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Chinese Physics Letters

Frequency Stability of Atomic Clocks Based on Coherent Population Trapping Resonance in 85 Rb

Lu Liu, Guo Tao, Ke Deng, Xinyuan Liu, Xuzong Chen, Wang Zhong

Published Jun 28, 2007• 9 citations
Publisher

Abstract

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.

DOI
10.1088/0256-307x/24/7/026

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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)•May 1, 2008•2 citations
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.
In this paper, the investigation of a 85 Rb coherent-population-trapping (CPT) atomic clock for low-power operation is described. The modulation profiles and CPT resonances for both 85 Rb and 87 Rb, the long-term frequency stabilities of the CPT resonance for both, and the feasibility of switching between low-power and high-stability operation modes are presented. The choice of 85 Rb rather than conventional 87 Rb is dictated by the lower ground-state hyperfine-splitting frequency, which leads to less than one-half the required RF power and more effective RF-related circuit designs. CPT resonances for both 85 Rb and 87 Rb can be observed separately in the same natural mixture Rb gas cell, and the linewidth is less than 120 Hz in both cases. The long-term frequency stabilities are <4.0 ×10 -12 /day for 85 Rb and <3.5 ×10 -12 /day for 87 Rb. These results indicate that a 85 Rb CPT atomic clock has a long-term stability of ∼4.0 ×10 -12 /day and consumes about one-half the RF power of a 87 Rb equivqlent. In addition, it is possible to switch between low-power and high-stability operation modes by changing the RF frequency and power.
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.
This letter presents for the first time a systematic study of an atomic frequency standard based on coherent population trapping (CPT) effect in87Rb vapour optically pumped with a multifrequency laser radiation formed with sidebands of a single-frequency laser whose injection current is modulated at a subharmonic frequency (1/2-1/6) relative to the frequency of hyperfine ground state splitting. Measured are the CPT resonance parameters and the atomic clock short-term stability at different subharmonic frequencies of microwave modulation. The possibility of CPT resonance light shift suppression is experimentally demonstrated at modulation frequencies between 1/3 and 1/5 of the resonant frequency for continuous wave optical pumping of the87Rb vapour D1 line. Analyzed are the capabilities and potential of reduced energy consumption of atomic clocks when CPT resonance in rubidium vapour is excited at subharmonic modulation frequencies.
This paper describes the investigation of a85Rb D1-line coherent population trapping (CPT) atomic clock with a Rb gas-cell containing a natural mixture of Rb atoms and buffer gas. The choice of85Rb rather than conventional87Rb is dictated by the lower ground-state hyperfine-splitting frequency, which leads to less than one-half the required RF power and more effective RF-related circuit designs. In this paper, the modulation profiles, the CPT resonances, and the long-term frequency stabilities of the CPT resonance for85Rb D1 and D2 lines are presented. The frequency deviations for D1 and D2 lines are measured using a same experimental setup except for VCSEL wavelength and RF power. The Allan deviations, estimated from the frequency deviations, are-12/day for D1 line and-12/day for D2 line. In the case of D1 excitation, required RF frequency is 1.5 GHz and its power is only -8.0 dBm.
The authors demonstrate a simple technique to significantly improve the long-term frequency stability of atomic clocks based on coherent population trapping (CPT). In this technique, the CPT fields are created by a modulated diode laser and a slow servo is used to actively tune the laser modulation index to a value where the light shift vanishes. The observed clock frequency at this modulation index is given by the rubidium hyperfine frequency when no light fields are present, and this makes the clock frequency largely insensitive to variations in laser properties. In addition to reducing stringent requirements on the long-term stabilities of laser temperature, laser frequency, and rf modulation power, this technique may also significantly reduce frequency drifts related to laser aging. In the experiment, they demonstrate improvement by over one order of magnitude in the stability of a clock that is limited by light-shift-induced frequency drifts.
This paper outlines the features of the85Rb vapor cell atomic clock, the coherent population trapping (CPT) resonance in an85Rb/87Rb mixed vapor cell, and the results of the frequency stabilities of a pilot system. The experiments were performed with a sealed vapor cell containing a natural mixture of Rb isotopes and a buffer gas. Excitation of the CPT resonance was accomplished using a vertical-cavity surface-emitting laser modulated at 1.5, 3.0, 3.4 and 6.8 GHz, which corresponded to the full- and half-ground-state hyperfine frequencies of85Rb and87Rb. The results show that the CPT resonances at both85Rb and87Rb can be observed separately. The frequency stabilities of the output signals for each CPT resonance were ≪ 10x10−12/day for85Rb and ≪ 6x10−12/day for87Rb. In addition, the RF modulation signal power for85Rb was 1.1 dB less than that of87Rb, which was 3.4GHz.