Fine frequency tuning of a microspherical cavity by temperature control

In this paper, resonant frequency control for the whispering-gallery modes (WGM) is demonstrated by adjusting the temperature. We used single-mode optical fiber for the fabrication of the microsphere. The polymer coating of the fiber was stripped, and then the tip was melted by a focused CO₂ laser beam to form a sphere with a stem. The temperature of the microsphere was monitored by a thermometer. A tunable external-cavity diode laser (linewidth<300 kHz) was illuminated on the microsphere. The scattered light from the microsphere was collected by a microscope objective and imaged onto a pinhole so that the edge of the microsphere was selectively observed by an avalanche photodiode. We scanned the laser frequency to measure the scattering spectra over tens of GHz range at 780 nm.