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European Frequency and Time Forum

Progress on a pulsed CPT clock: Reduction of the main noise source contributions

François Tricot, Peter Yun, Bruno François, Sinda Mejri, Jean-Marie Danet, M. Lours, Stéphane Guérandel, Emeric de Clercq

Published Apr 1, 2016• 4 citations
Publisher

Abstract

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.

DOI
10.1109/eftf.2016.7477813

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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.
This paper reports on our best performance achieved with a laboratory compact CPT caesium clock. The device is working in a pulsed regime with orthogonal linear polarizations arranged in the so-called double-lambda configuration using two separate phase-locked lasers tuned to the D1 line. The short term frequency stability has been measured at the level of 3.2×10−13τ−1/2. The mid-term stability is limited at a few 10−14after hundreds of seconds. The main noise sources impacting both short and mid terms are presented and discussed.
Toward the next generations of compact atomic clocks, clocks based on coherent population trapping (CPT) offer a very interesting alternative. Thanks to CPT, a quantum interfering process, this technology has made a decisive step in the miniaturization direction. Fractional frequency stability of 1.5x10-10 at 1 s has been demonstrated in commercial devices of a few cm3. The laboratory prototype presented here intends to explore what could be the ultimate stability of a CPT based device. To do so, an original double-Λ optical scheme and a pulsed interrogation have been implemented in order to get a good compromise between contrast and linewidth. A study of two main sources of noise, the relative intensity and the local oscillator (LO) noise, has been performed. By designing simple solutions, it led to a new fractional frequency limitation lower than 4x10-13 at 1 s integration. Such a performance proves that such a technology could rival with classical ones as double resonance clocks.
We demonstrate a new technique for the suppression of noise associated with the laser source in atomic clocks based on coherent population trapping (CPT). The technique uses differential detection of the transmission of linearly and circularly polarized beams that propagate through different parts of a single rubidium vapor cell filled with a buffer gas mixture. The common-mode noise associated with the laser frequency and amplitude noise is suppressed by the differential detection of the two laser beams. The CPT signal, which is present only in the circularly polarized laser beam, is unaffected. The implementation of the technique requires only a change of the polarization of part of the laser beam and an additional photodiode. The technique is simple and applicable to CPT frequency references where a major source of noise is the laser, such as compact and chip-scale devices.
We evaluate the use of Coherent Population Trapping (CPT) excited with parallely polarized laser fields in vapour cell atomic clocks. We study the resonance shape, the discriminator slope and signal-to-noise ratio as a function of relevant parameters such as the laser frequency detuning and the applied magnetic field. We show that a stability of 6⋅10−126 \cdot 10^{-12}6⋅10−12 τ−1/2τ^{-1/2}τ−1/2 may be reached in a compact system using a modulated VCSEL.
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.