STABLE MICROWAVE-FREQUENCY SOURCE BASED ON CASCADED BRILLOUIN LASERS

• US 20150311662A1
• Filed: 03/06/2015
• Published: 10/29/2015
• Est. Priority Date: 03/06/2014
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Vahala, Kerry, Li, Jiang
• Granted Patent: US 9,595,918 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: H01S 3/005 Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B
• CPC Class Codes: H01S 3/06791 Fibre ring lasers fibre laser gyrometers G01C19/72
• CPC Class Codes: H01S 3/0809 Two-wavelenghth emission
• 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 3/1312 by controlling the optical pumping
• CPC Class Codes: H01S 3/302 in an optical fibre
• CPC Class Codes: H01S 5/0085 for modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H03B 17/00 Generation of oscillations using radiation source and detector, e.g. with interposed variable obturator

Abstract

A microwave-frequency source, generating an output electrical signal at an output frequency f_M, comprises a pump laser source, an optical resonator, and a photodetector. Free spectral range v_FSR of the optical resonator equals an integer submultiple of a Brillouin shift frequency v_B of the optical resonator (i.e., v_B=Mv_FSR). The pump laser source is frequency-locked to a corresponding resonant optical mode of the optical resonator. Pumping the optical resonator with output of the pump laser source at a pump frequency v_pump results in stimulated Brillouin laser oscillation in the optical resonator at respective first, second, and third Stokes Brillouin-shifted frequencies v₁=v_pump−v_B, v₂=v_pump−2v_B, and v₃=v_pump−3v_B. The photodetector receives stimulated Brillouin laser outputs at the first and third Stokes Brillouin-shifted frequencies v₁ and v₃ and generates therefrom the output electrical signal at a beat frequency f_M=v₁−v₃=2v_B. The output electrical signal at the output frequency f_M exhibits exceptionally low phase noise.

Claims

1. A microwave-frequency source for generating an output electrical signal at an output frequency f_M, the microwave-frequency source comprising a pump laser source, an optical resonator, and a photodetector, wherein:

(a) a free spectral range v_FSR of the optical resonator is substantially equal to an integer submultiple of a Brillouin shift frequency v_B of the optical resonator so that v_B=Mv_FSR where M is an integer;

(b) the pump laser source is frequency-locked to a corresponding resonant optical mode of the optical resonator;

(c) the pump laser source and the optical resonator are arranged so that pumping the optical resonator with output of the pump laser source at a pump frequency v_pump results in stimulated Brillouin laser oscillation in the optical resonator at respective first, second, and third Stokes Brillouin-shifted frequencies v₁=v_pump−

v_B, v₂=v_pump−

2v_B, and v₃=v_pump−

3v_B; and

(d) the photodetector is arranged to receive stimulated Brillouin laser outputs at the first and third Stokes Brillouin-shifted frequencies v₁ and v₃ and to generate therefrom the output electrical signal at a beat frequency f_M=v₁−

v₃=2v_B.

2. The microwave-frequency source of claim 1 wherein the first and third Stokes stimulated Brillouin laser outputs co-propagate along a common optical path from the optical resonator to the photodetector.

3. The microwave-frequency source of claim 1 wherein the optical resonator is structurally arranged so as to suppress stimulated Brillouin laser oscillation at a fourth Stokes Brillouin-shifted frequency v₄=v_pump−

4v_B.

4. The microwave-frequency source of claim 1 wherein the free spectral range v_FSR of the optical resonator is substantially equal to the Brillouin shift frequency v_B of the optical resonator.

5. The microwave-frequency source of claim 1 wherein the optical resonator comprises silica and the Brillouin shift frequency v_B of the optical resonator is about 10.87 GHz.

6. The microwave-frequency source of claim 1 wherein the optical resonator comprises a ring optical resonator.

7. The microwave-frequency source of claim 6 wherein the ring optical resonator comprises a disk optical resonator.

8. The microwave-frequency source of claim 6 wherein the pump laser source and the ring optical resonator are arranged so that pumping the ring optical resonator with the output of the pump laser source at the pump frequency v_pump results in (i) backward-propagating stimulated Brillouin laser oscillation in the ring optical resonator at the respective first and third Stokes Brillouin-shifted frequencies v₁ and v₃ and (ii) forward-propagating stimulated Brillouin laser oscillation in the ring optical resonator at the second Stokes Brillouin-shifted frequency v₂.

9. The microwave-frequency source of claim 1 wherein the optical resonator comprises a fiber optical resonator.

10. The microwave-frequency source of claim 9 wherein the fiber optical resonator includes a fiber Bragg grating that is structurally arranged so as to suppress stimulated Brillouin laser oscillation at a fourth Stokes Brillouin-shifted frequency v₄=v_pump−

4v_B.

11. The microwave-frequency source of claim 9 wherein the fiber optical resonator comprises a fiber Fabry-Perot optical resonator.

12. The microwave-frequency source of claim 9 wherein the fiber optical resonator comprises a fiber-loop optical resonator.

13. The microwave-frequency source of claim 12 wherein the pump laser source and the fiber-loop optical resonator are arranged so that pumping the fiber-loop optical resonator with the output of the pump laser source at the pump frequency v_pump results in (i) backward-propagating stimulated Brillouin laser oscillation in the fiber-loop optical resonator at the respective first and third Stokes Brillouin-shifted frequencies v₁ and v₃ and (ii) forward-propagating stimulated Brillouin laser oscillation in the fiber-loop optical resonator at the second Stokes Brillouin-shifted frequency v₂.

14. The microwave-frequency source of claim 9 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 20 meters long.

15. The microwave-frequency source of claim 9 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 40 meters long.

16. The microwave-frequency source of claim 9 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 100 meters long.

17. The microwave-frequency source of claim 9 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 200 meters long.

18. The microwave-frequency source of claim 9 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 500 meters long.

19. The microwave-frequency source of claim 1 wherein the output electrical signal has a bandwidth less than about 100 Hz over about a 1 second timescale.

20. The microwave-frequency source of claim 1 wherein the output electrical signal has a bandwidth less than about 1 Hz over about a 1 second timescale.

21. The microwave-frequency source of claim 1 wherein the output electrical signal exhibits a phase noise level less than about −

30 dBc/Hz at 100 Hz offset frequency and less than about −

90 dBc/Hz at 10 kHz offset frequency.

22. The microwave-frequency source of claim 1 wherein the output electrical signal exhibits a phase noise level less than about −

70 dBc/Hz at 100 Hz offset frequency and less than about −

110 dBc/Hz at 10 kHz offset frequency.

23. The microwave-frequency source of claim 1 wherein the pump laser source is frequency-locked to the corresponding resonant optical mode of the optical resonator by a Pound-Drever-Hall mechanism.

24. A method for generating a microwave-frequency output electrical signal using the microwave-frequency source of claim 1, the method comprising:

(a) using the pump laser source, pumping the optical resonator so as to generate the stimulated Brillouin laser oscillation at the first, second, and third Stokes Brillouin-shifted frequencies; and

(b) directing the stimulated Brillouin laser outputs at the first and third Stokes Brillouin-shifted frequencies onto the photodetector so as to generate the microwave-frequency output electrical signal at the frequency f_M=2v_B.

25. A method for generating a microwave-frequency output electrical signal at an output frequency f_M, the method comprising:

(a) using a pump laser source, pumping at a pump frequency v_pump an optical resonator so as to generate stimulated Brillouin laser outputs at respective first and third Stokes Brillouin-shifted frequencies v₁=v_pump−

v_B and v₃=v_pump−

3v_B, where v_B is a Brillouin shift frequency of the optical resonator; and

(b) directing the stimulated Brillouin laser outputs at the first and third Stokes Brillouin-shifted frequencies onto a photodetector so as to generate an output electrical signal at a beat frequency f_M=v₁−

v₃=2v_B,

wherein;

(c) a free spectral range v_FSR of the optical resonator is substantially equal to an integer submultiple of the Brillouin shift frequency v_B of the optical resonator so that v_B=Mv_FSR where M is an integer;

(d) the pump laser source is frequency-locked to a corresponding resonant optical mode of the optical resonator;

(e) the pump laser source and the optical resonator are arranged so that pumping the optical resonator with output of the pump laser source at the pump frequency v_pump results in stimulated Brillouin laser oscillation in the optical resonator at the respective first, second, and third Stokes Brillouin-shifted frequencies v₁=v_pump−

v_B, v₂=v_pump−

2v_B, and v₃=v_pump−

3v_B; and

(d) the photodetector is arranged to receive the first and third Stokes stimulated Brillouin laser outputs and to generate therefrom the output electrical signal at the beat frequency f_M=v₁−

v₃=2v_B.

26. The method of claim 25 further comprising suppressing stimulated Brillouin laser oscillation in the optical resonator at a fourth Stokes Brillouin-shifted frequency v₄=v_pump−

4v_B.

27. The method of claim 26 wherein the optical resonator is structurally arranged so as to suppress stimulated Brillouin laser oscillation at the fourth Stokes Brillouin-shifted frequency v₄=v_pump−

4v_B.

28. The method of claim 26 further comprising limiting output power of the pump laser source so as to suppress stimulated Brillouin laser oscillation at the fourth Stokes Brillouin-shifted frequency v₄=v_pump−

4v_B.

29. The method of claim 25 further comprising limiting output power of the pump laser source so as to suppress multimode stimulated Brillouin laser oscillation in the optical resonator.