PRECISION PHOTONIC OSCILLATOR AND METHOD FOR GENERATING AN ULTRA-STABLE FREQUENCY REFERENCE USING A TWO-PHOTON RUBIDIUM TRANSITION

• US 20130215918A1
• Filed: 02/20/2012
• Published: 08/22/2013
• Est. Priority Date: 02/20/2012
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Wilkinson, Steven R., Price, Gabriel N., Clatterbuck, Todd O., Cashen, Matthew T., Sabala, Jeffrey L.
• Granted Patent: US 8,780,948 B2
• Time in Patent Office: Days
• US Class Current: 372/29.022
• CPC Class Codes: G02F 1/3526 using two-photon emission or absorption processes
• CPC Class Codes: G04F 5/14 using atomic clocks
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108
• CPC Class Codes: H01S 3/0092 Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity nonlinear frequency conversion per se G02F1/35
• CPC Class Codes: H01S 3/13 Stabilisation of laser output parameters, e.g. frequency or amplitude
• CPC Class Codes: H01S 3/1303 by using a passive reference, e.g. absorption cell H01S3/139 takes precedence
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase
• CPC Class Codes: H01S 3/139 by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity length
• CPC Class Codes: H01S 3/1392 by using a passive reference, e.g. absorption cell H01S3/1396, H01S3/1398 take precedence

Abstract

Embodiments of an ultra-stable frequency reference generating system and methods for generating an ultra-stable frequency reference using a two-photon Rubidium transition are generally described herein. In some embodiments, a cavity-stabilized reference laser comprising a laser source is locked to a stabilized cavity. A Rubidium cell is interrogated by a stabilized laser output to cause at least a two-photon Rubidium transition and a detector may detect fluorescence resulting from spontaneous decay of the upper state Rubidium transition. The output of the detector is provided at a wavelength of the fluorescence to lock the cavity-stabilized reference laser to generate a stabilized laser output. A frequency comb stabilizer may be locked to the stabilized laser output to generate a super-continuum of optical wavelengths for use in generating an ultra-stable frequency reference.

Claims

1. An ultra-stable frequency reference generating system comprising:

a cavity-stabilized reference laser comprising a laser source locked to a stabilized cavity to generate a stabilized laser output;

a Rubidium cell configured to be interrogated by the stabilized laser output to cause at least a two-photon Rubidium transition;

a detector to detect fluorescence resulting from spontaneous decay of the Rubidium transition; and

a frequency comb stabilizer locked to the stabilized laser output to generate an optical output for use in generating an ultra-stable frequency reference.

2. The ultra-stable frequency reference generating system of claim 1 wherein the frequency comb stabilizer includes:

a frequency comb;

a first control loop to stabilize the frequency comb relative to zero frequency; and

a second control loop to stabilize a frequency comb spacing of the frequency comb.

3. The ultra-stable frequency reference generating system of claim 2 wherein the frequency comb is a femtosecond frequency comb.

4. The ultra-stable frequency reference generating system of claim 3 wherein the frequency comb stabilizer is locked to the stabilized laser output to generate a super-continuum of optical wavelengths for use in generating the ultra-stable frequency reference, and

wherein the detector is configured to provide a detector output at a wavelength of the fluorescence to lock the cavity-stabilized reference laser to generate the stabilized laser output.

5. The ultra-stable frequency reference generating system of claim 2 wherein the two-photon Rubidium transition is a two-photon Rubidium transition from a 5 s state to a 5 d state, and

wherein the detected fluorescence results from the spontaneous decay from a 6 p state to the 5 s state, and

wherein the Rubidium cell is a Rubidium vapor cell comprising Rubidium 87.

6. The ultra-stable frequency reference generating system of claim 5 further comprising a wavelength divider to half the wavelength of the stabilized laser output,

wherein the halved stabilized laser output is to interrogate the Rubidium cell to generate an output for use in locking the cavity-stabilized reference laser to the Rubidium transition.

7. The ultra-stable frequency reference generating system of claim 2 wherein the stabilized cavity is a dimensionally-stable optical cavity and comprises an ultra-low expansion (ULE) glass Fabry-Perot cavity, and

wherein an output of the laser source is pre-stabilized to the optical cavity using a Pound-Drever-Hall stabilization technique.

8. The ultra-stable frequency reference generating system of claim 2 wherein the frequency comb comprises a fiber-based frequency comb that includes a non-linear fiber, and

wherein the fiber-based frequency comb is to generate a super-continuum of optical wavelengths comprising at least an octave span.

9. The ultra-stable frequency reference generating system of claim 2 further comprising RF generating circuitry to generate the ultra-stable frequency reference from the super-continuum of optical wavelengths, the ultra-stable frequency reference comprising one or more ultra-stable microwave or RF output signals.

10. The ultra-stable frequency reference generating system of claim 9 wherein the RF generating circuitry comprises:

a photo-detector to convert the super-continuum of optical wavelengths to a set of microwave signals; and

a microwave frequency comb to generate a set of microwave clock or reference signals from the output of the photo-detector, the set of microwave clock or reference signals having a stability approximating a stability of the stabilized laser output.

11. The ultra-stable frequency reference generating system of claim 2 further comprising optical reference signal generating circuitry to convert the super-continuum of optical wavelengths to a set of optical reference signals.

12. The ultra-stable frequency reference generating system of claim 1 wherein the stabilized laser output has a stability on the order of 10⁻

15.

13. A method to generate an ultra-stable frequency reference comprising:

locking a laser source to a stabilized cavity to generate a pre-stabilized laser output;

further locking the laser source to a decay of a two-photon Rubidium transition to generate a stabilized laser output; and

locking a frequency comb stabilizer to the stabilized laser output to generate an optical output for use in generating an ultra-stable frequency reference.

14. The method of claim 13 wherein locking the frequency comb stabilizer to the stabilized laser output generates a super-continuum of optical wavelengths, and

wherein the method further comprises generating the ultra-stable frequency reference from the super-continuum.

15. The method of claim 14 wherein the frequency comb stabilizer comprises a fiber-based frequency comb that includes a non-linear fiber, and

wherein the method comprises generating the super-continuum of optical wavelengths comprising at least an octave span with the fiber-based frequency comb.

16. The method of claim 15 further comprising:

interrogating a Rubidium cell with the pre-stabilized laser output to cause at least the two-photon Rubidium transition to an upper state; and

detecting fluorescence resulting from spontaneous decay of the upper state Rubidium transition to provide a detected output at a wavelength of the fluorescence for use in further locking the laser source.

17. The method of claim 16 wherein the Rubidium cell is a Rubidium vapor cell comprising Rubidium 87,

wherein the two-photon Rubidium transition is a two-photon Rubidium transition from a 5 s state to a 5 d state,

wherein the detected fluorescence results from the spontaneous decay from a 6 p state to the 5 s state.

18. The method of claim 17 further comprising doubling the frequency of the stabilized laser output prior to interrogating the Rubidium cell to generate an output for use in locking the cavity-stabilized reference laser to the Rubidium transition.

19. An ultra-stable frequency reference generating system comprising:

a cavity lock loop to lock a laser source to a stabilized cavity and generate a pre-stabilized laser output;

a frequency control loop to further lock the laser source to a decay of an upper state Rubidium transition using two photon excitation to generate a stabilized laser output; and

a frequency comb stabilizer having a first frequency comb stabilizer control loop to stabilize a frequency comb relative to zero frequency and a second frequency comb stabilizer control loop to stabilize a frequency comb spacing,

wherein the frequency comb stabilizer is locked to the stabilized laser output to generate optical wavelengths for use in generating an ultra-stable frequency reference.

20. The ultra-stable frequency reference generating system of claim 19 wherein the cavity lock loop comprises a cavity-stabilized reference laser comprising the laser source locked to the stabilized cavity,

wherein the frequency control loop comprises:

a Rubidium cell configured to be interrogated by the stabilized laser output to cause at least a two-photon Rubidium transition; and

a detector to detect fluorescence resulting from spontaneous decay of the transition,

wherein the detector is configured to provide a detector output at a wavelength of the fluorescence to lock the cavity-stabilized reference laser to generate the stabilized laser output.

21. The ultra-stable frequency reference generating system of claim 20 wherein the Rubidium cell is a Rubidium vapor cell comprising Rubidium 87,

wherein the two-photon Rubidium transition is a two-photon Rubidium transition from a 5 s state to a 5 d state, and

wherein the detected fluorescence results from the spontaneous decay from a 6 p state to the 5 s state.

22. The ultra-stable frequency reference generating system of claim 21 wherein the frequency control loop further comprises a wavelength divider to half the wavelength of the stabilized laser output,

wherein the halved stabilized laser output is to interrogate the Rubidium cell to generate an output for use in locking the cavity-stabilized reference laser to the Rubidium transition.

23. The ultra-stable frequency reference generating system of claim 20 wherein the frequency comb stabilizer comprises a fiber-based frequency comb that includes a non-linear fiber, and

wherein the fiber-based frequency comb is to generate the super-continuum of optical wavelengths comprising at least an octave span.

24. The ultra-stable frequency reference generating system of claim 19 wherein the frequency comb is a femtosecond frequency comb.

25. A method to generate an ultra-stable frequency reference comprising:

generating a stabilized laser output from a cavity-stabilized reference laser that includes a laser source locked to a stabilized cavity;

interrogating a Rubidium cell by the stabilized laser output to cause at least a two-photon Rubidium transition; and

detecting fluorescence resulting from spontaneous decay of the Rubidium transition to provide a detected output at a wavelength of the fluorescence;

a

locking the cavity-stabilized reference laser to the wavelength of the fluorescence; and

locking a frequency comb stabilizer to the stabilized laser output to generate a continuum of optical wavelengths.

26. The method of claim 25 further comprising generating an ultra-stable frequency reference from the continuum of optical wavelengths.

27. The method of claim 26 wherein locking the frequency comb stabilizer comprises:

stabilizing a frequency comb relative to zero frequency with a first control loop; and

stabilizing a frequency comb spacing of the frequency comb with a second control loop.