Coherent optimization of pulse, transmission, and detection enables a 250.36-km single-ended φ-OTDR with enhanced...

This study presents an innovative solution to overcome the key technical challenges in ultra-long-range and weak-vibration monitoring based on phase-sensitive optical time-domain reflectometry (φ-OTDR). These challenges include fundamental limitations on sensing distance arising from probe pulse width, optical fiber nonlinearities, and transmission losses, as well as the difficulty of detecting weak vibration signals at the far end due to low signal-to-noise ratio and inevitable trade-off between noise suppression and detection bandwidth, and the large electrical dynamic range encountered in long-distance sensing. To address these issues, the system incorporates a cascaded dual-acoustic-optic modulator (AOM) configuration, which enables a high extinction ratio (ER), minimal frequency shift, and sharp rising edges in probe pulses. This design effectively suppresses continuous-wave (CW) light leakage, reduces the required detection bandwidth, and achieves high photoelectric conversion gain. A logarithmic amplifier is integrated after photoelectric conversion to compress the electrical dynamic range, significantly alleviating the dynamic range burden on the ADC and improving the effective resolution for weak distal signals. For enhanced long-haul signal transmission, delayed forward distributed Raman amplification (DRA) is combined with a remotely pumped erbium-doped fiber amplifier (RP-EDFA) to provide optimized gain distribution and improved signal integrity over extended distances. Furthermore, a convolutional feature extraction algorithm is implemented to enhance the sensitivity of weak vibration signal detection without sacrificing the system's response bandwidth. Experimental results demonstrate a record-breaking sensing range of 250.36 km at a spatial resolution of 21 m under single-end operation, enabling robust detection of weak vibration signals buried in strong background noise and precise demodulation of both bandwidth-limited response vibrations and ultra-low-frequency vibrations. This work establishes a new benchmark for single-ended φ-OTDR-based distributed vibration sensing (DVS) using conventional constant-frequency pulses and provides a robust technical support for structural health monitoring of critical infrastructure.