Study On The Optical Fibe Vibration Sensing Technology Based On ?-OTDR

In distributed optical fiber sensing the optical fiber is used as both the transmission medium of the system signal and the sensing element simultaneously,and the sensing of environment parameters around the fiber can be realized by the change of some characteristics of light in the fiber.It has a vast field of applications because of its unique advantages that include high sensitivity,wide range of sensing,and immunity to electromagnetic interference.This paper mainly focuses on the theory and experimental research of the vibration sensing system based on ?-OTDR.The main contents of this paper are as follows:1.The basic theory of the ?-OTDR vibration sensing system is analyzed.Theoretical analysis of the changes of axial and radial stress to the refractive index of the optical fiber is firstly studied by elastic mechanics and photoelastic theory,obtaining the influence of the external vibration on the optical phase.Then the basic theory of the ?-OTDR sensor system is introduced,describing the formation of the Rayleigh scattering,and the relationship between the change of scattered light phase and the intensity of the scattering.2.The formation process of scattered light is discussed in detail,and a mathematical statistical model is established.The static and dynamic ?-OTDR scattering curves are simulated numerically.A spatial discrete approximation of the sensing fiber is performed.The intensity and position of scattering are used to describe the scattering unit on the fiber.The statistical models are established by the Rayleigh scattering mechanism,realizing the simulation of scattered light.Finally,based on the theory of ?-OTDR sensing,the scattered light is interfered and superimposed,the static scattering curve is obtained,in addition the dynamic scattering curve is simulated by adding the additional phase change to the scattered light in a specific region.3.A ?-OTDR vibration sensing system based on direct detection is established.Amplitude differential and two-dimensional Fourier transform method are used to extract the vibration information produced by PZT,analyzing the performance of the system.In addition,a vibration localization method based on the mixture Gaussian model is proposed,in which vibrating threshold is set for each location point,overcoming the misjudged caused by using a global threshold,Simultaneously,the threshold adaptive algorithm can also suppress the system error caused by laser power drift and improve the system's positioning accuracy.4.A heterodyne detection system based on self-mixing is proposed and built to improve the low frequency response of vibration sensing.And a ?-OTDR signal processing algorithm based on bilateral filtering is proposed.In the filtering process,the weight coefficients of the spatial domain and the value domain are introduced simultaneously.The edge details generated by the vibration are well preserved,while filtering out the background noise,improving signal-to-noise ratio by over 3dB.