U.S. patent application number 10/355360 was filed with the patent office on 2004-08-05 for laser range finding apparatus.
Invention is credited to Campbell, Blair F., Muenter, Steven E..
Application Number | 20040150810 10/355360 |
Document ID | / |
Family ID | 32770512 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040150810 |
Kind Code |
A1 |
Muenter, Steven E. ; et
al. |
August 5, 2004 |
LASER RANGE FINDING APPARATUS
Abstract
The laser range finding apparatus includes an optical relaxation
oscillator assembly, an outcoupling optics, a photodetector and a
controller. The optical relaxation oscillator assembly produces
relaxation oscillations. The relaxation oscillations are a series
of optical pulses having a controllable repetition rate. The
outcoupling optics receives the series of optical pulses and
redirects a minor portion of the energy of the series of optical
pulses. A major portion of the energy of the series of optical
pulses is adjusted in accordance with first desired beam
propagation parameters. A photodetector receives the minor portion
and converts the minor portion to an electrical signal
representative of the series of optical pulses. A controller
receives the electrical signal and determines the repetition period
between the optical pulses. The controller provides a controller
output to the optical relaxation oscillator assembly for adjusting
the controllable repetition rate of the series of optical pulses
produced by the optical relaxation oscillator assembly. During
operation, the major portion of the energy of the series of optical
pulses is directed to a reflecting target, reflected therefrom,
collected by the outcoupling optics, and directed back to the
optical relaxation oscillator assembly to stimulate subsequent
relaxation oscillations, thus locking the period of the relaxations
oscillations to the time of flight of the roundtrip path between
the laser finding apparatus and the reflecting target.
Inventors: |
Muenter, Steven E.; (Van
Nuys, CA) ; Campbell, Blair F.; (Malibu, CA) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
32770512 |
Appl. No.: |
10/355360 |
Filed: |
January 31, 2003 |
Current U.S.
Class: |
356/5.01 |
Current CPC
Class: |
G01S 7/484 20130101 |
Class at
Publication: |
356/005.01 |
International
Class: |
G01C 003/08 |
Claims
What is claimed and desired to be secured by Letters Patent of the
United States is:
1. A laser range finding apparatus, comprising: a) an optical
relaxation oscillator assembly for producing relaxation
oscillations, said relaxation oscillations being a series of
optical pulses having a controllable repetition rate; b) an
outcoupling optics for receiving said series of optical pulses and
redirecting a minor portion of the energy of said series of optical
pulses, a major portion of the energy of said series of optical
pulses being adjusted in accordance with first desired beam
propagation parameters; c) a photodetector for receiving said minor
portion and converting said minor portion to an electrical signal
representative of said series of optical pulses; and, d) a
controller for receiving said electrical signal and determining the
repetition period between said optical pulses, said controller
providing a controller output to said optical relaxation oscillator
assembly for adjusting said controllable repetition rate of said
series of optical pulses produced by said optical relaxation
oscillator assembly, wherein during operation said major portion of
said energy of said series of optical pulses is directed to a
reflecting target, reflected therefrom, collected by said
outcoupling optics and directed back to said optical relaxation
oscillator assembly to stimulate subsequent relaxation
oscillations, thus locking the period of said relaxation
oscillations to the time of flight of the roundtrip path between
the laser finding apparatus and the reflecting target.
2. The laser range finding apparatus of claim 1, wherein said
optical relaxation oscillator assembly, comprises: a) a power
source; and, b) a laser gain cavity operably associated with said
power source for producing said relaxation oscillations.
3. The laser range finding apparatus of claim 1, wherein said
optical relaxation oscillator assembly, comprises: a) a
controllable power source for pumping a gain medium of said optical
relaxation oscillator assembly, providing a means for controlling a
timing profile of a population inversion within said gain medium;
and, b) a laser gain cavity operably associated with said power
source for producing said relaxation oscillations, said laser gain
cavity operating in a perturbed mode to induce relaxation
oscillations at a repetition rate influenced by said power
source.
4. The laser range finding apparatus of claim 1, wherein said
outcoupling optics allows for the injection of photons to induce
said relaxation oscillations.
5. The laser range finding apparatus of claim 1, wherein said
outcoupling optics, comprises: a) a beamsplitter for said
redirecting of said minor portion; and, b) beam directing optics
for providing said adjustment of said major portion.
6. The laser range finding apparatus of claim 1, wherein said
controller comprises means for determining when said repetition
period results from said locking.
7. The laser range finding apparatus of claim 1, wherein said
controller comprises means for determining when said repetition
period results from said locking, by identifying a discontinuity in
the relationship between pump power and the oscillation period,
said discontinuity being defined by a oscillation period of
substantially constant value over a small range of pump power.
8. The laser range finding apparatus of claim 1, wherein said
controller comprises means for measuring said repetition
period.
9. The laser range finding apparatus of claim 1, wherein said
optical relaxation oscillator assembly, comprises: a) a power
source, said power source comprising a controllable electric
source; and, b) a laser gain cavity operably associated with said
power source for producing said relaxation oscillations.
10. The laser range finding apparatus of claim 1, wherein said
optical relaxation oscillator assembly, comprises: a) a power
source, said power source comprising an optical source; and, b) a
laser gain cavity operably associated with said power source for
producing said relaxation oscillations.
11. The laser range finding apparatus of claim 1, wherein said
optical relaxation oscillator assembly, comprises: a) a power
source, said power source comprising an optical source; and, b) a
laser gain cavity operably associated with said power source for
producing said relaxation oscillations.
12. The laser range finding apparatus of claim 1, wherein said
optical relaxation oscillator assembly, comprises: a) a power
source, said power source comprising a radio frequency (rf) source;
and, b) a laser gain cavity operably associated with said power
source for producing said relaxation oscillations.
13. A method for finding the range of a reflecting target,
comprising the steps of: a) producing relaxation oscillations, said
relaxation oscillations being a series of optical pulses at a
controllable repetition rate; b) receiving said series of optical
pulses and redirecting a minor portion of the energy of said series
of optical pulses, a major portion of the energy of said series of
optical pulses being adjusted in accordance with first desired beam
propagation parameters and being directed to a reflecting target,
said optical pulses being reflected therefrom; c) receiving said
minor portion and converting said minor portion to an electrical
signal representative of said series of optical pulses; d)
receiving said electrical signal, determining the repetition period
between said optical pulses and adjusting said controllable
repetition rate of said series of optical pulses; e) collecting
said reflected optical pulses to stimulate subsequent relaxation
oscillations; f) locking the period of said relaxation oscillations
to the time of flight of the roundtrip path between the laser
finding apparatus and the reflecting target; and, g) analyzing the
relationship between pump power and the relaxation oscillation
period for determining a period of locked oscillation, thus
determining the time of flight of the roundtrip path between the
laser finding apparatus and the reflecting target.
14. The method of claim 13, wherein said step of producing
relaxation oscillations, comprises the steps of: a) providing a
power source; and b) providing a laser gain cavity operably
associated with said power source for producing said relaxation
oscillations.
15. The method of claim 13, wherein said step of producing
relaxation oscillations, comprises: pumping a gain medium of an
optical relaxation oscillator assembly and controlling a timing
profile of a population inversion within said gain medium.
16. The method of claim 13, wherein said step of collecting said
reflected optical pulses comprises the step of injecting photons to
induce said subsequent relaxation oscillations.
17. The method of claim 13, wherein said step of analyzing said
relationship comprises identifying a discontinuity in the
relationship between pump power and the oscillation period, said
discontinuity being defined by a oscillation period of
substantially constant value over a small range of pump power.
18. A laser range finding apparatus, comprising: a) an optical
relaxation oscillator assembly for producing relaxation
oscillations, said relaxation oscillations being a series of
optical pulses having a controllable repetition rate, said optical
relaxation oscillator, comprising: a. a power source; and, b. a
laser gain cavity operably associated with said power source for
producing said relaxation oscillations; b) an outcoupling optics
for receiving said series of optical pulses and redirecting a minor
portion of the energy of said series of optical pulses, a major
portion of the energy of said series of optical pulses being
adjusted in accordance with first desired beam propagation
parameters; c) a photodetector for receiving said minor portion and
converting said minor portion to an electrical signal
representative of said series of optical pulses; and, d) a
controller for receiving said electrical signal and determining the
repetition period between said optical pulses, said controller
providing a controller output to said optical relaxation oscillator
assembly for adjusting said controllable repetition rate of said
series of optical pulses produced by said optical relaxation
oscillator assembly, wherein during operation said major portion of
said energy of said series of optical pulses is directed to a
reflecting target, reflected therefrom, collected by said
outcoupling optics, and directed back to said optical relaxation
oscillator assembly to stimulate subsequent relaxation
oscillations, thus locking the period of said relaxation
oscillations to the time of flight of the roundtrip path between
the laser finding apparatus and the reflecting target, said
controller determining when said repetition period results from
said locking, by identifying a discontinuity in the relationship
between pump power and the oscillation period, said discontinuity
being defined by a oscillation period having a substantially
constant value over a small range of pump power.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to laser range finding systems
and more particularly to the detection of return pulses utilizing
properties inherent to a laser cavity undergoing relaxation
oscillations.
[0003] 2. Description of the Related Art
[0004] Laser range finders operate on the principle of measuring
the time of flight of an intense, short duration pulse of energy
from the time it is produced by a transmitter assembly to the time
the reflected pulse from the downrange target is detected by a
receiver assembly. Since the speed of light is a known constant,
the time of flight of the pulse can be used to calculate the
distance to the downrange target. Laser range finders typically
consist of a collection of the following subassemblies: transmitter
assembly, receiver assembly and controller assembly.
[0005] Presently, many implementations exist for a transmitter
assembly to produce the desired high intensity, short duration
pulse of energy such as flashlamp pumping or Q-switching of the
laser cavity. Beam forming and directing optics are used to focus
the pulse on the downrange target. Characteristics of the
transmitted pulse, such as temporal profile, spatial profile and
wavelength, are preserved in the reflected pulse and may therefore
be used to differentiate the reflected pulse from background or
other interfering sources. The components of the transmitter
assembly are often expensive, bulky and sensitive to misalignment.
It would be desireable to eliminate many of these components while
still retaining the functionality of the transmitter assembly.
[0006] The function of the receiver assembly is to collect the
energy from the reflected pulse and detect its time of arrival.
This is typically implemented using beam collecting optics to focus
the incoming pulse on a photodetector such as a photomultiplier
tube or a semiconductor photodiode. The reflected pulse from the
downrange target is greatly attenuated due to such effects as
atmospheric absorption and scatter, range to the target, diffuse
scattering of the reflected pulse from the target and low
reflectivity of the target. The peak intensity of the transmitted
pulse must be great enough to insure detection of the attenuated
return pulse by the receiver assembly under the most stressing
conditions. The receiver assembly must also accommodate a wide
dynamic range of reflected pulse intensities due to the fact that
the intensity of the short time-of-flight return pulse from nearby
targets is greater than the long time-of-flight pulses from distant
targets. A desirable feature of the receiver assembly is the
ability to increase the sensitivity of the receiver detector as a
function of time-of-flight synchronized to the timing of the
transmitted pulses.
[0007] The receiving assembly must also discriminate the return
pulse from background interfering sources. The beam collection
optics limits the field of view of the detector to the region
illuminated by the transmitting assembly. This requires careful
alignment of the receiver optics to the transmitter optics. It is
more desirable to use the same optical system for both functions,
however, the backscattering and retroreflections of the transmitted
pulse from the optics may appear with great intensity at the
receiver detector, which may result in saturation of the
detector.
[0008] To further aid the receiver assembly in discriminating the
return pulse, narrow band optical filters are used to reject
signals that do not match the wavelength of the transmitted pulse.
These filters can be costly and may require precise alignment. It
would be desirable if the detector were inherently sensitive to
only the same wavelength as the transmitted pulse.
[0009] The generation of short optical pulses with long repetition
rates using electronic regeneration techniques in laser diodes is
disclosed by Hung-Tser Lin and Yao-Huang Kao in their article
entitled "A Possible Way for Low-power Short Distance Optical Range
Detector Using Regenerative Gain-Switched Laser Diode" from the
IEEE Lasers and Electro-Optics Society 1996 Annual Meeting
Conference Proceedings. However, the pulse regeneration method
described uses electronic means to sense the output pulse and
modulate the power to the laser diode to induce oscillations. No
direct optical feedback is employed in this method.
[0010] U.S. Pat. No. 4,928,152, issued to Jean-Pierre Gerardin,
discloses an apparatus in which the optical signal issued from a
laser cavity is reflected by a target and re-injected into the same
laser cavity using the same collimating and focusing optics. The
purpose of this configuration is to produce heterodyne beat signals
as the CW laser diode is frequency modulated. This apparatus uses
interferometery to determine distance, rather than measurement of
the time-of-flight of an optical pulse.
[0011] U.S. Pat. No. 5,359,404, issued to Jeremy G. Dunne,
discloses a laser rangefinder which determines the time-of-flight
of an infrared laser pulse reflected from a downrange target. This
apparatus is inherently sensitive to interfering signal sources and
therefore requires additional means for the detection and
discrimination of the return pulse. A digital logic-operated gate
for the "opening" and "closing" of a time window is required in the
optical receiver for the purpose of rejecting interfering optical
signal sources, such as internal reflections and atmospheric
backscatter. Further filtering is provided by a narrow band
interference filter tuned to the wavelength of the emitted laser
pulse. Additionally, separate collimating and focusing optics are
used in the transmitting and receiving portions of the
apparatus.
OBJECTS AND SUMMARY OF THE INVENTION
[0012] It is a principal object of the present invention to provide
the necessary functionality for a laser range finding apparatus
while significantly reducing the number of components and
subsequent cost and complexity by utilizing inherent properties of
a laser cavity.
[0013] It is another object of the invention to generate optical
pulses, suitable for the purposes of range finding, by utilizing a
laser cavity in a perturbed mode to induce relaxation
oscillations.
[0014] It is another object of the invention to minimize the number
of optical components by utilizing the same beam conditioning and
directing optical assembly for both the transmission of the optical
pulse and the collection of the return pulse from the downrange
target.
[0015] It is another object of the invention to eliminate the
high-gain photodetector amplifier electronics in the receiver
assembly by optically amplifying the collected return pulse
utilizing the gain medium of the same laser cavity used to produce
the outgoing pulse.
[0016] The present invention is a laser range finding apparatus. In
a broad aspect it includes an optical relaxation oscillator
assembly, outcoupling optics, a photodetector and a controller. The
optical relaxation oscillator assembly produces relaxation
oscillations. The relaxation oscillations are a series of optical
pulses having a controllable repetition rate. The outcoupling
optics receives the series of optical pulses and redirects a minor
portion of the energy of the series of optical pulses. A major
portion of the energy of the series of optical pulses is adjusted
in accordance with first desired beam propagation parameters. A
photodetector receives the minor portion and converts the minor
portion to an electrical signal representative of the series of
optical pulses. A controller receives the electrical signal and
determines the repetition period between the optical pulses. The
controller provides a controller output to the optical relaxation
oscillator assembly for adjusting the controllable repetition rate
of the series of optical pulses produced by the optical relaxation
oscillator assembly. During operation, the major portion of the
energy of the series of optical pulses is directed to a reflecting
target, reflected therefrom, collected by the outcoupling optics,
and directed back to the optical relaxation oscillator assembly to
stimulate subsequent relaxation oscillations, thus locking the
period of the relaxations oscillations to the time of flight of the
roundtrip path between the laser finding apparatus and the
reflecting target.
[0017] The present invention eliminates the need for variable gain
control of the receiving assembly detector by utilizing the
inherent time varying gain property of the relaxation oscillator.
The relaxation oscillator provides variable optical amplification
as a function of time, synchronized to the time of transmission of
the outgoing optical pulse. The amplification is at a minimum after
the generation and transmission of the outgoing pulse, thereby
preventing amplification and detection of backscatter and retro
reflections from the optical assembly. The optical amplification
monotonically increases, thereby providing higher gain for
typically less intense, longer time-of-flight return pulses from
more distant targets.
[0018] The present invention eliminates the need for optical
filters by utilizing the inherent narrow bandwidth amplification of
the laser cavity to amplify only return pulses with a wavelength
that is mode matched with the laser cavity. Since the same laser
cavity is used for generation of the outgoing pulse, the return
pulse is inherently mode matched with the laser cavity. This
extremely narrow band amplification effectively filters out all
out-of-band background noise and interference sources.
[0019] The present invention eliminates the requirement for a large
dynamic range of the photodetector amplifier used to detect the
return pulses. The return optical pulses are used to seed the
subsequent relaxation oscillation in the laser cavity. The
intensity of the optical pulse produced by the relaxation
oscillation is independent of the intensity of the seed pulse. The
photodetector detects the seeded pulses which are of uniform
intensity.
[0020] Other objects, advantages and novel features will become
apparent from the following detailed description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
[0021] FIG. 1 is a schematic representation of the laser range
finding apparatus of the present invention.
[0022] FIG. 2 is a graph of pump power vs. oscillation period
indicating the seeded oscillation region.
[0023] FIGS. 3a-3c are three graphs relating to the instance where
the pump power is too low for seeded oscillation. FIG. 3a
illustrates population inversion as a function of time. FIG. 3b
illustrates laser output as a function of time. FIG. 3c illustrates
target return as a function of time.
[0024] FIGS. 4a-4c are three graphs relating to the instance where
the pump power is correct for seeded oscillation. FIG. 4a
illustrates population inversion as a function of time. FIG. 4b
illustrates laser output as a function of time. FIG. 4c illustrates
target return as a function of time.
[0025] FIGS. 5a-5c are three graphs relating to the instance where
the pump power is too high for seeded oscillation. FIG. 5a
illustrates population inversion as a function of time. FIG. 5b
illustrates laser output as a function of time. FIG. 5c illustrates
target return as a function of time.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Referring now to the drawings and the characters of
reference marked thereon, FIG. 1 illustrates a preferred embodiment
of the laser finding apparatus of the present invention, designated
generally as 10. The laser finding apparatus 10 includes an optical
relaxation oscillator assembly, designated generally as 12, for
producing relaxation oscillations. The relaxation oscillations
produced are a series of optical pulses at a controllable
repetition rate. The optical relaxation oscillator assembly 12
preferably includes a power source 14 and a laser gain cavity 16
operably associated with the power source 14 for producing the
relaxation oscillations. The power source pumps a gain medium of
the optical relaxation oscillator assembly, providing a means for
controlling a timing profile of a population inversion within the
gain medium. The laser gain cavity 16 operates in a perturbed mode
to induce relaxation oscillations at a repetition rate influenced
by the power source 14. The power source 14 can be, for example, a
controllable electric source, an optical source or radio frequency
(rf) source.
[0027] Outcoupling optics 18 receives the series of optical pulses
from the optical relaxation oscillator assembly 12 and redirects a
minor portion of the energy of the series of optical pulses. A
major portion of the energy of the series of optical pulses is
adjusted in accordance with first desired beam propagation
parameters.
[0028] The outcoupling optics 18 preferably includes a beamsplitter
20 for redirecting the minor portion of the energy and beam
directing optics 22 for providing the adjustment of the major
portion. The first desired beam propagation parameters may include,
for example, collimation and focus.
[0029] The outcoupling optics 18 allows for the injection of
photons to induce the relaxation oscillations.
[0030] A photodetector 24 receives the minor portion and converts
that minor portion to an electrical signal representative of the
series of optical pulses. The photodetector 24 used may be, for
example, a semiconductor photodiode or a phototube.
[0031] A controller 26 receives the electrical signal from the
photodetector 24 and determines the repetition period between the
optical pulses. The controller 26 provides a controller output to
the optical relaxation oscillator assembly 12 for adjusting the
controllable repetition rate of the series of optical pulses
produced by the optical relaxation oscillator assembly 12. The
controller 26 includes means for determining when the repetition
period results from the locking. Additionally, it includes means
for measuring the repetition period. The controller 26 may be, for
example, a microcontroller, FPGA (field programmable gate array) or
an ASIC (application specific integrated circuit) along with a
precision timebase such as a crystal oscillator.
[0032] Referring now to FIG. 2, the relationship between the pump
power to the laser and the relaxation oscillation period can be
seen. In the absence of seed pulses inducing locking of the
relaxation oscillator, the oscillation period decreases with
increasing pump power. In the region where the relaxation
oscillation period is locked, due to the seeding of the laser
cavity, the relaxation oscillation period remains at a constant
value over a small range of pump power, as shown by the dashed
lines. The function of the controller 26 is to identify this
discontinuity in the relationship between the pump power and the
oscillation period, hence determining the period of the locked
oscillation and therefore the round-trip time of flight of the
optical pulse to the downrange target. There are multiple methods
for determining the presence of the discontinuity discussed above.
For example, the controller may gradually increase the pump power
from an initial low power to a high power while it simultaneously
measures the relaxation oscillation period. The controller may
mathematically identify the discontinuities in the relationship of
the oscillation period to the pump power.
[0033] Referring to FIG. 3a, a low pump power causes a slow rise in
the population inversion until the population inversion exceeds the
lasing threshold, defined as the point when the overall gain of the
cavity exceeds unity. A further rise in the population inversion
monotonically increases the cavity gain up to the point when a
pulse is spontaneously emitted from the cavity. A seed pulse is
injected into the cavity during the time that the population
inversion is above threshold, but before the occurrence of a
spontaneous emission pulse will induce a subsequent pulse from the
cavity. This period of time identified in FIG. 3a as the "Seeded
Emission Region" performs the function of range gating any return
pulses injected into the cavity. The distance from the laser
rangefinding apparatus to the reflecting target must lie within the
narrow distance range such that the time of flight of a transmitted
pulse results in its reception and injection into the laser cavity
during the seeded emission region period.
[0034] FIG. 3b shows the optical output as the cavity spontaneously
emits a pulse and extracts energy from the population inversion. As
the pump power again increases the population inversion, the first
spontaneous pulse is reflected from the downrange target and this
seed pulse is injected into the laser cavity.
[0035] FIG. 3c shows that the time of flight of this pulse is
shorter than the population inversion build up time required for
the laser cavity to reach lasing threshold. The seed pulse is
therefore unable to induce the subsequent output pulse from the
cavity. This condition indicates that the distance to the
reflecting target is too short to produce locked oscillations at
the selected pump power.
[0036] FIG. 4a shows the operation of the relaxation oscillation
when the pump power is adjusted to allow locked oscillation. Again,
a spontaneous pulse is emitted from the laser cavity. The return
seed pulse is injected into the cavity at a time when the
population inversion is greater than threshold, but before the
spontaneous build up time of the cavity shown by dashed lines. The
timing of the seed pulse is seen in FIG. 4c. The timing of the
seeded output from the cavity matches the injected seed pulse as
shown in FIG. 4b. This cycle is repeated for subsequent locked
pulses.
[0037] Finally, FIG. 5a shows the operation of the relaxation
oscillation when the pump power is too high for locked oscillation.
The seed pulse, resulting from the first spontaneous pulse, as
shown in FIG. 5c, is injected into the cavity after the cavity has
generated the subsequent spontaneous pulse shown in FIG. 5b. Since
the population inversion is below lasing threshold at the time the
seed pulse is injected, the seed pulse does not induce a subsequent
output pulse from the cavity. This condition indicates that the
distance to the reflecting target is too long to produce locked
oscillations at the selected pump power.
[0038] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is,
therefore, to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described.
* * * * *