Fast Location Method Of ?-OTDR System Event Based On Phase Shift Compensation

Phase-sensitive Optical Time Domain Reflectometer(?-OTDR)is a new type of fully distributed optical fiber sensing technology,which can withstand high pressure and has a wide detection range.However,due to the special optical fiber It is also resistant to corrosion and extremely sensitive.It has naturally become an important technology for research and development in various fields such as communications.Compared with other existing optical fiber sensing technologies,the main advantage of ?-OTDR is that it can detect weaker external vibration information.More importantly,we can demodulate the amplitude and phase information of the optical signal.Restore and even restore external vibration events.At present,in the ?-OTDR system,we generally use amplitude detection to analyze vibration events.This method is difficult to quantitatively analyze the events.In addition,some parameters of the ?-OTDR system,such as dynamic range and sensitivity,are limited by the line width,pulse,and frequency stability of the laser.Therefore,the laser plays a very important role in the ?-OTDR system.The frequency of the narrow linewidth laser has a great drift,which will distort the obtained curve,which will not only reduce the system signal-to-noise ratio,but also reduce the system's ability to sense weak,low-frequency events.The main principle of ?-OTDR is to locate by sensing the back-to-Rayleigh scattered light in the fiber,using a laser to emit light pulses into the fiber,and combining the return time of the back-to-Rayleigh scattered light in the fiber with the speed of light To determine the spatial position,according to the amplitude and phase of the scattered light back to Rayleigh,the disturbance information in the external environment can be obtained and restored.However,?-OTDR is a distributed sensor.The larger the detection range,the larger the amount of signal data that needs to be processed.This causes difficulties in real-time signal processing and data storage.In addition,in coherent detection ?-OTDR However,due to the existence of the signal carrier,higher requirements are put forward on the sampling rate of the data acquisition card,which doubles the amount of signal data and the difficulty of computer processing.These factors limit the application scope of the ?-OTDR optical fiber sensing system in engineering.Aiming at the above-mentioned shortcomings of ?-OTDR,this thesis proposes a fast locating method of ?-OTDR system events based on phase shift compensation,which can demodulate the phase change of Rayleigh backscattered light caused by interference in the fiber environment and restore the interference The amplitude and phase information of the signal can quantitatively detect the strength of the target signal.Through frequency drift compensation and improved data processing algorithm,the long-distance ?-OTDR system can be applied to engineering.The specific work content of this thesis is introduced as follows:(1)This thesis introduces the theoretical basis of optical fiber sensing technology,as well as the application of optical fiber scattering in sensing,communication and other fields.Based on the theoretical model of Rayleigh scattered light,it analyzes the amplitude and phase demodulation of ?-OTDR According to the basis,several commonly used ?-OTDR structures and corresponding demodulation algorithms are described,and their respective advantages and disadvantages are pointed out,and the factors affecting the performance of the?-OTDR system are briefly analyzed.(2)This thesis analyzes the principle of ?-OTDR quadrature demodulation and the method of phase extraction,studies and discusses the influence of quadrature demodulation on the spatial resolution of the ?-OTDR system,and explains in detail the phase of quadrature demodulation based on experiments.As a result,the phase unwrapping process was carried out,the deviation between the unwrapping result and the real disturbance information was analyzed,and the necessity of the second phase unwrapping was verified.(3)A phase compensation algorithm for ?-OTDR frequency drift is proposed to compensate the phase change caused by laser frequency drift,weaken or even eliminate the influence of laser frequency drift on the sensing performance of ?-OTDR system,and improve ?-OTDR restores the accuracy of vibration events.This algorithm has a small amount of calculation and a lighter load on the computer,and can compensate for laser frequency drift in real time.(4)A fast positioning method for ?-OTDR system events is proposed,which solves the problem of large amount of calculation for phase demodulation data of ?-OTDR optical fiber sensing system through the method of gradual positioning from large to small.