Fig. 1. Variation of Brillouin signal power P_S(L) and signal-to-noise ratio with fiber position L. (a) Brillouin signal power at different positions; (b) SNR at different positions when P₀=64 mW

Fig. 2. Flow chart of lifting wavelet threshold denoising

Fig. 3. Optimal wavelet basis under different signal-to-noise ratios. (a) R_SN=-6 dB; (b) R_SN=-15 dB; (c) R_SN=-20 dB

Fig. 4. Optimal threshold rules and decomposition layers under different signal-to-noise ratios. (a) R_SN=-6 dB; (b) R_SN=-15 dB; (c) R_SN=-20 dB

Fig. 5. Flow chart of noise reduction algorithm

Fig. 6. Schematic diagram of the BOTDR system

Fig. 7. Brillouin frequency shift distribution along the fiber length. (a) Cumulative average; (b) cumulative average + wavelet threshold denoising; (c) cumulative average + lifting wavelet threshold denoising