Locking a microsphere whispering-gallery mode to a laser.

We have locked a whispering-gallery resonance of a fused-silica microsphere to a frequency-scanning laser. The resonance frequency is modulated by axial compression of the microsphere, and phase-sensitive detection of the fiber-coupled optical throughput is used for locking. Using a laser wavelength of either 1570 nm or 830 nm, we demonstrate a locked tracking range exceeding 30 GHz for a microsphere of 120 GHz free spectral range. This performance has been enabled by an improved compression tuner design that allows coarse tuning over 1 THz and piezoelectric tuning over 80 GHz. Compression modulation rates of up to 13 kHz have also been achieved with this tuner.