ATOMIC CLOCK BASED ON AN OPTO-ELECTRONIC OSCILLATOR

• US 20040109217A1
• Filed: 04/09/2003
• Published: 06/10/2004
• Est. Priority Date: 04/09/2002
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Maleki, Luftollah, Yu, Nan
• Granted Patent: US 6,762,869 B2
• Time in Patent Office: Days
• US Class Current: 359/239
• CPC Class Codes: G04F 5/00 Apparatus for producing preselected time intervals for use as timing standards generating clock signals for electric digital computers G06F1/04
• CPC Class Codes: G04F 5/14 using atomic clocks
• CPC Class Codes: G04G 7/00 Synchronisation radio-controlled time-pieces G04R

Abstract

Opto-electronic oscillators having a frequency locking mechanism to stabilize the oscillation frequency of the oscillators to an atomic frequency reference. Whispering gallery mode optical resonators may be used in such oscillators to form compact atomic clocks.

Claims

1. A device, comprising:

an opto-electronic oscillator having an opto-electronic loop with an optical section and an electrical section, said oscillator operable to generate an oscillation at an oscillation frequency; and

an atomic reference module including an atomic frequency reference and coupled to receive and interact with at least a portion of an optical signal in said optical section to produce a feedback signal, wherein said oscillator is operable to respond to said feedback signal to stabilize said oscillation frequency with respect to said atomic frequency reference.

2. The device as in claim 1, wherein said oscillator includes an optical modulator to receive a laser carrier at a carrier frequency and to modulate said laser carrier at a modulation frequency to produce a modulated optical signal in response to an electrical output of said electrical section, wherein said optical section receives at least a portion of said modulated optical signal.

3. The device as in claim 2, wherein said optical section includes an optical delay element to produce a delay in said opto-electronic loop.

4. The device as in claim 3, wherein said optical delay element includes an optical resonator.

5. The device as in claim 4, wherein said optical resonator is a whispering gallery mode resonator.

6. The device as in claim 2, wherein said atomic reference module includes an atomic cell located in said optical section to filter optical energy.

7. The device as in claim 2, wherein said atomic reference module comprises a feedback loop having an atomic cell to provide said atomic frequency reference and to transmit said portion of said optical signal, an optical detector to convert optical transmission of said atomic cell into a monitor signal, and a feedback unit to produce said feedback signal by processing said monitor signal and to use said feedback to control said optical modulator.

8. The device as in claim 2, wherein said optical modulator includes an optical resonator within which said laser carrier is modulated, said optical resonator operable to produce an optical delay in said opto-electronic loop.

9. The device as in claim 8, wherein said optical resonator is a whispering gallery mode resonator.

10. The device as in claim 9, wherein said whispering gallery mode resonator is formed an electro-optical material.

11. The device as in claim 2, wherein said optical modulator is a phase modulator.

12. The device as in claim 2, wherein said optical modulator is an amplitude modulator.

13. The device as in claim 1, wherein said oscillator includes a laser coupled to receive an electrical signal from said electrical section and operable to modulate a laser gain at a modulation frequency in response to said electrical signal to produce a modulated optical signal having modulation bands in a laser carrier at a laser frequency, wherein said optical section receives at least a portion of said modulated optical signal.

14. The device as in claim 13, wherein said atomic reference module includes an atomic cell located in said optical section to filter optical energy.

15. The device as in claim 13, wherein said atomic reference module comprises a feedback loop having an atomic cell to provide said atomic frequency reference and to transmit said portion of said optical signal, an optical detector to convert optical transmission of said atomic cell into a monitor signal, and a feedback unit to produce said feedback signal by processing said monitor signal, said feedback unit operable to control said laser with said feedback signal to stabilize said oscillation frequency.

16. The device as in claim 1, wherein said atomic reference module comprises an atomic cell having atoms with an energy structure comprising three different energy levels that allow for two different optical transitions that share a common energy level, wherein one modulation band and another immediate adjacent band in said modulated optical signal are in resonance with said two different optical transitions, respectively

17. The device as in claim 16, wherein said atomic cell includes a solid-state material to form a matrix which hold said atoms.

18. A device, comprising:

an optical modulator to modulate an optical carrier signal at a modulation frequency in response to an electrical modulation signal to produce a plurality of modulation bands in said optical carrier signal;

an opto-electronic loop having an optical section coupled to receive a first portion of said optical carrier signal from said optical modulator, and an electrical section to produce said electrical modulation signal according to said first portion of said optical carrier signal, said opto-electronic loop causing a delay in said electrical modulation signal to provide a positive feedback to said optical modulator;

a frequency reference module having an atomic transition in resonance with a selected modulation band among said modulation bands and coupled to receive a second portion of said optical carrier signal, said second portion interacting with said atomic transition to generate an optical monitor signal; and

a feedback module to receive said optical monitor signal and to control said optical modulator in response to information in said optical monitor signal to lock said modulation frequency relative to an atomic reference frequency associated with said atomic transition.

19. The device as in claim 18, wherein said optical section includes an optical resonator.

20. The device as n claim 19, wherein said optical resonator has a structure to support at least one whispering gallery mode.

21. The device as in claim 18, wherein said optical section includes a fiber segment.

22. The device as in claim 18, wherein said opto-electronic loop includes an optical detector coupled between said optical and said electrical sections to convert said second portion into an electrical signal as an input to said electrical section.

23. The device as in claim 18, wherein said frequency reference module includes a second, different atomic transition that shares a common energy level with said atomic transition, and wherein said second atomic transition is in resonance with a spectral component in said optical carrier signal.

24. The device as in claim 18, wherein said spectral component is separated from said selected modulation band in frequency by said modulation frequency.

25. The device as in claim 18, wherein said feedback module comprises:

an optical splitter to couple a portion of optical energy in said optical section as a reference optical signal; and

a differential detector to convert said reference optical signal and said optical monitor signal into two detector signals and to produce a differential signal which controls said optical modulator to lock said modulation frequency.

26. The device as in claim 25, wherein said feedback module operates to use said differential signal to control a DC bias in said optical modulator.

27. A device, comprising:

an opto-electronic oscillator to receive an optical signal at an optical carrier frequency and to output a modulated optical signal having a carrier band at said optical carrier frequency and a plurality of modulation bands;

an atomic filter to receive and filter at least a portion of said modulated optical signal to produce an optical monitor signal, said atomic filter having atoms with an energy structure comprising three different energy levels that allow for two different optical transitions that share a common energy level, wherein one modulation band and another immediate adjacent band in said modulated optical signal are in resonance with said two different optical transitions, respectively; and

a feedback control coupled to receive said optical monitor signal and to control said opto-electronic oscillator to lock a frequency of each modulation band relative to an atomic frequency reference in said three different energy levels according to information in said optical monitor signal indicative of a variation in said frequency relative to said atomic frequency reference.

28. The device as in claim 27, wherein said opto-electronic oscillator comprises:

an optical resonator to support whispering gallery modes and formed of an electro-optic material;

an electrical control coupled to said optical resonator to apply a control electrical field to modulate a property of said electro-optic material;

an optical coupler positioned to couple said optical signal into said optical resonator in one whispering gallery mode and couple energy in said one whispering gallery mode out to produce said modulated optical signal;

an optical loop to receive said modulated optical signal; and

a photodetector coupled to said optical loop to convert optical energy in said optical loop into a detector signal, said photodetector coupled to send said detector signal to said electrical control.

29. The device as in claim 28, wherein said feedback control comprises an optical detector to convert said optical monitor signal into a bias control signal and to apply said bias control signal to control a DC bias in said control electrical field at said optical resonator.

30. The device as in claim 27, wherein said opto-electronic oscillator comprises:

a semiconductor electro-absorption modulator to modulate said optical signal in response to an electrical control signal;

a first optical waveguide to receive said modulated optical signal from said semiconductor electro-absorption modulator;

a whispering gallery mode resonator optically coupled to receive at least part of said modulated optical signal;

a second optical waveguide optically coupled to to receive an output optical signal from said whispering gallery mode resonator;

a photodetector to convert said output optical signal into an electrical signal; and

an electrical unit connected between said photodetector and said semiconductor electro-absorption modulator to apply a portion of said electrical signal as said electrical control signal.

31. A method, comprising:

modulating a coherent laser beam at a modulation frequency to produce a modulated optical beam;

transmitting a portion of the modulated optical beam through an optical delay element to cause a delay;

converting the portion from the optical delay element into an electrical signal;

using the electrical signal to control modulation of the coherent laser beam to cause an oscillation at the modulation frequency;

obtaining a deviation of the modulation frequency from an atomic frequency reference; and

adjusting the modulation of the coherent laser beam to reduce the deviation.

32. The method as in claim 31, further comprising:

using a tunable laser to produce the coherent laser beam; and

adjusting the frequency of the tunable laser in response to the deviation to stabilize the tunable laser.

33. A device, comprising:

an optical modulator to modulate an optical carrier signal at a modulation frequency in response to an electrical modulation signal to produce a plurality of modulation bands in said optical carrier signal; and

an opto-electronic loop having an optical section coupled to receive a portion of said optical carrier signal from said optical modulator, and an electrical section to produce said electrical modulation signal from said portion of said optical carrier signal, said opto-electronic loop causing a delay in said electrical modulation signal to provide a positive feedback to said optical modulator; and

an atomic cell having atoms with two atomic transitions sharing a common energy level and in resonance with two adjacent bands in said modulated optical signal to exhibit electromagnetically induced transparency, said atomic cell positioned in said optical section to transmit said first portion of said optical carrier signal to said electrical section.

34. The device as in claim 33, further comprising:

a laser to produce said optical carrier signal at a carrier frequency; and

a laser frequency control coupled to receive and process a portion of said electrical modulation signal indicative of a variation of said carrier frequency and operable to control said laser to reduce said variation.

35. The device as in claim 33, wherein said optical modulator includes a whispering gallery mode resonator formed of an electro-optical material and having electrodes to receive said positive feedback.

36. The device as in claim 33, wherein said optical modulator includes a semiconductor electro-absorption modulator and said optical section of said opto-electronic loop includes a whispering gallery mode resonator.

37. A device, comprising:

an optical resonator configured to support whispering gallery modes and formed of an electro-optical material;

an optical coupler near said optical resonator to evanescently couple an input optical signal into a whispering gallery mode in said optical resonator and to couple energy in said whispering gallery mode out of said optical resonator to produce an optical output signal;

electrodes formed on said optical resonator to apply an electrical control signal to said optical resonator to change a refractive index of said electro-optical material to modulate said optical output signal at a modulation frequency;

an atomic cell having atoms that interact with said modulated optical output signal to exhibit electromagnetically induced transparency, said atomic cell located to receive at least a portion of said modulated optical output signal to produce an optical transmission;

a photodetector to convert said optical transmission into a detector signal; and

a feedback control to produce said electrical control signal according to said detector signal to stabilize said modulation frequency relative to an atomic frequency reference in said atoms.

38. A device, comprising:

a substrate;

a semiconductor optical modulator formed on said substrate to modulate light in response to an electrical modulation signal;

a first waveguide on said substrate coupled to receive a modulated optical signal from said optical modulator;

an optical resonator to support whispering gallery modes and optically coupled to said first waveguide via evanescent coupling;

a second waveguide on said substrate having a first end optically coupled to said optical resonator via evanescent coupling and a second end;

a photodetector on said substrate to receive and convert an optical output from said second waveguide into an electrical signal;

an electrical link coupled between said photodetector and said optical modulator to produce said electrical modulation signal from said electrical signal;

a reflector located on one side of said semiconductor optical modulator to form an optical cavity with said second end of said second waveguide to include said semiconductor optical modulator, said optical resonator, said first and said second waveguides in an optical path within said optical cavity, wherein said first and second waveguides are doped to produce an optical gain for a laser oscillation in said optical cavity; and

an atomic cell on said substrate having atoms that interact with light in said optical cavity to exhibit electromagnetically induced transparency, said atomic cell located to receive at least a portion of said light to produce an optical transmission;

a second photodetector on said substrate to convert said optical transmission into a detector signal; and

a feedback control to control said optical modulator according to said detector signal to stabilize a modulation frequency in said light relative to an atomic frequency reference in said atoms.