Fiber based whispering gallery mode resonators

Abstract

The ability to confine light in small volume is a key challenge in modern optics. Optical microresonator supporting whispering gallery mode (WGM) is able to achieve over 109 Q-factor with micron-scale modal volume. Due to the high-Q and small-mode-volume characteristics, WGM resonators have become the basis of a wide range of applications including sensing, filtering and switching devices; the studies of quantum electrodynamics (QED) and optical nonlinear phenomena have grown to rely on WGM as it provides quantum mechanical analogy and low power-threshold. Despite the continuous effort to optimize WGM resonators for high-Q performance, the problem of efficient power delivery with robust mechanical stability remains unsolved. This thesis investigates the fiber based WGM resonators to address all the above mentioned challenges. Metallic diffraction grating as a coupler to the WGM resonator is proposed. It is the first time that such coupler is extended to the use of metallic material. With the employment of this unconventional structure, the new WGM resonator gives rise to interesting phenomena and performance enhancement. Unlike the fragile fiber taper coupler, the metallic diffraction grating is mounted on a much larger platform – the end facet of a single mode fiber (SMF) – so that the coupler has sound mechanical strength and is durable for repeated use. In contrast with dielectric grating, the metallic grating is able to provide higher diffraction efficiency and therefore higher coupling efficiency to the WGM resonator. The coupling utilizes the first order diffractions that bear transverse wavevector to align with the local wavevector of the WGM. Phase-matching conditions can therefore be tuned by the period of the grating. Due to the interaction of multiple scattering processes in the metallic diffraction grating coupled WGM resonator, Fano resonances are observed in both reflected and transmitted spectra. The Fano resonance is found tunable by the incident polarization, grating thickness and filling factor. Hence, the grating coupler generates tunable resonance shapes and provides an entirely new degree of freedom for the realization of different applications. Moreover, the metallic grating is the only known coupler for WGM resonators that is able to excite and manipulate modes of two propagation directions simultaneously. Such complex resonance system is modelled for power coupling calculation. The use of metal provides a reflection pathway so that single port coupling with tunable phase-matching design is realized for the first time. The grating coupled WGM resonator serves as a practical platform for study of linear and nonlinear applications that require mechanical robustness and simple configuration. Despite the use of a lossy material (gold), high Q-factor is observed for coupled silica microspheres. Moreover, four wave mixing (FWM) is demonstrated in the reflection spectra of a metallic grating coupled microsphere resonator at milliwatt power-threshold. With further optimization of the grating coupler…