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Applied Physics Letters

Coherent population trapping atomic clock by phase modulation for wide locking range

Yuichiro Yano, Masatoshi Kajita, T. Ido, Motoaki Hara

Published Nov 13, 2017• 15 citations
Publisher

Abstract

A method of detecting Coherent Population Trapping resonance by phase modulation (PM) was studied to expand the frequency locking range (LR). We calculated error signals by density matrix analysis using an eigenvector algorithm and verified the calculated result using an 87Rb vapor cell and a Rb-D1 vertical cavity surface-emitting laser. By comparing the error signal for PM with that for conventional frequency modulation (FM), it was found that the LR with PM is one order of magnitude wider than that with FM without degrading the frequency stability.

DOI
10.1063/1.4991560

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A method of detecting Coherent Population Trapping resonance by phase modulation (PM) was studied to expand the frequency locking range (LR). We calculated error signals by density matrix analysis using an eigenvector algorithm and verified the calculated result using an 87Rb vapor cell and a Rb-D1 vertical cavity surface-emitting laser. By comparing the error signal for PM with that for conventional frequency modulation (FM), it was found that the LR with PM is one order of magnitude wider than that with FM without degrading the frequency stability.
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 investigate the use of phase modulation to generate an error signal for frequency stabilization to a reference atomic transition. The study focuses on the high-frequency modulation regime. Our theoretical analysis shows that, in the case of coherent population trapping, the maximal slope of the error signal remains constant as the modulation frequency increases, whereas it steadily decreases for other types of resonances. Experimental results with 87Rb atoms confirm this behavior under comparison with the double radio-optical resonance.
Abstract Chip-scale atomic clocks based on coherent population trapping (CPT) face challenges due to miniaturized gas cells, which reduces the CPT resonance signal and the absorption signals of alkali atoms. To overcome this, we propose a method that simultaneously stabilizes the laser wavelength and microwave modulation frequency without relying on absorption line detection. Our approach enables stabilization at the point where the amplitude of the CPT resonance signal is maximized, making it suitable for compact atomic clocks. The system uses three lock-in amplifiers: one is used to stabilize the RF modulation frequency, and the other two are used for detecting the amplitude of the CPT resonance and stabilizing the laser wavelength. The Allan deviation measurements demonstrate a 1.4-fold improvement in short-term stability compared to conventional methods, suggesting that the proposed technique is a promising solution for enhancing the performance of small gas-cell atomic clocks.
We describe the optimization of a Frequency Locked Loop (FLL) in an atomic clock which is based on Coherent Population Trapping (CPT) in (87)Rb vapor using the D(2) transition. The FLL uses frequency modulation (FM) spectroscopy and we study the effect of FM parameters (modulation frequency and index) on the sensitivity and the signal to noise ratio of the feedback signal in the FLL. The clock which employs a small spherical glass cell containing (87)Rb atoms and a buffer gas, exhibits a short term stability of 3x10(-11)/ radicaltau. The long term relative frequency stability of the 10 MHz output is better than 10(-10) with a drift of 10(-11) per day.