Active frequency stabilization of an ultra-high Q whispering-gallery-mode microresonator

We report on active frequency stabilization of a whispering-gallery-mode microresonator with an ultra-high quality factor exceeding 2×108 to an atomic transition using the Pound–Drever–Hall technique. The critically coupled resonator is stabilized to 5.4% of its linewidth, or 206 kHz. Thermal bistability and mode-splitting are also observed and have a significant effect on the error signal.