 50, 100, 200, 500, 1000, 10,000, or any other suitable number
of carriers through an avalanche-multiplication process. The
carriers generated in an APD 400 may produce an electrical current
that is coupled to an electrical circuit which may perform 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.
[0140] The number of carriers generated from a single
photo-generated carrier may increase as the applied reverse bias V
is increased. If the applied reverse bias V is increased above a
particular value referred to as the APD breakdown voltage, then a
single carrier can trigger a self-sustaining avalanche process
(e.g., the output of the APD 400 is saturated regardless of the
input light level). The APD 400 that is operated at or above a
breakdown voltage may be referred to as a single-photon avalanche
diode (SPAD) and may be referred to as operating in a Geiger mode
or a photon-counting mode. The APD 400 that is operated below a
breakdown voltage may be referred to as a linear APD, and the
output current generated by the APD 400 may be sent to an amplifier
circuit (e.g., a transimpedance amplifier). The receiver 140 (see
FIG. 1) may include an APD configured to operate as a SPAD and a
quenching circuit configured to reduce a reverse-bias voltage
applied to the SPAD when an avalanche event occurs in the SPAD. The
APD 400 configured to operate as a SPAD may be coupled to an
electronic quenching circuit that reduces the applied voltage V
below the breakdown voltage when an avalanche-detection event
occurs. Reducing the applied voltage may halt the avalanche
process, and the applied reverse-bias voltage may then be re-set to
await a subsequent avalanche event. Additionally, the APD 400 may
be coupled to a circuit that generates an electrical output pulse
or edge when an avalanche event occurs.
[0141] In some implementations, the APD 400 or the APD 400 along
with transimpedance amplifier have a noise-equivalent power (NEP)
that is less than or equal to 100 photons, 50 photons, 30 photons,
20 photons, or 10 photons. For example, the APD 400 may be operated
as a SPAD and may have a NEP of less than or equal to 20 photons.
As another example, the APD 400 may be coupled to a transimpedance
amplifier that produces an output voltage signal with a NEP of less
than or equal to 50 photons. The NEP of the APD 400 is a metric
that quantifies the sensitivity of the APD 400 in terms of a
minimum signal (or a minimum number of photons) that the APD 400
can detect. The NEP may correspond to an optical power (or to a
number of photons) that results in a signal-to-noise ratio of 1, or
the NEP may represent a threshold number of photons above which an
optical signal may be detected. For example, if the APD 400 has a
NEP of 20 photons, then the input beam 410 with 20 photons may be
detected with a signal-to-noise ratio of approximately 1 (e.g., the
APD 400 may receive 20 photons from the input beam 410 and generate
an electrical signal representing the input beam 410 that has a
signal-to-noise ratio of approximately 1). Similarly, the input
beam 410 with 100 photons may be detected with a signal-to-noise
ratio of approximately 5. In some implementations, the lidar system
100 with the APD 400 (or a combination of the APD 400 and
transimpedance amplifier) having a NEP of less than or equal to 100
photons, 50 photons, 30 photons, 20 photons, or 10 photons offers
improved detection sensitivity with respect to a conventional lidar
system that uses a PN or PIN photodiode. For example, an InGaAs PIN
photodiode used in a conventional lidar system may have a NEP of
approximately 10.sup.4 to 10.sup.5 photons, and the noise level in
a lidar system with an InGaAs PIN photodiode may be 10.sup.3 to
10.sup.4 times greater than the noise level in a lidar system 100
with the InGaAs APD detector 400.
[0142] Referring back to FIG. 1, an optical filter may be located
in front of the receiver 140 and configured to transmit light at
one or more operating wavelengths of the light source 110 and
attenuate light at surrounding wavelengths. For example, an optical
filter may be a free-space spectral filter located in front of APD
400 of FIG. 12. This spectral filter may transmit light at the
operating wavelength of the light source 110 (e.g., between
approximately 1530 nm and 1560 nm) and attenuate light outside that
wavelength range. As a more specific example, light with
wavelengths of approximately 400-1530 nm or 1560-2000 nm may be
attenuated by any suitable amount, such as for example, by at least
5 dB, 10 dB, 20 dB, 30 dB, or 40 dB.
[0143] Next, FIG. 13 illustrates an APD 502 coupled to an example
pulse-detection circuit 504. The APD 502 can be similar to the APD
400 discussed above with reference to FIG. 10, or can be any other
suitable detector. The pulse-detection circuit 504 can operate in
the lidar system of FIG. 1 as part of the receiver 140. Further,
the pulse-detection circuit 504 can operate in the receiver 164 of
FIG. 2, the receiver 304 of FIG. 10, or any other suitable
receiver. The pulse-detection circuit 504 alternatively can be
implemented in the controller 150, the controller 306, or another
suitable controller. In some implementations, parts of the
pulse-detection circuit 504 can operate in a receiver and other
parts of the pulse-detection circuit 504 can operate in a
controller. For example, components 510 and 512 may be a part of
the receiver 140, and components 514 and 516 may be a part of the
controller 150.
[0144] The pulse-detection circuit 504 may include circuitry that
receives a signal from a detector (e.g., an electrical current from
the APD 502) and performs current-to-voltage conversion, 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. The pulse-detection circuit 504 may
determine whether an optical pulse has been received by the APD 502
or may determine a time associated with receipt of an optical pulse
by the APD 502. Additionally, the pulse-detection circuit 504 may
determine a duration of a received optical pulse. In an example
implementation, the pulse-detection circuit 504 includes a
transimpedance amplifier (TIA) 510, a gain circuit 512, a
comparator 514, and a time-to-digital converter (TDC) 516.
[0145] The TIA 510 may be configured to receive an
electrical-current signal from the APD 502 and produce a voltage
signal that corresponds to the received electrical-current signal.
For example, in response to a received optical pulse, the APD 502
may produce a current pulse corresponding to the optical pulse. The
TIA 510 may receive the current pulse from the APD 502 and produce
a voltage pulse that corresponds to the received current pulse. The
TIA 510 may also act as an electronic filter. For example, the TIA
510 may be configured as a low-pass filter that removes or
attenuates high-frequency electrical noise by attenuating signals
above a particular frequency (e.g., above 1 MHz, 10 MHz, 20 MHz, 50
MHz, 100 MHz, 200 MHz, or any other suitable frequency).
[0146] The gain circuit 512 may be configured to amplify a voltage
signal. As an example, the gain circuit 512 may include one or more
voltage-amplification stages that amplify a voltage signal received
from the TIA 510. For example, the gain circuit 512 may receive a
voltage pulse from the TIA 510, and the gain circuit 512 may
amplify the voltage pulse by any suitable amount, such as for
example, by a gain of approximately 3 dB, 10 dB, 20 dB, 30 dB, 40
dB, or 50 dB. Additionally, the gain circuit 512 may also act as an
electronic filter configured to remove or attenuate electrical
noise.
[0147] The comparator 514 may be configured to receive a voltage
signal from the TIA 510 or the gain circuit 512 and produce an
electrical-edge signal (e.g., a rising edge or a falling edge) when
the received voltage signal rises above or falls below a particular
threshold voltage V.sub.T. As an example, when a received voltage
rises above V.sub.T, the comparator 514 may produce a rising-edge
digital-voltage signal (e.g., a signal that steps from
approximately 0 V to approximately 2.5 V, 3.3 V, 5 V, or any other
suitable digital-high level). As another example, when a received
voltage falls below V.sub.T, the comparator 514 may produce a
falling-edge digital-voltage signal (e.g., a signal that steps down
from approximately 2.5 V, 3.3 V, 5 V, or any other suitable
digital-high level to approximately 0 V). The voltage signal
received by the comparator 514 may be received from the TIA 510 or
the gain circuit 512 and may correspond to an electrical-current
signal generated by the APD 502. For example, the voltage signal
received by the comparator 514 may include a voltage pulse that
corresponds to an electrical-current pulse produced by the APD 502
in response to receiving an optical pulse. The voltage signal
received by the comparator 514 may be an analog signal, and an
electrical-edge signal produced by the comparator 514 may be a
digital signal.
[0148] The time-to-digital converter (TDC) 516 may be configured to
receive an electrical-edge signal from the comparator 514 and
determine an interval of time between emission of a pulse of light
by the light source and receipt of the electrical-edge signal. The
output of the TDC 516 may be a numerical value that corresponds to
the time interval determined by the TDC 516. In some
implementations, the TDC 516 has an internal counter or clock with
any suitable period, such as for example, 5 ps, 10 ps, 15 ps, 20
ps, 30 ps, 50 ps, 100 ps, 0.5 ns, 1 ns, 2 ns, 5 ns, or 10 ns. The
TDC 516 for example may have an internal counter or clock with a 20
ps period, and the TDC 516 may determine that an interval of time
between emission and receipt of a pulse is equal to 25,000 time
periods, which corresponds to a time interval of approximately 0.5
microseconds. Referring back to FIG. 1, the TDC 516 may send the
numerical value "25000" to a processor or controller 150 of the
lidar system 100, which may include a processor configured to
determine a distance from the lidar system 100 to the target 130
based at least in part on an interval of time determined by a TDC
516. The processor may receive a numerical value (e.g., "25000")
from the TDC 516 and, based on the received value, the processor
may determine the distance from the lidar system 100 to a target
130.
Operating a Dual-Mode Lidar System
[0149] Now referring to FIGS. 14-16, an example vehicle 550 is
equipped with a dual-mode lidar system capable of determining the
distance to a target as well as a relative velocity of the target,
i.e., the velocity of the target relative to the vehicle 550. In
each of these scenarios, the vehicle 550 generates a pulse to
determine a distance to another vehicle and then a pulse train 554
in which successive pulses are separated by equal time intervals.
The pulse train in general need not be characterized by equal time
intervals, however, as discussed in more detail below.
[0150] When the target is traveling at approximately the same speed
as the vehicle 550 in approximately the same direction, the
relative velocity of the target is approximately 0 mph. When the
vehicle 550 is travelling at 30 mph and the target is travelling in
the same direction at 40 mph, for example, the relative velocity is
+10 mph. On the other hand, when an oncoming vehicle is travelling
at 20 mph in the opposite direction, the target in this case has
the relatively velocity of -50 mph, where the negative sign
indicates that the lidar system and the target are moving toward
each other.
[0151] In the scenario of FIG. 14, a vehicle 552 is moving
substantially toward the vehicle 550. The angles at which pulses
are emitted are illustrated in FIG. 4 in an exaggerated manner for
clarity. When the vehicle 552 scatters the light in the pulse train
554, the intervals between pulses in a return pulse train 556 are
compressed in accordance with the velocity of the vehicle 552
relative to the vehicle 550.
[0152] On the other hand, a vehicle 560 is moving substantially
away from the vehicle 550 in the scenario of FIG. 15. Accordingly,
when the vehicle 560 scatters the light in the pulse train 554, the
intervals between pulses in a return pulse train 562 are expanded
in accordance with the velocity of the vehicle 560 relative to the
vehicle 550.
[0153] In the scenario of FIG. 16, a vehicle 570 is moving in a
direction substantially perpendicular to the forward-facing
direction of the vehicle 550. The vehicle 570 can be moving along a
perpendicular street when the vehicle 550 is approaching, or is
stopped at, an intersection. The vehicle 550 emits the pulse train
554 at an angle .alpha. relative to direction of the vehicle 570.
The vehicle 570 scatters the pulse train 554, and the intervals
between adjacent pulses in a return pulse train 574 are compressed.
If the frequency shift between the pulse trains 554 and 574 yields
a velocity measurement v.sub.L along a straight line between the
vehicles 550 and 570, the velocity of the vehicle 570 in the
direction of travel can be calculated as
v=v.sub.L/cos(.alpha.).
[0154] Although the targets in the example scenarios of FIGS. 14-16
are vehicles, in general the lidar system of this disclosure can
determine the velocity of any target such as a person, a bicycle,
an animal, etc.
[0155] FIG. 17 is a timing diagram of an example ranging event 600
of duration T.sub.MAX. The time T.sub.MAX can correspond to the
time it takes a pulse to travel the maximum distance R.sub.MAX and
back to the dual-mode lidar system. The dual-mode lidar system
first operates in the distance-detection mode and generates an
outbound pulse 602. The target scatters some of the light in the
outbound pulse 602, and the system detects a return signal 604. The
lidar system then detects a distance to the target based on the
timing of the return signal 604. In this case, the return signal
604 arrives at the lidar system before T.sub.MAX/2, and thus there
is sufficient time for the lidar system to operate in the
velocity-detection mode within the same ranging event. Processing
this and other conditions for transitioning to the
velocity-measurement mode is discussed in more detail with
reference to the method of FIG. 24.
[0156] The dual-mode lidar system accordingly generates a series of
pulses 610, referred to below as the "pulse train." There is an
equal separation .tau..sub.s between successive pulses in this
example, defining a constant pulse repetition frequency (PRF). The
pulse train 610 can be emitted immediately upon receipt of the
return pulse 604 or in response to other conditions, as discussed
below. The dual-mode lidar system then can detect the velocity of
the target based on the difference between .tau..sub.s and the
equal separation .tau..sub.R between successive pulses of a return
pulse train 612.
[0157] In an example implementation, the pulse train 610 includes
ten pulses having a pulse repetition frequency of at least 10 MHz.
In another implementation, the pulse train 610 includes more than
ten pulses. Each of the pulses in the pulse train 610 can have the
pulse duration of less than 10 ns, for example.
[0158] In an example scenario, the relative speed between a target
and lidar system is v=45 m/s (=100 mph). The time interval between
successive pulses in pulse burst is .tau.=1 ns; the pulse
repetition frequency is f=1/.tau.=1 GHz; and the relative distance
moved between successive pulses: .DELTA.d=(45 m/s).times.(1 ns)=45
nm. The change in time interval between successive pulses:
.DELTA..tau.=(2.times..DELTA.d)/c=0.3 fs, and the change in
frequency between transmitted and received pulses:
.DELTA.f=(.DELTA..tau.)/.tau..sup.2=300 Hz. Because it is generally
easier to detect a frequency shift of 300 Hz than a time-domain
shift of 0.3 fs, a dual-model lidar system can process
pulse-frequency characteristics in frequency domain. However, both
frequency-domain and time-domain approaches are discussed
below.
[0159] For further clarity, FIGS. 18A-C illustrate several example
patterns according to which a pulse train can be transmitted. FIG.
18A first illustrates an example pulse train 610 in which
successive pulses are separated by a constant time interval
.tau..sub.s. FIG. 18B then illustrates an example "chirped" pulse
train with increasing intervals between successive pulses,
.tau..sub.s, .tau..sub.s+.DELTA., .tau..sub.s+2.DELTA.,
.tau..sub.s+3.DELTA., etc. More generally, a chirped signal can
have an increasing PRF or a decreasing PRF, and the intervals can
change according to any suitable principle, such as an exponential
increase in time intervals between successive pulses.
[0160] Next, FIG. 18C illustrates an example pulse train with
pseudo-random intervals between successive pulses, .tau..sub.s1,
.tau..sub.s2, .tau..sub.s3, etc. In one such implementation, a
dual-mode lidar system generates pseudo-random intervals using
certain properties of mode-locked lasers such as vertical external
cavity semiconductor lasers (VECSELs) or mode-locked integrated
external-cavity surface emitting lasers (MIXSELs). As a more
specific example, a MIXSEL can produce a pseudo-random pulse train
at a very high frequency (e.g., at 0.5 GHz to 50 GHz rate) upon
activation, and the dual-mode lidar system can store the
"signature," or the particular sequence of intervals in the first N
pulses of the sequence, of an outbound pulse train. The dual-mode
lidar system then can detect whether a return pulse train includes
this signature, in its original form or as transformed due to the
Doppler effect. In another implementation, the dual-mode lidar
system is configured with a particular sequence .tau..sub.s1,
.tau..sub.s2, .tau..sub.s3, etc. defining a persistent signature.
In any case, signatures imparted to pulse trains can reduce the
probability of cross-talk and otherwise improve reliability of
detection.
[0161] Generally speaking, a dual-mode lidar system can obtain
pulse-frequency characteristics of emitted and received pulse
trains using analog and/or digital signal processing techniques.
These techniques can be implemented in hardware, firmware,
software, or any suitable combination thereof. The calculations can
be carried out in frequency domain or time domain, depending on the
particular pulse-frequency characteristics used.
[0162] FIG. 19 illustrates an example circuit 650 that a lidar
system can implement to determine the velocity of a target in the
frequency domain using a heterodyne technique. The circuit 650
includes a mixer element 656 that receives an electrical signal 657
corresponding to the series of emitted pulses (e.g., the pulse
train 610 of FIG. 17). The mixer element 656 can receive this
electrical signal from an oscillator 654, for example. The mixer
element 656 also receives an input signal 652 that corresponds to
the series of scattered pulses (e.g., the return pulse train 612 of
FIG. 17). Referring back to FIGS. 12 and 13, the input signal 652
can correspond to the output of the gain circuit 512, i.e., the
output of the APD 502 amplified by the TIA 510 and processed by the
gain circuit 512. The mixer element 656 electrically mixes these
inputs to generate a mixed output signal 659.
[0163] The output signal 659 is an electrical signal with one or
more frequencies that corresponds to the velocity of the vehicle.
In the example of FIG. 19, the output signal 659 is provided to an
analog-to-digital converter (ADC) 658 and then to an FFT module
660. The FFT module 660 can generate a velocity signal 662 that
represents the relative velocity of the target.
[0164] For clarity, FIGS. 20A and 20B illustrate the relationships
between the signals of FIG. 19 in frequency domain: FIG. 20A
illustrates the signals 657 and 652 in frequency domain, and FIG.
20B illustrates the results of frequency mixing.
[0165] In other implementations, electrical signals corresponding
to received pulse train can be processed in time domain. Generally
speaking, a signal first can be digitized using comparators, TDCs,
etc., a time or frequency characteristic of the digitized
electrical signal can be determined, and the determined time or
frequency characteristic can be compared with the corresponding
characteristic of the emitted pulse train.
[0166] FIG. 21 is a diagram of an example circuit 700 that can be
implemented in the system of FIG. 1 or FIG. 3, for example, to
impart a signature to a pulse train, record the signature of the
outbound pulse train, use the recorded signature to detect return
light corresponding to the outbound pulse train, and detect the
change in pulse-frequency characteristics to determine the velocity
of the target.
[0167] A controller 701 can provide a control signal to a light
source 702 (or, in another implementation, an optical modulator
coupled to the light source 702) to cause the light source 702 to
output a pulse train having particular pulse-frequency
characteristics. The controller 701 in general can cause the light
source 702 to output a pulse train 704 with a constant PRF, an
increasing PRF, a decreasing PRF, a pseudo-random PRF, etc.
However, the implementation of FIG. 21 is particularly suitable for
those implementations where the PRF is pseudo-random.
[0168] Some of the energy of the output pulse train 704 can be
directed to a receiver 707 by an optical element 705. The receiver
707 includes a detector 708 and a sequence processing circuit 710.
The same detector 708 and sequence processing circuit 710 may be
used to process both the generated outbound pulse train and the
return pulse train. In another implementation, separate detectors
may be used for outbound and inbound signals. Yet in another
implementation, separate detectors as well as separate sequence
processing circuits may be used to process outbound and inbound
signals.
[0169] The sequence processing circuit 710 may determine time
intervals between successive pairs of the first N peaks of the
pulse train and store these intervals in a memory element such as a
sequence value register 712. For example, referring to FIG. 22A,
the sequence processing circuit 710 can detect the rising edge and
the falling edge for each of the pulses 750A-D. The processing in
this example can rely on a single threshold value 770. Upon
detecting the sequence of values {t.sub.1R, t.sub.1F, t.sub.2R,
t.sub.2F, t.sub.3R, t.sub.3RF, . . . }, the circuit 700 can
determine the time intervals between the successive pulses by
calculating the intervals between the respective rising or falling
edges, e.g., t.sub.1R-t.sub.2R, t.sub.2R-t.sub.3R, etc. and store
these values in a sequence value register 712.
[0170] Referring back to FIG. 21, when a light signal 720 arrives
at the detector 708, the processor 701 may generate a different
control signal to instruct the sequence processing circuit 710 to
not store the determined sequence in the sequence value register
712 and instead provide the output as a determined sequence 722 to
an FFT sequence comparator 730. The FFT sequence comparator 730 may
be implemented a separate processing unit that performs comparisons
using hardware, firmware, software, or any suitable combination of
hardware, firmware, or software. In a different implementation, the
FFT sequence comparator 730 may be a software routine stored in the
memory of the processor 701. The processor 714 processes the light
signal 720 as a return pulse train corresponding to the output
pulse train 704 only if the value stored in the sequence value
register 712 matches the determined sequence 722, adjusted for the
Doppler effect. The processor 701 can output an indication 724 of
whether the determined sequence 722 corresponds to the output pulse
train 704, and an indication of velocity 726.
[0171] FIG. 23 illustrates an example digital circuit 800 that can
detect time intervals between successive pulses in a sequence such
as the sequence of FIG. 22. An edge detection circuit 802 includes
an amplifier 810A and a comparator 812A configured to trigger when
the rising edge of a pulse in a pulse train reaches the threshold
value T, and an amplifier 810B and a comparator 812B configured to
trigger when the falling edge of the pulse reaches the threshold
value T. The output of the comparator 812A is coupled to a
time-to-digital converter (TDC) 814A to detect the number of clock
cycles since time t.sub.0, and the output of the comparator 812B is
coupled to a TDC 814B to detect the number of clock cycles since
time t.sub.0. A pulse frequency characteristic detector 804 can
include a register to store a certain number of values and the
logic to reset component 802.
[0172] In operation, the edge detection circuit 802 receives an
electric signal 806, such as the pulse train of FIG. 22. After the
edge detection circuit 802 detects the rising edge of the first
pulse in the input signal 806 reaching the threshold value T as
well as the falling edge of the first pulse in the input signal 806
reaching the threshold value T, the pulse frequency characteristic
detector 804 can store the corresponding time values (e.g.,
t.sub.1R and t.sub.1F illustrated in FIG. 22) in a memory and
supply a reset signal to the edge detection circuit 802, so that
the edge detection circuit 802 can detect the timing of the rising
and the falling edges of the next pulse in the pulse train. After
processing N pulses in the train, the pulse frequency
characteristic detector 804 can output a sequence 808 in the format
{t.sub.1R, t.sub.1F, t.sub.2R, t.sub.2F, t.sub.3R, t.sub.3RF, . . .
}, {t.sub.1R-t.sub.2R, t.sub.2R-t.sub.3R, . . . }, or any other
suitable format. Referring back to FIG. 21, the digital circuit 800
can operate as the sequence processing circuit 710, for example, to
output the sequence stored in the register 712 and the sequence
722.
[0173] Next, FIG. 24 illustrates a flow diagram of an example
method 800 for determining a distance to a target and the velocity
of the target. The method 800 can be implemented in the controller
150 of FIG. 1, the controller 306 of FIG. 10, the controller 372 of
FIG. 11, or another suitable processing hardware.
[0174] At block 822, a pulse of light is emitted and directed in
accordance with the scan pattern, in the distance-determination
operational mode. The distance to the target that scattered the
emitted pulse of light is determined at block 824, using the time
when the return from the emitted pulse of light is detected. Next,
if it is determined at block 826 that one or more conditions for
transitioning to the second mode to determine the velocity has been
satisfied, the flow proceeds to block 828.
[0175] In one example implementation, the dual-mode lidar system at
block 826 determines whether the distance to the target is greater
than a certain maximum distance D.sub.MAX. The maximum distance
D.sub.MAX can be one half of the R.sub.MAX of the dual-model lidar
system, when the system determines the distance and the velocity
during the same ranging event. When the target is disposed more
than D.sub.MAX meters away, there may not be sufficient time to
send out a pulse train and receive a return signal; moreover,
targets more than D.sub.MAX meters away may not be as important as
nearby objects. More generally, the dual-mode lidar system can be
configured to measure the velocity of the target only for certain
distance ranges.
[0176] In an embodiment, a dual-mode lidar system also can detect
whether the distance to the target is less than a certain minimum
distance D.sub.MIN. If the distance to the target is less than
D.sub.MIN, the system can reduce the pulse energy of the pulses in
the pulse train. For example, when it is determined that the target
within 20 m, the system can reduce the energy to approximately 10%
of the value applied to pulse train at other distances. The
dual-mode lidar system in this manner can improve eye safety as
well as power usage, without losing so long as the pulse train has
sufficient energy for the receiver when scattered from a nearby
target.
[0177] More generally, once the distance to the target is
determined at block 824, one or more parameters of the pulse train
to be used for velocity measurement can be selected in view of the
determined distance. In addition to modifying pulse energy as
discussed above, a dual-mode lidar system can select the number of
pulses in view of the distance: for example, the system can include
fewer pulses in each pulse train when the target is closer, and
more pulses in each pulse train when the target is farther away.
Further, the system can modify the width of pulses included in the
pulse train in view of the distance so that shorter-duration pulses
are transmitted when the target is closer, for example.
[0178] In another example implementation, the dual-mode lidar
system determines whether the time remaining in the ranging event
is sufficient to determine the velocity of the target using a pulse
train. For example, the dual-mode lidar system can determine
whether at least half of the maximum ranging event remains.
[0179] Additionally or alternatively, the dual-mode lidar system at
block 826 can determine, or receive an appropriate indication from
another controller, whether the target is of a type for which
velocity determination is relevant. For example, the vehicle
controller 372 (see FIG. 11) can determine that a certain region of
interest within the FOR includes a vehicle, a pedestrian, a
bicycle, or another object of a dynamic or moving type using a
classifier. When the dual-mode lidar system scans this region of
interest, both distance and velocity measurements can be conducted.
On the other hand, when the classifier in the vehicle controller
372 determines that a certain region within the FOR includes a
building, a tree, a sign, a mailbox, a highway divider, or another
object of a static type etc., the dual-mode lidar system can
conduct only distance measurements when scanning this region.
[0180] Still further, it can be determined at block 826 whether the
vehicle in which the dual-model lidar system operates is stopped.
For example, the dual-model lidar system can be configured to
determine velocity of cross traffic when the vehicle is stopped at
an intersection. More generally, a the dual-model lidar system can
be configured to carry out velocity measurements only at certain
speeds (e.g., 0 to 20 m