Enhanced Ultra-Long-Range Vibration Sensing Using an Asymmetric Interferometer With Bidirectional Raman Amplification

Ultra-long-range vibration monitoring via distributed fiber optic sensing technology is a critical advancement for modern engineering projects. However, the loss of light power during transmission is the main bottleneck constraining the sensing distance. In this paper, we construct an asymmetric interference model based on bidirectional Raman amplification (BRA) and propose an asymmetric interferometric vibration sensing scheme with BRA. We first analyze the transmission and signal-to-noise ratio (SNR) characteristics of the bidirectional signal lights along the sensing fiber by numerical simulation, providing a theoretical basis for the ultra-long-range all-fiber system based on the interference principle. The sensing system employs two Raman pump sources for non-relay amplification, enhancing the SNR along the fiber to extend the sensing distance. Additionally, we introduce a high-precision positioning method based on wavelet-based synchroextracting transform (WSET), which has superior time-frequency resolution and noise immunity. By applying a cross-correlation method to the phase variation characteristics of bidirectional interference signals extracted by the WSET, we determine the time delay for position demodulation. We conduct a series of field tests, and the experimental results demonstrate that the sensing system can accurately locate vibrations at a sensing distance of 200 km, achieving a positioning accuracy of approximately 71.88 m. The SNR of interference signals can reach up to 27 dB. This scheme shows significant potential for applications in linear engineering monitoring.