US-12625263-B2 - Digital electro-optical phase locked loop in a LiDAR system

Abstract

The FMCW LiDAR system includes an optical drive electronic circuit to receive a reference frequency signal and a beat frequency signal to generate a drive signal. The optical drive electronic circuit includes a TDC to calculate a phase difference between the reference frequency signal and the beat frequency signal and a digital ramp control to: provided the phase difference is a positive value, produce a ramp down control signal to increase a current chirp rate to an increased chirp rate; provided the phase difference is a negative value, produce a ramp up control signal to decrease the current chirp rate to a decreased chirp rate. The optical drive electronic circuit includes a digital integrator to generate a digital output based on at least one of the ramp down control signal or the ramp up control signal and a DAC to convert the digital output to an analog output.

Inventors

• Eric Bohannon
• Garret Phillips
• Bryce Bradford

Assignees

• Aeva, Inc.

Dates

Publication Date

20260512

Application Date

20230131

Claims (20)

1. 1 . A frequency modulated continuous wave (FMCW) light detection and ranging (LiDAR) system, comprising: an optical source to receive a drive signal to cause an optical beam to be transmitted according to a current chirp rate along a target path and a reference path; a photodetector to receive, via the reference path, a portion of the optical beam transmitted through an optical interferometer to generate a beat frequency signal; and an optical drive electronic circuit to receive a reference frequency signal and the beat frequency signal to generate the drive signal, the optical drive electronic circuit comprising: a time-to-digital convertor (TDC) to calculate a phase difference between the reference frequency signal and the beat frequency signal; a digital ramp control to: provided the phase difference is a positive value, produce a ramp down control signal to increase the current chirp rate to an increased chirp rate; provided the phase difference is a negative value, produce a ramp up control signal to decrease the current chirp rate to a decreased chirp rate; a digital integrator to generate a digital output based on at least one of the ramp down control signal or the ramp up control signal; and a digital to analog convertor to convert the digital output to an analog output to produce the drive signal.
2. 2 . The FMCW LiDAR system of claim 1 , wherein the reference frequency signal and the beat frequency signal form a closed loop feedback signal to allow the beat frequency signal to be locked at a predetermined reference frequency.
3. 3 . The FMCW LiDAR system of claim 1 , wherein the phase difference is a time-averaged phase difference.
4. 4 . The FMCW LiDAR system of claim 1 , wherein the optical interferometer is a Mach-Zender-Interferometer.
5. 5 . The FMCW LiDAR system of claim 1 , wherein the optical drive electronic circuit is to control the frequency change of the optical source to be linear.
6. 6 . The FMCW LiDAR system of claim 1 , further comprising: a digital loop filter coupled to the time digital convertor to generate an output based on the phase difference between the reference frequency and the beat frequency signal.
7. 7 . The FMCW LiDAR system of claim 6 , wherein at least one of the time digital convertor, the digital loop filter, the digital ramp control, or the digital integrator is programmable by a processor.
8. 8 . The FMCW LiDAR system of claim 6 , wherein a bandwidth of the digital loop filter is adjustable by a processor.
9. 9 . A method of operating a frequency modulated continuous wave (FMCW) light detection and ranging (LiDAR) system, comprising: receiving a drive signal to cause an optical beam to be transmitted according to a current chirp rate along a target path and a reference path; receiving, via the reference path, a portion of the optical beam transmitted through an optical interferometer to generate a beat frequency signal; and receiving a reference frequency signal and the beat frequency signal to generate the drive signal, comprising: calculating a phase difference between the reference frequency signal and the beat frequency signal; provided the phase difference is a positive value, producing a ramp down control signal to increase the current chirp rate to an increased chirp rate; provided the phase difference is a negative value, producing a ramp up control signal to decrease the current chirp rate to a decreased chirp rate; generating a digital output based on at least one of the ramp down control signal or the ramp up control signal; and converting the digital output to an analog output to produce the drive signal.
10. 10 . The method of claim 9 , further comprising: forming a closed loop feedback signal to allow the beat frequency signal to be locked at a predetermined reference frequency.
11. 11 . The method of claim 9 , wherein the phase difference is a time-averaged phase difference.
12. 12 . The method of claim 9 , further comprising: controlling the frequency change of the optical source to be linear.
13. 13 . The method of claim 9 , further comprising: generating an output based on the phase difference between the reference frequency and the beat frequency signal; and adjusting a bandwidth of the output by a processor.
14. 14 . An electro-optical system, comprising: an optical drive electronic circuit to receive a reference frequency signal and a beat frequency signal to generate a drive signal for an optical source, the optical drive electronic circuit comprising: a time-to-digital convertor (TDC) to calculate a phase difference between the reference frequency signal and the beat frequency signal; a digital ramp control to: provided the phase difference is a positive value, produce a ramp down control signal to increase the current chirp rate to an increased chirp rate; provided the phase difference is a negative value, produce a ramp up control signal to decrease the current chirp rate to a decreased chirp rate; a digital integrator to generate a digital output based on at least one of the ramp down control signal or the ramp up control signal; and a digital to analog convertor to convert the digital output to an analog output to produce the drive signal.
15. 15 . The electro-optical system of claim 14 , wherein the reference frequency signal and the beat frequency signal form a closed loop feedback signal to allow the beat frequency signal to be locked at a predetermined reference frequency.
16. 16 . The electro-optical system of claim 14 , wherein the phase difference is a time-averaged phase difference.
17. 17 . The electro-optical system of claim 14 , wherein the optical drive electronic circuit is to control the frequency change of the optical source to be linear.
18. 18 . The electro-optical system of claim 14 , further comprising: a digital loop filter coupled to the time digital convertor to generate an output based on the phase difference between the reference frequency and the beat frequency signal.
19. 19 . The electro-optical system of claim 18 , wherein at least one of the time digital convertor, the digital loop filter, the digital ramp control, or the digital integrator is programmable by a processor.
20. 20 . The electro-optical system of claim 18 , wherein a bandwidth of the digital loop filter is adjustable by a processor.

Description

TECHNICAL FIELD The present disclosure relates generally to light detection and ranging (LiDAR) systems, and more particularly to an electro-optical phase locked loop (EOPLL) in a LIDAR system. BACKGROUND In coherent LiDAR techniques such as Frequency-Modulated Continuous-Wave radar (FMCW) LiDAR, both the distance and the speed of a target affects the mixing frequency between the Local Oscillator (LO) and the return signal. To sense both the distance and the speed, LiDAR systems may use frequency modulation signals referred to as a down-chirp and an up-chirp. The down-chirp and the up-chirp can be carried within the same optical beam. Often a laser diode may be used as an optical source for generating the optical beam. The laser diode generates the optical beam at a wavelength that is proportional to the magnitude of the current through it. Modulating the current in turn modulates the frequency of the optical beam and generates the chirps. The laser diver, which is the circuitry used to generate and control the optical beam, may include an EOPLL. Conventionally, the EOPLL is an analog EOPLL. However, the analog EOPLL may occupy a large physical area. In order to make changes to the analog EOPLL, an old analog component may have to be physically removed and a new analog component may have to physically installed, which may be time consuming. Furthermore, it is difficult to increase the scalability and flexibility with the analog EOPLL. SUMMARY The present disclosure describes various examples of a digital EOPLL in LiDAR systems, e.g., in a FMCW LiDAR system. In some examples, disclosed herein is an optical drive control circuit (e.g., a laser diode control circuit) of a FMCW LiDAR system with a digital EOPLL. The digital EOPLL may include a time-to-digital converter (TDC) and a digital loop filter, which may be significantly smaller than a conventional analog loop filter. The digital EOPLL may include a digital ramp control and a digital integrator. The output of the digital integrator may drive a digital to analog converter (DAC), which may then drive the laser diode to adjust the modulating current. The digital EOPLL may have a small area footprint and allow for the increased portability. In addition, the digital EOPLL may increase the scalability and flexibility of the laser diode control circuit. The digital EOPLL is easily scalable due to the small geometry complementary metal-oxide-semiconductor (CMOS) technologies, for example, with external power field effect transistor (FET). The digital EOPLL is flexible because it is programmable and configurable, for example, by a processor. In some examples, an FMCW LiDAR system is provided herein. The FMCW LiDAR system includes an optical source to receive a drive signal to cause an optical beam to be transmitted according to a current chirp rate along a target path and a reference path. The FMCW LiDAR system includes a photodetector to receive, via the reference path, a portion of the optical beam transmitted through an optical interferometer to generate a beat frequency signal. The FMCW LiDAR system includes an optical drive electronic circuit to receive a reference frequency signal and the beat frequency signal to generate the drive signal. The optical drive electronic circuit includes a time-to-digital convertor (TDC) to calculate a phase difference between the reference frequency signal and the beat frequency signal. The optical drive electronic circuit includes a digital ramp control to: provided the phase difference is a positive value, produce a ramp down control signal to increase the current chirp rate to an increased chirp rate; provided the phase difference is a negative value, produce a ramp up control signal to decrease the current chirp rate to a decreased chirp rate. The optical drive electronic circuit includes a digital integrator to generate a digital output based on at least one of the ramp down control signal or the ramp up control signal. The optical drive electronic circuit includes a digital to analog convertor to convert the digital output to an analog output to produce the drive signal. In some examples, a method of operating a FMCW LiDAR system is provided herein. The method includes receiving a drive signal to cause an optical beam to be transmitted according to a current chirp rate along a target path and a reference path. The method includes receiving, via the reference path, a portion of the optical beam transmitted through an optical interferometer to generate a beat frequency signal. The method includes receiving a reference frequency signal and the beat frequency signal to generate the drive signal. The method further includes calculating a phase difference between the reference frequency signal and the beat frequency signal. The method further includes that, provided the phase difference is a positive value, producing a ramp down control signal to increase the current chirp rate to an increased chirp rate; provided the phase d