Narrow-linewidth brillouin laser stabilized by optical self-injection locking

1 . An apparatus comprising:

a ring optical resonator characterized by a Brillouin shift frequency Ω a and a free spectral range ν FSR ; and

a first pump laser source that arranged so as to (i) produce a first pump optical signal at a first pump optical frequency ν 1P , and (ii) launch at least a portion of the first pump optical signal into the ring optical resonator to propagate in a forward direction around the ring optical resonator,

the ring optical resonator and the first pump laser source being arranged so that:

a portion of the first pump optical signal backscattered by the ring optical resonator propagates back to the first pump laser source and injection-locks the first pump laser source so that the first pump optical frequency ν 1P is locked to a first resonant mode optical frequency of the ring optical resonator; and

a portion of the first pump optical signal circulating in the ring optical resonator results in a first stimulated Brillouin laser (SBL) optical signal at a first SBL optical frequency ν 1 =ν 1P −Ω a that resonantly propagates in a backward direction around the ring optical resonator and is emitted from a resonant optical mode of the ring optical resonator at a resonant mode optical frequency that is lower than the first resonant mode optical frequency.

2 . The apparatus of claim 1 further comprising a second pump laser source that arranged so as to (i) produce a second pump optical signal at a second pump optical frequency ν 2P , and (ii) launch at least a portion of the second pump optical signal into the ring optical resonator to propagate in a forward direction around the ring optical resonator, the ring optical resonator and the second pump laser source being arranged so that:

a portion of the second pump optical signal backscattered by the ring optical resonator propagates back to the second pump laser source and injection-locks the second pump laser source so that the second pump optical frequency ν 2P is locked to a second resonant mode optical frequency of the ring optical resonator different from the first resonant mode optical frequency; and

a portion of the second pump optical signal circulating in the ring optical resonator results in a second SBL optical signal at a second SBL optical frequency ν 2 =ν 2P −Ω a that resonantly propagates in a backward direction around the ring optical resonator and is emitted from a resonant optical mode of the ring optical resonator at a resonant mode optical frequency that is lower than the second resonant mode optical frequency.

3 . The apparatus of claim 2 wherein Ω a is about equal to ν FSR or an integer multiple of ν FSR .

4 . The apparatus of claim 2 wherein the optical resonator comprises silica and the Brillouin shift frequency Ω a of the optical resonator is about 10.9 GHz.

5 . The apparatus of claim 2 further comprising an optical detector arranged so as (i) to receive at least a portion of the first SBL optical signal and at least a portion of the second SBL optical signal, and (ii) to generate therefrom an electrical output signal at a difference frequency ν DIFF =|ν 2 −ν 1 | that is less than 300 GHz.

6 . The apparatus of claim 5 wherein the difference frequency ν DIFF =|ν 2 −ν 1 | is greater than 0.3 GHz.

7 . The apparatus of claim 5 wherein (i) the difference frequency ν DIFF =|ν 2 −ν 1 | exhibits fluctuations over a 0.1 second timescale only within a bandwidth less than 100 Hz, or (ii) the output electrical signal exhibits phase noise less than −80 dBc/Hz at 10 kHz offset frequency.

8 . The apparatus of claim 5 wherein (i) the difference frequency ν DIFF =|ν 2 −ν 1 | exhibits fluctuations over a 0.1 second timescale only within a bandwidth less than 1 Hz, or (ii) the output electrical signal exhibits phase noise less than −100 dBc/Hz at 10 kHz offset frequency.

9 . The apparatus of claim 2 further comprising an optical frequency divider arranged so as (i) to receive at least a portion of the first SBL optical signal and at least a portion of the second SBL optical signal, and (ii) to generate therefrom an electrical output signal at a divided frequency about equal to |ν 2 −ν 1 |/N, with N being an integer and |ν 2 −ν 1 | being greater than 100 GHz.

10 . The apparatus of claim 9 wherein the divided frequency is greater than 0.3 GHz.

11 . The apparatus of claim 9 wherein (i) the difference frequency ν DIFF =|ν 2 −ν 1 | exhibits fluctuations over a 0.1 second timescale only within a bandwidth less than 100 Hz, or (ii) the output electrical signal exhibits phase noise less than −80−20·log N dBc/Hz at 10 kHz offset frequency.

12 . The apparatus of claim 9 wherein (i) the difference frequency ν DIFF =|ν 2 −ν 1 | exhibits fluctuations over a 0.1 second timescale only within a bandwidth less than 1 Hz, or (ii) the output electrical signal exhibits phase noise less than −100−20·log N dBc/Hz at 10 kHz offset frequency.

13 . The apparatus of claim 2 wherein one or both of the first or second pump lasers sources are semiconductor diode lasers.

14 . The apparatus of claim 2 wherein one or both of the first or second pump optical frequencies ν 1P and ν 2P are greater than 75 THz or less than 750 THz.

15 . The apparatus of claim 2 wherein the ring optical resonator comprises a ring waveguide optical resonator on a substrate.

16 . The apparatus of claim 2 wherein the ring optical resonator comprises a disk optical resonator.

17 . The apparatus of claim 2 wherein the ring optical resonator comprises a fiber-loop optical resonator.

18 . The apparatus of claim 2 wherein the optical resonator exhibits a Q-factor greater than 10 6 .

19 . The apparatus of claim 1 wherein the ring optical resonator comprises a ring waveguide optical resonator on a substrate.

20 . The apparatus of claim 1 wherein the ring optical resonator comprises a disk optical resonator.

21 . The apparatus of claim 1 wherein the ring optical resonator comprises a fiber-loop optical resonator.