Design And Implementation Of Distributed Fiber Vibration Sensing System Based On Analog Heterodyne Demodulation

Distributed optical fiber vibration sensing system,which is based on the ?-OTDR principle,is used to detect the vibration signal along the optical fiber by detecting the Rayleigh backscattering light modulated by the vibration in the optical fiber.In an ?-OTDR system,the optical fiber can be used as transmission medium and sensing medium at the same time,so it can be used to realize long-range distributed vibration sensing.There're many incomparable advantages and wide application in the field of long-range rail traffic safety monitoring and long-range pipeline safety monitoring.At present,the research of the ?-OTDR system is mainly focused on the research of the digital signal processing algorithm for the phase demodulation of the back scattered light.But in this thesis,a method of demodulating the phase of the backscattering light,which is based on the analog heterodyne phase detection circuit,is applied to the ?-OTDR system.The main content and innovation of the thesis focus on the following points:First of all,according to the ?-OTDR principle,a hardware platform of ?-OTDR system with heterodyne demodulation is set up.During the process of building hardware platform,some critical system parameters are designed and optimized to meet the requirement of the expected performance,such as data acquisition ratio and length,optical pulse modulation,coupler splitting ratio and etc.Then the thesis analyzed and verified the influence of the optical power and the linewidth of laser on the performance of the ?-OTDR system.By carrying out an observational study on the optical power of injected laser pulse,the optimal detection pulse power is found.After the power optimization,the signal to noise ratio(SNR)of the system is enhanced.The dual AD8302 reference phase detection circuit is designed on the basis of phase discrimination function of the AD8302 analog phase detector.With this schematic design,the dual AD8302 reference phase detection PCB module is designed and manufactured.After the testing of the module,it proved the dual AD8302 reference phase detection module has the ability to detect the phase error of two sinusoidal signals from 0 degree to 360 degrees,while the single AD8302 can only detected the phase error of two sinusoidal signals from 0 degree to 180 degrees.What's more,the phase demodulation program can also correct the nonlinearity error with reference phase detection principle,which greatly reduce the AD8302 phase detection error,maximal phase detection error phase error decreased from about 7 degrees to 1.1795 degrees when it works at 80 MHz.Then,the dual AD8302 reference phase detection circuit module was added to the ?-OTDR system for heterodyne demodulation experiment,and it successfully detected the vibration signal at 5.09 km position.When the analog phase demodulation results are compared with the results of digital heterodyne demodulation scheme,the results show that the demodulation accuracy of the analogue heterodyne demodulation is a little lower than digital heterodyne demodulation.The main reason is the mixed signal of the heterodyne type ?-OTDR system has some sideband,and the amplitude of the signal has some fluctuations caused by polarization fading and other factors.These two main factors make the dual AD8302 reference phase detection circuit has some inaccuracy.However,it should be noticed that the mixed signal's frequency conversion is achieved by the analog phase detection circuit,which makes the data sampling rate reduced from 1GSa / s(digital heterodyne demodulation)to 5MSa / s(analog heterodyne demodulation),the costs of ?-OTDR system with analog heterodyne demodulation scheme has been significantly reduced.Finally,in order to improve the accuracy of the analog phase demodulation,the AOM feedback control and the analog square wave phase discrimination technique are introduced into the?-OTDR system.