Fig. 1. Principle diagram of BOTDR

Fig. 2. BGS at room temperature and local temperature variation. (a) BGS three-dimensional diagram at room temperature; (b) schematic diagram of BGS with end temperature rise

Fig. 3. Schematic diagram of search window and similar window principle for NLM filtering algorithm

Fig. 4. Schematic diagrams of vertical gradient of BGS. (a) Three-dimensional schematic diagram of vertical gradient of BGS; (b) two-dimensional schematic diagram after taking the absolute value

Fig. 5. Noise reduction effects of three algorithms when the noise standard deviation is 5 dB. (a) Front view of 3D BGS diagram before noise reduction; (b) noise reduction effect of NLM algorithm; (c) noise reduction effect of BM3D algorithm; (d) noise reduction effect of GNLM algorithm

Fig. 6. Noise reduction effects of three algorithms when the noise standard deviation is 10 dB. (a) Front view of 3D BGS diagram before noise reduction; (b) noise reduction effect of NLM algorithm; (c) noise reduction effect of BM3D algorithm; (d) noise reduction effect of GNLM algorithm

Fig. 7. Noise reduction effects of three algorithms when the noise standard deviation is 15 dB. (a) Front view of 3D BGS diagram before noise reduction; (b) noise reduction effect of NLM algorithm; (c) noise reduction effect of BM3D algorithm; (d) noise reduction effect of GNLM algorithm

Fig. 8. Noise reduction effects of three algorithms when the noise standard deviation is 20 dB. (a) Front view of 3D BGS diagram before noise reduction; (b) noise reduction effect of NLM algorithm; (c) noise reduction effect of BM3D algorithm; (d) noise reduction effect of GNLM algorithm

Fig. 9. Schematic diagram of BODTR system device

Fig. 10. Schematic diagram of experimental design

Fig. 11. Schematic diagrams of two-dimensional BGS with noise reduction after heating at 50 ℃ with a pulse width of 50 ns. (a) Schematic diagram of two-dimensional BGS without noise reduction; (b) noise reduction effect of NLM algorithm; (c) noise reduction effect of BM3D algorithm; (d) noise reduction effect of GNLM algorithm

Fig. 12. BFS curves under the condition of a pulse width of 50 ns and heating at 30 ℃. (a) BFS curve without noise reduction; (b) BFS curve after NLM denoising; (c) BFS curve after BM3D denoising; (d) BFS curve after GNLM denoising

Fig. 13. Comparison of BFS curves of the front and the rear heating sections under the condition of a pulse width of 50 ns and heating at 30 ℃. (a) Comparison of noise reduction results of three algorithms in the front heating section; (b) comparison of noise reduction results of three algorithms in the rear heating section

Fig. 14. Comparison of BFS curves for the 50 °C heating section at the fiber rear end under different pulse widths. (a) Comparison of BFS without noise reduction; (b) comparison of BFS after NLM denoising; (c) comparison of BFS after BM3D denoising; (d) comparison of BFS after GNLM denoising