Research On Fault Detection Technology Of Few-mode Fiber Based On Higher-order Mode

With the development of Internet plus,5G technologies,and the rapid rise of new generation of information technologies represented by artificial intelligence,big data,and cloud computing,the demand for network bandwidth has increased dramatically.The capacity of communication network using single-mode fiber?SMF?as transmission medium has approached its nonlinear Shannon limit,it has been unable to meet the increasing capacity demand of people.To break through the capacity bottleneck of the traditional communication network,the mode division multiplexing?MDM?system based on the few-mode fiber?FMF?has become the most promising and competitive expansion scheme with the advantage of doubling the transmission capacity,which has been widely concerned by researchers.Thus,the MDM optical network based on FMF faces a new challenge in monitoring the transmission performance.At present,fiber fault detection technology is only for SMF,however,FMF has multiple transmission modes,and each mode has unique transmission characteristics and interaction with each other,which makes the FMF and SMF have certain differences in fault detection.To ensure the stable and reliable operation of the MDM optical network based on FMF,studying the fault location technology of the FMF link is a very important work.It is of great significance to realize the simple,accurate and fast fault monitoring and maintenance of the transmission performance of the system.This paper focuses on the fault detection of each spatial mode of FMF link.On the basis of theoretical analysis of the Rayleigh backscattering model and fault loss characteristics of FMF,the fusion splice loss characteristics of FMF and the change of detection sensitivity of each spatial mode are measured experimentally,the feasibility of the method for fault detection and location of FMF is studied in depth.The specific research work in this paper is as follows:Firstly,the basic mode theory of FMF is derived from Maxwell equations,the orthogonality of spatial modes is proved by Lorentz's reciprocity theorem.Based on the above,the measurement principle of Rayleigh backscattering of FMF is described in detail.The reflective and non-reflective fault events of the FMF are briefly introduced.Secondly,the theoretical simulation analysis of the coupling loss of the fundamental mode and the higher-order mode at the fusion splice point is performed.According to the Rayleigh backscattering theoretical model,an experimental system for fault detection and location of FMF is built.Using the fundamental mode as the excitation mode,the splice loss characteristic of each spatial mode of the 6-mode step-index fiber and graded-index fiber are measured,and the detection sensitivity of each space mode is evaluated.The experimental results show that it is not feasible to use the fundamental mode fault loss characteristics only as the detection criterion to judge the fault events of the FMF,while the high-order modes with high fault detection sensitivity are easier to realize the characterization of the fault loss events of FMF.Next,in order to further explore the change of detection sensitivity when different spatial modes are excited,we use high-order mode as excitation mode to study the fault detection sensitivity of each spatial mode of FMF.The results show that when the higher-order mode is excited,the detection sensitivity of the LP₀₁,LP_11a,LP_11b,LP_21a and LP_21b modes increases slightly,while the detection sensitivity of the LP₀₂ mode decreases slightly,but the higher-order mode still has higher detection sensitivity.Among them,in consideration of the influence of the detection sensitivity caused by the low coupling power of the non-excitation mode,the problem of the sensitivity of the excitation mode for fault detection is further discussed and analyzed.It shows that the fault loss amplitude of excitation mode increases with the increase of mode order,the detection sensitivity of high-order mode is significantly higher than that of fundamental mode.Then,aiming at the problem of low sensitivity of Rayleigh backscattering of fundamental mode to fault detection,a method of fault detection and accurate location for FMF based on Rayleigh backscattering of high-order modes is proposed and experimentally validated.Fault detection is performed for 9.2 km six-mode fiber with multiple different quality fault points,the detection results of each mode are compared and analyzed.The results show that all fault points can be detected successfully by the high-order mode,the loss events of different fault amplitudes can be represented accurately.Among them,the LP₀₂ mode successfully detects all fault points,which are located at 2.002 km,4.004 km,4.192 km and 8.196 km of the fiber,respectively,while the fundamental mode can only detect one fault point,which is located at4.201km of the fiber.Finally,in order to further reduce the influence of inherent noise on the fault detection accuracy of the Rayleigh backscattered signals,an optimization scheme using Kalman filtering to improve the fault detection accuracy is proposed.The filtering process of the Rayleigh backscattering signals of each mode is implemented effectively,the fault point that have not been detected before is detected successfully.This method has good denoising effect,improves the detection accuracy to a certain extent,reduces the difficulty of fault identification.It provides a powerful guarantee for the accurate and fast localization of the FMF fault detection method based on Rayleigh backscattering light of high-order mode.