Distributed fiber-optic sensing using double-loop Sagnac interferometer

The significant advances of Fiber optic technology are making its comprehensive applications, including security monitoring, pipeline maintenance and communication cable protection, etc. Many of these applications operate in harsh environments over long distances. This leads to low SNR, making high-precision positioning a great challenge. In this paper, a distributed double-loop Sagnac interferometer system based fiber optic sensor is designed and validated in field for detecting and locating intruders. First, the double-loop structure is improved by adding a certain length of transmission fiber in the start and end of each loopwhich enhances the linear relationship between amplitude and distance. Second, in order to enhance the SNR, a spectral subtraction pre-processing method is provided. Through the above methods, precise location of a disturbance is guaranteed. Results of field tests show that the SNR is greatly improved and the system could achieve spatial resolution of +/- 15m per kilometer.