Fig. 1. Structural diagram of in-fiber WGM microsphere resonator and lightpath schematic. (a) Structural diagram; (b) lightpath schematic

Fig. 2. Lightpath simulation of optical fiber structures and the relationship between reflected light intensity of hollow core fiber end face M₁ and inner diameter d of the fiber. (a) Lightpath simulation; (b) reflected light intensity on the end face M₁ versus inner diameter d of the HCF

Fig. 3. Propagation constants of optical fiber structure with different radial mode numbers and the relationship between propagation constants of optical fiber structure and microsphere resonator. (a) Propagation constants of optical fiber structure with different radial mode numbers; (b) relationship between propagation constants of optical fiber structure and microsphere resonator

Fig. 4. Images of the functional relationship between $P_{\mathrm{R}}$ and θ in different ranges of δ. (a) 0-0.25$\mathrm{\pi }$; (b) 0.25$\mathrm{\pi }$-0.50$\mathrm{\pi }$; (c) 0.50$\mathrm{\pi }$-0.75$\mathrm{\pi }$; (d) 0.75$\mathrm{\pi }$-1.00$\mathrm{\pi }$

Fig. 5. Simulation results of Fano line shape movement at different temperatures and linear fitting results of Fano resonance peak wavelengths. (a) Simulation results; (b) linear fitting results

Fig. 6. Sensing structure preparation process. (a)(b) Splicing of SMF and HCF; (c) etching of HCF; (d) placing the microsphere resonator into HCF

Fig. 7. Microscope images of in-fiber microsphere resonator. (a) Collapse angle between SMF and HCF at splicing; (b) cone after etching the HCF; (c) microsphere embedded in resonator structure

Fig. 8. Platform of optical fiber probe preparation

Fig. 9. Reflection spectra of in-fiber WGM microsphere resonator and the enlarged Fano resonance and relevant parameter definitions. (a) Reflection spectra; (b) the enlarged Fano resonance and relevant parameter definitions

Fig. 10. Fano reflection spectra and linear fitting results in the temperature sensing experiments. (a) Reflection spectra; (b) linear fitting results

Fig. 11. Lorentz reflection spectra and linear fitting results in the temperature sensing experiments. (a) Reflection spectra; (b) linear fitting results

Fig. 12. Reflection spectra and linear fitting results in the refractive index sensing experiments. (a) Reflection spectra; (b) linear fitting results

Fig. 13. Spatial frequency spectrum obtained by FFT and schematic of coupled lightpath of in-fiber WGM microsphere resonator. (a) Spatial frequency spectrum; (b) schematic of coupled lightpath