Method and system for laser phase tracking for internal reflection subtraction in phase-encoded lidar

1. An autonomous vehicle control system comprising:

one or more processors; and

one or more computer-readable storage mediums storing instructions that are operable, when executed by the one or more processors, cause the one or more processors to perform operations including:

receiving a plurality of electrical signals that are respectively associated with (i) a plurality of optical signals provided by a laser and (ii) a plurality of returned optical signals that are responsive to the plurality of optical signals provided by the laser;

determining a phase difference between a first electrical signal of the plurality of electrical signals and a second electrical signal;

determining an internal reflection signal based on the phase difference;

determining a range to an object by adjusting a third electrical signal of the plurality of electrical signals using the internal reflection signal; and

operating a vehicle based on the determined range to the object.

2. The autonomous vehicle control system of claim 1 , wherein the operations further comprises:

adjusting the first electrical signal of the plurality of electrical signals based on the phase difference.

3. The autonomous vehicle control system of claim 2 , wherein the one or more processors are further configured to:

determine, responsive to adjusting the first electrical signal, the internal reflection signal by averaging the plurality of electrical signals.

4. The autonomous vehicle control system of claim 1 , further comprising:

a modulator that is configured to modulate the plurality of optical signals provided by the laser to generate a plurality of modulated optical signals prior to the one or more processors receiving the plurality of electrical signals.

5. The autonomous vehicle control system of claim 4 , further comprising:

one or more optical components; and

an optical coupler that is configured to transmit the plurality of modulated optical signals through the one or more optical components for a first plurality of measurements, and receive the plurality of returned optical signals for the first plurality of measurements.

6. The autonomous vehicle control system of claim 5 , further comprising:

an optical mixer that is configured to generate a plurality of mixed optical signals for the first plurality of measurements; and

an optical detector that is configured to generate, based on the plurality of mixed optical signals, a respective electrical signal of the plurality of electrical signals for each of the first plurality of measurements.

7. The autonomous vehicle control system of claim 1 , wherein the phase difference is a complex number representing an angle in a range between −π and π.

8. The autonomous vehicle control system of claim 1 , wherein in determining the phase difference, the one or more processors are further configured to:

compute a cross correlation between the first electrical signal and the second electrical signal; and

determine the phase difference based on a time lag at which the cross correlation has a maximum absolute value.

9. The autonomous vehicle control system of claim 1 , wherein in determining the phase difference, the one or more processors are further configured to:

shift the first electrical signal to generate a shifted first electrical signal based on a time lag;

compute a dot product between the shifted first electrical signal and the second electrical signal; and

determine the phase difference based the dot product.

10. The autonomous vehicle control system of claim 1 , wherein the one or more processors are further configured to:

adjust the third electrical signal of the plurality of electrical signals by subtracting the internal reflection signal from the third electrical signal.

11. A method comprising:

receiving, by one or more processors, a plurality of electrical signals associated with a (i) plurality of optical signals provided by a laser and (ii) a plurality of returned optical signals that are responsive to the plurality of optical signals provided by the laser;

determining, by one or more processors, a phase difference between a first electrical signal of the plurality of electrical signals and a second electrical signal;

determining, by one or more processors, an internal reflection signal based on the phase difference;

determining, by one or more processors, a range to an object by adjusting a third electrical signal of the plurality of electrical signals using the internal reflection signal; and

operating, by one or more processors, a vehicle based on the determined range to the object.

12. The method of claim 11 , further comprising:

adjusting, by the one or more processors, the first electrical signal of the plurality of electrical signals based on the phase difference; and

determining, by the one or more processors responsive to adjusting the first electrical signal, the internal reflection signal by averaging the plurality of electrical signals.

13. The method of claim 11 , wherein a modulator modulates the plurality of optical signals provided by the laser to generate a plurality of modulated optical signals prior to the one or more processors receiving the plurality of electrical signals.

14. The method of claim 13 , wherein an optical coupler transmits the plurality of modulated optical signals through the one or more optical components for each of a first plurality of measurements and receives each of the plurality of returned optical signals for each of the first plurality of measurements, and wherein an optical mixer generates a plurality of mixed optical signals for each of the first plurality of measurements, wherein an optical detector generates a respective electrical signal of the plurality of electrical signals for each of the first plurality of measurements.

15. The method of claim 11 , wherein the phase difference is a complex number representing an angle in a range between −π and π.

16. The method of claim 11 , wherein determining the phase difference comprises:

computing a cross correlation between the first electrical signal and the second electrical signal; and

determining the phase difference based on a time lag at which the cross correlation has a maximum absolute value.

17. A light detection and ranging (LIDAR) system comprising one or more processors configured to:

receive a plurality of electrical signals that are respectively associated with (i) a plurality of optical signals provided by a laser (ii) a plurality of returned optical signals that are responsive to the plurality of optical signals provided by a laser;

determine a phase difference between a first electrical signal of the plurality of electrical signals and a second electrical signal;

determine an internal reflection signal; and

determine a range to an object by adjusting a third electrical signal of the plurality of electrical signals using the internal reflection signal.

18. The LIDAR system of claim 17 , wherein the one or more processors are further configured to:

adjust the first electrical signal of the plurality of electrical signals based on the phase difference.

19. The LIDAR system of claim 18 , wherein the one or more processors are further configured to:

determine, responsive to adjusting the first electrical signal, the internal reflection signal by averaging the plurality of electrical signals.

20. The LIDAR system of claim 17 , wherein the phase difference is a complex number representing an angle in a range between −π and π.