Research On Distributed Optical Fiber Vibration Sensing System Based On ?-OTDR

Distributed optical fiber sensing technology takes optical fiber as both sensor and signal transmission medium.And it has a broad research and application prospect in the field of health monitoring of perimeter security,tunnel and bridge and large buildings because of its excellent characteristic of corrosion resistance,easy integration and antielectromagnetic interference.Sensing system based on ?-OTDR technology is sensitive to vibration disturbance with a simple structure and has the ability to identify multiple disturbance events at the same time.However,the system may have some problems such as disturbance misjudgment and locating difficulty when affected by noise and laser energy attenuation in practical applications.In this paper,the theoretical model and data processing method based on ?-OTDR system were studied,the experimental system based on ?-OTDR was established and a vibration disturbance discrimination and locating method based on PCA-SVDD was proposed.The main research content of this thesis is as follows:(1)The sensing mechanism of distributed optical fiber vibration sensor based on ?-OTDR is studied and the mathematical model which describing the response of Rayleigh scattering intensity affected by external disturbance is constructed.The sensitivity,spatial resolution,frequency response range and other parameters of the ?-OTDR system are analyzed.(2)The ?-OTDR experimental system is built based on the theoretical research above and the control software of this experimental system is also developed according to the experimental requirements.The software uses C++ language and Qt framework to construct a graphical interface,with functions of signal acquisition,storage,sliding average-difference processing and real-time waveform display.On this basis,the methods of denoising,locating and analyzing to signal data are discussed and the time-frequency analysis of signal is realized by using Fourier transform and wavelet transform.(3)Undisturbed samples and three different types of vibration disturbance samples(continuous vibration,pressing,knocking)are constructed using the data collected by the ?-OTDR system.In order to face the reality that undisturbed samples are easy to be collected and disturbance signal samples are complex and difficult to be collected,the SVDD method is proposed for the discrimination of disturbance types.The recognition rate of the method for the undisturbed signals is 99.62%,and the average recognition rate of the three disturbance signals is 96.06%.(4)The method of PCA is used to optimize the sample characteristics.The recognition rate of the undisturbed signals is 99.98%,and the average recognition rate of the three disturbance signals is 98.13%.The PCA-SVDD disturbance discrimination model is applied to the disturbance location of the sensing system,and simultaneous positioning of multiple disturbance points is realized at both the front and rear ends of the 11km-long sensing fiber.