Research On Distributed Optical Fiber Acoustic Vibration Sensing Technology Based On OFDR

Optical Frequency Domain Reflectometry(OFDR)has great value in military,large-scale construction,precision instrument manufacturing and other fields,which has been popular in recent years.In this paper,a theoretical model of the OFDR system is established firstly.Based on this theoretical model,a experimental platform is built secondly.Thirdly,we study the tunable laser nonlinearity and the methods to compensate it,which improves the spatial resolution of the system.Finally,the vibration event is located by the cross-correlation algorithm,furthermore the vibration signal is successfully recovered by the phase demodulation method.The main contents of the paper are exhibited as follows:1.According to the Rayleigh scattering principle and the theory of FBG sensing,we study the theory of OFDR and establish the mathematical model of the system.The nonlinearity of frequency sweeping caused by the tunable laser and the effect of the nonlinearity is analyzed.We set up the model of laser phase noise and simulate its impact.2.The OFDR distributed acoustic vibration sensing system is designed and built.First the modulation effect of the acoustic vibration frequency on the optical fiber is analyzed,which is the theoretical basis for the OFDR distributed acoustic vibration sensing system.The main parameters of OFDR sensing system,such as spatial resolution,measuring distance,and frequency response range are analyzed,which are related to the scanning time,sweeping frequency range,frequency scanning rate and sampling rate of the acquisition card The OFDR system is designed and built,and the oscilloscope is used to beat the auxiliary interferometer to verify the correctness of the theory.The selection of system hardware is introduced in detail.3.The demodulation algorithm of OFDR distributed acoustic vibration sensing system is researched.Three methods to compensate the nonlinearity are proposed and demonstrated,as NUFFT?deskew filter and Cubic spline interpolation.We propose two demodulation schemes for OFDR signals,including phase demodulation algorithms and cross-correlation demodulation algorithms.The cross-correlation algorithm can locate the vibration event and estimate the vibration frequency,and the phase demodulation algorithm can recover the vibration signal at different vibration frequencies and vibration amplitudes.4.The distributed fiber vibration sensing experiment based on OFDR is carried out.The spatial resolution reaches 0.32 cm at a fiber sensing length of 100 m after cubic spline interpolation algorithm,and the spatial resolution of the system reaches 025 m after being compensated by the deskew filter algorithm at 440 m.Through the cross-correlation algorithm,the location of the vibration event is accurately realized,and the vibration spatial resolution reaches 0.4m at 120 m.The vibration signal is successfully recovered by the phase demodulation method,and the vibration frequency measurement range is 100Hz-2k Hz.