Thermal-noise limited laser stabilization to a crystalline whispering-gallery mode resonator

The quality of many laser-based measurements critically depends on the spectral purity of the employed laser source. For this reason, researchers have tried to reduce phase and frequency noise of lasers ever since its invention 50 years ago. These efforts have culminated in laser oscillators with linewidths below 1 Hz, and a relative Allan deviation below 10^-15, by locking a laser to a ~1 dm³-sized Fabry-Perot (FP) cavity, whose geometry and materials has been engineered to reduce the sensitivity to thermal and acoustic perturbations.