We propose and demonstrate a tapered hollow annular core fiber (HACF) coupler for excitation of whispering-gallery modes (WGMs) of an embedded microsphere resonator. The coupler is simply fabricated by fusion splicing of a segment of HACF with the single-mode fiber (SMF), and then improved by tapering the splicing joint to reduce the cone-apex angle. Therefore, the coupling efficiency from the SMF to the HACF is enhanced to excite various WGMs via evanescent field coupling. Normal positive, negative symmetrical Lorentzian and asymmetric Fano line shapes can be obtained by varying the resonator size and location. Another interesting phenomenon is observed that a higher Q-factor mode in a lower Q-factor mode has a contrast as high as 58. Temperature sensing with good stability is also demonstrated. This embedded WGM microsphere resonator in the tapered HACF is expected to promote environmental adaptability in practical applications due to its simplicity and robustness.

In this paper, a detailed theoretical study on the characteristics of cone-shaped inwall capillary-based microsphere resonators is described and demonstrated for sensing applications. The maximum, minimum, slope, contrast, and width of the Fano resonance are analyzed. As the transmission coefficient of the capillary resonator increases, the absolute value of the slope of Fano resonances increases to reach its maximum, which is useful for sensors with an ultra-high sensitivity. There occurs another phenomenon of electromagnetically induced transparency when the reflectivity at the capillary–environment interface is close to 100%. We also experimentally demonstrated its capability for temperature and refractive index sensing, with a sensitivity of 10.9 pm/°C and 431 dB/RIU based on the Fano resonance and the Lorentzian line shape, respectively.