Reflection-type vapor cell for micro atomic clocks using local anodic bonding of 45^◦ mirrors

This Letter reports the design, fabrication, and evaluation of reflection-type planar vapor cells for chip-scale atomic clocks. The cell with 2–8 mm cavity length contains two 45^◦ Bragg reflector mirrors assembled using a local anodic bonding. Coherent population trapping resonance of Rb atoms is observed, realizing an atomic clock operation. Allan deviations at an averaging time of 1 s are 2.2 × 10⁻10 and 9.5 × 10⁻¹¹ for 2 mm long and 6 mm long vapor cells, respectively. These results show that planar vapor cells compatible with a system-in-package are feasible without degradation of clock stabilities compared to conventional vertically stacked cells.