Fig. 1. Schematic of photo-elastic effect

Fig. 2. Schematic of MZ-Sagnac optical fiber interferometric structure

Fig. 3. Schematic of MZ-Sagnac interferometric distributed optical fiber vibration sensor based on PGC

Fig. 4. 3×3 coupler and MZ-Sagnac structure^[41]. (a) Cross section of 3×3 coupler; (b) schematic of 3×3 coupler; (c) schematic of MZ-Sagnac distributed optical fiber vibration sensor based on 3×3 coupler

Fig. 5. Power spectrum of the phase signal^[43]

Fig. 6. Auto-correlation of the phase difference signal

Fig. 7. MZ-Sagnac interferometric distributed optical fiber vibration sensor based on chaotic laser^[48]. (a) Schematic of the optical structure; (b) null-frequency curve under 15 GHz bandwidth chaotic signal; (c) null-frequency curve under 11 GHz bandwidth chaotic signal

Fig. 8. MZ-Sagnac interferometric distributed optical fiber vibration sensor based on nested pulses^[49]

Fig. 9. MZ-Sagnac interferometric distributed optical fiber vibration sensor based on dual-wavelength EDFRL^[47]. (a) Structure of EDFRL; (b) phase demodulation signal of SLD; (c) phase demodulation signal of EDFRL

Fig. 10. Long distance MZ-Sagnac interferometric distributed optical fiber vibration sensor based on cascaded Bi-EDFA^[50]

Fig. 11. Long distance MZ-Sagnac interferometric distributed optical fiber vibration sensor based on reflective EDFA^[51]

Fig. 12. MZ-Sagnac interferometric distributed optical fiber vibration sensor based on cascaded Bi-EDFA and DCF^[52]

Fig. 13. Multi-channel MZ-Sagnac distributed optical fiber vibration sensor based on WDM^[45]

Fig. 14. Schematic diagram of the star-shaped sensor with multiple branches^[57]. (a) Based on the original MZ-Sagnac system; (b) based on branch MZ-Sagnac structure; (c) fiber optic vibration sensor system based on MZ-Sagnac interferometer for branch localization

Fig. 15. MZ-Sagnac structure based on PGC at the end of the sensing fiber^[58]. (a) Optical structure; (b) separation of the effective interference signal and the scattered light parasitic interference

Fig. 16. Schematic of light path with the presence of reflection point^[59]

Fig. 17. MZ-Sagnac interferometric distributed optical fiber vibration sensor based on triple detection of 3×3 coupler^[62]

Fig. 18. Schematic of low-frequency compensation^[63]

Fig. 19. Self-calibration MZ-Sagnac system based on a reference interferometer^[65]. (a) Schematic of the optical structure; (b) flow chart of elliptic fitting self-calibration algorithm; (c)(d) demodulation results and power spectral density (PSD) in the small signal case using traditional Arctan, DCM, and self-calibration demodulation methods

Fig. 20. Processing results of the twice-FFT algorithm^[66]. (a) Single-FFT; (b) twice-FFT and Gaussian fitting

Fig. 21. Improvement of the frequency domain method^[29]. (a) Power spectrum of the time series using the FFT algorithm; (b) power spectrum estimated by the ARMA model

Fig. 22. Dual MZ-Sagnac interferometric distributed optical fiber vibration sensor based on wavelength division multiplexing^[26]. (a) Schematic of optical structure; (b) two signals Δϕ1t and Δϕ2t; (c) cross-correlation of Δϕ1t and Δϕ2t

Fig. 23. Schematic of dual MZ-Sagnac interferometric distributed optical fiber vibration sensor. (a) Bidirectional dual MZ-Sagnac structure^[69]; (b) asymmetrical dual MZ-Sagnac structure^[42]

Fig. 24. Schematic of MZ-Sagnac interferometric distributed optical fiber sound sensor

Fig. 25. Schematic of perimeter security system based on MZ-Sagnac interferometric distributed optical fiber vibration sensor and pattern recognition

Fig. 26. Pipeline monitoring system based on MZ-Sagnac interferometric distributed optical fiber sensor^[85]