Research On Distributed Optical Fiber Vibration Sensing Technology Based On Phase Demodulation

Distributed optical fiber sensor,as one of the most competitive technique in the current optical fiber sensor technique,has the advantages of anti-electromagnetic interference,anti-corrosion,no external field power supply,low loss,high sensitivity,and distributed multi-point real-time detection.The research on distributed optical fiber vibration sensing technique to detect vibration signals has profound developing value in many fields related to people's livelihood and industrial production,such as national security,transportation,and construction.This thesis carries out related theoretical and experimental research on distributed optical fiber vibration sensing technique based on ?-OTDR(phase sensitive optical time domain reflectometer),and analyzes this sensor system to develop a phase demodulation program to complete the vibration signal information extraction.The main research content of this thesis can be summarized into the following aspects:1.The basic principles of ?-OTDR technique,focusing on the basic working mechanism and structure of ?-OTDR distributed optical fiber vibration sensing system to realize vibration sensing are analyzed,and also the external influences that cause signal phase changes are discussed in this thesis.After that,the conclusion that phase demodulation can be used to quantify the measurement of external disturbance is obtained.Then,a mathematical model of coherent back Rayleigh scattered light received by the ?-OTDR system is established.Finally,this thesis analyzes the basic working principle of the ?-OTDR distributed optical fiber vibration sensing system with heterodyne coherent detection structure.2.In the process of ?-OTDR system vibration positioning,Savitzky-Golay filtering algorithm is proposed that uses low-order polynomial fitting and linear least squares to smooth the data points in the convolution process to improve the signal-to-noise ratio of the signal.The relationship between filter parameters(fitting order and filter window length)and filter effect is studied.Using this algorithm to filter the original scattered signal and then combining it with the amplitude difference method,the signal-to-noise ratio of the positioning signal obtained is much larger than the signal-to-noise ratio processed by the common differential sliding average algorithm.3.For the ?-OTDR system based on the direct detection structure,a technical scheme for phase demodulation combining iteration and least squares method is proposed: after randomly setting the initial phase value,using the least squares fit between the known time-domain light intensity values of multiple vibration points and the phase to be measured,then iteratively updating the initial phase value and phase distribution,and finally reaching the convergence condition to obtain the phase to be measured.A ?-OTDR vibration sensing system for direct detection is built,which successfully locates single-point and multi-point vibration events and simulates the vibration events of different vibration frequencies(100Hz~1000Hz)and amplitudes(2.5V~10V)by PZT on the system to perform intensity and phase demodulation respectively.The intensity demodulation results show that its extraction of vibration information will be restricted under the condition of low frequency and strong amplitude vibration.Finally,iterative random phase shift method is used to demodulate the vibration phase information of 100 Hz and 7.5V successfully.4.Utilizing the Hilbert transform method to phase demodulate the vibration signals collected by the ?-OTDR vibration sensing system of the heterodyne coherent detection structure and extract relevant information,demodulate the phase distribution of the 200 Hz,5V vibration signals.