Unidirectional Ultra-Long Distributed Optical Fiber Sensor

In this paper, we propose and experimentally demonstrate an ultra-long distributed fiber vibration sensing system using unidirectional forward transmission of a continuous-wave signal and coherent detection with digital signal processing. Two optical fibers, which are close to each other, are deployed for sensing. A loop-back configuration is formed by splicing these two optical fibers at the far end of these two fibers. The location of the vibration event is identified by analyzing the null points in the frequency spectrum of the extracted phase signal. Thanks to the nature of unidirectional forward transmission, the Rayleigh backscattering noise can be avoided. Meanwhile, forward transmission enables optical amplifiers to compensate for the signal loss and hence fundamentally overcome the sensing range limit. We successfully demonstrate the localization of single point and multi-point vibrations with measurement errors of less than ±100 m and ±200 m, respectively, over a 500-km sensing range. The proposed scheme opens new possibilities in ultra-long haul distributed optical sensing applications.