Self-Raman laser for lidar system

• US 9,905,992 B1
• Filed: 03/16/2017
• Issued: 02/27/2018
• Est. Priority Date: 03/16/2017
• Status: Active Grant

• Current Assignee: Luminar Technologies, Inc.
• Original Assignee: Luminar Technologies, Inc.
• Inventors: Welford, David, Jaspan, Martin A., Eichenholz, Jason M., Campbell, Scott R., Martin, Lane A., Weed, Matthew D.
• Primary Examiner(s): Ratcliffe, Luke D
• Application Number:
  US15/461,394
• Time in Patent Office: 348 Days
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G01S 17/42 Simultaneous measurement of distance and other co-ordinates indirect measurement G01S17/46
• CPC Class Codes: G01S 17/88 Lidar systems specially adapted for specific applications
• CPC Class Codes: G01S 7/4814 of transmitters alone
• CPC Class Codes: G01S 7/484 Transmitters
• CPC Class Codes: H01S 3/0092 Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity nonlinear frequency conversion per se G02F1/35
• CPC Class Codes: H01S 3/061 with elliptical or circular cross-section and elongated shape, e.g. rod
• CPC Class Codes: H01S 3/0612 Non-homogeneous structure H01S3/07 takes precedence
• CPC Class Codes: H01S 3/0621 Coatings on the end-faces, e.g. input/output surfaces of the laser light
• CPC Class Codes: H01S 3/0627 the resonator being monolithic, e.g. microlaser
• CPC Class Codes: H01S 3/08 Construction or shape of optical resonators or components thereof
• CPC Class Codes: H01S 3/094076 Pulsed or modulated pumping H01S3/1024 takes precedence
• CPC Class Codes: H01S 3/094084 with pump light recycling, i.e. with reinjection of the unused pump light, e.g. by reflectors or circulators
• CPC Class Codes: H01S 3/0941 of a laser diode
• CPC Class Codes: H01S 3/09415 the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
• CPC Class Codes: H01S 3/1022 by controlling the optical pumping
• CPC Class Codes: H01S 3/1115 using intracavity saturable absorbers
• CPC Class Codes: H01S 3/113 using intracavity saturable absorbers
• CPC Class Codes: H01S 3/1611 neodymium
• CPC Class Codes: H01S 3/1643 YAG
• CPC Class Codes: H01S 3/1673 : YVO4 [YVO]
• CPC Class Codes: H01S 3/30 : using scattering effects, e.g. stimulated Brillouin or Raman effects

Abstract

In one embodiment, a lidar system includes a self-Raman laser that includes a Raman-active gain medium and a Q-switch. The self-Raman laser is configured to: produce Q-switched pulses of light at a lasing wavelength of the self-Raman laser; Raman-shift, in the Raman-active gain medium, at least a portion of the Q-switched pulses to produce Raman-shifted pulses of light, where the Raman-shifted pulses have a Raman-shifted wavelength that is longer than the lasing wavelength; and emit at least a portion of the Raman-shifted pulses. The lidar system further includes a scanner configured to scan the emitted pulses of light across a field of regard and a receiver configured to detect at least a portion of the scanned pulses of light scattered by a target located a distance from the lidar system. The lidar system also includes a processor configured to determine the distance from the lidar system to the target.

Claims

1. A lidar system comprising:

a self-Raman laser comprising a Raman-active gain medium and a Q-switch, wherein the self-Raman laser is configured to;

produce Q-switched pulses of light at a lasing wavelength of the self-Raman laser;

Raman-shift, in the Raman-active gain medium, at least a portion of the Q-switched pulses to produce Raman-shifted pulses of light, wherein the Raman-shifted pulses have a Raman-shifted wavelength that is longer than the lasing wavelength; and

emit at least a portion of the Raman-shifted pulses, wherein the emitted Raman-shifted pulses of light have optical characteristics comprising;

a pulse duration less than or equal to 20 nanoseconds;

a duty cycle less than or equal to 1%;

a pulse energy greater than or equal to 10 nanojoules; and

a peak power greater than or equal to 1 watt;

a scanner configured to scan the emitted pulses of light across a field of regard;

a receiver configured to detect at least a portion of the scanned pulses of light scattered by a target located a distance from the lidar system; and

a processor configured to determine the distance from the lidar system to the target based at least in part on a round-trip time of flight for an emitted pulse of light to travel from the lidar system to the target and back to the lidar system.

2. The lidar system of claim 1, wherein the self-Raman laser is an actively Q-switched self-Raman laser and the Q-switch is an active Q-switch.

3. The lidar system of claim 1, wherein the self-Raman laser is a passively Q-switched (PQSW) self-Raman laser and the Q-switch is a saturable absorber.

4. The lidar system of claim 3, wherein the saturable absorber comprises vanadium-doped yttrium aluminum garnet (V:

YAG), cobalt-doped MgAl₂O₄ (Co;

spinel), or neodymium-doped strontium fluoride (Nd;

SrF₂).

5. The lidar system of claim 1, wherein the Raman-active gain medium comprises a rare-earth-doped orthovanadate crystal or a rare-earth-doped tungstate crystal.

6. The lidar system of claim 1, wherein the Raman-active gain medium comprises neodymium-doped yttrium orthovanadate (Nd:

YVO₄) comprising neodymium ions in a yttrium orthovanadate (YVO₄) host crystal, wherein;

the lasing wavelength of the Q-switched pulses of light produced by the Nd;

YVO₄ material is approximately 1342 nm; and

the YVO₄ host crystal is configured to Raman shift the portion of the Q-switched pulses to produce the Raman-shifted pulses of light, wherein the Raman-shifted wavelength is approximately 1525 nm.

7. The lidar system of claim 1, wherein the gain medium is pumped at a wavelength between approximately 800 nm and approximately 1000 nm by an edge-emitter laser diode or a vertical-external-cavity surface-emitting laser.

8. The lidar system of claim 1, wherein the self-Raman laser further comprises an end cap coupled to the gain medium, wherein:

the end cap is substantially free of gain-material dopants; and

the end cap is positioned to receive light from a pump laser so that the pump-laser light propagates through the end cap before entering the gain medium.

9. The lidar system of claim 1, wherein the self-Raman laser further comprises a back surface with a dielectric coating having low reflectivity at a pump-laser wavelength, high reflectivity at the lasing wavelength of the self-Raman laser, and high reflectivity at the Raman-shifted wavelength.

10. The lidar system of claim 1, wherein the self-Raman laser further comprises an output surface with a dielectric coating having high reflectivity at the lasing wavelength of the self-Raman laser and high reflectivity, partial reflectivity, or low reflectivity at the Raman-shifted wavelength.

11. The lidar system of claim 1, wherein the self-Raman laser further comprises a mirror separated from the gain medium by an air gap, wherein the mirror comprises a concave surface that forms a cavity mirror of the self-Raman laser, wherein the concave surface comprises a back surface with a dielectric coating having low reflectivity at a pump-laser wavelength, high reflectivity at the lasing wavelength of the self-Raman laser, and high reflectivity at the Raman-shifted wavelength.

12. The lidar system of claim 1, wherein the self-Raman laser is an eye-safe laser with an operating wavelength between approximately 1400 nm and approximately 1600 nm.

13. The lidar system of claim 1, wherein the pulses of light emitted by the self-Raman laser have a pulse repetition frequency greater than or equal to 20 kHz.

14. The lidar system of claim 1, further comprising a splitter configured to receive the pulses of light emitted by the self-Raman laser and split each received pulse of light into two or more angularly separated pulses of light which are scanned by the scanner across the field of regard.

15. The lidar system of claim 14, wherein:

the angularly separated pulses of light are scanned along a scanning direction; and

the angularly separated pulses of light are split along a direction that is approximately orthogonal to the scanning direction.

16. The lidar system of claim 14, wherein the receiver comprises an array of two or more detector elements, wherein each detector element is configured to detect scattered light from a respective pulse of the two or more angularly separated pulses of light which are scanned across the field of regard.

17. The lidar system of claim 1, wherein the field of regard comprises:

a horizontal field of regard greater than or equal to 25 degrees; and

a vertical field of regard greater than or equal to 5 degrees.

18. The lidar system of claim 1, wherein the scanner comprises one or more mirrors, wherein each mirror is mechanically driven by a galvanometer scanner, a resonant scanner, a microelectromechanical systems (MEMS) device, or a voice coil motor.

19. The lidar system of claim 1, wherein:

an output beam of the lidar system comprises the emitted pulses of light which are scanned across the field of regard;

an input beam of the lidar system comprises the portion of the scanned pulses of light detected by the receiver; and

the input and output beams are substantially coaxial.

20. The lidar system of claim 19, further comprising an overlap mirror configured to overlap the input and output beams so that they are substantially coaxial, wherein the overlap mirror comprises:

a hole, slot, or aperture which the output beam passes through; and

a reflecting surface that reflects at least a portion of the input beam toward the receiver.

21. The lidar system of claim 1, wherein:

scanning the emitted pulses of light across the field of regard comprises scanning a field of view of the self-Raman laser across the field of regard; and

the scanner is further configured to scan a field of view of the receiver across the field of regard, wherein the self-Raman-laser field of view and the receiver field of view are scanned synchronously with respect to one another.