Stabilized microwave-frequency source

• US 10,523,214 B1
• Filed: 04/28/2018
• Issued: 12/31/2019
• Est. Priority Date: 04/28/2017
• Status: Active Grant

• Current Assignee: Hqphotonics Inc.
• Original Assignee: Hqphotonics Inc.
• Inventors: Li, Jiang, Vahala, Kerry
• Primary Examiner(s): Kinkead, Arnold M
• Application Number:
  US15/965,911
• Time in Patent Office: 612 Days
• Field of Search: 331 2, 331 10, 331 46, 331 47, 372 32, 398192, 370503
• CPC Class Codes: H01S 3/0085 Modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H01S 3/06708 Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering optical fibres as passive waveguides G02B6/02
• CPC Class Codes: H01S 3/094049 Guiding of the pump light
• CPC Class Codes: H01S 3/094096 Multi-wavelength pumping
• CPC Class Codes: H01S 3/1109 Active mode locking
• CPC Class Codes: H01S 3/1303 by using a passive reference, e.g. absorption cell H01S3/139 takes precedence
• CPC Class Codes: H01S 3/1304 by using an active reference, e.g. second laser, klystron or other standard frequency source H01S3/139 takes precedence; automatic control of electronic generators H03L7/00
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/2391 emitting at different wavelengths
• CPC Class Codes: H01S 3/30 using scattering effects, e.g. stimulated Brillouin or Raman effects
• CPC Class Codes: H01S 3/302 in an optical fibre
• CPC Class Codes: H01S 5/065 Mode locking; Mode suppression; Mode selection ; Self pulsating
• CPC Class Codes: H03L 5/00 Automatic control of voltage, current, or power
• CPC Class Codes: H03L 7/06 using a reference signal applied to a frequency- or phase-locked loop
• CPC Class Codes: H03L 7/087 using at least two phase detectors or a frequency and phase detector in the loop
• CPC Class Codes: H03L 7/235 Nested phase locked loops
• CPC Class Codes: H04B 10/00 Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
• CPC Class Codes: H04B 10/2507 for the reduction or elimination of distortion or dispersion
• CPC Class Codes: H04B 10/25891 Transmission components H04B10/40 takes precedence
• CPC Class Codes: H04B 10/503 Laser transmitters
• CPC Class Codes: H04B 2210/006 : Devices for generating or processing an RF signal by optical means

Abstract

A voltage-controlled oscillator generates a VCO output signal at frequency f_M. A dual optical-frequency source generates optical signals at frequencies v_1S and v_2S. An electro-optic frequency divider (EOFD) generates multiple optical sidebands spaced by f_M, and from two sidebands generates a beat signal at beat frequency δf. A first control circuit generates an error signal from the beat signal and a first reference signal at frequency f_REF1, and couples the VCO and the EOFD in a negative feedback arrangement that stabilizes the output frequency f_M. A second control circuit generates an error signal from the frequency-divided output signal and a second reference signal at frequency f_REF2, and couples the VCO and one or both of the dual source or the first reference signal in a negative feedback arrangement that stabilizes, or compensates for fluctuations of, a difference frequency v_2S−v_1S.

Claims

1. A microwave-frequency source for generating an output electrical signal at an output frequency f_M, the microwave-frequency source comprising:

(a) a voltage-controlled electrical oscillator arranged so as to generate a VCO output electrical signal at the frequency f_M, a first portion of which forms the output electrical signal of the microwave-frequency source;

(b) a dual optical-frequency reference source arranged so as to generate first and second optical reference signals at respective first and second optical reference frequencies v_1S and v_2S, with v_2S>

v_1S;

(c) an electro-optic frequency divider arranged so as to (i) receive a second portion of the VCO output electrical signal at the frequency f_M, (ii) receive the first and second optical reference signals, (iii) generate a set of multiple sideband optical signals at respective sideband optical frequencies spaced by f_M, and (iv) generate from two of the sideband optical signals a beat electrical signal at a beat frequency δ

f;

(d) first and second electrical reference oscillators arranged so as to generate respective first and second reference electrical signals at respective first and second reference frequencies f_REF1 and f_REF2;

(e) a first electrical control circuit arranged so as to (i) generate a first electrical error signal dependent on relative phase of the beat electrical signal and the first reference electrical signal, and (ii) couple the voltage-controlled oscillator and the electro-optic frequency divider in a negative feedback arrangement, based on the first electrical error signal, that stabilizes the output frequency f_M;

(f) an electrical frequency divider characterized by a division factor m and arranged so as to receive a third portion of the VCO output electrical signal at the frequency f_M, and generate therefrom a divided VCO output electrical signal at a frequency f_M/m; and

(g) a second electrical control circuit arranged so as to (i) generate a second electrical error signal dependent on relative phase of the divided VCO output electrical signal and the second reference electrical signal, and one or both of (ii) couple the voltage-controlled oscillator and the dual optical-frequency reference source in a negative feedback arrangement, based on the second electrical error signal, that stabilizes an optical reference difference frequency v_2S−

v_1S, or (iii) couple the voltage-controlled oscillator and the first electrical reference oscillator in a negative feedback arrangement, based on the second electrical error signal, that alters the first reference frequency f_REF1 so as to at least partly compensate for fluctuations of the optical reference difference frequency v_2S−

v_1S.

2. The microwave-frequency source of claim 1 wherein the second electrical control circuit arranged so as to couple the voltage-controlled oscillator and the first electrical reference oscillator in the negative feedback arrangement, based on the second electrical error signal, that alters the first reference frequency f_REF1 so as to at least partly compensate for fluctuations of the optical reference difference frequency v_2S−

v_1S.

3. The microwave-frequency source of claim 2 wherein the second electrical control circuit includes a corresponding comparator portion arranged so as to generate the second electrical error signal and a corresponding loop filter portion arranged so as to process the second electrical error signal, and the voltage-controlled oscillator and the first electrical reference oscillator are coupled by the second electrical control circuit so as to function as a phase-locked loop.

4. The microwave-frequency source of claim 1 wherein the second electrical control circuit is arranged so as to couple the voltage-controlled oscillator and the dual optical-frequency reference source in the negative feedback arrangement, based on the second electrical error signal, that stabilizes the optical reference difference frequency v_2S−

v_1S.

5. The microwave-frequency source of claim 4 wherein the second electrical control circuit includes a corresponding comparator portion arranged so as to generate the second electrical error signal and a corresponding loop filter portion arranged so as to process the second electrical error signal, and the voltage-controlled oscillator and the dual optical-frequency reference source are coupled by the second electrical control circuit so as to function as a phase-locked loop.

6. The microwave-frequency source of claim 4 wherein (i) the dual optical-frequency reference source includes an optical resonator, (ii) one or both of the first or second optical reference frequencies v_1S or v_2S are resonant in corresponding resonant optical modes of the optical resonator, and (iii) the second electrical control circuit is arranged so as to alter corresponding optical frequencies of resonant optical modes of the resonator, thereby altering the optical reference difference frequency v_2S−

v_1S.

7. The microwave-frequency source of claim 6 wherein (i) the dual optical-frequency reference source includes first and second pump laser sources and the optical resonator is characterized by a Brillouin shift frequency v_B, (ii) one or both of the first and second pump laser sources is frequency-locked at one or both corresponding first and second pump optical frequencies v₁ and v₂ to corresponding resonant optical modes of the optical resonator, (iii) the first and second optical reference signals at the first and second optical reference frequencies v_1S and v_2S comprise stimulated Brillouin laser output generated by optical pumping of the optical resonator simultaneously by the first and second pump laser sources, with v₁−

v_1S and v₂−

v_2S each being substantially equal to v_B or an integer multiple thereof, and (iv) the first and second optical reference frequencies v_1S and v_2S are resonant in corresponding resonant optical modes of the optical resonator.

8. The microwave-frequency source of claim 6 wherein the optical resonator comprises a fiber optical resonator, and the second electrical control circuit is connected to a fiber stretcher arranged so as to enable tuning of the optical reference difference frequency v_2S−

v_1S by altering length of the fiber optical resonator.

9. The microwave-frequency source of claim 8 wherein the fiber optical resonator comprises a fiber-loop optical resonator.

10. The microwave-frequency source of claim 9 wherein the optical resonator is arranged so as to support resonant optical modes propagating in both directions.

11. The microwave-frequency source of claim 9 wherein the optical resonator is arranged so as to support resonant optical modes propagating in only one direction.

12. The microwave-frequency source of claim 1 wherein the electro-optic frequency divider is arranged so as to (i) generate from the first optical reference signal a first subset of the multiple sideband optical signals at respective sideband optical frequencies centered at v_1S and spaced by f_M, (ii) generate from the second optical reference signal a second subset of the multiple sideband optical signals at respective sideband optical frequencies centered at v_2S and spaced by f_M, and (iii) combine on a photodetector one sideband of each subset so as to generate the beat electrical signal at the beat frequency δ

f.

13. The microwave-frequency source of claim 1 wherein the electro-optic frequency divider is arranged so as to (i) combine on a first photodetector the first optical reference signal and a first one of the multiple sidebands so as to generate a first sideband beat electrical signal, (ii) combine on a second photodetector the second optical reference signal and a second one of the multiple sidebands so as to generate a second sideband beat electrical signal, and (ii) mix electrically the first and second sideband beat electrical signals so as to generate the beat electrical signal at the beat frequency δ

f.

14. The microwave-frequency source of claim 1 wherein (i) the electro-optic frequency divider includes one or more electro-optic phase modulators driven by the second portion of the VCO output electrical signal at the frequency f_M, and (ii) the one or more phase modulators are arranged so as to transmit the first optical reference signal, the second optical reference signal, or a sideband-generating optical signal so as to generate the multiple sideband optical signals.

15. The microwave-frequency source of claim 14 wherein (i) the electro-optic frequency divider further includes an intensity modulator driven by the second portion of the VCO output electrical signal at the frequency f_M, a dispersion compensator, an optical amplifier, and a nonlinear optical medium, and (ii) the one or more phase modulators, the intensity modulator, the dispersion compensator, the optical amplifier, and the nonlinear optical medium are arranged in series so as to sequentially, in order, transmit the first optical reference signal, the second optical reference signal, or the sideband-generating optical signal so as to generate the multiple sideband optical signals.

16. The microwave-frequency source of claim 1 wherein the set of sideband optical signals includes at least 10 different sideband optical frequencies.

17. The microwave-frequency source of claim 1 wherein the set of sideband optical signals includes at least 100 different sideband optical frequencies.

18. The microwave-frequency source of claim 1 wherein the frequency f_M is between about 0.3 GHz and about 300 GHz.

19. The microwave-frequency source of claim 1 wherein both the first reference frequency f_REF1 and the beat frequency δ

f are between about 1.0 MHz and about 1.0 GHz.

20. The microwave-frequency source of claim 1 wherein both the second reference frequency f_REF2 and the frequency f_M/m are between about 1.0 MHz and about 10.0 GHz.

21. The microwave-frequency source of claim 1 wherein the optical reference difference frequency v_2S−

v_1S is greater than about 0.10 THz.

22. The microwave-frequency source of claim 1 wherein the optical reference difference frequency v_2S−

v_1S is greater than about 1.0 THz.

23. The microwave-frequency source of claim 1 wherein phase noise of the output electrical signal of the microwave-frequency source is reduced by a factor of approximately ((v_2S−

v_1S)/f_M)² relative to phase noise of an optical reference difference frequency signal at the optical reference difference frequency v_2S−

v_1S of the dual optical-frequency reference source.

24. The microwave-frequency source of claim 1 wherein (v_2S−

v_1S)/f_M is greater than or equal to 10.

25. The microwave-frequency source of claim 1 wherein (v_2S−

v_1S)/f_M is greater than or equal to 50.

26. The microwave-frequency source of claim 1 wherein the first and second optical reference frequencies v₁ and v₂ are each between about 75 THz and about 750 THz.

27. The microwave-frequency source of claim 1 wherein the output frequency f_M is stabilized so as to (i) maintain fluctuations of the output frequency f_M within an operationally acceptable output bandwidth or (ii) maintain phase noise of the output electrical signal within an operationally acceptable reference phase noise level.

28. The microwave-frequency source of claim 27 wherein the operationally acceptable reference bandwidth is less than about 1.0 Hz for time periods greater than about 10. seconds.

29. The microwave-frequency source of claim 27 wherein the operationally acceptable reference bandwidth is less than about 1.0 Hz for time periods greater than about 1.0 hour.

30. The microwave-frequency source of claim 1 wherein the dual optical-frequency reference source is stabilized so as to (i) maintain fluctuations of the optical reference difference frequency v_2S−

v_1S within an operationally acceptable output bandwidth or (ii) maintain phase noise of the optical reference difference frequency signal within an operationally acceptable reference phase noise level.

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

(a) using a voltage-controlled electrical oscillator, generating a VCO output electrical signal at the frequency f_M, a first portion of which forms the output electrical signal of the microwave-frequency source;

(b) using a dual optical-frequency reference source, generating first and second optical reference signals at respective first and second optical reference frequencies v_1S and v_2S, with v_2S>

v_1S;

(c) using an electro-optic frequency divider, (i) receiving a second portion of the VCO output electrical signal at the frequency f_M, (ii) receiving the first and second optical reference signals, (iii) generating a set of multiple sideband optical signals at respective sideband optical frequencies spaced by f_M, and (iv) generating from two of the sideband optical signals a beat electrical signal at a beat frequency δ

f;

(d) using first and second electrical reference oscillators, generating respective first and second reference electrical signals at respective first and second reference frequencies f_REF1 and f_REF2;

(e) using a first electrical control circuit, (i) generating a first electrical error signal dependent on relative phase of the beat electrical signal and the first reference electrical signal, and (ii) coupling the voltage-controlled oscillator and the electro-optic frequency divider in a negative feedback arrangement, based on the first electrical error signal, thereby stabilizing the output frequency f_M;

(f) using an electrical frequency divider characterized by a division factor m, receiving a third portion of the VCO output electrical signal at the frequency f_M, and generating therefrom a divided VCO output electrical signal at a frequency f_M/m; and

(g) using a second electrical control circuit, (i) generating a second electrical error signal dependent on relative phase of the divided VCO output electrical signal and the second reference electrical signal, and one or both of (ii) coupling the voltage-controlled oscillator and the dual optical-frequency reference source in a negative feedback arrangement, based on the second electrical error signal, thereby stabilizing an optical reference difference frequency v_2S−

v_1S, or (iii) coupling the voltage-controlled oscillator and the first electrical reference oscillator in a negative feedback arrangement, based on the second electrical error signal, thereby altering the first reference frequency f_REF1 so as to at least partly compensate for fluctuations of the optical reference difference frequency v_2S−

v_1S.