Research On The Bending Characteristics Of Two-core Fiber And Its Application In Subgrade Settlement

In recent years,as the living standard of people has been improved and the pace of life has been accelerating,people tend to choose efficient and convenient ways when they will travel,such as high-speed rail.The railway roadbed is the important guarantee that affects the safety and fast driving of high-speed rails,and their safety issues have become more and more important.In order that we can be able to monitoring the subgrade condition in real-time and effective way,many experts and scholars at home and abroad have made great efforts for this purpose.Among them,the use of optical sensors to measure subgrade settlement has become a research hotspot in recent years.Due to its special structure,the two-core fiber has the advantages of large transmission capacity,strong sensitivity to temperature and bending,and the advantage of being able to work in a relatively harsh environment.Therefore,this paper proposes the use of force bending characteristics of two-core fiber to monitor the subgrade condition.The main research work and results are as follows:1.The paper introduces the monitoring methods of subgrade settlement problems,the working principle of optical fiber sensing systems,and the research status of two-core optical fibers around the world,it also analyzed the advantages and disadvantages of various subgrade subsidence monitoring methods.2.Starting from Maxwell's equations,the generalized coupled-mode equations are derived and applied to the derivation of the coupled-mode equations of the two-core fiber.The coupled-mode equations of the two-core fiber are obtained and the solutions to the equations are obtained.Then using Matlab simulation software to analyze the coupling characteristics of the two-core fiber,the conclusion is:in the ideal coupling,the energy between the two cores can be completely converted to each other and have the same mutual coupling coefficient;in non-ideal coupling,?2/K12K21 can be used to reflect the coupling efficiency,and the coupling efficiency is affected by the core distance,core radius,core refractive index,and cladding refractive index.3.Based on the bending theory of two-core fiber,the bending characteristics of two-core fiber when it is bent by force are analyzed.The characteristics of.two-core fiber bending are influenced by many performance parameters,mainly including effective refractive index,bending radius and placement angle,and through COMSOL Multiphysics simulation software we can simulate and analyze these parameters.In this paper,we analyze the effects of changes in bending radius,wavelength,and placement angle on the bending loss when the core radius is 4?m,5?m,and 6?m,and we also analyze the effect of bending radius,wavelength and placement angle on bending loss when the distance between two cores of fiber is 10?m,14?m,18?m.Finally,we analyze the effect of bending radius,wavelength and placement angle on bending loss.From the above analysis,the following conclusions can be drawn:The bending loss of a two-core fiber is affected by the core radius,the core distance,and the refractive index difference between the core and the cladding.The larger the core distance,the smaller the refractive index difference between the core and the cladding,and the smaller the core radius,the greater the change in bending loss.In addition,when the structure parameters such as fiber core radius and fiber core distance are certain,the two core fibers with smaller bending radius and longer wavelength has larger bending loss,Based on the two-core optical fiber bending theory,in the paper we analysis the features of two core optical fiber to verify the two-core fiber can be applied to the embankment settlement measurement,and according to the mathematical models of embankment settlement,we can calculate the height of subgrade subsidence.4.Based on the analysis of the bending characteristics of the two-core fiber,we analyze the relationship between the bending loss of each pair of two-core fibers and the settlement of the roadbed,compare the sensitivity of each group of fibers,and verify that they can be applied to the measurement of subgrade settlement.