Fig. 1. BOTDR system diagram^[14]

Fig. 2. Fitting method based on equivalent optical pulse^[15]. (a) Rectangular optical pulse with duration τ; （b） schematic generation of backscattered Brillouin signal with a duration of τ′； （c） equivalent optical pulse when τ≥τ′； （d） equivalent optical pulse when τ≤τ′

Fig. 3. Probe pulse and Brillouin signal in the fiber (shaded segment represents fiber which generates the Brillouin signal)^[16]

Fig. 4. Generation of the EDD (shaded segment represents fiber which generates the Brillouin signal)^[16]

Fig. 5. Pulse subdivision superposition principle for backscattering spectrum^[17]

Fig. 6. Pulse subdivision analysis method^[18]. (a) Pumping pulse and resulting Brillouin signal in optical fiber; (b) dividing the pump pulse into m sub-pulses and then subdividing the Brillouin spectrum into m sub-spectra using the energy weights of subdivided sub-pulses

Fig. 7. Brillouin gain spectrum superposition process (black indicates the temperature/strain change area)^[19]. (a) Head of the probe pulse generates a sub-Brillouin signal in the (m-1)th section; (b)‒(e) generation and superposition processes of the sub-Brillouin signal of each segment for each L/m long distance of the probe pulse propagation, respectively

Fig. 8. Process of the Brillouin scattering information weights contained in the DFT transform unit changed by the 4th-order HSCW^[22]

Fig. 9. Original BGS distribution without applying Wiener filtering, and those after applying the filtering^[23]. (a) Original measured BGS distribution using 40 ns pulses, and Wiener filtering recovered BGS distributions to retrieve; (b) 1 m spatial resolution; (c) 0.5 m spatial resolution; (d) 0.2 m spatial resolution; (e) 0.1 m spatial resolution

Fig. 10. Schematic of quick positioning^[24]

Fig. 11. Frequency spectra constructed by equal division FFT^[24]. (a) Frequency spectra by FFT processing of 0‒1250 m and 1250‒2500 m time domain signals; (b) frequency spectra by FFT processing of 1563‒1719 m and 1719‒1875 m time domain signals; (c) frequency spectra by FFT processing of 1641‒1680 m and 1680‒1719 m time domain signals; (d) frequency spectra by FFT processing of 1719‒1758 m and 1758‒1797 m time domain signals

Fig. 12. Brillouin frequency shift distribution at different conditions^[24]. (a) Traditional peak searching algorithm & 40 ℃ optical fiber; (b) traditional peak searching algorithm & 50 ℃ optical fiber; (c) maximum seeking method & 40 ℃ optical fiber; (d) maximum seeking method & 50 ℃ optical fiber