Design Of Dual Mach-zehnder Distributed Optical Fiber Sensing System Based On 3*3 Coupler

The distributed optical fiber sensing technology which integrates disturbance sensing and signal transmission into one has great potential for application in areas such as the monitoring of water pipelines,regional boundary security and real-time detection of bridge and dam with increasing safety requirements.Compared to scattering-type or hybrid-type optical fiber sensing systems,Mach-Zehnder distributed optical fiber sensing system basing on interferometry has high sensitivity,good real-time performance,zero-sensing dead zone,simple structure,low cost and therefore good engineering application potential.At present,the main factors restricting the application of long-range Mach-Zehnder distributed optical fiber sensing system are as follows.The system is vulnerable to environmental noise caused false alert or missed alert;the traditional positioning scheme has a high demand on the stability of the light source;the polarized degradation of transmitted light caused by birefringence leads to positioning errors.A dual Mach-Zehnder distributed fiber-optic sensing system based on 3*3 coupler is designed in this paper.It combines optical structure and signal processing algorithm and can effectively solve the problems above.The main works are as follows: Based on the analysis of the advantages and disadvantages of several distributed optical fiber sensing systems,the working mechanism of the interferometric fiber-optic sensing system is described;the principle of the cross-correlation delay positioning algorithm based on the dual Mach-Zehnder structure is analyzed;the demodulation algorithm based on 3*3 coupler is used to solve the effects of polarization degradation;the adaptive noise reducer based on LMS algorithm is used to reduces the time-varying ambient noise;system structure is designed and parameters of each device are selected reasonably to the instrumentation of the system;the experiments are designed to test the demodulation,noise reduction and positioning performance of the system.Experimental results demonstrate that the system has high sensitivity and quick response,can eliminate the effects of polarization degradation and can suppress ambient noise significantly.The designed system whose average positioning error is about 15.12 meters has reference value in engineering application.