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International Conference on Optoelectronics and Microelectronics

Dependence of Cs atomic clock CPT signal on magnetic intensity

Hongsong Mei, Qiang Huang, Junhai Zhang, Weimin Sun, Zongjun Huang

Published Sep 1, 2013• 0 citations
Publisher

Abstract

This work demonstrates the CPT (Coherent Population Trapping) signal of Cs atomic clock can be affected by magnetic field theoretically. The amplitude and the width of the resonance line of the CPT signal are the functions of the magnetic intensity. When both the coherent lasers have a specific rate of intensity the amplitude of the CPT signal keeps the most stable. Compared with four lambda models in theory, research shows that the scheme of the translation from ground states |Fg=4, mF=0> and |Fg=3, mF=0> to excited state |Fe=4, mF=-1> is the most suitable for Cs CPT atomic clock.

DOI
10.1109/icoom.2013.6626528

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This work demonstrates the CPT (Coherent Population Trapping) signal of Cs atomic clock can be affected by magnetic field theoretically. The amplitude and the width of the resonance line of the CPT signal are the functions of the magnetic intensity. When both the coherent lasers have a specific rate of intensity the amplitude of the CPT signal keeps the most stable. Compared with four lambda models in theory, research shows that the scheme of the translation from ground states |Fg=4, mF=0> and |Fg=3, mF=0> to excited state |Fe=4, mF=-1> is the most suitable for Cs CPT atomic clock.
Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs D1 line transmission, using a double Λ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters.
We have studied the effects of the intensity difference of the two laser fields on the performance of the coherent population trapping (CPT) atomic clocks. Our experimental results show that there is a ratio range for the intensity of the two laser fields, between 1:2 and 2:1, in which the CPT resonance has larger amplitude and narrower line width. If we can adjust the ratio close to the center (1: 1), the signal quality (ratio of amplitude to line width) of CPT resonance will be efficiently improved. Our results also show that using full hyperfine frequency to modulate the laser has its advantages.
We have experimentally demonstrated that magnetic-field-insensitive coherent-population-trapping (CPT) resonances are generated between the ground hyperfine levels on the D1{D}_{1}D1​ line of 133Cs^{133}\mathrm{Cs}133Cs using a two-photon $\mathrm{\ensuremath{\Lambda}}$ scheme excited by lin||lin polarizations. The frequency shift of the CPT resonance is 0.04 Hz for the deviation of $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ at a ``magic'' magnetic field of $139\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ and is 50 times smaller than that of the conventional clock transition at a typical bias magnetic field for the clock operation. The amplitude of the CPT spectrum excited by lin||lin polarizations is enhanced as the excitation intensity increases and is well explained by the three-level model without trap levels. Thus, the CPT resonance on the D1{D}_{1}D1​ line of 133Cs^{133}\mathrm{Cs}133Cs excited by a simple lin||lin scheme will be one of the best candidates for frequency reference of miniature 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.
Based on a Λ-type three-level system consists of cesium (Cs) 6S1/2 (Fg = 3) and 6S1/2 (Fg = 4) long-lived ground states and 6P3/2 (Fe = 4) excited state, we have experimentally measured and theoretically analyzed the characters of coherent population trapping (CPT) resonances in Cs vapor cells with different partial pressure of neon (Ne) as buffer gas around room temperature. The impact of some experimental parameters, such as the relative intensity ratio between two phase-locked laser beams, the laser intensity, with or without buffer gas and the partial pressure of Ne, the temperature and the longitudinal magnetic field, on the CPT resonance was studied in details. With the optimized parameters, we got typical CPT signal with the full-width half-maximum (FWHM) linewidth as narrow as ~ 181 Hz in a Cs vapor cell filled with 30 Torr of Ne as buffer gas.
According to the Λ -type three-level system consisting of cesium 6S1/2 (F=3 and F=4) long-lived ground states and 6P3/2 (F'=4) excited state, we experimentally investigate and theoretically analyze the parameters of coherent population trapping (CPT) spectra in a cesium atomic vapor cell with or without neon as buffer gas. The CPT of the full width at half maximum (FWHM), the signal amplitude as a function of optical intensity of the two phased-locked laser beams with a frequency difference of 9.19263177 GHz (the hyperfine splitting in the cesium ground state 6S1/2), relative intensity ratio, beam spot size, residual magnetic field of the magnetic shielding, and the influence of with and without the cesium vapor cell containing neon as buffer gas are investigated experimentally. With the optimized parameters, we obtain the CPT signal FWHM to be as narrow as ～ 340 Hz.