Fig. 1. Schematic of experimental setup for fused tapering by hydrogen oxygen flame

Fig. 2. Structure of tapered grating

Fig. 3. Transmission spectrum of MZI with FBG after fused tapering

Fig. 4. Λ(z) after tapering

Fig. 5. Relationship between n(z) and z

Fig. 6. Transmission spectra of MZI under different NaCl solution concentrations. (a) 1%; (b) 5.76%; (c) 7.88%; (d) 10.16%; (e) 13.11%; (f) 16.22%

Fig. 7. Transmission spectra under different solution concentrations of NaCl

Fig. 8. Relationship between bandwidth and refractive index as well as its corresponding linear fitting curve

Fig. 9. Transmission spectra of four hypothetical structures. (a) Simulated and experimental transmission spectra of tapered-drawing fiber grating; (b) transmission spectra of SMF with tapering lengths of 0, 19.831, 22.244 mm; (c) reflectance spectra of long grating FBG before and after tapering; (d) simulated transmission spectrum of LPG-SMF-LPG