Fig. 1. Working principle of Φ-OTDR^[1]

Fig. 2. Exponential attenuation factor values at different positions under different line widths ^[5]

Fig. 3. Comparison of relationship between signal-to-noise ratio and line width of experimental detection and theoretical derivation ^[5]

Fig. 4. Schematic diagram of UWB fiber grating enhanced SNR^[18]. (a) A single OTDR trajectory; (b) 299^th and 300^th UWB fiber grating signals

Fig. 5. Variation diagram of Φ-OTDR pulse train^[41]. (a) Traditional Φ-OTDR pulse train; (b) Φ-OTDR pulse train under frequency divsion multiplexing; (c) frequency variation of pulse train

Fig. 6. Φ-OTDR system installation diagram^[9]. (a) Based on direct detection structure; (b) based on heterodyne coherence detection structure; (c) based on 3-port coupler detection structure; (d) based on phase generation carrier (PGC) detection structure

Fig. 7. Based on DAS intrusion detection device^[60]

Fig. 8. Deformation simulation results of fiber composite phase line dancing^[68]