Fig. 1. Random sequence and its time-domain feature. (a) Sequence distribution of 512 bit random code; (b) frequency distribution of “0” and “1” in 800 kbit random code

Fig. 2. Self-correlation characteristic curves. (a) Normalized η(t) waveforms under different numbers of coding groups M and coding lengths L; (b) variation of PSLR with coding group; (c) variation of PSLR with coding length

Fig. 3. Schematic diagram of random-sequence-based pulse code modulation

Fig. 4. Procedure diagram of BOTDR decoding using random-sequence-based pulse code modulation

Fig. 5. Single-frequency time-domain decoding simulation results in BOTDR system without noise. (a) Backscattering response curve of single-pulse signal; (b) a set of cross-correlation curve in the decoding process; (c) accumulated average calculation result among multiple sets of cross-correlation curves in the decoding process

Fig. 6. Simulation curves at different coding lengths. (a) Single-frequency power curves; (b) MAE curve

Fig. 7. Simulation curves at different numbers of coding groups. (a) Single-frequency power curves; (b) MAE curve

Fig. 8. Procedure diagram of long-distance BOTDR using random-sequence-based pulse code modulation

Fig. 9. Configuration of the fiber in the experiment

Fig. 10. Self-correlation result of encoding pulse sequences from SOA at different encoding lengths

Fig. 11. Brillouin backscattering spectra. (a) Brillouin backscattering spectra at different encoding lengths; (b) difference between Stokes peak and anti-Stokes peak

Fig. 12. Brillouin transmission spectra

Fig. 13. Measurement results of random-sequence-based pulse code modulation at different coding lengths. (a) BFS distribution; (b) RMSE distribution; (c) effective sensing distance

Fig. 14. Self-correlation result of encoding pulse sequences output by SOA at different encoding groups numbers

Fig. 15. Measurement results and accuracy evaluation of random-sequence-based pulse code modulation at different numbers of coding groups. (a) BFS distribution curve; (b) RMSE and time-consumption in data updating

Fig. 16. Measured single-frequency curve and scanning-frequency 3D-BGS of decoding results. (a) Single-frequency curve; (b) scanning-frequency 3D-BGS (local temperature variation section)

Fig. 17. BFS measurement results and linear fitting curve at different temperatures

Fig. 18. Comparison between spatial resolutions of different methods. (a) Random-sequence-based pulse code modulation method; (b) single-pulse method

Fig. 19. RMSE distribution of BFS curve at room temperature