Probing 10 μK stability and residual drifts in the cross-polarized dual-mode stabilization of single-crystal ultrahigh- Q optical resonators

The thermal stability of monolithic optical microresonators is essential for many mesoscopic photonic applications such as ultrastable laser oscillators, photonic microwave clocks, and precision navigation and sensing. Their fundamental performance is largely bounded by thermal instability. Sensitive thermal monitoring can be achieved by utilizing cross-polarized dual-mode beat frequency metrology, determined by the polarization-dependent thermorefractivity of a single-crystal microresonator, wherein the heterodyne radio-frequency beat pins down the optical mode volume temperature for precision stabilization. Here, we investigate the correlation between the dual-mode beat frequency and the resonator temperature with time and the associated spectral noise of the dual-mode beat frequency in a single-crystal ultrahigh-Q MgF2 resonator to illustrate that dual-mode frequency metrology can potentially be utilized for resonator temperature stabilization reaching the fundamental thermal noise limit in a realistic system. We show a resonator long-term temperature stability of 8.53 μK after stabilization and unveil various sources that hinder the stability from reaching sub-μK in the current system, an important step towards compact precision navigation, sensing, and frequency reference architectures. Researchers in California have improved the thermal stability of tiny optical microresonators for use in high-precision timing and global navigation technologies. Ultrahigh-quality whispering gallery optical microresonators work by guiding the light from two differently-polarized lasers around the resonator circumference, which is carefully designed to have particular resonant frequencies. However, microresonators are extremely sensitive to temperature changes, and the impact of laser-induced heating, heat diffusion, and thermal expansion over time is detrimental to performance. Jinkang Lim and Chee Wei Wong at the University of California, US, and co-workers have shown that, by locking the dual-mode beat frequency of the lasers to a radio-frequency clock, the resulting suppression of thermal noise and frequency drift can enhance the long-term thermal stability of optical microresonators. This novel solution could result in microresonators stable enough to be used in space.