Rising-Edge-Assisted Decoding for Brillouin Optical Time-Domain Reflectometer

Long-distance distributed fiber-optic sensing based on optical pulse coding is widely applied in structural health monitoring and geological disaster early warning. However, erbium-doped fiber amplifiers' (EDFA) transient effect causes attenuation and power fluctuations in the coded optical pulse sequence, compromising decoding accuracy and sensing range. This paper proposes a rising-edge-assisted decoding (REA-D) for the encoded Brillouin optical time-domain reflectometer (BOTDR). Based on the staircase-like characteristics of the rising edge in the encoded BOTDR signal, a more accurate decoding function is constructed that eliminates the EDFA transient effect, yielding a higher-precision decoded signal. Unlike existing coding schemes with fixed-duty cycles, a linearly decreasing-duty-cycle coding scheme has been developed to match the REA-D. In the experiment, the signal-to-noise ratio (SNR) of the decoded signal improved by 3.93dB compared to conventional methods, while the 3σ value of the BFS error over a 25km sensing fiber decreased from 3.96 MHz to 2.61 MHz, and the BFS uncertainty at the tail was reduced to 2.5 MHz, representing a 1.4-fold improvement. The spatial resolution remains 2 meters, surpassing that of the conventional method. We believe the proposed REA-D is better suited to distributed fiber-optic sensing than traditional fixed-code schemes derived from communication, offering greater flexibility and potential for further development.