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Journal of the Optical Society of America B

Characterization of coherent population-trapping resonances as atomic frequency references

Svenja A. Knappe, R. Wynands, John E. Kitching, H. G. Robinson, Leo W. Hollberg

Published Nov 1, 2001• 144 citations
Publisher

Abstract

A low-cost, potentially compact and robust microwave frequency reference can be constructed by use of vertical-cavity surface-emitting lasers and coherent population-trapping resonances in Cs vapor cells. Fractional frequency instabilities of 2×10-11/τ/s have been achieved with a minimum of 7×10-13 at τ=1000 s. The performance of this device as a function of external parameters such as light intensity, optical detuning, and cell temperature is discussed. The dependence of the dark-line resonance signal on these parameters can be understood largely by means of a simple, three-level model. The short-term stability depends critically on the optical detuning, whereas the long-term stability is limited currently by line shifts due to drifts in cell temperature.

DOI
10.1364/josab.18.001545

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A low-cost, potentially compact and robust microwave frequency reference can be constructed by use of vertical-cavity surface-emitting lasers and coherent population-trapping resonances in Cs vapor cells. Fractional frequency instabilities of 2×10-11/τ/s have been achieved with a minimum of 7×10-13 at τ=1000 s. The performance of this device as a function of external parameters such as light intensity, optical detuning, and cell temperature is discussed. The dependence of the dark-line resonance signal on these parameters can be understood largely by means of a simple, three-level model. The short-term stability depends critically on the optical detuning, whereas the long-term stability is limited currently by line shifts due to drifts in cell temperature.
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.
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.
This article reports on the development and short-term fractional frequency stability of a continuous-regime (CW) Cs vapor cell atomic clock based on coherent population trapping (CPT). The push-pull optical pumping technique is used to increase the number of atoms that participate to the clock transition, yielding a typical CPT resonance contrast of 25% for a CPT linewidth of about 450 Hz. The clock short-term fractional frequency stability is measured to be 3 x 10 -13 τ -1/2 up to 100 seconds averaging time, in correct agreement with the signal-to-noise ratio limit. The mid-term frequency stability results are currently mainly limited by laser power effects. The detection of high-contrast narrow Raman-Ramsey fringes is demonstrated with this setup by making the atoms interact with a light pulse sequence.
We demonstrate a vapor-cell atomic-clock prototype based on a continuous-wave interrogation and double-modulation coherent population trapping (DM-CPT) technique. The DM-CPT technique uses a synchronous modulation of polarization and the relative phase of a bichromatic laser beam in order to increase the number of atoms trapped in a dark state, i.e., a nonabsorbing state. The narrow resonance, observed in the transmission of a Cs vapor cell, is used as a narrow frequency discriminator in an atomic clock. A detailed characterization of the CPT resonance versus numerous parameters is reported. A short-term fractional-frequency stability of 3.2×10−13τ−1/2 up to a 100-s averaging time is measured. These performances are more than one order of magnitude better than industrial Rb clocks and are comparable to those of the best laboratory-prototype vapor-cell clocks. The noise-budget analysis shows that the short- and midterm frequency stability is mainly limited by the power fluctuations of the microwave used to generate the bichromatic laser. These preliminary results demonstrate that the DM-CPT technique is well suited for the development of a high-performance atomic clock, with the potential compact and robust setup due to its linear architecture. This clock could find future applications in industry, telecommunications, instrumentation, or global navigation satellite systems.
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.