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 a phase variance at a laser source comprising:
a phase difference detector configured to receive a reference arm signal and to detect a phase difference of the reference arm signal, the phase difference corresponding to a difference in the phase of the reference arm signal at two points in time;
a phase correction estimator configured to receive the phase difference from the phase detector and to estimate a phase correction to be applied to a target arm signal;
a phase modulator configured to receive the phase correction from the phase correction estimator, to modulate the target arm signal with the phase correction, and to output a phase corrected target arm signal, wherein the phase correction compensates the target arm signal for the phase variance of the laser source; and
a delay coupled either to either the reference arm signal and configured to introduce a delay time into the reference arm signal prior to being received by the phase difference detector or to the target arm signal prior to being received by the phase modulator, wherein
the delay time comprises at least a round trip path delay.
2. The system of claim 1 , wherein the delay is coupled to the target arm signal prior to being received by the phase modulator.
3. The system of claim 1 , wherein the delay time is adjustable.
4. A method for compensating for a phase variance of a laser source, the method comprising:
determining, via a phase difference detector, a phase difference of a reference arm signal, the phase difference corresponding to a difference in the phase of the reference arm signal at two points in time;
estimating a phase correction from the phase difference, the phase correction to be applied to a target arm signal to compensate for the phase variance of the laser source;
modulating, via a phase modulator, the target arm signal with the phase correction to produce a phase corrected target arm signal which compensates the target arm signal for the phase variance of the laser source; and
either delaying the reference arm signal by a delay time prior to the determining or delaying the target arm signal by the delay time prior to the modulating.
5. The method of claim 4 , wherein the delay time is adjustable.