Optical Fiber Vibration Sensing System For Perimeter Monitoring

Optical fiber sensing system is widely used in many fields,such as key regional security monitoring,pipeline maintenance and communication cable protection.It has the advantages of anti-electromagnetic interference,high sensitivity,no power required in the field,intrinsic safety in volatile environments and cost effectiveness over large distances.Developing an efficient perimeter monitoring sysytem is a meaningful work.Sagnac interferometric fiber sensors,with high reliability and low noise,play excellent roles in wide applications.Based on the Sagnac techniques,two perimeter monitoring sysytems are improved.First,based on the single-loop Sagnac sensor,the zone-perimeter monitoring sysytem is achieved.It has simple structure and stable performance,implements multipoint monitoring,and links to video monitoring system easily.Secend,the distributed-perimeter monitoring sysytem is presented by improving the double-loop Sagnac structure.Three depolarizers are appended to maintain reliability;at the start and end of each loop a certain length of transmission fiber is added,then the signal blind zone is eliminated and the linear relationship between amplitude and distance is enhanced;vibration signal is located by amplitude ratio method and the accuracy is ± 15m.Third,a comprehensive event detection solution is proposed,including signal denoising using spectral subtraction method,feature extraction using amplitude-ascend method and event classification using BP neural network.Based on five characteristic parameters(DB,s,θ,LB,CV),the solusion can detect four different signals(driving,climbing,knocking,shaking).Fourth,the hardware and software platforms,which include signal acquisition,display,storage and alarm,are set up for both of the zone-perimeter monitoring system and distributed system.