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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

A fully functional Ramsey-CPT atomic clock

Qun Zhou, Chao Guan, Xiaosong Zhu, Yeqing Li

Published May 1, 2011• 1 citation
Publisher

Abstract

The abstract presents a fully functional Ramsey-CPT atomic clock, which has the characteristics of compact structure, small volume, low cost, strong adaptability, high stability, convenient for adjustment, unique design, and easy-to-commercialization. The Ramsey-CPT atomic clock combines the method of separated oscillatory fields and the phenomenon of coherent population trapping, so as to obtain narrower line width and better signal-to-noise ratio of the resonance signal for improving the frequency stability. It not only keeps the advantages of miniaturization and low power of the CPT atomic clock, but also can obtain higher frequency stability than CPT atomic clock about one order of magnitude.

DOI
10.1109/fcs.2011.5977803

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The abstract presents a fully functional Ramsey-CPT atomic clock, which has the characteristics of compact structure, small volume, low cost, strong adaptability, high stability, convenient for adjustment, unique design, and easy-to-commercialization. The Ramsey-CPT atomic clock combines the method of separated oscillatory fields and the phenomenon of coherent population trapping, so as to obtain narrower line width and better signal-to-noise ratio of the resonance signal for improving the frequency stability. It not only keeps the advantages of miniaturization and low power of the CPT atomic clock, but also can obtain higher frequency stability than CPT atomic clock about one order of magnitude.
Based on the operating principle and the comprehensive analysis of the coherent population trapping (CPT) atomic clock system, the paper presents an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system which uses vertical-cavity surface-emitting laser (VCSEL) as optical source, selects85Rb as operating atoms, and is in resonance with the D1transitions. The system can be used for experimental study of correlation of each parameter and study of the Ramsey-CPT atomic clock. The whole system, which adequately combines the advantages of the ordinary desktop CPT experimental system and the chip-scale atomic clock (CSAC), is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.
A Coherent Population Trapping(CPT) atomic clock servo control circuit is designed based on the CPT phenomenon of 85 Rb. Taking MSP430 chip as a platform, combining with the physical device made up of Vecsel laser, photoelectric detector, and small 85 Rb vapor cell, a miniature atomic clock with the high-performance, high stability, low power consumption and small size is implemented. The volume is 31 cm3 and the power consumption is 660 m W. The frequency stability of 10 MHz measured is 2×10-10/s, 4×10-11/1 000 s. Full-hyperfine-splitting frequency modulation is used to improve the frequency stability.
An atomic clock system based on coherent population trapping (CPT) resonance in 85 Rb is reported, while most past works about the CPT clock are in 87 Rb. A new modulation method (full-hyperfine-frequency-splitting modulation) is presented to reduce the effect of light shift to improve the frequency stability of the CPT clock in 85 Rb. The experimental results show that the short-term frequency stability of the CPT clock in 85 Rb is in the order of 10 −10 /s and the long-term frequency stability can achieve 1.5 × 10 −11 /80000 s, which performs as well as 87 Rb in CPT resonance. This very good frequency stability performance associated with the low-cost and low-power properties of 85 Rb indicates that an atomic clock based on CPT in 85 Rb should be a promising candidate for making the chip scale atomic clock.
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 high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1/(2TR), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3×10−13 τ−1/2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained in the conventional continuous wave regime, with the use of an additional laser power stabilization setup. The pulsed interaction allows to reduce significantly the clock frequency sensitivity to laser power variations, especially for high values of TR. This pulsed CPT clock, ranking among the best microwave vapor cell atomic frequency standards, could find applications in telecommunication, instrumentation, defense or satellite-based navigation systems.
We propose a two-step pulse observation method to enhance the frequency stability of coherent population trapping (CPT) atomic clocks. The proposed method is a Raman–Ramsey scheme with low light intensity at resonance observation and provides a Ramsey-CPT resonance with both reduced frequency sensitivity to the light intensity and a high signal-to-noise ratio by reducing the repumping into a steady dark state. The resonance characteristics were calculated using density matrix analysis of a Λ-type three-level system that was modeled on the 133 Cs D 1 line, and the characteristics were also measured using a vertical-cavity surface-emitting laser and a Cs vapor cell.