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Acta Physica Sinica

Study on characteristics of coherent population trapping spectral line for chip-scale atomic clock

Yi Yin, Yi Zhang, Tan Bo-Zhong, Jie‐Hua Chen, Sihong Gu

Published Jan 1, 2015• 1 citation
Publisher

Abstract

To Obtain an appropriate coherent population trapping (CPT) signal through a miniature atomic vapor cell is one of the key steps to implement a chip-scale atomic clock (CSAC). In the present experiment, the high S/N differential CPT spectral line has been achieved with a miniature atomic vapor cell through modulating the laser and extracting 87Rb atom laser interacting signal using the phase sensitive demodulation technique. With the spectral line, the dependence of CPT signal on the working parameters and the dependence of frequency stability of CSAC on the quality of the CPT signal have been studied; the obtained experimental results agree well with the theoretical prediction, which can be used as the recommended working parameters for CSAC. The methods exploited in the experiment can be implemented in the resource of a CSAC, therefore this experiment explores for CSAC the practical approaches of performance optimization.

DOI
10.7498/aps.64.034207

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To Obtain an appropriate coherent population trapping (CPT) signal through a miniature atomic vapor cell is one of the key steps to implement a chip-scale atomic clock (CSAC). In the present experiment, the high S/N differential CPT spectral line has been achieved with a miniature atomic vapor cell through modulating the laser and extracting 87Rb atom laser interacting signal using the phase sensitive demodulation technique. With the spectral line, the dependence of CPT signal on the working parameters and the dependence of frequency stability of CSAC on the quality of the CPT signal have been studied; the obtained experimental results agree well with the theoretical prediction, which can be used as the recommended working parameters for CSAC. The methods exploited in the experiment can be implemented in the resource of a CSAC, therefore this experiment explores for CSAC the practical approaches of performance optimization.
Research on chip-scale atomic clocks (CSACs) based on coherent population trapping (CPT) is reviewed. The background and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the reliability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance's collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.
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.
We report our measurement of coherent-population-trapping (CPT) signal linewidths in small vapor cells, which are desirable for chip scale atomic clock (CSAC) applications. The linewidths are measured in both rubidium vapor cells and cesium vapor cells. Using a micro-machined silicon absorption cell (diameter = 1.7 mm, length = 2 mm), we demonstrate a sub-kHz linewidth (FWHM) of the clock transition in the /sup 133/Cs atom (|F = 3, m/sub F/ = 0>/spl harr/|F = 4, m/sub F/ = 0>). The measured intrinsic linewidths (FWHM = 777 Hz for Cs and FWHM = 1496 Hz for Rb) of the cells agree with the calculated results.
A universal method for fabrication of miniature cells for frequency standards and quantum magnetometers containing 87Rb atoms in the atmosphere of inert gas neon based on integrated technologies is considered. The results of experimental studies of coherent population trapping signals observed for a series of cells which provided recovery of vapors of an alkali metal from the rubidium dichromate salt with the help of laser radiation are presented. The coherent population trapping signals with a typical linewidth of 2–3 kHz and a signal-to-noise ratio of 1500 in the 1-Hz bandwidth were observed, which allows one to provide a relative frequency stability of atomic clock of 10−11 at 100 s.
We report the characterization of dark line resonances observed in Cs vapor microcells filled with a unique neon (Ne) buffer gas. The impact on the coherent population trapping (CPT) resonance of some critical external parameters such as laser intensity, cell temperature, and microwave power is studied. We show the suppression of the first-order light shift by proper choice of the microwave power. The temperature dependence of the Cs ground state hyperfine resonance frequency is shown to be canceled in the 77–80 °C range for various Ne buffer gas pressures. The necessity to adjust the Ne buffer gas pressure or the cell dimensions to optimize the CPT signal height at the frequency inversion temperature is pointed out. Based on such Cs–Ne microcells, we preliminary demonstrate a 852 nm vertical cavity surface emitted laser (VCSEL)-modulated based CPT atomic clock exhibiting a short term fractional frequency instability σy(τ)=1.5×10−10τ−1/2 until 30 s. These results, similar to those published in the literature by others groups, prove the potential of our original microcell technology in view of the development of high-performance chip scale atomic clocks.
Spectroscopy studies of coherent population trapping (CPT) with constructive polarization modulation are presented. In these studies, the effects of modulation frequency and laser intensity on the CPT contrast of the clock transition are investigated. We experimentally show that no CPT transition with ΔmF=2 is excited. Moreover, the constructive polarization modulation CPT signals are observed in both cases, with excited levels F'=3 and F'=4 in the Dl line of Cesium. Our studies show that this scheme, with potential to implement a compact and high performance clock, could also be applied to high pressure buffer gas cell, e.g., chip scale atomic clock.