Fig. 1. The setup of DVS-Φ-OTDR system based on direct detection.

Fig. 2. The setup of DAS-Φ-OTDR system with different demodulation methods.(a) Heterodyne detection + IQ phase demodulation. (b) Heterodyne detection + Hilbert transform phase demodulation. (c) Direct detection + phase demodulation based on a 3×3 coupler. (d) Direct detection + phase demodulation based on PGC.

Fig. 3. Operation principle of Φ-OTDR based VSP monitoring system. (a) Zero-offset VSP. (b) Walk-away VSP.

Fig. 4. (a) Normalized strain (green curve) recorded during an Mb ~6.2 (USGS) earthquake (Kota Ternate, Indonesia, 2015-03-17 22:12:28 UTC, 1.669°N; 126.522°E, 44 km depth) superimposed with the normalized velocity record (red curve) from the broadband station RAH (80 m from the optical cable). (b) Zoom-view from (a) showing a good phase correspondence between seismometer velocity record and DAS strain records at a 20s period. (c) Short record (6 s) of strain phases from a local earthquake trapped in the fault damage zone. Waves inside and outside the fault zone have different apparent velocities. Figures reproduced from ref.⁹⁸, under a Creative Commons Attribution 4.0 International License.

Fig. 5. (a) Stacked DAS beam trace (black) filtered to various bands between 0.02 and 1 Hz compared with amplitude-normalized particle velocity from a broadband seismometer rotated into the mean azimuth of the DAS array (red). (b) Separation of ocean and seismic waves in the first quadrant of the logarithmic space of the Φ-OTDR signal frequency-wave number domain. Figures reproduced from ref.¹⁰¹, under a Creative Commons Attribution 4.0 International License.

Fig. 6. Ambient noise based, cross-correlation computed between all Φ-OTDR traces of the cable with respect to one arbitrary trace (at position ~11.5 km) showing several geological features. Figure reproduced from supplementary material of ref.⁹⁸ under a Creative Commons Attribution 4.0 International License.

Fig. 7. Experimental site layout. Figure reproduced from ref.¹⁰³, under a Creative Commons Attribution 4.0 International License.

Fig. 8. Fiber cable layout and operation principle of 1D-CNN. Figure redrawn after ref.¹⁹².

Fig. 9. An illustration of the railway safety monitoring experiments. Figure reproduced from ref.¹⁰⁸, under the OSA Open Access Publishing Agreement.

Fig. 10. Fiber cable layout alongside railways. Figure redrawn after ref.¹¹³.

Fig. 11. Integration of Φ-OTDR system in the DWDM communication network. Figure reproduced with permission from ref.¹¹⁶, IEEE.

Fig. 12. Experimental setup for discharge detection with two acoustic transducers attached to the 40 kV joint. (Figure redrawn after ref. ¹¹⁹)

Fig. 13. Fiber deployment inside or outside electrical cable. (Figure redrawn after ref. ¹²⁰)

Fig. 14. (a) Fiber coil transducer deployment on the GIS device. (b) Detected discharge signal of transducer #1, #2 and #3 at pulse repetition rate of 10 kHz. Figure reproduced from ref. ¹²¹, under a Creative Commons Attribution 4.0 International License.

Fig. 15. (a) Φ-OTDR system configuration. (b) Tree infestation sensing results. (Figure reproduced from ref.¹⁸², under a Creative Commons Attribution 4.0 International License.

Fig. 16. (a) Cross section of the MCF. The circled fiber cores were selected for bending direction analysis. (b) Fiber bending direction analysis in the x-y-ε space. The x-yplane corresponds to one cross section of the MCF. Figure reproduced from ref.¹⁸¹, under the OSA Open Access Publishing Agreement.

Fig. 17. (a) Operation principle of the Φ-OTDR based solar irradiance sensing system. (b) Temperature difference between the black and reference fiber vs. the applied solar irradiance. Figure reproduced from ref.¹⁸³, under a Creative Commons Attribution 4.0 International License.

Table 0. Research progress on improving the SNR in Ф-OTDR systems

Table 0. Research progress on improving sensing distance in Ф-OTDR systems.

Table 0. Research progress on event discrimination in Ф-OTDR systems.

Table 0. Research progress on improving frequency response range in Ф-OTDR systems.

Table 0. Research progress on improving spatial resolution in Ф-OTDR systems.