Thermal Nonlinear Effect And Mode Coupling In Fiber-coupled Whispering Gallery Mode Optical Resonators

The ultrahigh quality factor(Q-factor) and ultrasmall mode volume of a whispering gallery mode(WGM) resonator make it a typical optical element for studying light-matter interaction. In this dissertation, mode spectrum characteristics and manufactory technique of microspherical and bottle-shaped WGM resonators were presented. Based on the microresonator-tapered fiber coupling system, thermal nonlinear effect and mode coupling phenomenon were theoretically and experimentally demonstrated, and an all-optical thermo-optic effect-based resonant frequency tuning method was introduced and analyzed. The work accomplished and main points in this thesis are shown as follows:(1) The WGM distribution and mode spectrum characteristics in microspherical and bottle-shaped resonators were calculated with both analytical method and finite element method. Tuning methods suitable for these two resonators were also investigated. Based on the coupling equations describing microresonator-tapered fiber coupling system, the transmission characteristics under different coupling conditions were analyzed.(2) Based on standard telecom fibers, tapered fibers, microspherical and bottle-shaped WGM resonators with high uniformity were produced using a tapered fiber pulling machine and a fiber splicer. Their mode spectrum characteristics excited by a tapered fiber were investigated and their resonant frequency response under temperature and external strain were also exploited.(3) The thermal nonlinear effect was theoretically analyzed based on the model considering the two thermal relaxation processes, and the results were verified in microspherical resonators. An all-optical thermo-optic effect-based resonant wavelength tuning method was introduced and its responses to the tuning frequency was modeled and analyzed; it is found that the thermal relaxation rate ultimately limits the tuning frequency, and this presents instructions for thermo-optic effect-based all-optical switching devices.(4) Based on the coupled mode equations, different resonance line-shapes resulting from mode coupling were discussed. Experimentally, mode coupling was easier achieved in bottle-shaped resonators than microspheres when excited by a tapered fiber and modes as many as five were found coupled in a bottle-shaped resonator.