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Quantum Electronics and Laser Science Conference

An atomic clock based on a VCSEL-driven CPT resonance and a small 87Rb vapor cell

Matan Kahanov, Ido Ben-Aroya, G. Eisenstein

Published May 1, 2007• 1 citation
Publisher

Abstract

We report a high performance frequency standard based on Coherent Population Trapping, incited by a modulated VCSEL in the87Rb-D2line. Short-term stabilities of σy=3×10-11/√τ and relative frequency deviations below 10-11/day were demonstrated.
DOI
10.1109/qels.2007.4431588

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•Apr 14, 2019•0 citations
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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.
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 work reports for the first time development and experimental study of a laboratory prototype of an atomic clock with stability of 2×10–11τ –1/2 (Allan deviation) over time 1 s < τ < 1000 s (7×10–13 over 1000 s) on the basis of a coherent population trapping (CPT) resonance in 87Rb in a compact spherical cell having 1.3-cm diameter and without any buffer gas. Demonstrated are dependencies of the width of the studied CPT resonance upon the amplitude and modulation frequency of optical frequency difference (3.417 GHz) of a bichromatic circularly polarised pump field. For the first time, a considerable non-linearity was discovered in the dependence upon the reference difference frequency of the spectral position of the CPT resonance when the modulation frequency of the optical frequency difference is varied within a broad range.
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 demonstrate a vapor-cell atomic-clock prototype based on a continuous-wave interrogation and double-modulation coherent population trapping (DM-CPT) technique. The DM-CPT technique uses a synchronous modulation of polarization and the relative phase of a bichromatic laser beam in order to increase the number of atoms trapped in a dark state, i.e., a nonabsorbing state. The narrow resonance, observed in the transmission of a Cs vapor cell, is used as a narrow frequency discriminator in an atomic clock. A detailed characterization of the CPT resonance versus numerous parameters is reported. A short-term fractional-frequency stability of 3.2×10−13τ−1/2 up to a 100-s averaging time is measured. These performances are more than one order of magnitude better than industrial Rb clocks and are comparable to those of the best laboratory-prototype vapor-cell clocks. The noise-budget analysis shows that the short- and midterm frequency stability is mainly limited by the power fluctuations of the microwave used to generate the bichromatic laser. These preliminary results demonstrate that the DM-CPT technique is well suited for the development of a high-performance atomic clock, with the potential compact and robust setup due to its linear architecture. This clock could find future applications in industry, telecommunications, instrumentation, or global navigation satellite systems.