Fig. 1. Schematic of BOTDA^[21]

Fig. 2. BOTDA experimental setup based on reference fiber method^[6]

Fig. 3. BOTDA experimental setup based on FBG auxiliary method^[7]

Fig. 4. BOTDA experimental setup based on multi-parameter demodulation method^[9]

Fig. 5. Measurement results of the PCF as a sensing fiber^[10]. (a) Brillouin loss spectrum of PCF with a partially Ge-doped core measured at 24 ℃and in a loose state; (b) Brillouin frequency shift of peaks a and c as function of strain; (c) Brillouin frequency shift of peaks a and c as function of temperature

Fig. 6. BOTDA experimental setup with PMF fiber as a sensing fiber^[11]

Fig. 7. BGS of LEAF at room temperature and relaxed state^[12]

Fig. 8. Principle diagrams of mode launchers^[13]. (a) Schematic diagram of a free-space mode launcher; (b) SLM phase patterns and the corresponding spatial modes

Fig. 9. Experimental setup for the FMF multi-parameter sensor based on BOTDA technique^[13]

Fig. 10. Cross-section of dual-core optical fiber and frequency shifts of cores 1 and 2^[14]. (a) Cross-section of dual-core optical fiber; (b) measured frequency shifts of cores 1 and 2

Fig. 11. Experimental set-up and results based on dual wavelength sensing demodulation method^[15]. (a) Experimental set-up used for dual wavelength BOTDA measurements; (b) temperature (blue line) and strain (red line) profile reconstructions

Fig. 12. Experimental setup of hybrid Raman/Brillouin optical time domain analysis distributed optical fiber sensors^[16]

Fig. 13. Experimental setup of Rayleigh/Brillouin hybrid distributed optical fiber sensing^[17]

Fig. 14. Schematic diagram of the proposed solution based on ANN^[18]. (a) Off-line stage: definition of pre-processing and ANN parameters and architecture; (b) online stage: real-time temperature-strain discrimination

Fig. 15. Principle of using DNN for simultaneous temperature and strain measurement from double-peak BGS in LEAF^[19]

Fig. 16. Experimental results based on deep neural network algorithm^[19]. (a) Measured BGS distributed along LEAF sensing fiber; (b) measured double-peak BGS of LEAF under room temperature of 23.5 ℃ and stain of 0 με; (c) measured BFS-temperature relations for Peak1 and Peak2; (d) measured BFS-strain relations for Peak1 and Peak2

Table 1. Principles and performance characteristics of each program