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

Low power 85Rb CPT atomic clock

Shigeyoshi Goka, Okura Taisuke, Moroyama Munetake, Yasuaki Watanabe

Published May 1, 2008• 2 citations
Publisher

Abstract

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.

DOI
10.1109/freq.2008.4622966

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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.
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.
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 85Rb D1-line coherent-population-trapping (CPT) atomic clock with a natural-mixture Rb gas cell is demonstrated. D1-line excitation can generate a larger CPT resonance amplitude, and the choice of 85Rb rather than conventional 87Rb is because of its lower ground-state hyperfine-splitting frequency. The frequency deviation for the D1 line was measured using the same experimental setup as that for the D2 line except for the vertical-cavity surface-emitting laser (VCSEL) wavelength and RF power. In the case of D1-line excitation, the CPT resonance amplitude was 1.5 times that for the D2 line, and the light shift sensitivity was one-half that for the D2 line. The Allan standard deviations for the 85Rb D1 line, estimated from the frequency deviations, are <2.5×10-12 per day. In addition, the required RF is 1.5 GHz and its power is only -8.0 dBm.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
We report on the performance of the first table-top prototype atomic clock based on coherent population trapping (CPT) resonances with parallel linearly polarized optical fields (lin‖lin configuration). Our apparatus uses a vertical-cavity surface-emitting laser (VCSEL) tuned to the D1 line of 87Rb with the current modulation at the 87Rb hyperfine frequency. We demonstrate cancellation of the first-order light shift by the proper choice of rf modulation power and further improve our prototype clock stability by optimizing the parameters of the microwave lock loop. Operating in these optimal conditions, we measured a short-term fractional frequency stability (Allan deviation) 2×10−11τ−1/2 for observation times 1 s ≤ τ ≤ 20 s. This value is limited by large VCSEL phase noise and environmental temperature fluctuation. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated lin‖lin CPT resonance clock with environmental impacts minimized.
An all-optical microwave frequency standard based on coherent population trapping (CPT) in 85Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10-11 τ-1/2(1 s<τ<2000 s). The best stability of 6.4×10-13 is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.