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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Reduction of optical field noise by differential detection in atomic clocks based on coherent population trapping

Vladislav P. Gerginov, Svenja A. Knappe, Vishal Shah, John E. Kitching, Leo W. Hollberg

Published Mar 2, 2007• 1 citation
Publisher

Abstract

We present preliminary results showing that some noise sources in vapor cell atomic clocks based on coherent population trapping (CPT) can be suppressed with differential detection. The scheme we propose differs from more conventional differential detection in that both optical fields pass through the alkali vapor cell but have different polarizations, one circular and one linear. Because CPT resonances are only excited by the circularly polarized beam, the linearly polarized beam can be used to reduce several important sources of noise. With this technique, we demonstrate reduction of the short-term frequency instability of a CPT atomic frequency reference by a factor of about 1.5.

DOI
10.1117/12.726893

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We present preliminary results showing that some noise sources in vapor cell atomic clocks based on coherent population trapping (CPT) can be suppressed with differential detection. The scheme we propose differs from more conventional differential detection in that both optical fields pass through the alkali vapor cell but have different polarizations, one circular and one linear. Because CPT resonances are only excited by the circularly polarized beam, the linearly polarized beam can be used to reduce several important sources of noise. With this technique, we demonstrate reduction of the short-term frequency instability of a CPT atomic frequency reference by a factor of about 1.5.
We demonstrate a new technique for the suppression of noise associated with the laser source in atomic clocks based on coherent population trapping (CPT). The technique uses differential detection of the transmission of linearly and circularly polarized beams that propagate through different parts of a single rubidium vapor cell filled with a buffer gas mixture. The common-mode noise associated with the laser frequency and amplitude noise is suppressed by the differential detection of the two laser beams. The CPT signal, which is present only in the circularly polarized laser beam, is unaffected. The implementation of the technique requires only a change of the polarization of part of the laser beam and an additional photodiode. The technique is simple and applicable to CPT frequency references where a major source of noise is the laser, such as compact and chip-scale devices.
We present a digital differential detection scheme using a single photodiode to suppress the diode laser frequency noise and improve the short-term stability in atomic clocks based on coherent population trapping (CPT) resonance. One laser light is divided into two beams via modulation of an acousto-optic modulator. One beam induces CPT resonance with circularly polarized light, and the other beam induces only absorption with linearly polarized light. A differential algorithm counteracts common-mode noise from the two beams. A stability of 6.86×10−12 at 100 s and 2.26×10−12 at 1000 s has been demonstrated. The short-term stability is improved by a factor of 2.4 at 100 s. The scheme promises to eliminate laser-related noise and explore potential applications in chip-scale atomic clocks.
We demonstrate the excitation and low-noise differential detection of a coherent population trapping (CPT) resonance with two modulated optical fields with orthogonal circular polarizations. When a microwave phase delay of lambda/4 is introduced in the optical path of one of the fields, the difference in the power transmitted through the cell in each polarization shows a narrow, dispersive resonance. The differential detection allows a high degree of suppression of laser-induced noise and will enable nearly shot-noise-limited operation of atomic frequency references and magnetometers based on CPT.
We propose and investigate a scheme for differential detection of the magneto-optic rotation (MOR) effect, where a linearly polarized bichromatic laser field is coherent population-trapping (CPT)-resonant with alkali atoms, and discuss the application of this effect to CPT-based atomic clocks. The results of our study indicate that laser noise in a vertical cavity surface-emitting laser-based CPT atomic clock can be effectively suppressed by the proposed scheme. The proposed scheme promises to realize a packaged MOR-CPT atomic clock that has significantly better frequency stability coupled with similar power consumption, volume, and cost when compared with currently available packaged CPT atomic clocks.
Rb vapor cell with differential detection. Elliptically polarized light is used to interact with alkali atoms to generate coherent population trapping (CPT) resonance, and the CPT signal is obtained by detecting the Faraday rotation effect with differential detection technology. To move closer to an actual chip-scale atomic clock (CSAC), we reduce the volume of the experimental apparatus and use a divergent laser beam to interact with the atoms. We obtain the short-term frequency stability of the CSAC based on the differential detection scheme and compare it with that of a conventional CSAC. The results show that the frequency stability is more than two times better than that of current commercial CSAC devices with the same power consumption and volume.
A scheme is investigated for a coherent population trapping (CPT) atomic clock, wherein the polarization of a beam produced by a vertical-cavity surface-emitting laser is converted to an elliptically polarized beam that interacts with alkali atoms, where the CPT signal is extracted by differentially detecting the magneto-optically rotated light within the transmitted beam. The scheme eliminates the spin-polarized trap state of the atoms and the unwanted background signal, and suppresses in the CPT signal the noise converted from the laser noise. This result reveals the promise of this scheme for realizing a compact CPT atomic clock possessing a significantly improved frequency stability compared to current compact CPT atomic clock devices, coupled with similar power consumption, volume, and cost therewith.