Research On Performance Improvement Technology Of Phase Sensitive Optical Time Domain Reflectometer

Distributed fiber vibration sensing technology has the advantages of anti-electromagnetic interference,large monitoring range,intrinsic safety and high sensitivity,and it has shown an important application prospect in the fields of cable condition monitoring,structural health monitoring,traffic track monitoring and oil and gas pipeline monitoring.As a typical distributed optical fiber vibration sensing technology,phase sensitive optical time domain reflectometer(?-OTDR)uses the backscattered Rayleigh light to sense the external vibration signal,which is widely used in the field of long-distance multi-point vibration monitoring.The ?-OTDR system uses the pulse light as detection signal.The sensing distance of the system will be affected by the incident power of the pulse light,and the high power will lead to the nonlinear effect in the fiber.At the same time,the repetition rate of pulse light will restrict the frequency response range and sensing distance of the system.The above problems will limit the detection ability of the system in different scenarios.Therefore,expanding the frequency response range and sensing distance is of great significance to improve the detection performance of ?-OTDR system.Based on the study of ?-OTDR system,in this paper a composite optical fiber vibration sensing system is proposed by using wavelength division multiplexing technology,which can realize the spatial positioning and waveform restoration of external vibration signals,so as to enhance the frequency response range of the system.At the same time,based on the phase demodulation principle through quadrature demodulation algorithm,a phase autocorrelation algorithm in the whole sensing fiber is proposed.By distinguishing the autocorrelation curves of vibration signal and noise signal to achieve positioning and enhance the sensing distance of the system.The research content mainly includes the following three aspects:Firstly,the optical fiber vibration sensing principle of ?-OTDR system is studied.The factors affecting the performance of ?-OTDR system are analyzed,including frequency response and sensing distance.The mechanisms of wavelength division multiplexing technology and quadrature demodulation technology is discussed to improve the performance of ?-OTDR system.Secondly,based on wavelength division multiplexing technology,a composite optical fiber vibration sensing system is proposed.Three optical structures of ?-OTDR,Michelson interferometer(MI)and Mach-Zehnder interferometer(MZI)are combined by using the 3×3fiber coupler to realize the spatial positioning and phase reduction of external vibration signals.At the same time,the MZI structure can be used to detect the e ambient noise,and with the help of a digital filter,the ambient noise component in the external vibration information can be removed.The composite system can accurately recover and locate the external vibration signal of 20 k Hz in the ambient noise with the frequency of 1k Hz.Thirdly,based on the quadrature demodulation technology,the positioning principle and limitations of the differential accumulation average algorithm are analyzed,and the feasibility of phase restoration and autocorrelation positioning in ?-OTDR system is discussed.On the sensing optical fiber of 90 km,the sinusoidal vibration signal of 5Hz is detected by using autocorrelation algorithm without any additional optical amplifier.