85Rb D1-line CPT atomic clock with low power consumption

This paper describes the investigation of a ⁸⁵Rb 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 of ⁸⁵Rb rather than conventional ⁸⁷Rb 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 for ⁸⁵Rb 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 < 2.5times10^-12/day for D1 line and < 4.0times10^-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.