Optical frequency divider based on an electro-optical-modulator frequency comb

• US 9,905,999 B2
• Filed: 02/25/2016
• Issued: 02/27/2018
• Est. Priority Date: 02/26/2015
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Li, Jiang, Vahala, Kerry
• Primary Examiner(s): Nguyen, Hai L
• Application Number:
  US15/053,876
• Publication Number:
  US 20160254646A1
• Time in Patent Office: 733 Days
• CPC Class Codes: H01S 3/0085 Modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H01S 3/0627 the resonator being monolithic, e.g. microlaser
• CPC Class Codes: H01S 3/06791 Fibre ring lasers fibre laser gyrometers G01C19/72
• 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 3/1307 Stabilisation of the phase
• 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 5/0078 for frequency filtering
• CPC Class Codes: H01S 5/041 Optical pumping
• CPC Class Codes: H01S 5/0623 using the beating between two closely spaced optical frequencies, i.e. heterodyne mixing
• CPC Class Codes: H01S 5/06817 Noise reduction
• CPC Class Codes: H01S 5/0687 Stabilising the frequency of the laser
• CPC Class Codes: H01S 5/40 Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30 H01S5/50 takes precedence
• CPC Class Codes: H03B 17/00 Generation of oscillations using radiation source and detector, e.g. with interposed variable obturator
• CPC Class Codes: H03L 7/00 Automatic control of frequency or phase; Synchronisation

Abstract

Microwave-frequency signal generation by generating multiple sideband optical signals separated by phase-modulation frequency f_M, generating beat signals between one or two sidebands and one or two optical reference signals, generating a loop-filtered error signal by comparing an electrical reference signal to one of the beat signals or their difference, and controlling with the error signal in a phase-locked loop arrangement a voltage-controlled oscillator (VCO) that drives the sideband generation at the frequency f_M. A portion of the VCO output is the generated 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 a third optical reference signal at a third optical reference frequency v₀ 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 v₀±

nf_M, wherein n is an integer;

(c) one or both of (i) a first optical bandpass filter arranged so as to transmit the first optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₀−

N₁f_M, wherein N₁ is an integer, or (ii) a second optical bandpass filter arranged so as to transmit the second optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₀+N₂f_M, wherein N₂ is an integer;

(d) one or both of (i) a first optical detector arranged so as to receive the optical signals transmitted by the first optical bandpass filter and to generate therefrom a first optical detector electrical signal at a first beat frequency f_BEAT1=|v₁−

(v₀−

N₁f_M)|, or (ii) a second optical detector arranged so as to receive the optical signals transmitted by the second optical bandpass filter and to generate therefrom a second optical detector electrical signal at a second beat frequency f_BEAT2=|v₂−

(v₀+N₂f_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 reference oscillator electrical signal and only one of (A) the first optical detector electrical signal, (B) the second optical detector electrical signal, or (C) a difference electrical signal generated from the first and second optical detector electrical signals by an electrical frequency mixer at an electrical difference frequency f_DIFF=f_BEAT2−

f_BEAT1=|v₂−

v₁−

(N₁+N₂)·

f_M|, (ii) generate therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the electrical signals received by the electrical circuit, 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:

(b′

) the third optical reference frequency v₀ differs from the first optical reference frequency v₁ and from the second optical reference frequency v₂;

(c′

) the first optical bandpass filter is arranged so as to transmit the first optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at the frequency v₀−

N₁f_M, and the second optical bandpass filter is arranged so as to transmit the second optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at the frequency v₀+N₂f_M;

(d′

) the first optical detector arranged so as to receive the optical signals transmitted by the first optical bandpass filter and to generate therefrom the first optical detector electrical signal at the first beat frequency f_BEAT=|v₁−

(v₀−

N₁f_M)|, and the second optical detector arranged so as to receive the optical signals transmitted by the second optical bandpass sideband optical signals and to generate therefrom the second optical detector electrical signal at the second beat frequency f_BEAT1=|v₂−

(v₀+N₂f_M)|; and

(f′

) the electrical circuit is arranged so as to (i) receive the reference oscillator electrical signal and the difference electrical signal at the electrical difference frequency f_DIFF=f_BEAT2−

f_BEAT1=|v₂−

v₁−

(N₁+N₂)·

f_M|, and (ii) generate therefrom, using the comparator portion of the electrical circuit, the electrical error signal dependent on the relative phase of the reference oscillator electrical signal and the difference electrical signal received by the electrical circuit, and (iii) process the electrical error signal using the loop-filter portion of the electrical circuit.

3. The microwave-frequency source of claim 2 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 optical difference frequency v₂−

v₁ of the dual optical-frequency reference source.

4. The microwave-frequency source of claim 1 wherein:

(b′

) the electro-optic sideband generator is arranged so as to (i) receive a portion of the first optical reference signal as the third optical reference signal so that v₀=v₁ and (ii) generate therefrom multiple sideband optical signals at respective sideband optical frequencies of the form v₁±

nf_M, wherein n is an integer;

(c′

) the second optical bandpass filter is arranged so as to transmit the second optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₁+N₂f_M;

(d′

) the second optical detector is arranged so as to receive the optical signals transmitted by the second optical bandpass filter and to generate therefrom the second optical detector electrical signal at the second beat frequency f_BEAT2=|v₂−

(+N₂f_M)|; and

(f′

) the electrical circuit is arranged so as to (i) receive the reference oscillator electrical signal and the second optical detector electrical signal, and (ii) generate therefrom, using the comparator portion of the electrical circuit, the electrical error signal dependent on the relative phase of the reference oscillator electrical signal and the second optical detector electrical signal received by the electrical circuit, and (iii) process the electrical error signal using the loop-filter portion of the electrical circuit.

5. The microwave-frequency source of claim 4 wherein phase noise of the output electrical signal of the microwave-frequency source is reduced by a factor of about (N₂)² relative to phase noise of a reference difference frequency signal at a reference optical difference frequency v₂−

v₁ of the dual optical-frequency reference source.

6. The microwave-frequency source of claim 1 wherein:

(b′

) the electro-optic sideband generator is arranged so as to (i) receive a portion of the second optical reference signal as the third optical reference signal so that v₀=v₂ and (ii) generate therefrom multiple sideband optical signals at respective sideband optical frequencies of the form v₂±

nf_M, wherein n is an integer;

(c′

) the first optical bandpass filter is arranged so as to transmit the first optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₂−

N₁f_M;

(d′

) the first optical detector is arranged so as to receive the optical signals transmitted by the first optical bandpass filter and to generate therefrom the first optical detector electrical signal at the first beat frequency f_BEAT1=|v₁−

(v₂−

N₁f_M)|; and

(f′

) the electrical circuit is arranged so as to (i) receive the reference oscillator electrical signal and the first optical detector electrical signal, and (ii) generate therefrom, using the comparator portion of the electrical circuit, the electrical error signal dependent on the relative phase of the reference oscillator electrical signal and the first optical detector electrical signal received by the electrical circuit, and (iii) process the electrical error signal using the loop-filter portion of the electrical circuit.

7. The microwave-frequency source of claim 6 wherein phase noise of the output electrical signal of the microwave-frequency source is reduced by a factor of about (N₁)² relative to phase noise of a reference difference frequency signal at a reference optical difference frequency v₂−

v₁ of the dual optical-frequency reference source.

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

9. The microwave-frequency source of claim 8 wherein the output frequency f_M is between about 1 GHz and about 100 GHz.

10. The microwave-frequency source of claim 1 wherein the reference oscillator frequency is between about 1 MHz and about 1 GHz.

11. The microwave-frequency source of claim 10 wherein the reference oscillator frequency is between about 10 MHz and about 100 MHz.

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

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

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

v₁ is greater than about 100 GHz.

15. The microwave-frequency source of claim 14 wherein a reference optical difference frequency v₂−

v₁ is greater than about 1 THz.

16. The microwave-frequency source of claim 14 wherein a reference optical difference frequency v₂−

v₁ is greater than about 10 THz.

17. The microwave-frequency source of claim 14 wherein a reference optical difference frequency v₂−

v₁ is greater than about 100 THz.

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

19. The microwave-frequency source of claim 18 wherein N₁+N₂ is greater than or equal to 50.

20. The microwave-frequency source of claim 18 wherein N₁+N₂ is greater than or equal to 100.

21. The microwave-frequency source of claim 18 wherein N₁+N₂ is greater than or equal to 1000.

22. 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.

23. The microwave-frequency source of claim 22 wherein the first and second optical reference frequencies v₁ and v₂ are each between about 120 THz and about 430 THz.

24. The microwave-frequency source of claim 22 wherein the first and second optical reference frequencies v₁ and v₂ are each between about 150 THz and about 300 THz.

25. 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 optical difference frequency v₂−

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

26. The microwave-frequency source of claim 25 wherein the operationally acceptable reference bandwidth is less than about 100 Hz over about a 1 second timescale.

27. The microwave-frequency source of claim 25 wherein the operationally acceptable reference bandwidth is less than about 1 Hz over about a 1 second timescale.

28. The microwave-frequency source of claim 25 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.

29. The microwave-frequency source of claim 25 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.

30. 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 a Brillouin shift frequency of the optical resonator or an integer submultiple thereof, (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.

31. The microwave-frequency source of claim 30 wherein the third optical reference signal is comprises a portion of the optical output of one of the first or second pump laser sources.

32. The microwave-frequency source of claim 30 wherein the frequency f_M is about equal to the free spectral range of the optical resonator or an integer submultiple thereof.

33. The microwave-frequency source of claim 30 wherein the free spectral range of the optical resonator is substantially equal to the Brillouin shift frequency of the optical resonator.

34. The microwave-frequency source of claim 30 wherein the optical resonator comprises silica and the Brillouin shift frequency of the optical resonator is about 10.9 GHz.

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

36. The microwave-frequency source of claim 35 wherein the ring optical resonator comprises a disk optical resonator.

37. The microwave-frequency source of claim 30 wherein the optical resonator comprises a fiber optical resonator.

38. The microwave-frequency source of claim 37 wherein the optical resonator comprises a fiber Fabry-Perot optical resonator.

39. The microwave-frequency source of claim 37 wherein the fiber optical resonator comprises a fiber-loop optical resonator.

40. The microwave-frequency source of claim 30 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.

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

42. The microwave-frequency source of claim 41 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.

43. The microwave-frequency source of claim 42 wherein the third optical reference signal is provided by a third laser source, and the third laser source is frequency-locked to a corresponding resonant optical mode, distinct from the optical modes to which are frequency-locked the first and second laser sources, of the common optical reference cavity.

44. The microwave-frequency source of claim 42 wherein the frequency f_M is about equal to a free spectral range of the common optical reference cavity or an integer submultiple thereof.

45. 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 third optical reference signal so as to generate the multiple optical sideband signals.

46. The microwave-frequency source of claim 45 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 third optical reference signal so as to generate the multiple optical sideband signals.

47. The microwave-frequency source of claim 46 wherein pairs of sideband optical signals are generated with n ranging from 2 up to at least 30.

48. The microwave-frequency source of claim 46 wherein pairs of sideband optical signals are generated with n ranging from 2 up to at least 100.

49. The microwave-frequency source of claim 46 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 third optical reference signal so as to generate the multiple optical sideband signals.

50. The microwave-frequency source of claim 49 wherein pairs of sideband optical signals are generated with n ranging from 2 up to at least 100.

51. The microwave-frequency source of claim 49 wherein pairs of sideband optical signals are generated with n ranging from 2 up to at least 1000.

52. The microwave-frequency source of claim 49 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.

53. 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 a third optical reference signal at a third optical reference frequency v₀ 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 v₀±

nf_M, wherein n is an integer;

(c) using one or both of a first optical bandpass filter or a second optical bandpass filter, (i) transmitting through the first optical bandpass filter the first optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₀−

N₁f_M, wherein N₁ is an integer, or (ii) transmitting through the second optical bandpass filter the second optical reference signal and a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v₀+N₂f_M, wherein N₂ is an integer;

(d) using one or both of a first optical detector or a second optical detector, (i) receiving at the first optical detector the optical signals transmitted by the first optical bandpass filter and generating therefrom a first optical detector electrical signal at a first beat frequency f_BEAT1=|v₁−

(v₀−

N₁f_M)|, or (ii) receiving at the second optical detector the optical signals transmitted by the second optical bandpass filter and generating therefrom a second optical detector electrical signal at a second beat frequency f_BEAT2=|v₂−

(v₀+N₂f_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 reference oscillator electrical signal and only one of (A) the first optical detector electrical signal, (B) the second optical detector electrical signal, or (C) a difference electrical signal generated from the first and second optical detector signals by an electrical frequency mixer at an electrical difference frequency f_DIFF=f_BEAT2−

f_BEAT1=|v₂−

v₁−

(N₁+N₂)·

f_M|, (ii) generating therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the electrical signals received by the electrical circuit, and (iii) processing the electrical error signal using a loop-filter portion of the electrical circuit; 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 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.