Atomic clock with enhanced oscillator regulation
A rubidium optical atomic clock uses a modulated 778 nanometer (nm) probe beam and its reflection to excite rubidium 87 atoms, some of which emit 758.8 nm fluorescence as they decay back to the ground state. A spectral filter rejects scatter of the 778 nm probe beams while transmitting the 775.8 nm fluorescence so that the latter can be detected with a high signal-to-noise ratio. Since the spectral filter is only acceptably effective at angles of incidence less than 8° from the perpendicular, the atoms are localized by a magneto-optical trap so that most of the atoms lie within a conical volume defined by the 8° angle so that the resulting fluorescence detection signal has a high signal-to-noise ratio. The fluorescence detection signal can be demodulated to provide an error signal from which desired adjustments to the oscillator frequency can be calculated.
1. A method, comprising:
exciting molecular entities in a cell using a first probe beam having a first probe wavelength, the molecular entities fluorescing at a first fluorescence wavelength in response to the exciting by the first probe beam;
filtering a fluorescence of the first fluorescence wavelength so as to transmit the fluorescence of the first fluorescence wavelength and reject light of the first probe wavelength, a fluorescence detection signal being provided based on the fluorescence of the first fluorescence wavelength transmitted by the filtering;
adjusting an oscillator frequency of an oscillator signal based on the fluorescence detection signal; and
generating a clock output based on the oscillator frequency of the oscillator signal.
2. The method of claim 1 , wherein the first fluorescence wavelength is 775.8±0.5 nm.
3. The method of claim 1 , further comprising:
generating a comb input signal based on the oscillator signal having the oscillator frequency, wherein the generating the clock output further includes:
comb filtering the comb input signal to yield a comb output, and wherein the generating further includes generating the clock output based on the comb output.
4. The method of claim 3 , wherein the generating the comb input signal further includes:
splitting the oscillator signal having the oscillator frequency to yield the comb input signal and a converter input signal; and
converting the converter input signal into the first probe beam.
5. The method of claim 1 , further comprising:
iterating the exciting, filtering, adjusting, and generating such that the oscillator frequency is constrained to a range that results in a stable comb output.
6. The method of claim 1 , wherein the filtering further includes:
spectral filtering, using a spectral filter, the fluorescence of the first fluorescence wavelength.
7. The method of claim 6 , wherein the spectral filter has an effective angle of incidence on the spectral filter within which the spectral filter can effectively reject the first probe wavelength and transmit the first fluorescence wavelength.
8. The method of claim 7 , wherein at least half of the molecular entities are within a conical volume defined by the effective angle of incidence.
9. The method of claim 7 , wherein the effective angle of incidence is at least six degrees and not more than ten degrees.
10. The method of claim 1 , wherein the molecular entities include rubidium 87 atoms, the oscillator frequency is 1556±1 nanometer (nm), the first probe wavelength is 778±0.5 nm, and the first fluorescence wavelength is 775.8±0.5 nm.
11. A system, comprising:
an oscillator for generating an oscillator signal having an oscillator frequency;
a cell having molecular entities therein, the molecular entities being excited using a first probe beam having a first probe wavelength, the molecular entities fluorescing at a first fluorescence wavelength in response to being excited by the first probe beam;
a filter configured to filter the fluorescence of the first fluorescence wavelength so as to transmit the fluorescence of the first fluorescence wavelength and reject light of the first probe wavelength, a fluorescence detection signal being provided based on the fluorescence of the first fluorescence wavelength transmitted by the filter; and
a frequency controller configured to adjust the oscillator frequency based on the fluorescence detection signal,
wherein a clock output signal for an atomic clock is based on the oscillator frequency of the oscillator signal.
12. The system of claim 11 , further comprising:
a fluorescence detector for collecting filtered fluorescence of the first fluorescence wavelength and providing the fluorescence detection signal,
wherein frequency controller is configured to adjust the oscillator frequency based on the fluorescence detection signal.
13. The system of claim 12 , further comprising:
a beam splitter for splitting the oscillator signal into a comb input signal and a converter input signal;
a frequency comb for providing a comb output based on the comb input signal; and
a counter for generating the clock output signal based on the comb output.
14. The system of claim 11 , wherein the filter is a spectral filter.
15. The system of claim 14 , wherein the spectral filter has an effective angle of incidence on the spectral filter within which the spectral filter can effectively reject the first probe wavelength and transmit the first fluorescence wavelength.
16. The system of claim 15 , wherein at least half of the molecular entities are within a conical volume defined by the effective angle of incidence.
17. The system of claim 15 , wherein the effective angle of incidence is at least six degrees and not more than ten degrees.
18. The system of claim 11 , wherein the molecular entities include rubidium 87 atoms, the oscillator frequency is 1556±1 nanometer (nm), the first probe wavelength is 778±0.5 nm, and the first fluorescence wavelength is 775.8±0.5 nm.
19. The system of claim 14 , further comprising an error-signal generator for deriving an error signal from the fluorescence detection signal, the frequency controller adjusting the oscillator frequency based on the error signal.
20. A system, comprising:
an oscillator for generating an oscillator signal having an oscillator frequency;
a cell having a trap therein, the trap for localizing molecular entities in the cell, the molecular entities being excited using a first probe beam having a first probe wavelength, the molecular entities fluorescing at a first fluorescence wavelength in response to being excited by the first probe beam;
a spectral filter for filtering the fluorescence of the first fluorescence wavelength so as to transmit the fluorescence of the first fluorescence wavelength and reject light of the first probe wavelength, a fluorescence detection signal being provided based on the fluorescence of the first fluorescence wavelength transmitted by the filtering; and
a frequency controller for adjusting the oscillator frequency based on the fluorescence detection signal,
wherein a clock output for an atomic clock is based on the oscillator frequency of the oscillator signal.