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Applied Optics

High-performance coherent population-trapping configuration with polarization conversion and multiple constructive interference

Qu suping, Yi Yin

Published Oct 8, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1364/ao.613788

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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.
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.
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.
Coherent population trapping (CPT) is extensively studied for future vapor cell clocks of high frequency stability. In the constructive polarization modulation CPT scheme, a bichromatic laser field with polarization and phase synchronously modulated is applied on an atomic medium. A high contrast CPT signal is observed in this so-called double-modulation configuration, due to the fact that the atomic population does not leak to the extreme Zeeman states, and that the two CPT dark states, which are produced successively by the alternate polarizations, add constructively. Here, we experimentally investigate CPT signal dynamics first in the usual configuration, a single circular polarization. The double-modulation scheme is then addressed in both cases: one pulse Rabi interaction and two pulses Ramsey interaction. The impact and the optimization of the experimental parameters involved in the time sequence are reviewed. We show that a simple seven-level model explains the experimental observations. The double-modulation scheme yields a high contrast similar to the one of other high contrast configurations like push-pull optical pumping or crossed linear polarization scheme, with a setup allowing a higher compactness. The constructive polarization modulation is attractive for atomic clock, atomic magnetometer, and high precision spectroscopy applications.
With interfering enhanced coherent population trapping (CPT) signals, a CPT atomic clock with improved frequency stability performance can be realized. We explore an optical device that converts single-polarized bichromatic light to left and right circularly polarized superposed bichromatic light to generate interfering enhanced CPT resonance with atoms. We have experimentally studied a tabletop CPT atomic clock apparatus with a microfabricated 87Rb atomic chip-scale cell, and the study results show that it is promising to realize a compact CPT atomic clock, even a chip-scale CPT atomic clock through microfabrication, with improved frequency stability performance.
Coherent population trapping (CPT) atomic clocks are prevailingly realized by exciting CPT resonance with circularly polarized bichromatic coherent light (σ+-σ+ scheme), while atom utilization efficiency is relatively low in the scheme. For improving efficiency, we experimentally study the scheme of realizing CPT resonance through the interaction between atoms and parallel linearly polarized bichromatic coherent light field (lin//lin scheme). In the same experimental conditions the experimental results show that the signal-noise ratio of the CPT resonance signal is twice, and the differential slope is 1.65 time higher than those of σ+-σ+ scheme. The experimental results reveal that the scheme is an ideal candidate for low power consuming atomic clock and magnetometer.
A scheme is studied for a coherent population trapping (CPT) atomic clock with counterpropagating left and right circularly polarized lasers, wherein the vertical-cavity surface-emitting laser and the partially reflecting mirror form an external cavity of an established length. With cancellation of spin-polarized dark states and interaction repeated with atoms, a CPT signal is obtained that is eight times larger than that of the conventional scheme. For the improvement of modulation efficiency, CPT resonance still can be excited with a lower microwave power compared with the conventional scheme. The experimental results suggest that it is an alternative for low-power atomic clocks.