Research On Signal Demodulation Method Of Coherent Optical Time Domain Reflectometer System

Traditional facilities monitoring methods are mainly based on piezoelectric detection technology,but this technology is difficult to monitor the safety of infrastructure in real time.The distributed optical fiber sensing technology uses optical fiber as the sensing medium,which can realize the real-time demodulation of vibration signal.It has the advantages of long detection distance,high precision,intrinsic safety,corrosion resistance and so on.It is widely used in transportation,petrochemical,perimeter security and other infrastructure areas.As a typical distributed optical fiber sensing technology,phase sensitive time domain reflectometer(?-OTDR)has high sensitivity to external vibration signals.It can obtain the vibration information of the whole optical fiber through a single measurement,and has the ability to distinguish the characteristics of vibration events.Therefore,it has unique advantages in vibration detection.Coherent optical time domain reflectometer(COTDR)introduces coherent detection technology based on ?-OTDR,which mainly processes beat frequency signal of intrinsic signal and backward Rayleigh scattering signal.It can sense position and phase information of external vibration signal,and has higher sensitivity and signal-to-noise ratio(SNR).In this paper,the signal demodulation method of COTDR system is studied,including quadrature demodulation technology,filter design and laser frequency drift suppression.Then,the host computer software is developed based on COTDR system,and an engineering prototype is built,and the amplitude and phase information of vibration signal are monitored in real time by using the prototype.The specific research work of this paper is as follows:(1)Based on the Rayleigh scattering theory,the optical time domain reflection mechanism is analyzed,the related principle of vibration information demodulation is described,and the parameters of the selected optical instruments are introduced.According to this,the optical path of the COTDR system is designed and built,and the key performance parameters of the system are analyzed.(2)The orthogonal demodulation method of COTDR system is deeply analyzed,the phase demodulation is studied in detail,and the influence of filter on COTDR is expounded.Finally,several groups of experiments on demodulating vibration signal are designed,and the experimental results are analyzed in detail.(3)The method of difference fitting based on COTDR system is proposed.The quantitative selection of phase difference region is discussed.The experiment shows that the difference fitting method can effectively suppress the influence of laser frequency drift.Finally,the low frequency signal sensing ability of COTDR system is detected to 0.02 Hz.(4)A coherent optical time domain reflectometer is designed and developed.This paper introduces the advantages of MFC development and the overall framework of COTDR upper computer software,analyzes the development process and implementation scheme of the main modules in the software,designs the graphical interface of the upper computer,and finally builds an engineering prototype.