System and method for increasing coherence length in lidar systems
Various implementations of the invention compensate for “phase wandering” in tunable laser sources. Phase wandering may negatively impact a performance of a lidar system that employ such laser sources, typically by reducing a coherence length/range of the lidar system, an effective bandwidth of the lidar system, a sensitivity of the lidar system, etc. Some implementations of the invention compensate for phase wandering near the laser source and before the output of the laser is directed toward a target. Some implementations of the invention compensate for phase wandering in the target signal (i.e., the output of the laser that is incident on and reflected back from the target). Some implementations of the invention compensate for phase wandering at the laser source and in the target signal.
1. A system for compensating for phase variance of a laser source comprising:
a laser output path comprising:
a delay line configured to delay an output of the laser source, and
a phase modulator configured to receive an output of the delay line and an output of a phase corrector; and
a phase error path comprising:
a phase detector configured to determine an estimated phase error of the laser source based on not greater than one half of the coherence time of the laser source, and
the phase corrector configured to receive the output of the phase detector and determine a phase correction as the output of the phase corrector, wherein the phase modulator outputs a phase corrected laser output.
2. The system of claim 1 , wherein the phase detector is an optical phase detector.
3. The system of claim 2 , wherein the optical phase detector comprises:
a splitter configured to receive the output of the laser source and output a first output and a second output;
a delay line configured to delay the first output; and
a phase detector configured to determine a phase difference between the delayed first output and the second output,
wherein the optical phase detector outputs the phase error derived from the phase difference as the estimated phase error.
4. The system of claim 1 , wherein the phase corrected laser output has an increased coherence length over the output of the laser source.
5. The system of claim 1 , wherein the phase detector configured to determine an estimated phase error of the laser source based on not greater than one half of the coherence time of the laser source comprises a phase detector configured to determine the estimated phase error of the laser source based on not greater than one third of the coherence time of the laser source.
6. A method for increasing coherence length of a lidar system comprising:
delaying an output of a laser source utilized by the lidar system, wherein the output of the laser source includes a phase variance, and wherein the output of the laser source is delayed by an amount not greater than one half of a coherence time of the laser source;
determining a phase difference between the output of the laser source and the delayed output of the laser source;
estimating a phase error correction to correct the phase variance in the output of the laser source based on the phase difference;
modulating the output of the laser source based on the estimated phase error correction; and
outputting the modulated output of the laser source as a phase corrected laser output.
7. The method of claim 6 , further comprising delaying an output of the laser source by a second delay, and wherein modulating the output of the laser source based on the estimate phase error correction comprises modulating the output of the laser source after the second delay based on the estimated phase error correction.
8. The method of claim 6 , wherein the output of the laser source is delayed by an amount not greater than one third of the coherence time of the laser source.
9. A system for increasing coherence length in a lidar system having a laser source, the system comprising:
a laser output path comprising a phase modulator configured to receive an output of the laser source, to receive a phase correction, and to output a phase corrected laser output; and
a phase error path comprising:
a phase detector configured to determine an estimated phase error of the output of the laser source by an amount not greater than one half of a coherence time of the laser source, and
the phase corrector configured to receive the estimated phase error, to determine the phase correction based on the estimated phase error, and to output the phase correction.
10. The system of claim 9 , wherein the phase detector configured to determine an estimated phase error of the output of the laser source by an amount not greater than one half of a coherence time of the laser source comprises a phase detector configured to determine the estimated phase error of the output of the laser source by an amount not greater than one third of the coherence time of the laser source.