Research Of Distributed Optical Fiber Vibration Sensor System Based On ?-OTDR And Mach-zenhder

The normal operation of China's large infrastructure(such as Bridges,rail transit,underground transmission pipelines,etc.)is an important guarantee for China's economic development,so the health monitoring of the above large infrastructure is the top priority.At present,the most advanced monitoring method is mainly distributed optical fiber sensing technology,which determines the fault location by detecting vibration signals.The interferometric system can measure the high frequency vibration,but it is difficult to accurately locate;Backscatter systems can accurately locate the vibration points,but can only measure the lower frequencies.The current operating environment of infrastructure structure is relatively complex.When a failure occurs,different frequencies of vibration will be generated,with the highest frequency up to MHz.Therefore,it is impossible to identify fault signals accurately and comprehensively by only using one of the sensing technologies.For this question,we design a system by using Phase-sensitive optical time domain reflectometry and Mach-Zenhder interferometer.?-OTDR is used to locate fault points,and MZI is used to obtain fault wide spectrum frequency domain information,realizing the distributed optical fiber monitoring system has the ability of accurate positioning and broadband measurement.The main work of this paper is as follows:1.Studied the basic sensing theory and signal processing method of ?-OTDR system and MZI system deeply,and the factors affecting system performance index are analyzed.Independently designed the physical structure based on the fusion of ?-OTDR and MZI.The preliminary performance test of the composite system shows that the composite system can achieve vibration positioning and broadband response,its sensing range can reach 20 km,the optimal spatial resolution is 15 m,and the frequency response range is 50Hz? 3.5mhz.But the dynamic range and spatial resolution of the system are not ideal.2.In order to improve the dynamic range of ?-OTDR subsystem,this paper further studies the generation principle and decoding method of pulse coding technology.Considering the difficulty of coding generation,decoding time and gain effect,we finally decided to apply Simplex coding in the ?-OTDR subsystem.Through the Simplex coding experiments of three orders,seven orders and fifteen orders,it is proved that Simplex coding pulse light can improve the dynamic range without increasing the pulse width.And the higher the order of Simplex coding,the higher the dynamic range.3.The composite system is designed integrally and the engineering prototype is built.Visual C++6.0 was used to design monitoring software with functions of real-time monitoring and alarm positioning.The sensing distance and spatial resolution of the seven orders Simplex coding engineering prototype were measured.The results show that based on the seven orders Simplex coding ?-OTDR machzede optical fiber vibration sensing system,which can realize vibration monitoring with long sensing distance,high spatial resolution and wide frequency response.Its sensing range can reach 41 km,the optimal spatial resolution is 10 m.Finally,the prototype was sent to the water pipeline for field test.The test results show that the prototype has the ability of broadband measurement and accurate fault location.