Atomic Optical Reference System

Abstract

One example includes an atomic optical reference system. The system includes an optical system with a laser configured to generate a light beam. The system includes a vapor cell including alkali metal atoms stimulated in response to a modulated beam corresponding to an amplitude modulated version of the light beam. The system includes a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell and generate at least one feedback signal in response to the at least one detection signal. The system further includes a beam modulator configured to amplitude modulate the light beam to generate the modulated beam and frequency shift the light beam in response to the at least one feedback signal to generate an output beam having a stable frequency.

Description

[Technical Field]

[0001] The present invention relates generally to atomic systems, and more particularly to atomic optical reference systems. [Background technology]

[0002] Atomic systems implementing the interaction of light beams with atoms have been implemented for various applications, such as sensors and atomic clocks. As an example, one or more light beams may be fed through a vapor cell containing alkali metal vapor, causing the excitation of atoms in the alkali metal vapor to exhibit an optical response based on the amplitude, frequency, and / or polarization of the one or more light beams. Thus, the optical response of a given atomic system can indicate parameters (e.g., time, rotation, magnetic field, electric field, acceleration) with very high precision. Maintaining the precision of an atomic system typically requires stability of the output power of the light beams. The output power of lasers generating the light beams can be unstable and affected by various factors, such as those caused by environmental fluctuations and aging of the laser, power supply, and / or monitoring system. Summary of the Invention

[0003] One example includes an atomic optical reference system. The system includes an optical system with a laser configured to generate a light beam. The system includes a vapor cell containing alkali metal atoms stimulated in response to a modulated beam corresponding to an amplitude-modulated version of the light beam. The system includes a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell and to generate at least one feedback signal in response to the at least one detection signal. The system further includes a beam modulator configured to amplitude-modulate the light beam to generate the modulated beam and to frequency-shift the light beam in response to the at least one feedback signal to generate an output beam having a stable frequency.

[0004] Another example includes a method for generating an output beam having a stable frequency reference. The method includes generating a light beam and amplitude modulating the light beam to generate a modulated beam. The method also includes passing the modulated beam through a vapor cell containing alkali metal atoms that are stimulated in response to the modulated beam and monitoring at least one detection signal corresponding to light emitted from or absorbed by the vapor cell. The method further includes generating at least one feedback signal in response to the at least one detection signal and frequency-shifting the light beam in response to the at least one feedback signal to generate the output beam.

[0005] Another example includes an atomic clock system. The atomic clock system includes an optical system including a laser configured to generate a light beam and a vapor cell containing alkali metal atoms stimulated in response to a modulated beam corresponding to an amplitude-modulated version of the light beam. The system also includes a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell and to generate at least one feedback signal in response to the at least one detection signal. The system includes an amplitude modulator configured to amplitude-modulate the light beam between a first amplitude and a second amplitude to generate the modulated beam. The system further includes a frequency shifter configured to shift the frequency of the light beam approximately equal to and inverse to the frequency shift between the first amplitude and the second amplitude of the modulated beam in response to the at least one feedback signal to generate an output beam having a stable frequency. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 illustrates an example of an atomic optical reference system. [Figure 2] FIG. 1 illustrates an example of a detection system. [Figure 3] FIG. 1 illustrates an example of a beam modulator. [Figure 4]FIG. 1 shows an example of a graph of light shift as a function of light output. [Figure 5] FIG. 1 illustrates an example of a method for generating an output beam with a stable frequency reference. DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention relates generally to atomic systems, and more particularly to atomic optical reference systems configured to generate an output optical beam having a highly stable frequency that is immune to aging and / or other instability factors. As an example, the output optical beam may be implemented as a frequency reference, such as an atomic clock.

[0008] The atomic optical reference system includes an optical system and a vapor cell. The optical system includes a laser configured to generate a light beam, and the vapor cell contains alkali metal atoms (e.g., cesium). The alkali metal vapor atoms can be stimulated (e.g., excited) in response to a modulated beam corresponding to an amplitude-modulated version of the light beam. As described herein, the terms "amplitude" and "power" with respect to the light beam are used interchangeably. The optical system can also include optical components that can polarize the light beam (e.g., circularly or linearly polarized) and pass the modulated beam twice (e.g., in antiparallel directions) through the vapor cell to provide two-photon excitation of the alkali metal vapor atoms based on the frequency of the modulated beam. Thus, the alkali metal vapor can exhibit fluorescence based on the energy of the alkali metal vapor atoms decaying from an excited state (e.g., from a second energy state to a first energy state).

[0009] The atomic optical reference system also includes a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell. The detection signal can include a frequency reference signal corresponding to fluorescence emitted from the alkali metal atoms and an amplitude detection beam corresponding to the modulated beam exiting the vapor cell. The detection system can be configured to generate at least one feedback signal in response to the one or more detection signals, such as a frequency-shift feedback signal based on the frequency reference signal and an amplitude feedback signal based on the amplitude detection beam. The atomic optical reference system further includes a beam modulator configured to amplitude-modulate the light beam to generate the modulated beam and frequency-shift the light beam in response to the one or more feedback signals to generate an output beam having a stable frequency.

[0010] As an example, the beam modulator can modulate the optical beam between a first amplitude and a second amplitude of the optical output to generate the modulated beam. As an example, the second amplitude of the optical output can be twice the first amplitude of the optical output. The detection system can include a frequency detector configured to monitor the frequency reference signal at different times for each of the first and second amplitudes of the modulated beam. Thus, the detection system can generate a frequency shift feedback signal corresponding to a frequency shift of the modulated beam at each of the first and second amplitudes of the modulated beam.

[0011] The frequency shift feedback signal can be provided to a frequency shifter configured to shift the frequency of the optical beam approximately equal to and inverse to the frequency shift between the first and second amplitudes of the modulated beam to generate an output beam. As an example, a laser output of any amplitude can induce an optical shift (e.g., an AC Stark shift), which can cause instability in the laser output when providing the optical beam. However, by locking the frequency of the optical beam to a frequency corresponding to the maximum probability of excitation of alkali metal atoms at the first amplitude of the modulated beam, the frequency shift between the first and second amplitudes of the modulated beam can correspond to a frequency shift from the first amplitude of the modulated beam to zero optical shift. Thus, by continuously measuring and removing the optically induced frequency shift of the optical beam, aging effects and other instabilities that cause optical frequency shifts can be counteracted from the output beam.

[0012] 1 shows an example of an atomic optical reference system 100. The atomic optical reference system 100 can be implemented in any of a variety of applications requiring a stable frequency optical beam. For example, the atomic optical reference system 100 can be implemented in an atomic clock system.

[0013] The atomic optical reference system 100 includes an optical system 102. The optical system 102 receives a light beam OPT. BM 1, the beam modulator 108 modulates the optical beam OPT. BM is amplitude modulated to obtain the modulated beam OPT MOD As an example, the optical beam OPT BM The amplitude modulation of the beam OPT can be between a first amplitude and a second amplitude that is greater than the first amplitude by a predetermined amplitude difference. For example, the second amplitude can be approximately twice the first amplitude. As described in more detail herein, the modulation of the beam OPT from the first amplitude to the second amplitude can be performed. MOD The increase in amplitude of the modulated beam OPTMOD A frequency shift of

[0014] The atomic optical reference system 100 also includes a vapor cell 110 containing an alkali metal vapor (e.g., cesium). MOD can interact with the alkali metal vapor in the vapor cell 110 to excite the alkali metal atoms from a first energy state (e.g., a ground state) to a second energy state higher than the first energy state. BM can be polarized (e.g., circularly or linearly polarized) and the modulated beam OPT MOD is passed twice (e.g., antiparallel) through the vapor cell to obtain the modulated beam OPT. MOD can be used to perform two-photon excitation of the alkali metal atoms in the vapor cell 110 based on the frequency of the laser beam. Thus, the alkali metal vapor can exhibit fluorescence based on the alkali metal atoms emitting a photon to return to an initial energy state (e.g., from the second energy state back to the first energy state).

[0015] Additionally, atomic optical reference system 100 includes a detection system 112. Detection system 112 detects a signal OPT provided from vapor cell 110, in the example of FIG. DET The optical fiber optics 100 is configured to monitor at least one detection signal indicated as a detection beam OPT. DET is the modulated beam OPT emitted from the vapor cell 110 MOD , and may include a frequency reference signal corresponding to the fluorescence emitted from the alkali metal atoms. In the example of FIG. 1, the detection system 112 includes one or more detection beams OPT DET The at least one feedback signal FDBK may be generated based on the modulation beam OPT. As an example, the one or more feedback signals FDBK may include an amplitude feedback signal and a frequency shift feedback signal. As described in more detail herein, the amplitude feedback signal may be generated based on the modulation beam OPT. MODmay be implemented to lock the amplitude of the optical beam OPT at each of the first and second amplitudes. Also, as described in more detail herein, the frequency shift feedback signal may be implemented to lock the amplitude of the optical beam OPT at each of the first and second amplitudes. BM For example, the optical beam OPT can be implemented to lock the frequency of BM The frequency of the modulated beam OPT can be locked to a frequency that maximizes the probability of excitation of the alkali metal atoms from a first energy state to a second energy state. MOD A first amplitude of 1000 .ANG. results in a maximum intensity of fluorescence from the energy of the alkali metal atom decaying back to the first energy state.

[0016] Furthermore, one or more frequency-shifted feedback signals of the feedback signal FDBK may be provided to the beam modulator 108. Thus, the beam modulator 108 modulates the optical beam OPT BM The frequency of the modulated beam OPT MOD and shifting the frequency of the output beam OPT OUT Thus, a frequency shift between the first amplitude and the second amplitude of the modulated beam can correspond to a frequency shift from the first amplitude of the modulated beam to a zero optical shift, which can be expressed as the optical beam OPT BM Therefore, the optical beam OPT BM By continuously measuring and removing the optically induced frequency shift, aging effects and other instabilities that cause optical frequency shifts can be counteracted from the output beam.

[0017] Figure 2 shows an example of a detection system 200. The detection system 200 may correspond to the detection system 112 in the example of Figure 1. Accordingly, in the following description of the example of Figure 2, reference will be made to the example of Figure 1.

[0018] The detection system 200 includes an optical detector 202 and an amplitude controller 204. The detection system 200 outputs a detection signal OPT DET Receives the detection signal OPT DETIn the example of Figure 2, the output detection beam is opt PWR and frequency reference signal OPT REF The output detection beam OPT PWR is the modulated beam OPT emitted from the vapor cell 110 MOD and is provided to the photodetector 202. The photodetector 202 therefore outputs an output detection beam OPT PWR , and hence the modulated beam OPT (e.g., after absorption of photons by alkali metal atoms in the vapor cell 110). MOD The voltage V indicates the amplitude of PWR The amplitude controller 204 is configured to generate a voltage V PWR and a given reference voltage V REF Therefore, the amplitude controller 204 receives the voltage V PWR and the reference voltage V REF and generating a control signal CNTL indicative of the difference between

[0019] As will be explained in more detail herein, the control signal CNTL controls the optical beam OPT BM For example, a reference voltage V can be implemented to amplitude modulate the reference voltage V to generate a modulated beam. REF can be modulated between a first voltage amplitude corresponding to the first amplitude and a second voltage amplitude corresponding to the second amplitude. REF the first voltage swing and the reference voltage V REF Voltage V for the second voltage swing PWR Thus, the control signal CNTL may correspond to a portion of one or more feedback signals FDBK provided to the beam modulator 108.

[0020] The detection system 200 also includes a frequency controller 206. The frequency controller 206 controls a frequency reference signal OPT REF As an example, the frequency controller 206 may include a photodetector (e.g., a photodiode) and receive the frequency reference signal OPT as described above. REFis the fluorescence emitted from an alkali metal atom as its energy decays from a second energy state to a first (e.g., ground) energy state, or the optical beam OPTO by an alkali metal atom. BM Therefore, the frequency reference signal OPT REF The intensity of can indicate the probability of stimulation of the alkali metal atoms from a first energy state to a second energy state, and the probability can be determined by modulating the beam OPT MOD Therefore, the frequency reference signal OPT REF In response to detecting the strength of the frequency reference signal OPT REF Modulated beam OPT compared to the optimum peak corresponding to the highest intensity of MOD a frequency shift feedback signal FREQ corresponding to a frequency offset of the light beam OPT corresponding to the highest probability of stimulation of the alkali metal atoms from the first energy state to the second energy state; BM A frequency shift feedback signal FREQ may be provided to the laser 104 to lock onto the frequency of the optical beam OPT, as described in more detail herein. BM OPT provides a frequency shift of the optical beam BM To offset an optical shift (e.g., an AC Stark shift) due to the output of the beam modulator 108, a frequency shift feedback signal FREQ may be provided to the beam modulator 108. Thus, the frequency shift feedback signal FREQ may correspond to a portion of one or more feedback signals FDBK provided to the beam modulator 108.

[0021] Figure 3 shows an example of a beam modulator 300. The beam modulator 300 may correspond to the beam modulator 108 in the example of Figure 1. Therefore, in the following description of the example of Figure 3, reference will be made to the example of Figure 1.

[0022] The beam modulator 300 includes an amplitude modulator 302 and a frequency shifter 304. Each of the amplitude modulator 302 and the frequency shifter 304 modulates the optical beam OPT. BMThe amplitude modulator 302 also receives a control signal CNTL as an input. The amplitude modulator 302 controls the optical beam OPT BM is amplitude modulated to obtain the modulated beam OPT MOD As an example, a modulated beam OPT MOD is the reference voltage V REF a first amplitude based on the first voltage amplitude of the reference voltage V REF As another example, the second amplitude may be modulated between a first amplitude based on a second voltage amplitude of a reference voltage V REF The second voltage swing is the reference voltage V REF Therefore, the amplitude of the first voltage amplitude of the modulated beam OPT can be approximately twice that of the first voltage amplitude of the modulated beam OPT. MOD The second amplitude (e.g., output) of the modulated beam OPT MOD The first amplitude (e.g., output) of the first input signal may be approximately twice the amplitude of the first input signal.

[0023] As mentioned above, the optical beam OPT BM output, hence the modulated beam OPT MOD The output of the vapor cell 110 is a modulated beam of alkali atoms. MOD This affects the optical shift associated with the absorption of the photon in the modulated beam. MOD The change in the amplitude of may result in an observable change in the optical shift of the absorption of the alkali metal atom photons between the first amplitude and the second amplitude, as will be explained in more detail with respect to the example of FIG. 4.

[0024] 4 shows an example graph 400 of optical shift as a function of optical power. Graph 400 shows a graph of the optical shift as a function of optical power for a modulated beam OPT. MOD This corresponds to the effect of optical shifts in the absorption of photons by alkali metal atoms in vapor cell 110 caused by fluctuations in the output of the vapor cell 110. Therefore, the following description of the example of Figure 4 will refer to the examples of Figures 1-3.

[0025] The graph 400 begins at a zero optical shift point 404 and continues through the modulated beam OPT MOD4 includes a line 402 that extends with a negative slope as the amplitude of the light output increases. A zero light shift point 404 corresponds to a point where the light output is approximately zero, and therefore any light output will result in some light shift, with the light shift increasing with a negative slope (e.g., based on negative polarity) as the light output increases further. In the example of FIG. 4, line 402 is shown as being approximately straight.

[0026] Graph 400 also shows a first output "A" and a second output "2A." For example, amplitude modulator 306 may be configured to modulate the optical beam between the first output "A" at a first time and the second output "2A" at a second time. As discussed above, the modulated beam OPT MOD Since any output of δ will experience an optical shift, the first output "A" will be shifted from the zero optical shift point 404 to δ f 4, the second output "2A" is shown as having approximately twice the power of the first output "A." Therefore, based on the linearity of line 402, the second output "2A" is shown as having a frequency shift of δ from the first output "A." f , and therefore has a frequency shift of about 2δ from the zero optical shift point 404. f has a frequency shift of

[0027] Frequency shift δ f is the frequency reference signal OPT REF For example, based on the known frequency of excitation of the alkali metal atoms from a first energy state to a second energy state, and based on the frequency reference signal OPT relative to the on-resonance frequency and detuning frequency. REF Based on the known spectral density of the intensity of the modulated beam, MOD The offset frequency of δ can be determined by the frequency controller 206. f may be communicated to the frequency shifter 304 via the frequency shift feedback signal FREQ.

[0028] 3, the frequency shifter 304 is configured to receive a frequency-shifted feedback signal FREQ corresponding to one of the one or more feedback signals FDBK. The frequency shifter 304 is configured to receive a frequency-shifted feedback signal FREQ corresponding to one of the one or more feedback signals FDBK. BM , the frequency offset δ f and a frequency offset δ f and the output beam OPT OUT Therefore, the output beam OPT OUT can be provided at a frequency that exhibits near zero optical shift despite the presence of instability factors.

[0029] For example, the difference in optical shift between the first output "A" and the second output "2A" is the frequency offset δ f , the difference between the first output "A" and the zero optical shift point 404 is also determined to be approximately equal to the frequency offset δ f Therefore, the laser 104 modulated the beam OPT MOD When the frequency shifter 304 is locked to the frequency corresponding to the first output "A" of the f The optical beam OPT BM By giving the output beam OPT OUT can be provided with nearly zero optical shift. In other words, the optical shift is MOD and such observable optical shift is sampled based on an observable difference between the first amplitude and the second amplitude of the optical beam OPT. BM If the light beam OPT is locked to the frequency at which absorption of photons at the first amplitude "A" is most likely, BM Therefore, the measured light shift is approximately equal to the light shift of the light beam OPT BM output beam OPT, which has zero optical shift regardless of the instability factor. OUT As a result, the output beam OPT OUTis provided at a very stable frequency and can therefore be used as a very stable frequency reference.

[0030] As an example, the instability factor of the optical shift may cause a change in the slope of the line 402 and / or a fluctuation in the optical power applied through the amplitude modulator 302. However, the offset δ between the first output and the second output f Since θ is observable and equals the frequency offset between the zero optical shift point 404 and the first output, these instability factors are OUT When generating a light beam, OPT BM Furthermore, the examples of FIGS. 1-4 herein are not related to compensating for the optical shift of the modulated beam OPT. MOD When generating the light beam OPT BM 1 illustrates an amplitude modulation between the two outputs, with the second output being approximately twice the amplitude of the first output. However, other configurations may be implemented in atomic optical reference system 100.

[0031] For example, amplitude modulation may occur between three or more amplitudes that are not limited to being even multiples of each other. As an example, amplitude modulator 306 may be configured to modulate the optical beam between a first output "A" at a first time, a second output "2A" at a second time, and a third output at a third time. The third output may be between the first output "A" and the second output "2A," or may be an output outside the range of the first output "A" and the second output "2A." Thus, nonlinearities in line 402 and optical shifts associated with the optical output may be detected and better modeled, providing a more accurate observation of the output-induced optical shifts and thus the output beam OPT. OUT This can provide a more stable frequency.

[0032] In view of the structural and functional features discussed above, methods according to various embodiments of the present disclosure may be better understood with reference to Figure 5. It should be understood and appreciated that, according to the present disclosure, some aspects may occur in a different order and / or concurrently with other aspects than illustrated and described herein, and therefore the method for Figure 5 is not limited by the illustrated order. Furthermore, not all illustrated features may be required to practice a method according to one embodiment of the present example.

[0033] FIG. 5 shows an output beam with a stable frequency reference (e.g., output beam OPT OUT 5 shows an example of a method 500 for generating a light beam (e.g., a light beam OPT BM At 504, the light beam is amplitude modulated to generate a modulated beam (e.g., modulated beam OPT MOD ) is generated. At 506, the modulated beam is fed through a vapor cell (e.g., vapor cell 110). The vapor cell can contain alkali metal atoms that are stimulated in response to the modulated beam. At 508, at least one detection signal (e.g., detection signal OPT) corresponding to light emitted from or absorbed by the vapor cell is generated. DET ) is monitored. At 510, at least one feedback signal (e.g., feedback signal FDBK) is generated in response to the at least one detection signal. At 512, the light beam is frequency shifted in response to the at least one feedback signal to generate an output beam.

[0034] The above description is an example of the present invention. It is, of course, not possible to describe every possible combination of elements or methodologies contemplated for illustrating the present invention, but those skilled in the art will recognize that many additional combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such changes, modifications, and variations that fall within the spirit and scope of the appended claims. Additionally, when this disclosure or claims recite "a," "an," "a first," or "another" element, or the equivalent thereof, it should be construed as including one or more such elements, and does not require or exclude two or more such elements. As used herein, the term "includes" means including, but not limited to, and the term "including" means including, but not limited to. The term "based on" means based at least in part on. The technical concepts that can be understood from the above-described embodiment will be described below as supplementary notes. [Appendix 1] 1. An atomic optical reference system comprising: an optical system including a laser configured to generate a light beam; a vapor cell containing alkali metal atoms stimulated in response to a modulated beam corresponding to an amplitude modulation of the light beam; a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell and to generate at least one feedback signal in response to the at least one detection signal; a beam modulator configured to amplitude modulate the optical beam to generate the modulated beam and to frequency shift the optical beam in response to the at least one feedback signal to generate an output beam having a stable frequency. [Appendix 2] 2. The atomic optical reference system of claim 1, wherein the beam modulator includes an amplitude modulator configured to amplitude modulate the optical beam between a first amplitude and a second amplitude to generate the modulated beam. [Appendix 3] 3. The atomic optical reference system of claim 2, wherein the at least one feedback signal comprises an amplitude feedback signal configured to lock the modulated beam to a first predetermined reference amplitude corresponding to the first amplitude at a first time and to a second predetermined reference amplitude corresponding to the second amplitude at a second time, the second amplitude being greater than the first amplitude by a predetermined amplitude difference. [Appendix 4] 3. The atomic optical reference system of claim 2, wherein the amplitude modulator is configured to amplitude modulate the optical beam between the first amplitude at a first time, the second amplitude at a second time, and a third amplitude at a third time to generate the modulated beam. [Appendix 5] 3. The atomic optical reference system of claim 2, wherein the at least one feedback signal comprises a frequency-shifted feedback signal, and the beam modulator comprises a frequency shifter configured to shift the frequency of the optical beam approximately equal to and inverse to a frequency shift between the first amplitude and the second amplitude of the modulated beam to produce the output beam. [Appendix 6] 2. The atomic optical reference system of claim 1, wherein the optical system includes an optical component configured to pass the modulated beam twice through the vapor cell to absorb two photons and excite the alkali metal atoms from a first energy state to a second energy state, and the at least one detection signal includes a frequency reference signal corresponding to fluorescence emitted from the alkali metal atoms in response to the decay of energy of the alkali metal atoms from the second energy state to the first energy state. [Appendix 7] 7. The atomic optical reference system of claim 6, wherein the detection system is configured to monitor the frequency reference signal, and the at least one feedback signal includes a frequency feedback signal corresponding to a frequency of the light beam, and the frequency feedback signal is configured to lock the frequency of the light beam to a predetermined frequency. [Appendix 8] 8. The atomic optical reference system of claim 7, wherein the beam modulator includes a frequency shifter configured to, in response to the frequency feedback signal, shift the frequency of the optical beam approximately equal to and inverse to a frequency shift between the first and second amplitudes of the modulated beam to produce the output beam. [Appendix 9] 7. The atomic optical reference system of claim 6, wherein the at least one detection signal includes an amplitude detection beam corresponding to the modulated beam exiting the vapor cell, the detection system includes a photodetector configured to monitor the amplitude of the amplitude detection beam to lock a first amplitude of the modulated beam to a predetermined amplitude, and the beam modulator is configured to modulate the modulated beam between the first amplitude and a second amplitude that is greater than the first amplitude by a predetermined amplitude difference. [Appendix 10] 10. An atomic clock comprising the atomic optical reference system of claim 1, the atomic clock configured to provide a time reference based on the output beam. [Appendix 11] 1. A method for generating an output beam having a stable frequency reference, comprising: generating a light beam; amplitude modulating the light beam to produce a modulated beam; providing the modulated beam through a vapor cell containing alkali metal atoms that are stimulated in response to the modulated beam; monitoring at least one detection signal corresponding to light emitted from or absorbed by the vapor cell; generating at least one feedback signal in response to the at least one detection signal; and frequency shifting the light beam in response to the at least one feedback signal to produce the output beam. [Appendix 12] 12. The method of claim 11, wherein the step of amplitude modulating the light beam includes amplitude modulating the light beam between a first amplitude at a first time and a second amplitude at a second time to produce the modulated beam. [Appendix 13] 13. The method of claim 12, wherein generating at least one feedback signal includes generating a frequency-shifted feedback signal, and frequency-shifting the light beam includes shifting the frequency of the light beam approximately equal to and inverse to a frequency shift between the first amplitude and the second amplitude of the modulated beam to generate the output beam. [Appendix 14] 12. The method of claim 11, wherein providing the modulated beam through the vapor cell comprises exciting the alkali metal atoms from a first energy state to a second energy state in response to the modulated beam; monitoring at least one detection signal comprises monitoring a frequency reference signal corresponding to fluorescence emitted from the alkali metal atoms in response to energy decay of the alkali metal atoms from the second energy state to the first energy state; generating at least one feedback signal comprises generating a frequency feedback signal corresponding to a frequency of the light beam, the frequency feedback signal being configured to lock the frequency of the light beam to a predetermined frequency; and the method further comprises shifting the frequency of the light beam approximately equal to and inverse to a frequency shift between a first amplitude and a second amplitude of the modulated beam in response to the frequency feedback signal to generate the output beam. [Appendix 15] 12. The method of claim 11, wherein the step of monitoring at least one detection signal includes monitoring an amplitude detection beam corresponding to the modulated beam emitted from the vapor cell with a photodetector to lock a first amplitude of the modulated beam to a predetermined amplitude, and modulating the light beam includes modulating the modulated beam between the first amplitude and a second amplitude that is greater than the first amplitude by a predetermined amplitude difference. [Appendix 16] 1. An atomic clock system comprising: an optical system including a laser configured to generate a light beam; a vapor cell containing alkali metal atoms stimulated in response to a modulated beam corresponding to an amplitude modulation of the light beam; a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell and to generate at least one feedback signal in response to the at least one detection signal; an amplitude modulator configured to amplitude modulate the light beam between a first amplitude and a second amplitude to generate the modulated beam; a frequency shifter configured to, in response to at least one feedback signal, shift the frequency of the light beam approximately equal to and inverse to the frequency shift between the first amplitude and the second amplitude of the modulated beam to produce an output beam having a stable frequency. [Appendix 17] 17. The atomic clock system of claim 16, wherein the optical system includes an optical component configured to pass the modulated beam twice through the vapor cell to absorb two photons and excite the alkali metal atom from a first energy state to a second energy state, and the at least one detection signal includes a frequency reference signal corresponding to fluorescence emitted from the alkali metal atom in response to the decay of the energy of the alkali metal atom from the second energy state to the first energy state. [Appendix 18] 18. The atomic clock system of claim 17, wherein the detection system is configured to monitor the frequency reference signal, and the at least one feedback signal includes a frequency feedback signal corresponding to a frequency of the light beam, the frequency feedback signal being phased to lock the frequency of the light beam to a predetermined frequency. [Appendix 19] 19. The atomic clock system of claim 18, wherein the at least one detection signal includes an amplitude detection beam corresponding to the modulated beam exiting the vapor cell, the detection system includes a photodetector configured to monitor the amplitude of the amplitude detection beam to lock the first amplitude of the modulated beam to a predetermined amplitude, and a beam modulator configured to modulate the modulated beam between the first amplitude and the second amplitude that is greater than the first amplitude by a predetermined amplitude difference. [Appendix 20] 17. The atomic clock system of claim 16, wherein the amplitude modulator is configured to amplitude modulate the light beam between the first amplitude at a first time, the second amplitude at a second time, and a third amplitude at a third time to generate the modulated beam.

Claims

1. 1. An atomic optical reference system comprising: an optical system including a laser configured to generate a light beam; a vapor cell containing alkali metal atoms stimulated in response to a modulated beam corresponding to an amplitude modulation of the light beam; a detection system configured to monitor at least one detection signal corresponding to light emitted from or absorbed by the vapor cell and to generate at least one feedback signal in response to the at least one detection signal; a beam modulator configured to amplitude modulate the optical beam to generate the modulated beam and to frequency shift the optical beam in response to the at least one feedback signal to generate an output beam having a stable frequency.

2. 2. The atomic optical reference system of claim 1, wherein the beam modulator comprises an amplitude modulator configured to amplitude modulate the optical beam between a first amplitude and a second amplitude to generate the modulated beam.

3. 3. The atomic optical reference system of claim 2, wherein the at least one feedback signal comprises an amplitude feedback signal configured to lock the modulated beam to a first predetermined reference amplitude corresponding to the first amplitude at a first time and to a second predetermined reference amplitude corresponding to the second amplitude at a second time, the second amplitude being greater than the first amplitude by a predetermined amplitude difference.

4. 3. The atomic optical reference system of claim 2, wherein the amplitude modulator is configured to amplitude modulate the optical beam between the first amplitude at a first time, the second amplitude at a second time, and a third amplitude at a third time to generate the modulated beam.

5. 3. The atomic optical reference system of claim 2, wherein the at least one feedback signal comprises a frequency-shifted feedback signal, and the beam modulator comprises a frequency shifter configured to shift the frequency of the optical beam approximately equal to and inverse to a frequency shift between the first amplitude and the second amplitude of the modulated beam to produce the output beam.

6. 2. The atomic optical reference system of claim 1, wherein the optical system includes an optical component configured to pass the modulated beam twice through the vapor cell to absorb two photons to excite the alkali metal atoms from a first energy state to a second energy state, and the at least one detection signal includes a frequency reference signal corresponding to fluorescence emitted from the alkali metal atoms in response to the decay of their energy from the second energy state to the first energy state.

7. 7. The atomic optical reference system of claim 6, wherein the detection system is configured to monitor the frequency reference signal, and wherein the at least one feedback signal comprises a frequency feedback signal corresponding to a frequency of the light beam, the frequency feedback signal being configured to lock the frequency of the light beam to a predetermined frequency.

8. 8. The atomic optical reference system of claim 7, wherein the beam modulator includes a frequency shifter configured to, in response to the frequency feedback signal, shift the frequency of the optical beam approximately equal to and inverse to a frequency shift between the first and second amplitudes of the modulated beam to produce the output beam.

9. 7. The atomic optical reference system of claim 6, wherein the at least one detection signal includes an amplitude detection beam corresponding to the modulated beam exiting the vapor cell, the detection system includes a photodetector configured to monitor the amplitude of the amplitude detection beam to lock a first amplitude of the modulated beam to a predetermined amplitude, and the beam modulator is configured to modulate the modulated beam between the first amplitude and a second amplitude that is greater than the first amplitude by a predetermined amplitude difference.

10. 10. An atomic clock comprising the atomic optical reference system of claim 1, wherein the atomic clock is configured to provide a time reference based on the output beam.

11. 1. A method for generating an output beam having a stable frequency reference, comprising: generating a light beam; amplitude modulating the light beam to produce a modulated beam; providing the modulated beam through a vapor cell containing alkali metal atoms that are stimulated in response to the modulated beam; monitoring at least one detection signal corresponding to light emitted from or absorbed by the vapor cell; generating at least one feedback signal in response to the at least one detection signal; and frequency shifting the light beam in response to the at least one feedback signal to produce the output beam.

12. 12. The method of claim 11 , wherein amplitude modulating the light beam comprises amplitude modulating the light beam between a first amplitude at a first time and a second amplitude at a second time to produce the modulated beam.

13. 13. The method of claim 12, wherein generating at least one feedback signal comprises generating a frequency-shifted feedback signal, and frequency-shifting the light beam comprises shifting the frequency of the light beam approximately equal to and inverse to a frequency shift between the first amplitude and the second amplitude of the modulated beam to generate the output beam.

14. 12. The method of claim 11 , wherein providing the modulated beam through the vapor cell comprises exciting the alkali metal atoms from a first energy state to a second energy state in response to the modulated beam; monitoring at least one detection signal comprises monitoring a frequency reference signal corresponding to fluorescence emitted from the alkali metal atoms in response to their energy decaying from the second energy state to the first energy state; generating at least one feedback signal comprises generating a frequency feedback signal corresponding to a frequency of the light beam, the frequency feedback signal being configured to lock the frequency of the light beam to a predetermined frequency; and the method further comprises shifting the frequency of the light beam approximately equal to and inverse to a frequency shift between a first amplitude and a second amplitude of the modulated beam in response to the frequency feedback signal to generate the output beam.

15. 12. The method of claim 11, wherein the step of monitoring at least one detection signal includes monitoring an amplitude detection beam corresponding to the modulated beam emitted from the vapor cell with a photodetector to lock a first amplitude of the modulated beam to a predetermined amplitude, and wherein modulating the light beam includes modulating the modulated beam between the first amplitude and a second amplitude that is greater than the first amplitude by a predetermined amplitude difference.

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