Fig. 1. Operation principles of slope-assisted methods based on Brillouin gain, phase-shift, and phase-gain ratio

Fig. 2. The first coherent detection BOTDA fiber sensor^[9]. (a) Operation principle and experimental setup; Measured Brillouin gain and phase spectra of (b) dispersion-shifted fiber and (c) standard single-mode fiber

Fig. 3. A coherent detection BOTDA fiber sensor based on single-sideband modulation and analog I/Q demodulation^[28]. (a) Operation principle and (b) experimental setup; Measured (c) BGS and (d) BPS

Fig. 4. A coherent detection BOTDA fiber sensor based on single-sideband modulation and digital I/Q demodulation^[17]

Fig. 5. A coherent BOTDA fiber sensor based on acousto-optic-modulated single-sideband probe and digital I/Q demodulation^[18]

Fig. 6. Eliminating the influence of fiber group delay jitter on the measurements of Brillouin gain and phase^[34-35] based on (a) wavelength division multiplexing and (b) subcarrier multiplexing

Fig. 7. Measured Brillouin gain and phase spectra (a) before and (b) after eliminating fiber group delay jitter^[35]

Fig. 8. A BOTDA fiber sensor based on the phase-noise-insensitive coherent detection (PNI-CD)^[36]. (a) Operation principle and (b) experimental setup

Fig. 9. Performance comparison between (a) Brillouin gain- and (b) phase-based measurements in a Raman-assisted BOTDA^[36]

Fig. 10. Differential Brillouin gain and phase measurements using digital I/Q demodulation^[37]

Fig. 11. (a) Eliminating the impacts of phase noises of RF driving sources and laser frequency fluctuations; (b) Eliminating the impacts of dynamic pump power fluctuations^[37]

Fig. 12. Ultra-precise differential Brillouin gain and phase measurements based on a microwave-photonic interferometer (MPI) ^[38]. (a) Operation principle and (b) experimental setup of a BOTDA fiber sensor based on the MPI

Fig. 13. Eliminating relative intensity noises^[38]. (a) The balanced direct detection (BDD) versus the direct detection (DD) and (b) the microwave-photonic interferometer (MPI) versus the PNI-CD

Fig. 14. Mitigating non-local effects^[29]. (a) Measured RF phase-shift spectra and (b) Brillouin gain spectra under different probe powers (blue solid line 0.67 mW, red long dashed line 0.56 mW, green short dashed line 0.47 mW and orange dashed-dot line 0.37 mW)

Fig. 15. Mitigating the impact of pump power variation^[18]. (a) Measured Brillouin phase-shift spectra and (b) Brillouin phase-gain ratio spectra under two pump powers

Fig. 16. Extending the linear region^[29]. (a) BPS; (b) BGS; (c) Brillouin phase-gain ratio spectrum

Fig. 17. Schematic of the combination of the Brillouin phase-gain ratio technique with the image denoising^[38]. (a) BFS measurement uncertainty cannot be reduced by using image denoising for the spectrum-fitting-based method. (b) BFS measurement uncertainty is dramatically reduced by using image denoising for the Brillouin phase-gain ratio method