Reducing the number of false detections in a lidar system
A light source includes a laser configured to emit a ranging pulse including a sequence of fast pulses. A lidar system uses one or more properties of the sequence of fast pulses to determine a signature of the ranging pulse. A receiver includes a detector element configured to detect a light signal and a signature detection circuitry configured to determine whether the detected light signal corresponds to the signature of the emitted ranging pulse. The lidar system is configured to generate a pixel value based on the detected light signal if the detected light signal corresponds to the signature of the emitted ranging pulse.
1. A lidar system comprising:
a light source including a mode-locked laser that includes an optical pump, the light source configured to emit a ranging pulse including a pseudo-random sequence of fast pulses generated by the laser, wherein one or more properties of the sequence of the fast pulses within the ranging pulse defines a signature of the ranging pulse;
a processor configured to modulate the optical pump to emit the ranging pulse and restart the optical pump for every ranging pulse emitted by the light source; and
a receiver configured to detect light scattered by remote targets, the receiver comprising:
a detector element configured to detect a light signal, and
a signature detection circuitry configured to determine whether the detected light signal corresponds to the signature of the emitted ranging pulse;
wherein the lidar system is configured to generate a pixel value based on the detected light signal if the detected light signal corresponds to the signature of the emitted ranging pulse.
2. The lidar system of claim 1 , wherein the lidar system further comprises a scanner configured to direct the ranging pulse according to a scan direction to illuminate an instantaneous field of view (FOV) of the light source at a time when the ranging pulse is emitted.
3. The lidar system of claim 1 , further comprising a storage element configured to store the signature of the emitted ranging pulse, wherein the signature detection circuitry is configured to compare one or more properties of fast pulses included in the detected light signal to the stored signature, wherein the properties comprise one or more time intervals, pulse widths, or pulse amplitudes.
4. The lidar system of claim 3 , wherein the lidar system further comprises an optical element to direct a portion of energy of the emitted ranging pulse to the signature detection circuitry; and wherein the system is configured to replace the signature of the emitted ranging pulse in the storage element for every emitted ranging pulse.
5. The lidar system of claim 3 , wherein the storage element is configured to store respective signatures for two or more most recently emitted ranging pulses, and wherein the signature detection circuitry is configured to compare time intervals between fast pulses included in the detected light signal to each of the stored signatures.
6. The lidar system of claim 1 , wherein time intervals between the fast pulses within the ranging pulse at least partially define the signature of the ranging pulse.
7. The lidar system of claim 1 , wherein the laser comprises a mirror that forms an external cavity of the laser, the mirror having an adjustable position.
8. The lidar system of claim 1 , wherein the signature detection circuitry is configured to perform frequency demodulation.
9. The lidar system of claim 1 , wherein the signature detection circuitry includes one or more time-to-digital converters (TDCs).
10. The lidar system of claim 9 , wherein the one or more TDCs are configured to trigger when rising edges of the fast pulses cross a certain threshold.
11. The lidar system of claim 9 , wherein the one or more TDCs are configured to trigger when rising or falling edges of the fast pulses cross a certain threshold.
12. The lidar system of claim 1 , wherein the laser operates in a passively mode-locked regime or an actively mode-locked regime.
13. The lidar system of claim 1 , wherein the laser includes a vertical external cavity surface-emitting laser (VECSEL) comprising a semiconductor saturable absorber mirror (SESAM).
14. The lidar system of claim 1 , wherein the laser includes a mode-locked integrated external-cavity surface emitting laser (MIXSEL).
15. The lidar system of claim 1 , wherein the laser includes a current-modulated laser diode or an optically-modulated laser diode.
16. The lidar system of claim 15 , wherein the laser further includes an optical amplifier, and light from the laser diode is amplified by the optical amplifier.
17. The lidar system of claim 1 , wherein the laser is configured to generate the fast pulses with an average repetition rate between 0.5 GHz and 25 GHz.
18. The lidar system of claim 1 , wherein the signature detection circuitry is further configured to disregard the detected light signal if the detected light signal does not correspond to the signature of the emitted ranging pulse.
19. The lidar system of claim 1 , wherein the signature detection circuitry is further configured to determine whether the detected light signal corresponds to a signature of a previously emitted ranging pulse.
20. The lidar system of claim 1 , wherein generating the pixel value based on the detected light signal comprises determining a distance to a remote target, a direction of the target, a reflectivity of the target, an intensity of the detected light signal, or a polarization of the detected light signal.
21. A method comprising:
emitting, using a mode-locked laser, a ranging pulse including a pseudo-random sequence of fast pulses, including modulating an optical pump to emit the ranging pulse and restarting the optical pump for every N ranging pulses emitted by the light source, wherein N≥2, wherein one or more properties of the sequence of the fast pulses within the ranging pulse defines a signature of the ranging pulse;
detecting a light signal using a detector element;
determining whether the detected light signal corresponds to the signature of the emitted ranging pulse; and
in response to determining that the detected light signal corresponds to the signature of the emitted ranging pulse, generating a pixel value based on the detected light signal;
otherwise, in response to determining that the detected light signal does not correspond to the signature of the emitted ranging pulse, discarding the detected light signal.
22. The method of claim 21 , further comprising directing, using a scanner, the ranging pulse according to a scan direction to illuminate an instantaneous field of view (FOV) of the light source at a time when the ranging pulse is emitted.
23. The method of claim 21 , further comprising determining whether the detected light signal corresponds to a signature of a previously emitted ranging pulse.
24. The method of claim 21 , further comprising storing the signature of the emitted ranging pulse in a storage element, wherein determining whether the light signal corresponds to the signature of the emitted ranging pulse includes using the stored signature.
25. The method of claim 24 , wherein determining whether the light signal corresponds to the signature of the emitted ranging pulse includes using a signature detection circuitry, the method further comprising directing a portion of energy of the emitted ranging pulse to the signature detection circuitry to replace the signature of the emitted ranging pulse in the storage element for every emitted ranging pulse.