Waveguide Connected With Optical Microcavity And Its Application

The ability of trapping light in a small volume has triggered intense researchattention on optical microcavities. Compared with other schemes such as planermicrocavity, whispering-gallery mode (WGM) based microcavity can confine thelight with total internal reflection for a long time, and thus give ultrahigh quality (Q)factor and small mode volume. The light-matter interaction within WGMs basedmicrocavities can be significantly increased. In the past few years, this enhancementhas been successfully applied in label-free optical detectors to improve theirsensitivities. In general, a WGM microcavity based optical sensor consists ofhigh-Q microcavity and a tapered driving microfiber. The light is coupled frommicrofiber to microcavity and from microcavity back to microfiber via evanescentwaves. Therefore, the separation distance between microcavity and driving fibershould be precisely controlled and is usually less than a few hundred nanometers.However, precise control of the separation distance is very difficult in practicalapplications and it also faces severe challenge from the system vibration. In order tosolve all these problems, in this thesis a direct coupling mechanism has been createdto excite the high-Q modes and also shown high detection performance.The following achievements have been accomplished in this thesis:A new waveguide-microcavity structure without the tapered-fibers is designed.By connecting a waveguide to microdisk, the light can be directly coupled into themicrodisk and excite its high-Q WGMs. A simple model named as resonant assistedcoupling has been built to explain the high coupling efficiency and it is furtherconfirmed with both finite element method (FEM) and finite difference time domainmethod (FDTD) based numerical calculations.Then, the waveguide-microcavity structures with effective refractive indicesn=3.3and n=1.45are taken for examples to test their sensing properties. Byinputting the light through the connected waveguide, the high-Q modes can beexcited and utilized to detect refractive index change or nanoparticles. From thesimulation results, the waveguide-microcavity structure shows high sensitivity.Refractive index changes on the order of10-7and single nanoparticle with radius5nm can be easily detected. In addition, this structure is also robust to the refractiveindex, waveguide width and waveguide shift, showing high tolerance of practicalfabrications.Moreover, based on the microcavity-waveguide structure, a resonant filter hasbeen realized by adding a coupling waveguide nearby the microcavity. And the relative output intensity ratio of the output waveguides can be modulated bychanging local refractive index of the microcavity.The direct coupling based microcavity-waveguide structure has overcome theoverall problems of conventional coupling via evanescent waves. And the numericalsimulation results can shed light on the researches on optical sensors and filters.