Fig. 1. Principle of acoustic wave detection based on optical phase

Fig. 2. Typical heterodyne coherent detection scheme^[22]

Fig. 3. Rayleigh scattering model of the standard optical fiber^[26]

Fig. 4. Schematic diagram of equivalent scattering position in a pulse interference

Fig. 5. Typical scheme of continuous scattering enhanced optical fiber^[31-33]. (a) Backscattering signal distribution of typical continuous scattering enhanced fiber; (b) scheme for improve the Rayleigh scattering through changing the fiber doping; (c) scattering spectrum of high Rayleigh scattering fiber

Fig. 6. Schematic diagram of discrete scattering enhanced fiber

Fig. 7. Simulation results of discrete enhanced scattering model. (a) Reflection distribution of discrete enhanced fiber; (b) reflection distribution of single mode fiber; (c) phase demodulation results of discrete scattering enhanced fiber and single mode fiber

Fig. 8. Typical UWFBG preparation method. (a) Online UWFBG preparation system^[45]; (b) OTDR measurement results of UWFBG array^[46]; (c) reflection spectrum of UWFBG^[45]

Fig. 9. Preparation method of colorless microstructure array based on UV exposure. (a) Automatic colorless microstructure preparation system^[50]; (b) OTDR measurement results of colorless microstructure array^[52]; (c) spectra of colorless microstructure arrays^[50]; (d) temperature stability of colorless microstructure arrays^[52]

Fig. 10. Preparation method of weak reflection array based on femtosecond laser^[39]. (a) Weak reflection array preparation system; (b) schematic diagram of weak reflection array preparation; (c) OTDR measurement results of weak reflection array

Fig. 11. Representative DAS scheme based on UWFBG. (a) 3×3 coupler scheme^[54]; (b) PGC scheme^[55]; (c) heterodyne coherent detection scheme^[29]

Fig. 12. Low frequency noise suppression scheme for discrete scattering fiber DAS. (a) Auxiliary interferometer compensation^[60-61]; (b) reference fiber compensation^[62]

Fig. 13. Representative scheme for polarization noise suppression. (a) Orthogonal polarization scheme ^[64-65]; (b) multi-parameter scheme^[67]

Fig. 14. Representative scheme for pulse width compression. (a) Swept frequency pulse scheme ^[71-72]; (b) pulse coding scheme^[73］

Fig. 15. Time-slot multiplexing scheme^[75]. (a) Principle; (b) demodulation algorithm

Fig. 16. VSP test results of micro structure fiber DAS system. (a) "Zero bias" test results; (b) schematic diagram of signal consistency; (c) "non-zero bias" test results; (d) "zero bias" spectrum diagram; (e) "non-zero bias" spectrum diagram^[76]

Fig. 17. DAS system based on UWFBG array. (a) VSP test site; (b) VSP test result^[77]

Fig. 18. Schematic diagram and result diagram of pipeline measurement^[80]. (a) DAS system structure and layout of scattering-enhancing fibers; (b) architectures of neural network; (c) confusion matrix of four acoustic events

Fig. 19. Rail defect detection based on DAS system^[81]. (a) Process of detecting and analyzing sound waves based on fiber DAS; (b) photographs of the field test environment; (c) sound distribution measured by DAS

Fig. 20. Experimental results. (a) Tunnel steel loop detection system based on DAS; (b) schematic of invalid steel loop; (c) BP neural network; (d) relationship between invalid degree and frequency deviation; (e) relationship between invalid degree and energy distribution; (f) power spectrum under different invalid degrees; (g) wavelet energy transformation under different invalid degrees; (h) recognition rate

Fig. 21. Experimental results. (a) Fully distributed underwater acoustic sensor system; (b) lightweight fully distributed underwater acoustic fiber optic cable based on microstructure scattering enhanced fiber; (c) relationship between the demodulated phase change and acoustic pressure; (d) frequency response curve within the frequency range of 100-2000 Hz^[84]

Fig. 22. Schematic diagram and result diagram of underwater acoustic test^[85]. (a) Structure of sensitized optical cable; (b) picture of sensitized optical cable; (c) setup of the field test; (d) motion trajectory tracking of the sound source