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IEEE photonics journal

Properties of Rb CPT Atomic Clock at Subharmonic Microwave Modulation Frequencies

Daba A. Radnatarov, Sergey M Kobtsev, Valeriy Andryushkov, Sergey Khripunov, E. V. Baklanov, Arkadii Yakovlev

Published Jun 26, 2019• 2 citations
Publisher

Abstract

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.

DOI
10.1109/jphot.2019.2925012

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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.
Presented for the first time is our research into reduction of energy consumption of miniaturised Rb atomic clocks based on coherent population trapping (CPT) effect at lowered microwave modulation frequency. We studied properties of a CPT-based atomic frequency standard pumped with multi-frequency radiation from sidebands of a single-frequency semiconductor laser whose injection current was modulated at subharmonics (1/2–1/6) of the frequency of Rb hyperfine ground state splitting. Pumping by third-order sidebands is the optimum when the atomic clock stability drops only slightly (by 11%), whereas energy consumption of the microwave components is reduced by a factor of 1.5.
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.
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.
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 active suppression of light shift of the reference coherent population trapping (CPT) resonance in87Rb vapor to improve the long-term stability of compact atomic clocks has been demonstrated experimentally. The approach is based on the phase jump technique used to synthesize an error signal, which is proportional to the light shift of the CPT resonance, from an optical transmission signal. The use of this error signal in an additional feedback loop allows one to stabilize the amplitude of a microwave signal for the phase modulation of laser radiation near a value at which the light shift of the CPT resonance is absent. This technique has made it possible to reduce the long-term instability of CPT atomic clocks in our experiments by a factor of 15 at an integration time of 10 000–20 000 s.