Fig. 1. Composition of sandwich multimode fiber interferometer. (a) Schematic diagram of SIMMF-GIMMF-SIMMF structure; (b) refractive index distribution of multimode fiber cross-section

Fig. 2. Fabrication process of sandwich multimode fiber interferometer based on SIMMF-GIMMF-SIMMF structure

Fig. 3. Experimental setup of curvature characteristic experiment

Fig. 4. Transmission spectra and function curves of characteristic interference peak intensity and curvature of sandwich multimode fiber interferometer with different GIMMF lengths. (a)(d) L_GIMMF=1 mm; (b)(e) L_GIMMF =2 mm; (c)(f) L_GIMMF=3 mm

Fig. 5. Breath monitoring system based on sandwich multimode fiber interferometer. (a) Experimental setup; (b) elastic waistband; (c) wearing method; (d) spectral response

Fig. 6. Respiratory signal waveforms before and after filtering. (a) Time-domain waveform of original signal; (b) frequency-domain waveform of original signal; (c) time-domain waveform of denoised signal; (d) frequency-domain waveform of denoised signal

Fig. 7. Time-domain waveform and frequency-domain waveform of respiratory signal. (a)(b) Time-domain waveform; (c)(d) frequency-domain waveform

Fig. 8. Respiratory signals of volunteer A in different states. (a) Stationary; (b) slow respiratory; (c) after strenuous exercise; (d) during sleep

Fig. 9. Respiratory signals of different volunteers monitored by system under normal respiratory. (a) Volunteer C; (b) volunteer D; (c) volunteer E; (d) volunteer F

Fig. 10. Analysis of unsteady respiratory signals. (a) Time-domain waveform of respiratory signal from fast to slow; (b) time-domain waveform of respiratory arrest signal; (c) time-frequency distribution of respiratory signal from fast to slow; (d) time-frequency distribution of respiratory arrest signal