Temperature measurements using a microoptical sensor based on whispering gallery modes

Temperature measurements were made using a novel microoptical sensor based on dielectric microspheres that are excited by coupling light from optical fibers. The technique exploits the morphology-dependent shifts in resonant frequencies that are commonly referred to as the whispering gallery modes. A change in the temperature of the microsphere leads to a change in both the size and the index of refraction of the sphere which results in a shift of the resonant frequency. By monitoring this shift, the temperature of the environment surrounding the sphere can be determined. The whispering gallery mode shifts are observed by scanning a tunable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photodiode on the other. When the microsphere is in contact with a bare section of the fiber, the optical modes are observed as dips in the intensity of the light transmitted through the fiber. Temperature measurements were made in both air and water using this novel technique. Measurements by the microoptical sensor were compared to those by thermocouples with good agreement between the two sets of results.