In-fiber zigzag excitation for whispering-gallery modes via evanescent wave and free space coupling

In this paper, we demonstrate a simple capillary-based coupler with optical zigzag transmission to excite whispering-gallery modes (WGMs) in a passive deformed microcavity. The coupler is just composed sections of a single mode fiber, a microsphere and a capillary, while neither corrosion nor tapering process is necessary. Based on the ray optics model, we obtain the criterion between the cone-apex angle and capillary wall thickness for the zigzag beams to excite WGMs effectively. This proper cone-apex angle range can be obtained by only setting the appropriate welding parameters. The smaller and larger cone-apex angles correspond to evanescent wave and free space coupling, respectively. The former has a clear free spectral range (FSR) envelope of WGMs, while the latter has no but compact and fairly intensity-uniform WGM peaks because it first directly excites chaotic sea which then couples into WGMs by dynamical tunneling. Moreover, the modified equations of motion for the photon counts are brought forward to analyze the energy distribution for the free space coupling. The excitation unit can work as a pure and basic building block, which is a simple and flexible coupling scheme for WGMs and has great potential in photonic integrated devices.