Research On Phi-OTDR Optical Fiber Sensing Technology Based On Chirp Pulse Modulation

The distributed optical fiber sensing technology based on phase-sensitive optical time domain reflectometer(Phi-OTDR)has the characteristics of high sensitivity,large positioning range and high positioning accuracy.For those advantages,it has a great application.However,in the traditional Phi-OTDR system,the spatial resolution is mainly determined by the pulse width.Using the linear chirp pulse as input signal can solve the problem between spatial resolution and the pulse width.But,the relation between the chirp pulse parameters and system performance,as well as the optimization of system parameters still need to be resolved.This paper analyzes the relation between the chirp pulse parameters and system performance,and provides reference for the pulse parameter selection.The improvement of the signal-to-noise ratio(SNR)at the disturbance point and the suppression of coherent fading are studied when the chirp pulse parameters are determined.This article is mainly divided into the following parts:1.Analyzed the relation between the chirp rate,pulse width,SNR and phase noise,and the simulation and experimental verification are carried out.Both of the results show that when the peak power of the input pulse signal and the local oscillator signal are unchanged,the SNR is only related to the chirp rate,the chirp rate increases,the SNR decreases.The system phase noise is directly proportional to the laser line width,chirp rate,and pulse width.When the sweep frequency range keeps unchanged,increasing the pulse width,the SNR goes up,and the phase noise decreases firstly and then increases.There is a combination of chirp rate and pulse width to minimize the system phase noise.These conclusions provide a basis for system parameter setting.2.The collected signals are synthesized into a two-dimensional time-distance image,and the image is filtered by using the Bi-Exponential Edge-Preserving Smoother(BEEPS).The experimental results show that after using BEEPS,the SNR at the disturbance point can be increased without losing the spatial resolution.3.In order to realize the quantitative analysis of the vibration signal,it's necessary to demodulate the optical phase information,but due to the existence of the coherent fading,the phase demodulation may be wrong.This article introduces two methods to suppress the coherent fading: rotated-vector-sum and setting the intensity threshold to reconstruct the phase.The experimental results show that both of the two methods can effectively suppress the coherent fading.