Tapered hollow annular core fiber coupled whispering-gallery mode microsphere resonators

A tapered hollow annular core fiber (HACF) coupler for excitation of whispering-gallery modes (WGMs) of an embedded microsphere resonator is proposed and demonstrated. Using HACF as the coupling medium can avoid the complicated fabrication process such as chemical corrosion and improve the robustness and stability of the device. The coupling efficiency from the SMF to HACF is enhanced after tapering the joint section to excite various WGMs. In both theory and experiment, we observe symmetrical Lorentzian and asymmetric Fano line shapes by varying the microsphere resonator size and location. Finally, the temperature sensing sensitivity and stability of the device are tested. The sensitivity of the device reaches 10.8 pm/°C. This novel tapered HACF based microsphere resonator is expected to promote the environmental adaptability in the practical application.