Fig. 1. Structural diagram of FMF MI

Fig. 2. Structural diagram of FMF-SMF FPI

Fig. 3. Structure diagram of sensor and system

Fig. 4. Effective refractive index of modes in FMF and effect of core-offset on mode excitation efficiency. (a) Effective refractive indexes of modes in FMF; (b) excitation efficiency of modes under different core-offsets in SMF-FMF

Fig. 5. Interference spectrum of FPI and its spatial frequency spectrum. (a) Interference spectrum; (b) spatial frequency spectrum

Fig. 6. Interference spectra of MI with different LR. (a) LR=5 cm; (b) LR=10 cm; (c) LR=20 cm; (d) LR=30 cm

Fig. 7. Vernier demodulation results of MI simulated signal. (a) Refractive index response of FPI; (b) interference spectrum of simulation MI; (c) cascade spectrum; (d) spatial frequency spectrum of cascade interferometer; (e) refractive index response of cascade interferometer; (f) sensitivity fitting with and without MI vernier sensitization

Fig. 8. Refractive index response test results of MI-FPI composite interferometer. (a) Interference spectrum and spatial frequency spectrum of MI; (b) interference spectrum and spatial frequency spectrum of MI-FPI; (c) refractive index response of MI-FPI; (d) sensitivity fitting with and without MI vernier sensitization