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Optics Express

First-order cancellation of the Cs clock frequency temperature-dependence in Ne-Ar buffer gas mixture

Rodolphe Boudot, D. Miletic, P. Dziuban, C. Affolderbach, Paweł Knapkiewicz, Jan Dziuban, G. Mileti, Vincent Girodano, Christophe Gorecki

Published Feb 2, 2011• 16 citations
Publisher

Abstract

Through the detection of Coherent Population Trapping (CPT) resonances, we demonstrate the temperature-dependence cancellation of the Cs clock frequency in microfabricated vapor cells filled with a mixture of Ne and Ar. The inversion temperature at which the Cs clock frequency temperature sensitivity is greatly reduced only depends on the partial pressure of buffer gases and is measured to be lower than 80°C as expected with simple theoretical calculations. These results are important for the development of state-of-the-art Cs vapor cell clocks with improved long-term frequency stability.

DOI
10.1364/oe.19.003106

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Through the detection of Coherent Population Trapping (CPT) resonances, we demonstrate the temperature-dependence cancellation of the Cs clock frequency in microfabricated vapor cells filled with a mixture of Ne and Ar. The inversion temperature at which the Cs clock frequency temperature sensitivity is greatly reduced only depends on the partial pressure of buffer gases and is measured to be lower than 80°C as expected with simple theoretical calculations. These results are important for the development of state-of-the-art Cs vapor cell clocks with improved long-term frequency stability.
We report on the characterization of Cs vapor microfabricated cells filled with a Ne-He buffer gas mixture using coherent population trapping (CPT) spectroscopy. The temperature dependence of the Cs clock frequency is found to be canceled at the first order around a so-called inversion temperature higher than 80°C whose value depends on the buffer gas partial pressure ratio. This buffer gas mixture could be well-adapted for the development of miniature atomic clocks devoted to be used in specific applications such as defense and avionic systems with high operating temperature environment (typically higher than 85°C). This solution suggests an alternative to buffer gas mixtures generally used in optically-pumped vapor cell atomic clocks.
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.
We report the characterization of Coherent Population Trapping (CPT) resonances in Cs vapor microcells filled with Neon (Ne) buffer gas. The impact on the atomic hyperfine resonance of some external parameters such as laser intensity and cell temperature 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. We preliminary demonstrate a 852 nm VCSEL-modulated based CPT atomic clock exhibiting a short term fractional frequency instability σy(τ) = 1.5×10-10τ-1/2until 200 s.
Pulsed coherent population trapping (CPT) spectroscopy is applied in buffer-gas filled Cs–Ne vapor microfabricated cells. The properties of the Ramsey-CPT clock transition central fringe (linewidth, signal amplitude, and contrast) are studied versus several experimental parameters, including the Ramsey sequence and the input laser intensity, and are compared to those obtained in the conventional continuous (CW) interrogation mode. In the pulsed case and for short Ramsey-free evolution times TR, the central fringe linewidth is found to exhibit a small but visible power broadening and is measured to be narrower than the value of 1/(2TR) that is reached for long TR. The microwave hyperfine coherence lifetime T2 in the Cs–Ne microcells is measured to be in the 50–500 μs range. Its dependence to the buffer gas pressure or cell temperature is briefly studied.
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.
Controlling buffer-gas conditions in vapor cells is essential for frequency stabilization and atomic-clock performance. This study investigated the dependence of coherent population trapping (CPT) signal characteristics on nitrogen (N 2{}_{2}2​ ) buffer-gas pressure in micro-fabricated vapor cells containing natural-abundance rubidium (Rb) generated from Rb azide (RbN 3{}_{3}3​ ) for chip-scale atomic clocks. A wafer-level fabrication method enabling individual control of N2 pressure was developed by inkjet patterning of RbN3 and subsequent krypton fluoride (KrF) excimer laser decomposition. An increasing N2 pressure was found to reduce the CPT linewidth and cause a significant decrease in the signal amplitude, which resulted in monotonic degradation in the CPT figure of merit with higher pressures. A CPT-based laser frequency stabilization method that locked the laser wavelength to the CPT resonance peak yielded significantly larger signal amplitudes than a conventional absorption-based method in the low-pressure regime below 8 kPa. These results show that CPT performance in micro-fabricated Rb vapor cells is more restricted by the signal amplitude rather than the linewidth. They also offer practical buffer-gas design guidelines for enhancing short-term frequency stability in chip-scale atomic clocks.