A Fiber-Attached Coupler for Transmission Bandpass Whispering Gallery Mode Resonator

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C.