Research On Linear Birefringence Quantitative Strategy For Metro Stray Current Fiber Sensing System

The metro is an important part of urban public transport system.However,the stray current leakage can corrode the buried metal pipeline seriously in the process of subway operation.Therefore,the stray current corrosion monitoring must be taken into consideration.At present,the metro system adopts stray current corrosion monitoring method based on reference electrode.However,the long-term measurement accuracy of this method is not high.Base on that,in this paper,stray current fiber sensing system is proposed to monitor the discharge current.And linear birefringence quantitative strategy of the sensing optical fiber is researched,which provides basis for the suppression and compensation of linear birefringence error and ensures the accuracy of stray current fiber sensing system in the practical application.The following studies are contented:Firstly,the generation cause of stray current is analyzed.And then,the stray current mathematical model for for“Rail-Buried metallic pipeline-Ground”structure is established by using the resistive network model.On this basis,the dynamic characteristic of stray current is simulated.Then,problems of stray current corrosion monitoring method based on reference electrode is analyzed.In response to these problems,stray current corrosion monitoring method based on fiber sensing is presented.And the feasibility of this monitoring method is proved from types of measurable current,measurement accuracy,measurement range and other several aspects.Secondly,stray current fiber sensing system based on polarization modulation is proposed.The mathematical model of this sensing system is established by Jones vector method.On this basis,the influences of error sources on the system output are simulated.And these error sources include the degree of polarization for SLD light source,the extinction ratio of the polarizer,the axis deviation angle of optical element,the modulation deviation angle of polarization controller and the linear birefringence of sensing optical fiber.According to simulation results,the control methods for above errors are proposed.Then,the experimental platform for stray current fiber sensing system is constructed,and its main performance are tested.The test results are shown as follow:the accuracy is not more than 0.4%;the sensitivity is 7.26×10^-5rad/A;the linearity is 0.33%;the repetitive error is 0.86%.Next,according to the classification of linear birefringence,the theoretical calculation method of linear birefringence with different types are studied.Then,linear birefringence measurement system based on polarizer rotation is proposed.The mathematical model of this measurement system is established by Jones vector method.On this basis,the influence of fiber optic axis on the measurement result is eliminated by rotating double polarizers.At the same time,the equations are constructed by using different output equations corresponding to different currents,and then a global optimization algorithm based on particle swarm optimization algorithm and multi-starting point optimization algorithm is proposed to solve these equations.To verify the feasibility of this linear birefringence measurement system,the advantages and disadvantages of four internationally recognized linear birefringence measurement methods are compared and analyzed.From these methods,Poincare Sphere method is selected.Finally,the experimental platforms for linear birefringence measurement systems based on polarizer rotation and Poincare Sphere are constructed separately.The linear birefringence of four different fiber rings is measured by using above two measurement systems separately.And then,measurement results of two measurement systems are compared,which to to verify the feasibility of linear birefringence measurement system based on polarizer rotation.