Fig. 1. Composition of sandwich multimode fiber MZI. (a) Structure diagram of SIMMF-GIMMF-SIMMF; (b) refractive index profiles of GIMMF and SIMMF

Fig. 2. Simulation results of light conduction mechanism of SIMMF-GIMMF-SIMMF. (a) Simulation results of optical transmission in sandwich multimode fiber MZI; (b) electromagnetic field distributions of core modes of GIMMF

Fig. 3. Schematic diagram of production process of sandwich multimode fiber MZI (SIMMF-GIMMF-SIMMF)

Fig. 4. Experimental spectra and corresponding spatial spectra of sandwich multimode fiber MZI with different GIMMF lengths. (a) Transmission spectra; (b) spatial spectra

Fig. 5. Diagram of temperature sensing system based on sandwich multimode fiber MZI using visual vernier effect

Fig. 6. Steps of extracting reference spectrum by frequency conversion processing of experimental spectrum. (a) Transmission spectrum of sandwich multimode fiber MZI when L_GIMMF=20 mm; (b) FFT result of experimental spectrum; (c) spectrum corresponding to dominant frequency component (peak 1) in experimental spectra; (d) FFT result of spectrum corresponding to peak 1; (e) spectrum after modulating experimental spectrum with modulation function IM (L_M=20 μm); (f) FFT result of modulated spectrum; (g) designed reference spectrum (spectrum corresponding to peak 2); (h) FFT result of reference spectrum

Fig. 7. Visual vernier spectra formed by superimposing reference and experimental spectra with different modulation lengths L_M. (a) L_M=20 μm; (b) L_M=15 μm; (c) L_M=10 μm; (d) L_M=4.3 μm

Fig. 8. Temperature response of a single sandwich multimode fiber MZI without visual vernier effect. (a) Spectral response; (b) relationship between characteristic wavelength and temperature

Fig. 9. Temperature response of virtual vernier envelopes for different FSRs. (a) Spectral response; (b) relationship between characteristic wavelength and temperature