Mitigation of Temperature-Induced Light-Shift Effects in Miniaturized Atomic Clocks

The demonstration of miniature atomic clocks (MACs) based on coherent population trapping (CPT) with improved mid- and long-term frequency stability benefits from the implementation of additional stabilization loops to reduce temperature-induced light-shift effects. In this article, we report and highlight the individual and combined benefits of such servo loops on the frequency stability of a CPT-based MAC. The first loop stabilizes the actual temperature of the vertical-cavity surface-emitting laser (VCSEL) chip using a compensation method in which the reading of external temperature variations is derived from the atomic vapor output signal. The second loop maintains the total microwave power absorbed by the laser to a value that maximizes the optical absorption and significantly reduces the laser power dependence of the clock frequency. Experimental tests are performed onto a miniaturized CPT-clock physics package using a chip-VCSEL tuned on the Cs D-1 line (lambda = 895 nm). The VCSEL temperature compensation technique improves, by a factor of 4, the Allan deviation of the clock at 10(4) s. The simultaneous operation of both servo loops improves, by a factor of 7, the clock fractional frequency stability at 10(4) s. The clock demonstrates a fractional frequency stability of 7.5 x 10(-11) at 1 s and better than 2 x 10(-11) at 1 day.