Fig. 1. (a) Spatial resolution model; (b) Vibration sensing model

Fig. 2. (a) Double pulse type; (b) Two-port Michelson Interferometer (MI) type; (c) Three-port Mach-Zender interferometer (MZI) type; (d) Three-port Michelson Interferometer (MI) type; (e) Heterodyne detection type (conventional pulse); (f) Heterodyne detection type (chirp pulse)

Fig. 3. Beat signals of different frequency ^[42]

Fig. 4. Experimental setup^[44]

Fig. 5. Schematic of RVS ^[44]. (a) Addition of two complex vectors; (b) RVS without strain; (c) RVS with strain

Fig. 6. (a) Seven frequency components; (b) Probability density distribution ^[57]

Fig. 7. Ф-OTDR experimental setup based on FSDL^[59]

Fig. 8. DAS experimental setup based on dual frequency comb^[65]

Fig. 9. Experimental setup of the few-mode fiber based Ф-OTDR system^[77]

Fig. 10. The phase demodulation and synthesis process of wavelength multiplexed double-pulse DAS based on an alternating arranged dual-wavelength UWFBG array^[87]

Fig. 11. Spectrum extraction process of rectangular pulses^[91]

Fig. 12. Experimental setup^[105]

Fig. 13. Processing methods for comparison^[105]. (a) BPF; (b) SERM; (c) PST; (d) MRVA; (e)-(h) IFS