US 2009/0289728 A1Published application

Atomic frequency standard based on phase detection

AssigneesAccuBeat Ltd., Bar-Ilan University, Technion Research & Development Foundation Limited

InventorsIdo Ben-Aroya, Gad Eisenstein, Avinoam Stern, Michael Rosenbluh, Benny Levy

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Abstract

This invention concerns the realization of a Coherent-Population-Trapping (CPT) atomic frequency standard by utilization of both the phase delay and the absorption of the light transmitted through an atomic vapor. The invented method enables the use of high modulation frequency and a fast lock of a low quality oscillator to the atomic hyperfine transition.

Claims16

Dependent claims

An atomic frequency standard comprising:

a. A cell containing a mixture of alkali metal atoms and a mixture of buffer gases non reactive with said alkali metal atoms, said mixture being selected to minimize the temperature coefficient within said cell;
b. A laser whose emitted wavelength corresponds to either of the D1 or the D2 transition of the said alkali metal atoms.
c. A combination of optical components that set the state of polarization of the light to be circularly polarized.
d. An RF generator that generates a frequency whose value is the hyperfine ground state 0-0 transition frequency of said alkali metal atoms (the “clock transition”) divided by an integer number. The RF output is phase modulated (PM) or amplitude modulated (AM) at a low frequency (below 100 kHz).
e. The said generator output is used to amplitude modulate (AM) the current of the said laser. The laser emission is thus amplitude, frequency and phase modulated. The modulated RF drive current is modulated at a rate lower than the said RF frequency but could be much higher than the resonance width. As a result the laser light is AM and PM modulated containing AM sidebands each of which carries its own low frequency FM sidebands. The number of AM sidebands is controlled by the RF power level while the number of FM sided bands is controlled by the FM modulation index, namely the amplitude of the electrical FM modulation signal.
f. A photo-detector placed behind the vapor cell is used to detect the variations in the intensity of the light transmitted through said cell.
g. A circuit that demodulates the variations in the light intensity. The demodulation is performed in-phase or in quadrature to the said low frequency FM modulation, or in a mixture of both so that the demodulated signal is mostly related to a phase shift difference of the transmitted light.
h. A frequency control loop that uses the said phase shift difference signal to lock the RF generator to the zero crossing point of the phase difference. The said RF generator frequency is tuned so that the clock transition frequency corresponds to a difference in frequency between one of the high and one of the low side bands.

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Dependencies are inferred from the claim text.

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Abstract

An atomic frequency standard comprising:

a. A cell containing a mixture of alkali metal atoms and a mixture of buffer gases non reactive with said alkali metal atoms, said mixture being selected to minimize the temperature coefficient within said cell;
b. A laser whose emitted wavelength corresponds to either of the D1 or the D2 transition of the said alkali metal atoms.
c. A combination of optical components that set the state of polarization of the light to be circularly polarized.
d. An RF generator that generates a frequency whose value is the hyperfine ground state 0-0 transition frequency of said alkali metal atoms (the “clock transition”) divided by an integer number. The RF output is phase modulated (PM) or amplitude modulated (AM) at a low frequency (below 100 kHz).
e. The said generator output is used to amplitude modulate (AM) the current of the said laser. The laser emission is thus amplitude, frequency and phase modulated. The modulated RF drive current is modulated at a rate lower than the said RF frequency but could be much higher than the resonance width. As a result the laser light is AM and PM modulated containing AM sidebands each of which carries its own low frequency FM sidebands. The number of AM sidebands is controlled by the RF power level while the number of FM sided bands is controlled by the FM modulation index, namely the amplitude of the electrical FM modulation signal.
f. A photo-detector placed behind the vapor cell is used to detect the variations in the intensity of the light transmitted through said cell.
g. A circuit that demodulates the variations in the light intensity. The demodulation is performed in-phase or in quadrature to the said low frequency FM modulation, or in a mixture of both so that the demodulated signal is mostly related to a phase shift difference of the transmitted light.
h. A frequency control loop that uses the said phase shift difference signal to lock the RF generator to the zero crossing point of the phase difference. The said RF generator frequency is tuned so that the clock transition frequency corresponds to a difference in frequency between one of the high and one of the low side bands.

5 more elements