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European Frequency and Time Forum

Recent progress on the pulsed CPT Cs clock at SYRTE

Jean-Marie Danet, Olga Kozlova, Stéphane Guérandel, Emeric de Clercq

Published Apr 1, 2012• 2 citations
Publisher

Abstract

Recent progress are reported on a compact cesium cell atomic frequency standard based on pulsed Coherent Population Trapping interaction in the Dicke regime. Main short term and long term limiting parameters have been identified. Last improvements have permitted to reach a stability of 7×10-13at 1sec and 2×10-14at 2000 s. The noise characterization process is presented in this article.

DOI
10.1109/eftf.2012.6502341

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Recent progress are reported on a compact cesium cell atomic frequency standard based on pulsed Coherent Population Trapping interaction in the Dicke regime. Main short term and long term limiting parameters have been identified. Last improvements have permitted to reach a stability of 7×10-13at 1sec and 2×10-14at 2000 s. The noise characterization process is presented in this article.
This paper reports on progress on a compact clock based on Coherent Population Trapping (CPT) using caesium vapour cell. The clock is working in a pulsed regime with two orthogonal linear polarizations made by two phase-locked lasers tuned to 895 nm and separated by 9.2 GHz. The short-term frequency stability is mainly limited by the Local Oscillator (LO) noise contribution and the laser relative intensity noise (RIN) at the level of 3.2×10-13 at 1 s. We present in this paper the work done to improve our experimental device by reducing the frequency noise source contributions.
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.
This article reports on the characterization of a compact coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique. This method allows to detect high-contrast CPT resonances on the 0-0 magnetic-field insensitive clock transition. The clock short-term fractional frequency stability is measured to be 3×10−13 τ−1/2 up to 100 s averaging time, in correct agreement with the signal-to-noise ratio limit. These results are among the best performances ever reported for a CPT-based clock. The noise budget is detailed. A preliminary study is reported to evaluate limitations to the clock mid-term frequency stability. The clock performances are found to be presently mainly limited in the mid-term by laser power effects.
A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the D1 line. A preliminary short-term stability of3.5×10−12t−1/23.5 \times 10^{-12}t^{-1/2}3.5×10−12t−1/2is reported. The CPT specificity allows a new interrogation–detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence.