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Review of Scientific Instruments

Selected detection of polarization for simultaneous laser locking and high-contrast coherent population trapping resonance

Suping Qu, Yi Xin Yin, Xianyang Lu

Published Dec 1, 2025• 0 citations
Publisher

Abstract

We present a new polarization-selective detection scheme that enables simultaneous dual-frequency laser stabilization and implementation of high-contrast coherent population trapping (CPT) resonance. The proposed configuration splits elliptically polarized light into two optical paths after atomic interaction. The component reflected from the polarizing beam splitter, containing Doppler-broadened spectral features, is employed for precise laser frequency locking. The reflected signal exhibits a frequency discrimination slope nine times steeper than that of the transmitted polarizing beam splitter component. Simultaneously, the transmitted component isolates the CPT resonance signals from the positive and negative first-order sidebands for microwave frequency stabilization. Experiment demonstrates excellent agreement with theoretical simulations derived from the Liouville equation. Compared with conventional circular polarization methods, this approach achieves a fourfold enhancement in short-term frequency stability. These advancements make the technique particularly promising for developing compact CPT atomic clocks with improved metrological performance.

DOI
10.1063/5.0274015

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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)•Jul 8, 2004•27 citations
We present a new polarization-selective detection scheme that enables simultaneous dual-frequency laser stabilization and implementation of high-contrast coherent population trapping (CPT) resonance. The proposed configuration splits elliptically polarized light into two optical paths after atomic interaction. The component reflected from the polarizing beam splitter, containing Doppler-broadened spectral features, is employed for precise laser frequency locking. The reflected signal exhibits a frequency discrimination slope nine times steeper than that of the transmitted polarizing beam splitter component. Simultaneously, the transmitted component isolates the CPT resonance signals from the positive and negative first-order sidebands for microwave frequency stabilization. Experiment demonstrates excellent agreement with theoretical simulations derived from the Liouville equation. Compared with conventional circular polarization methods, this approach achieves a fourfold enhancement in short-term frequency stability. These advancements make the technique particularly promising for developing compact CPT atomic clocks with improved metrological performance.
Traditional coherent population trapping (CPT) atomic clocks use single circularly polarized coherent multichromatic light to interact with atoms for preparing CPT dark states. While this configuration is simple, it suffers from the trap states and involves only the ±1st-order sidebands in the CPT resonance, excluding the carrier and other sidebands. This results in a lower CPT signal amplitude and higher laser noise, limiting the frequency stability of CPT atomic clocks. To address this issue, we developed and experimentally validated a scheme utilizing left- and right-circularly polarized light based on multiple coherent superpositions. This scheme significantly enhances the amplitude of the CPT resonance. Furthermore, by incorporating a Michelson interferometer, the polarization direction of the ±1st-order sidebands in the coherent multichromatic light is adjusted from parallel to perpendicular, enabling the filtering of one sideband to be detected while eliminating other unwanted sidebands and effectively suppressing laser noise. The results show that, under identical conditions, the short-term frequency stability of this method is theoretically improved by a factor of seven compared to the traditional circularly polarized light scheme, providing an effective pathway for the development of high-performance CPT atomic clocks.
This scheme is based on a Michelson interferometer in which the relative optical path difference between the two arms is adjusted to control the relative phase between the copropagating left- and right-handed ( σ + - σ − ) circularly polarized light. This configuration converts the dominant ±1 st sidebands from parallel to orthogonal linear polarizations, while the 0 th sideband becomes circularly polarized light. The ±1 st sidebands, forming a double-Λ configuration, interact with 87 Rb atoms to generate an interference-enhanced coherent population trapping (CPT) resonance. A partial reflector placed behind the vapor cell enables multiple interactions between the σ + - σ − circularly polarized light and the atoms. After the polarizer, only the +1 st sideband resonance is detected; the −1 st sideband is completely suppressed, and the 0 th sideband is partially attenuated, thereby reducing the light background. Under the same conditions, the multi-pass configuration improves the CPT contrast fourfold and reduces noise by roughly half an order of magnitude compared with the single-pass case. With only a few additional optical components, this scheme improves both contrast and noise, positioning it as a promising candidate for high-performance CPT atomic clocks.
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.
In order to suppress the effects of carrier and high-order sidebands on the coherent population trapping (CPT) signal, a scheme of interaction between an elliptically polarized beam and alkali atoms using the magneto-optic rotation detection is examined. By canceling the spin-polarized dark state in the CPT state preparation, a CPT resonance signal with a contrast of 32.5% is obtained. Our scheme requires minor modifications of the optical path in the prevalent scheme, and the short-term frequency stability of our scheme is five times better than that of the prevalent scheme, making it ideal for miniaturized CPT atomic clock applications.
Abstract Chip-scale atomic clocks based on coherent population trapping (CPT) face challenges due to miniaturized gas cells, which reduces the CPT resonance signal and the absorption signals of alkali atoms. To overcome this, we propose a method that simultaneously stabilizes the laser wavelength and microwave modulation frequency without relying on absorption line detection. Our approach enables stabilization at the point where the amplitude of the CPT resonance signal is maximized, making it suitable for compact atomic clocks. The system uses three lock-in amplifiers: one is used to stabilize the RF modulation frequency, and the other two are used for detecting the amplitude of the CPT resonance and stabilizing the laser wavelength. The Allan deviation measurements demonstrate a 1.4-fold improvement in short-term stability compared to conventional methods, suggesting that the proposed technique is a promising solution for enhancing the performance of small gas-cell atomic clocks.