The Design Of Distributed Fiber Optic Vibration Sensing System

Distributed optical fiber sensing technology is the application of strain measurementcharacteristics of the optical fiber. It has been obtaining a wide range of applications in theaircraft, pipeline leak detection, military and civilian security and other fields. This thesismainly researched distributed optical fiber vibration sensors.Based on the detailedtheoretical analysis for the basic Sagnac structure, we proposed modified Sagnac distributedoptical fiber vibration sensor design: dual Sagnac loop structure distributed optical fibervibration sensing system, hybrid Mach-Zehnder and Sagnac distributed optical fibervibration sensing system, Distributed Sagnac loop with fiber delay optical fiber vibrationsensing system.Firstly, The model of dual Sagnac ring fiber optic distributed vibration sensing systemare proposed, and the Matlab-Simulink simulation of model for the PGC phase generatedcarrier demodulation are undertook; also, we researched the choice basis of cutoff frequencyof low–pass filter (LPF) for PGC demodulation (ie, the relationship of LPF cut-offfrequency, the carrier frequency, frequency of the probed signal); And we studieddemodulation signal corresponding to different amplitude of the probed signal. When thedetection signal is too large, the demodulation signal will be deformed; we also studied thedemodulation waveform of frequency differential detection signal, and demonstrate thelocation analysis and simulation while there were many vibration points on the system; also,we build a sensing system for experimental verification, the experimental results tend to thetheoretical values.Secondly, we advanced a remote distance line structure hybrid Mach-Zehnder andSagnac distributed optic fiber vibration sensing system model, and analyzed the positionaccuracy of the system; We researched the impact on the first null frequency of differentrelationship of the length of delay fiber and the length of sensing fiber; We alsodemonstrated the analysis of position characteristics when there existed many vibrationsource; The system design was verified experimentally, the results showed the experimentsresults were consistent with theoretical values.Thirdly, we established the model of distributed Sagnac loop with fiber delay vibration sensing system; Analysis and simulation of the response of vibration were made on bothsides of the midpoint of the Sagnac loop. From the simulation, we can see the frequencyresponse clearly while the vibration source on both sides of midpoint (i.e. the distribution ofnull frequency); Experimental verification of the system design was conducted, validationresults showed that experimental values conformed to theoretical ones exactly.