Fig. 1. Schematic of Φ-OTDR system

Fig. 2. Interference visibility of the system at different pulse widths^[9]

Fig. 3. Law of signal to noise ratio vs pulse width^[10]

Fig. 4. Numerical simulation of standard deviation of the strain level and pulse width^[11]

Fig. 5. FRC of different frequency drift rates and pulse widths^[12]

Fig. 6. Backscattering power traces from different pulse shapes with magnification of traces^[14]

Fig. 7. Rayleigh backscattering traces of 1.5 km FUT. (a) High extinction-ratio pulse; (b) Direct-modulation pulses^[16]

Fig. 8. Schematic of the pulse-width multiplexing method^[21]

Fig. 9. 0-π binary phase shift pulse. (a) Phase-shifted double pulse method^[24]; (b) Differential phase shift pulsing technology^[25]

Fig. 10. Composite double probe pulse^[27]

Fig. 11. Spectrum of NLFM optical pulse^[28]

Fig. 12. Linear frequency modulates light pulses. (a) Time-domain profile; (b) Compressed profile^[29]

Fig. 13. Optical pulse train. (a) Pulse shape for Φ-OTDR and MZI multiplexing system^[33]; (b) Pulse shape for Φ-OTDR and polarization diversity multiplexing system ^[33-34]

Fig. 14. (a) Pulse train in conventional Φ-OTDR; (b) Pulse train in frequency division multiplexing Φ-OTDR; (c) Frequency variation of pulse train^[35]

Fig. 15. Three groups of consequent double pulses^[40]

Fig. 16. Dual-pulse using frequency division multiplexing. (a) Modulation on tunable laser source; (b) Modulation on AOM^[44]