Lidar system

1. A lidar system comprising:

a light source configured to emit a pulse of light into a field of view;

a receiver configured to detect a return pulse of light which comprises at least a portion of the emitted pulse of light which is reflected or scattered by a target in the field of view, the receiver comprising:

an avalanche photodiode configured to generate an electrical-current pulse corresponding to the return pulse of light,

a transimpedance amplifier configured to produce a voltage pulse that corresponds to the electrical-current pulse,

a voltage amplifier configured to amplify the voltage pulse to produce an amplified voltage pulse,

a first comparator configured to produce a first edge signal when the amplified voltage pulse exceeds a first threshold, and

a time-to-digital converter configured to determine a first time interval based on an emission time of the pulse of light and based on the first edge signal wherein the emission time of the pulse of light is determined from an optical trigger signal associated with detection by the receiver of a portion of the emitted pulse of light which is scattered or reflected from a surface located within the lidar system; and

a processor configured to determine a distance to the target using the first time interval.

2. The lidar system of claim 1 , further comprising a sensor head located remotely from the light source, wherein:

the sensor head comprises the receiver; and

the sensor head is coupled to the light source by an optical link.

3. The lidar system of claim 2 , wherein the lidar system further comprises one or more additional sensor heads, wherein:

each of the additional sensor heads comprises a respective receiver; and

the light source is coupled to each of the additional sensor heads by a respective optical link.

4. The lidar system of claim 3 , wherein the lidar system is incorporated into a vehicle, and wherein the sensor head and the one or more additional sensor heads of the lidar system are positioned about the vehicle to provide at least a 180-degree view of an environment around the vehicle.

5. The lidar system of claim 1 , wherein the lidar system has a horizontal resolution of at least 100 pixels and a vertical resolution of at least 4 pixels.

6. The lidar system of claim 1 , wherein the lidar system is configured to generate point clouds of distance measurements at a rate between 0.1 frames per second and 1,000 frames per second.

7. The lidar system of claim 1 , wherein:

the light source emits the pulse into a light source field of view;

the receiver detects the return pulse from a receiver field of view;

the light-source field of view and the receiver field of view are scanned along a scanning direction; and

the receiver field of view is offset from the light-source field of view in a direction opposite the scanning direction.

8. The lidar system of claim 1 , wherein:

the light source emits the pulse into a light source field of view;

the receiver detects the return pulse from a receiver field of view; and

an angular extent of the light source field of view is equal to an angular extent of the receiver field of view.

9. The lidar system of claim 1 , wherein:

the light source emits the pulse into a light source field of view;

the receiver detects the return pulse from a receiver field of view;

the light-source field of view has an angular extent of less than or equal to 50 milliradians; and

the receiver field of view has an angular extent of less than or equal to 50 milliradians.

10. The lidar system of claim 1 , wherein the receiver further comprises:

a second comparator configured to produce a second edge signal when the amplified voltage pulse falls below a second threshold,

wherein the time-to-digital converter is configured to determine a second time interval based on the emission time of the pulse of light by the light source and based on the second edge signal.

11. The lidar system of claim 10 , further comprising a processor configured to determine the distance to the target based at least in part on the first and second time intervals.

12. The lidar system of claim 1 , further comprising an optical filter, wherein the optical filter is configured to transmit the return pulse and attenuate other light by at least 10 dB.

13. The lidar system of claim 1 , wherein the receiver comprises an array of two or more avalanche photodiodes.

14. The lidar system of claim 1 , wherein the receiver comprises:

an avalanche photodiode configured to operate as a single-photon avalanche diode (SPAD); and

a quenching circuit configured to reduce a reverse-bias voltage applied to the SPAD when an avalanche event occurs in the SPAD.

15. The lidar system of claim 1 , wherein the receiver comprises:

two or more avalanche photodiodes (APDs); and

one or more logic gates coupled to the APDs, wherein the logic gates are configured to produce an output indicating that the receiver has detected the return pulse only if each of the APDs produces an electrical signal corresponding to detection of the return pulse.

16. The lidar system of claim 1 , wherein the processor is configured to determine the distance to the target based at least in part on a round-trip time of flight for the pulse of light emitted by the light source to travel from the lidar system to the target and back to the lidar system.

17. The lidar system of claim 16 , wherein the round-trip time of flight is determined based at least in part on a rising edge or a falling edge associated with the return pulse of light detected by the receiver.

18. The lidar system of claim 1 , further comprising a plurality of comparators, the plurality of comparators being configured to determine when the amplified voltage pulse reaches a respective plurality of different reference values so as to provide information about the shape of the amplified voltage pulse.