The characteristics of a microring resonator in optical waveguide displacement sensors

Pumping a single-mode, tunable, external-cavity laser to excite the whispering gallery modes (WGM) of microring resonators (MRRs) allows the observation of a series of equidistant, wide free spectrum ranges (FSR). A frequency shift of an FSR can be used to feedback external changes, making MRRs very promising for high-sensitivity sensing. Furthermore, sensor performance is intimately related to the resonance condition and resonator quality factor, Q. In this paper, a novel displacement sensor composed of a microring resonator and a waveguide is presented. Materials selection is discussed. The two most important influencing factors, the optimized resonance condition and Q, are demonstrated especially. Based on mathematical analysis and finite-difference time-domain simulation, the optimized resonance is obtained when the radius and coupling distance are around 20 mm and 0.2 mm, respectively, with the FSR of the transmission spectrum about 13.6 nm. Correspondingly, the ultimate Q is approximately 10 2 in the preliminary experiment.