Whispering gallery modes silica resonators: a platform for optical sensing

Optical Whispering Gallery Mode (WGM) microresonators are an ideal platform for miniature and high sensitive sensors. They exhibit very small mode volumes and extremely high Q factors of the order of 10⁶ - 10⁹ providing a platform with low nonlinearity thresholds and low detection limits to environmental changes. Here we present and discuss some of the WGM-based applications we have been developing in our lab. Specifically, we present a WGM strain sensor based on a silica stretchable microsphere, with a Q factor of 108 . Tension stress is applied along a microsphere with two fiber tails by pulling on one tail. Consequently, the microsphere's WGMs are shifted and the measurement may serve as a strain sensor. Based on this strain sensor, a WGM magnetic sensor is suggested where the mechanical response of a magnetostrictive material to a varying magnetic field applies the tension stress. In addition, a narrow linewidth laser source for these sensors is described here as well, where a multimode diode laser is locked to a single WGM of the microsphere via self-injection feedback from the microsphere. This optical locking is accompanied with a substantial spectral narrowing of the multimode diode laser.