Temperature measurements using a micro-optical sensor based on whispering gallery modes

Temperature measurements were made using a novel micro-optical sensor based on dielectric micro-spheres 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 (WGM). A small change in the size, shape or optical constants of the micro-sphere causes a shift in the resonant frequency (or the WGM). For example, a change in the temperature of the micro-sphere leads to a change in both the size and the index of refraction of the sphere which results in a shift of the WGM. By monitoring this shift, the temperature of the environment surrounding the sphere can be determined. The WGM shifts are observed by scanning a tuneable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photo diode on the other end. When the micro-sphere is in contact with a bare section of the fiber, the optical modes are observed as dips (due to destructive interference) in the intensity of the light transmitted through the fiber. In the present, temperature measurements were made in both air and water using this novel technique. The results obtained by the micro-optical sensor were compared to those simultaneously obtained by thermocouples. The good agreement between the two sets of results demonstrated the viability of this novel micro-optical sensor concept.