Fig. 1. Schematic of BOTDA system

Fig. 2. General schematic of BOTDA assisted by fiber Raman amplifier[8]

Fig. 3. Schematic of distributed fiber sensing based on random laser[15]

Fig. 4. SNR degradation of signal detected by BOTDA as a function of RIN level of Raman pump light[16]

Fig. 5. Experimental setup for the Simplex coded-BOTDA sensor[18]

Fig. 6. Experimental validation of the Simplex coded-BOTDA assisted by EDFA[23]. (a) Measured profile of BGS; (b) measured profile of BFS versus distance

Fig. 7. Schematic of conventional sequences and hybrid sequences[34]

Fig. 8. Comparison in BFS distribution between measurements with Walsh code and the conventional PSP-BOTDA system[36]

Fig. 9. Schematic of Simplex coding pre-pumped pulse[40]. (a) Conventional pump pulse and pre-pumped pulse; (b) conventional pump pulse and pre-pumped pulse of 3 bit Simplex coding

Fig. 10. Brillouin interaction and frequency modulation in the dual-probe BOTDA[46]. (a) Brillouin interaction of the conventional dual-probe BOTDA; (b) Brillouin interaction of the dual-probe BOTDA with the pump shifted in frequency; (c) combination of BGS and BLS for the dual-probe BOTDA with the pump shifted in frequency; (d) frequency modulation in the dual-probe BOTDA

Fig. 11. Experimental setup of BOTDA based on coherent detection[48]

Fig. 12. Experimental setup of BOTDA with balanced detection[49]

Fig. 13. Schematic of the frequency modulation and time multiplexing of multi-frequency pump light[51]

Fig. 14. Experimental setup for BOTDA based on multi-frequency time-shifted pump pulses[51]