Stabilized microwave-frequency source

• US 9,450,673 B2
• Filed: 01/26/2015
• Issued: 09/20/2016
• Est. Priority Date: 01/24/2014
• Status: Active Grant

• Current Assignee: California Institute of Technology, THE GOVERNMENT OF THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY OF COMMERCE
• Original Assignee: California Institute of Technology, The United States of America As Represented By The Secretary of Commerce
• Inventors: Vahala, Kerry, Diddams, Scott, Li, Jiang, Yi, Xu, Lee, Hansuek
• Primary Examiner(s): Payne, David
• Assistant Examiner(s): Motsinger, Tanya
• Application Number:
  US14/605,977
• Publication Number:
  US 20150236784A1
• Time in Patent Office: 603 Days
• Field of Search: 398/182, 398/195
• US Class Current: 1/1
• CPC Class Codes: H03B 17/00 Generation of oscillations using radiation source and detector, e.g. with interposed variable obturator
• CPC Class Codes: H03L 7/08 Details of the phase-locked loop
• CPC Class Codes: H03L 7/16 Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop
• CPC Class Codes: H04B 10/2507 for the reduction or elimination of distortion or dispersion
• CPC Class Codes: H04B 10/2575 Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
• 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
• CPC Class Codes: H04L 7/0075 with photonic or optical means
• CPC Class Codes: H04L 7/0091 Transmitter details

Abstract

A microwave-frequency source at frequency f_M comprises: a dual optical-frequency reference source, an electro-optic sideband generator, an optical bandpass filter, an optical detector, a reference oscillator, an electrical circuit, and a voltage-controlled oscillator (VCO). The sideband generator modulates dual optical reference signals at v₂ and v₁ to generate sideband signals at v₁±n₁f_M and v₂±n₂f_M. The bandpass filter transmits sideband signals at v₁+N₁f_M and v₂−N₂f_M. The optical detector generates a beat note at (v₂−N₂f_M)−(v₁+N₁f_M). The beat note and a reference oscillator signal are processed by the circuit to generate a loop-filtered error signal to input to the VCO. Output of the VCO at f_M drives the sideband generator and forms the microwave-frequency output signal. The resultant frequency division results in reduced phase noise on the microwave-frequency signal.

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 dual optical-frequency reference source arranged so as to generate (i) a first optical reference signal at a first optical reference frequency v₁ and (ii) a second optical reference signal at a second optical reference frequency v₂>

v₁;

(b) an electro-optic sideband generator arranged so as to (i) receive the first and second optical reference signals and a sideband generator input electrical signal at the frequency f_M and (ii) generate therefrom multiple sideband optical signals at respective sideband optical frequencies of the form v₁±

n₁f_M and v₂±

n₂f_M, wherein n₁ and n₂ are integers;

(c) an optical bandpass filter arranged so as to transmit a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₁+N₁f_M and the sideband optical signal at a frequency v₂−

N₂f_M, wherein N₁ and N₂ are integers;

(d) an optical detector arranged so as to (i) receive the transmitted sideband optical signals and (ii) generate an optical detector electrical signal at a beat frequency f_BEAT=(v₂−

N₂f_M)−

(v₁+N₁f_M);

(e) a reference oscillator arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency f_R;

(f) an electrical circuit arranged so as to (i) receive the optical detector electrical signal and the reference oscillator electrical signal, (ii) generate therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) process the electrical error signal using a loop-filter portion of the electrical circuit; and

(g) a voltage-controlled electrical oscillator arranged so as to (i) receive the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generate a VCO output electrical signal at the frequency f_M, wherein a first portion of the VCO output electrical signal is received by the electro-optic sideband generator as the sideband generator input electrical signal and a second portion of the VCO output electrical signal forms the output electrical signal of the microwave-frequency source,

(h) wherein reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop.

2. 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 about (N₁+N₂)² relative to phase noise of a reference difference frequency signal at a reference difference frequency v₂−

v₁ of the dual optical-frequency reference source.

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

4. The microwave-frequency source of claim 1 wherein the output frequency f_M is between about 1 GHz and about 100 GHz.

5. The microwave-frequency source of claim 1 wherein the reference oscillator frequency and the beat frequency are between about 1 MHz and about 500 MHz.

6. The microwave-frequency source of claim 1 wherein the reference oscillator frequency and the beat frequency are between about 5 MHz and about 100 MHz.

7. The microwave-frequency source of claim 1 wherein the reference oscillator frequency and the beat frequency are between about 10 MHz and about 50 MHz.

8. The microwave-frequency source of claim 1 wherein the reference oscillator comprises a crystal oscillator.

9. The microwave-frequency source of claim 1 wherein the reference oscillator comprises an electrical oscillator.

10. The microwave-frequency source of claim 1 wherein a reference difference frequency v₂−

v₁ is greater than about 100 GHz.

11. The microwave-frequency source of claim 1 wherein a reference difference frequency v₂−

v₁ is greater than about 1 THz.

12. The microwave-frequency source of claim 1 wherein a reference difference frequency v₂−

v₁ is greater than about 10 THz.

13. The microwave-frequency source of claim 1 wherein a reference difference frequency v₂−

v₁ is greater than about 100 THz.

14. The microwave-frequency source of claim 1 wherein N₁+N₂ is greater than or equal to 10.

15. The microwave-frequency source of claim 1 wherein N₁+N₂ is greater than or equal to 50.

16. The microwave-frequency source of claim 1 wherein N₁+N₂ is greater than or equal to 100.

17. The microwave-frequency source of claim 1 wherein N₁+N₂ is greater than or equal to 1000.

18. 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.

19. The microwave-frequency source of claim 1 wherein the first and second optical reference frequencies v₁ and v₂ are each between about 120 THz and about 430 THz.

20. The microwave-frequency source of claim 1 wherein the first and second optical reference frequencies v₁ and v₂ are each between about 150 THz and about 300 THz.

21. The microwave-frequency source of claim 1 wherein the dual optical-frequency reference source is stabilized so as to (i) maintain fluctuations of a reference difference frequency v₂−

v₁ within an operationally acceptable optical reference bandwidth or (ii) maintain phase noise of a reference difference frequency signal within an operationally acceptable reference phase noise level.

22. The microwave-frequency source of claim 21 wherein the operationally acceptable reference bandwidth is less than about 100 Hz over about a 1 second timescale.

23. The microwave-frequency source of claim 21 wherein the operationally acceptable reference bandwidth is less than about 1 Hz over about a 1 second timescale.

24. The microwave-frequency source of claim 21 wherein the operationally acceptable reference phase noise level is about −

40 dBc/Hz at 100 Hz offset frequency and about −

80 dBc/Hz at 10 kHz offset frequency.

25. The microwave-frequency source of claim 21 wherein the operationally acceptable reference phase noise level is about −

80 dBc/Hz at 100 Hz offset frequency and about −

125 dBc/Hz at 10 kHz offset frequency.

26. The microwave-frequency source of claim 1 wherein (i) the dual optical-frequency reference source comprises first and second pump laser sources and an optical resonator, (ii) a free spectral range of the optical resonator is substantially equal to an integer submultiple of a Brillouin shift frequency of the optical resonator, (iii) each one of the first and second pump laser sources is frequency-locked to a corresponding resonant optical mode of the optical resonator, and (iv) the first and second optical reference signals comprise stimulated Brillouin laser output generated by optical pumping of the optical resonator simultaneously by the first and second pump laser sources, respectively.

27. The microwave-frequency source of claim 26 wherein the free spectral range of the optical resonator is substantially equal to the Brillouin shift frequency of the optical resonator.

28. The microwave-frequency source of claim 26 wherein the optical resonator comprises silica and the Brillouin shift frequency of the optical resonator is about 10.9 GHz.

29. The microwave-frequency source of claim 26 wherein the optical resonator comprises a ring optical resonator.

30. The microwave-frequency source of claim 29 wherein the ring optical resonator comprises a disk optical resonator.

31. The microwave-frequency source of claim 26 wherein the optical resonator comprises a fiber optical resonator.

32. The microwave-frequency source of claim 31 wherein the optical resonator comprises a fiber Fabry-Perot optical resonator.

33. The microwave-frequency source of claim 31 wherein the fiber optical resonator comprises a fiber-loop optical resonator.

34. The microwave-frequency source of claim 26 wherein each one of the first and second pump laser sources is frequency-locked to the corresponding resonant optical mode of the optical resonator by a Pound-Drever-Hall mechanism.

35. The microwave-frequency source of claim 1 wherein the dual optical-frequency reference source comprises a dual-mode laser source.

36. The microwave-frequency source of claim 1 wherein the dual optical-frequency reference source comprises first and second reference laser sources, wherein the first and second laser sources are each frequency-locked to a corresponding distinct resonant optical mode of a common optical reference cavity.

37. The microwave-frequency source of claim 1 wherein the dual optical-frequency reference source comprises first and second reference laser sources, wherein the first and second laser sources are each frequency-locked to a corresponding distinct atomic transition.

38. The microwave-frequency source of claim 1 wherein (i) the electro-optic sideband generator comprises one or more electro-optic phase modulators each driven by a corresponding portion of the sideband generator input electrical signal at the frequency f_M, and (ii) the one or more phase modulators are arranged so as to transmit the first and second optical reference signals so as to generate the multiple optical sideband signals.

39. The microwave-frequency source of claim 1 wherein (i) the electro-optic sideband generator comprises two or more electro-optic phase modulators each driven by a corresponding portion of the sideband generator input electrical signal at the frequency f_M, and (ii) the two or more phase modulators are arranged in series so as to sequentially transmit the first and second optical reference signals so as to generate the multiple optical sideband signals.

40. The microwave-frequency source of claim 39 wherein pairs of sideband optical signals are generated with n₁+n₂ ranging from 2 up to at least 30.

41. The microwave-frequency source of claim 39 wherein pairs of sideband optical signals are generated with n₁+n₂ ranging from 2 up to at least 100.

42. The microwave-frequency source of claim 1 wherein (i) the electro-optic sideband generator further comprises one or more electro-optic phase modulators each driven by a corresponding portion of the sideband generator input electrical signal at the frequency f_M, an intensity modulator driven by a corresponding portion of the sideband generator input 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 and second optical reference signals so as to generate the multiple optical sideband signals.

43. The microwave-frequency source of claim 42 wherein pairs of sideband optical signals are generated with n₁+n₂ ranging from 2 up to at least 100.

44. The microwave-frequency source of claim 42 wherein pairs of sideband optical signals are generated with n₁+n₂ ranging from 2 up to at least 10000.

45. The microwave-frequency source of claim 42 wherein (i) the intensity modulator comprises an electro-optic Mach-Zehnder modulator, (ii) the dispersion compensator comprises a suitably dispersive optical fiber, (iii) the optical amplifier comprises a doped optical fiber amplifier and (iv) the nonlinear optical medium comprises a nonlinear optical fiber.

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

(a) using a dual optical-frequency reference source, generating (i) a first optical reference signal at a first optical reference frequency v₁ and (ii) a second optical reference signal at a second optical reference frequency v₂>

v₁;

(b) using an electro-optic sideband generator, (i) receiving the first and second optical reference signals and a sideband generator input electrical signal at the frequency f_M and (ii) generating therefrom multiple sideband optical signals at respective sideband optical frequencies of the form v₁±

n₁f_M and v₂±

n₂f_M, wherein n₁ and n₂ are integers;

(c) using an optical bandpass filter, transmitting a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₁+N₁f_M and the sideband optical signal at a frequency v₂−

N₂f_M, wherein N₁ and N₂ are integers;

(d) using an optical detector, (i) receiving the transmitted sideband optical signals and (ii) generating an optical detector electrical signal at a beat frequency f_BEAT=(v₂−

N₂f_M)−

(v₁+N₁f_M);

(e) using a reference oscillator, generating a reference oscillator electrical signal at a reference oscillator frequency f_R;

(f) using an electrical circuit, (i) receiving the optical detector electrical signal and the reference oscillator electrical signal, (ii) using a comparator portion of the electrical circuit, generating, from the optical detector electrical signal and the reference oscillator electrical signal, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) using a loop-filter portion of the electrical circuit, processing the electrical error signal; and

(g) using a voltage-controlled electrical oscillator, (i) receiving the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generating a VCO output electrical signal at the frequency f_M, wherein a first portion of the VCO output electrical signal is received by the electro-optic sideband generator as the sideband generator input electrical signal and a second portion of the VCO output electrical signal forms the microwave-frequency output electrical signal,

(h) wherein reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop.