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European Frequency and Time Forum

Optimization of laser radiation for CPT-based miniature frequency standard

К. А. Баранцев, A. N. Litvinov, Evgeniy Popov, Igor M. Sokolov

Published Apr 1, 2016• 2 citations
Publisher

Abstract

The present work is dedicated to optimization of the laser radiation parameters and optically thickness of the gas cell for improvement of short-term stability of quantum frequency standard based on coherent population trapping effect in the cell with 87Rb working gas and buffer gas. The effects associated with the optical density of the medium are investigated, such as laser spectrum distortion and appearance of the light shifts of the coherent population trapping resonance due to different absorption of frequency components.

DOI
10.1109/eftf.2016.7477818

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The present work is dedicated to optimization of the laser radiation parameters and optically thickness of the gas cell for improvement of short-term stability of quantum frequency standard based on coherent population trapping effect in the cell with 87Rb working gas and buffer gas. The effects associated with the optical density of the medium are investigated, such as laser spectrum distortion and appearance of the light shifts of the coherent population trapping resonance due to different absorption of frequency components.
Coherent population trapping (CPT) on 87 Rb atoms in neon atmosphere has been studied in small-size glass absorption cells under conditions of pumping with narrow-band laser radiation at the D 2 line of the main doublet. Parameters of the absorption signal have been measured in 3-mm-diameter cells at buffer gas (Ne) pressures varied within 200–400 Torr, cell temperatures within 65–120°C, and pumping radiation power densities within 30–400 μW/cm 2 . Optimum values of the buffer gas pressures, cell temperature, and pumping power are determined at which the short-term instability of the resonance line is at minimum. Orientational shifts of the CPT resonance signal in gas-filled cells and small-size cells with antirelaxation coating have been compared.
We investigated the influence of some critical parameters and operating conditions such as cell temperature, laser intensity, and interrogation technique affecting the performances of a gas cell Cs frequency standard based on coherent population trapping (CPT). Thanks to an original experimental setup, the atoms can be trapped in the dark state and interrogated using continuous wave (CW) or pulsed coherent optical radiations. Using a double-lambda scheme, a signal contrast as high as 52% has been measured in the continuous regime for an optimum cell temperature of 35 degrees C. Compared with the conventional continuous CPT interrogation, the pulsed interrogation technique reduces the light shift by a factor of 300 and allowed it to reach high-frequency stability for higher laser intensities. The frequency stability has been measured to be 9 x 10(-13) for a 1 s integration time. Main noise contributions limiting the short-term and medium-term frequency stability are reviewed and estimated.
The paper shows the presents results of studies of small-size rubidium cells with argon and neon buffer gases, produced by a patent pended technique of laser welding [Fishman et al. (2014)]. Cells were designed for miniature frequency standard. Temperature dependence of the frequency of the coherent population trapping (CPT) resonance was measured and used to optimize the ratio of partial pressures of buffer gases. The influence of duration and regime of annealing on the CPT-resonance frequency drift was investigated. The parameters of the FM modulation of laser current for two cases which correspond to the highest amplitude of CPT resonance and to the smallest light shifts of the resonance frequency were determined. The temperature dependences of the CPT resonance frequency were found to be surprisingly different in the two cases. A non-linear dependence of CPT resonance frequency on the temperature of the cell with the two extremes was revealed for one of these cases.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
The theory of coherent population trapping resonance is developed for the finite linewidth of the laser radiation spectrum in an optically dense medium of Λ atoms in a cell with a buffer gas. Equations are derived for the atomic density matrix and laser emission spectrum transfer in a cell with working and buffer gases at a finite temperature. The dependence of the quality factor of coherent population trapping resonance on the linewidth of the laser radiation spectrum is studied by measuring transmitted radiation and fluorescence signals.