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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Spectroscopy of micro-fabricated Cs vapours cells for miniature atomic clocks

D. Miletic, C. Affolderbach, G. Mileti, Madoka Hasegawa, Christophe Gorecki

Published Oct 8, 2010• 1 citation
Publisher

Abstract

In this paper we present our spectroscopic studies on Coherent Population Trapping (CPT) in micro-fabricated Caesium cells and our evaluation of its application in miniature atomic frequency standards (atomic clocks). We observe the CPT signal on the Cs D1-line by coupling two hyperfine ground-state Zeeman sub-levels to a common excited state using two coherent electromagnetic fields created with a modulated DFB laser. Contrarily to double resonance, CPT does not require any microwave cavity, which should facilitate the miniaturization of a future atomic clock device. We study and report here on the light shift phenomena at different cell temperatures and laser wavelength. We also present resonance shifts due to cell temperature variations and clock frequency stability measurements. To the best of our knowledge, this article is the first report on light shift with Cs D1 line in a CPT vapour-cell atomic clock.

DOI
10.1117/12.882194

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In this paper we present our spectroscopic studies on Coherent Population Trapping (CPT) in micro-fabricated Caesium cells and our evaluation of its application in miniature atomic frequency standards (atomic clocks). We observe the CPT signal on the Cs D1-line by coupling two hyperfine ground-state Zeeman sub-levels to a common excited state using two coherent electromagnetic fields created with a modulated DFB laser. Contrarily to double resonance, CPT does not require any microwave cavity, which should facilitate the miniaturization of a future atomic clock device. We study and report here on the light shift phenomena at different cell temperatures and laser wavelength. We also present resonance shifts due to cell temperature variations and clock frequency stability measurements. To the best of our knowledge, this article is the first report on light shift with Cs D1 line in a CPT vapour-cell atomic clock.
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.
State-of-the-art miniature atomic clocks (MACs) are based on the phenomenon of coherent population trapping (CPT) in alkali-metal atomic vapors (Rb\mathrm{Rb}Rb or Cs\mathrm{Cs}Cs). Improving the stability of the clock frequency is a crucial issue; resolving it would result in significant progress in numerous applications. Here, we examine the configuration of the light field, which is composed of two bichromatic beams with opposing handedness of circular polarization. The beams are in resonance with optical transitions in the Cs\mathrm{Cs}Cs D1{D}_{1}D1​ line ($\ensuremath{\lambda}\ensuremath{\approx}895$ nm). This configuration has previously been shown to produce observed CPT resonances with higher contrast than a standard single-beam scheme; however, our scheme implements pump and probe beams that operate independently, with the transmission of the probe beam being monitored separately. This approach differs from previous studies. The experiments are carried out using a cubic $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\phantom{\rule{0.2em}{0ex}}{\mathrm{mm}}^{3}$ vapor cell filled with buffer gas. It is shown that the line shape of the resonance acquires an asymmetry that can be efficiently controlled by a microwave (Raman) phase between the beams. As a proof of concept, we study the way in which this asymmetry can help to mitigate the influence of the ac Stark (light) shift on the long-term frequency stability of CPT clocks based on vapor microcells ($V\ensuremath{\ll}1\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{3}$). Experimental verification is performed with both a distributed-Bragg-reflector laser and a vertical-cavity surface-emitting laser. The latter is of particular importance for the development of MACs. The experimental results are in qualitative agreement with the analytical theory based on a double-$\mathrm{\ensuremath{\Lambda}}$ scheme of atomic energy levels.
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.
We report on measurements of frequency shifts in a microwave cesium vapor cell atomic clock based on coherent population trapping (CPT). The dependence of the clock frequency on numerous experimental parameters, such as the laser power, the laser frequency, the microwave power, the cell temperature, the static magnetic field, but also the temperature of some key components, or the translation and rotation of critical wave plates and optical elements, is investigated. The stability budget of the clock frequency at one day is reported and discussed. This study constitutes a solid database for the future demonstration of a CPT-based cell clock with enhanced mid- and long-term stability performances.
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.