Method and system for enhanced velocity resolution and signal to noise ratio in optical phase-encoded range detection

1. A method of determining a range of an object to a vehicle, the method comprising:

receiving an electrical signal generated by mixing a first optical signal and a second optical signal, wherein the first optical signal is generated by modulating an optical signal and wherein the second optical signal is received in response to transmitting the first optical signal toward the object;

determining a Doppler frequency shift of the second optical signal;

generating a corrected electrical signal by removing the Doppler frequency shift from the electrical signal; and

determining the range to the object based on a cross correlation of the corrected electrical signal with a radio frequency (RF) signal that is associated with the first optical signal.

2. The method as recited in claim 1 , further comprising:

determining an expected phase that depends on the Doppler frequency shift and the range.

3. The method as recited in claim 2 , further comprising:

determining a sign of the Doppler frequency shift based on the expected phase for a given range and an observed phase for the given range based on the cross correlation.

4. The method as recited in claim 3 , further comprising:

operating the vehicle based on the range and the sign of the Doppler frequency shift.

5. The method as recited in claim 1 , further comprising:

determining a spectrum over a first time interval of a first duration of the electrical signal, wherein the Doppler frequency shift is determined based on the spectrum.

6. The method as recited in claim 5 , wherein the spectrum is a cross spectrum of an in-phase component of the electrical signal and a quadrature component of the electrical signal.

7. The method as recited in claim 5 , wherein at least two successive time periods of the first duration overlap in time.

8. The method as recited in claim 5 , further comprising:

determining the cross correlation, over a second time interval of a second duration, between (i) a first Fourier transform of the RF signal and (ii) a second Fourier transform of the corrected electrical signal.

9. The method as recited in claim 8 , wherein the first optical signal is generated by modulating a code that indicates a sequence of phases for a phase encoded signal with reference to an optical signal.

10. The method as recited in claim 9 , wherein the first duration is longer than the second duration or wherein the first duration is an integer multiple greater than 1 of a duration of the code.

11. The method as recited in claim 9 , wherein the second duration is about equal to the duration of the code.

12. The method as recited in claim 9 , wherein the vehicle comprises a detector that generates the electrical signal and a laser that generates the optical signal.

13. The method as recited in claim 12 , further comprising:

determining the first duration based on at least one of a speed of the vehicle relative to a surface external to the vehicle, a location of the vehicle relative to the surface external to the vehicle, and a scan angle relative to the surface external to the vehicle.

14. A light detection and ranging (LIDAR) system, the LIDAR system comprising:

one or more processors; and one or more computer-readable storage mediums storing instructions which, when executed by the one or more processors, cause the one or more processors to:

receive an electrical signal generated by mixing a first optical signal and a second optical signal, wherein the first optical signal is generated by phase-modulating an optical signal and wherein the second optical signal is received in response to transmitting the first optical signal toward an object;

determine a spectrum over a first duration of the electrical signal;

determine a Doppler frequency shift of the second optical signal based on the spectrum;

generate a corrected electrical signal by removing the Doppler frequency shift from the electrical signal; and

determine a range to the object based on a cross correlation over the first duration of the corrected electrical signal and a second duration of a phase-encoded radio frequency (RF) signal associated with the first optical signal, the first duration is different from the second duration.

15. An autonomous vehicle control system comprising:

one or more processors; and one or more computer-readable storage mediums storing instructions which, when executed by the one or more processors, cause the one or more processors to:

receive an electrical signal generated by mixing a first optical signal and a second optical signal, wherein the first optical signal is generated by modulating an optical signal and wherein the second optical signal is received in response to transmitting the first optical signal toward an object;

determine a Doppler frequency shift of the second optical signal;

generate a corrected electrical signal by removing the Doppler frequency shift from the electrical signal;

determine a range to the object based on a cross correlation of the corrected electrical signal with a radio frequency (RF) signal that is associated with the first optical signal; and

operate an autonomous vehicle based on the range to the object.

16. The autonomous vehicle control system as recited in claim 15 , wherein the one or more computer-readable storage mediums store instructions that cause the one or more processors to further:

determine an expected phase that depends on the Doppler frequency shift and the range.

17. The autonomous vehicle control system as recited in claim 16 , wherein the one or more computer-readable storage mediums store instructions that cause the one or more processors to further:

determine a sign of the Doppler frequency shift based on the expected phase for a given range and an observed phase for the given range based on the cross correlation.

18. The autonomous vehicle control system as recited in claim 17 , wherein the one or more computer-readable storage mediums store instructions that cause the one or more processors to further:

operate the vehicle based on the range and the sign of the Doppler frequency shift.

19. The autonomous vehicle control system as recited in claim 15 , further comprising:

a laser that provides the optical signal;

a modulator that modulates the optical signal to generate the first optical signal;

an optical mixer that mixes the first optical signal with the second optical signal to generate a mixed optical signal; and

an optical detector that detects the mixed optical signal to generate the electrical signal from the mixed optical signal.

20. The autonomous vehicle control system as recited in claim 15 , wherein the autonomous vehicle comprises the autonomous vehicle control system.