Research On Differential Mode Group Delay Measurement Technology Of Few-mode Fiber

With the continuous development of network information technology,the Internet of Things,big data centers,5G and other technologies continue to emerge,data traffic continues to increase,and the demand has increased from 100 Gb/s to 100 Tb/s.The capacity of single-mode fiber-based optical communication network is approaching its Shannon's limit theorem,and a new breakthrough technology is urgently needed to break the current network capacity crisis.The emergence of mode-division multiplexing technology has solved the problem of capacity limit of single-mode fiber and become one of the hot solutions for communication system expansion.Mode-division multiplexing technology uses the few-mode fiber as the propagation medium,and uses the mutual orthogonality of the spatial modes to transmit information as independent channels that do not affect each other,which can exponentially increase the capacity of optical communication systems.There are some unique damages in the few-mode fiber itself,such as mode-dependent loss,mode coupling,and differential mode group delay.These damages make the modes no longer strictly orthogonal,which affects the transmission capacity of the mode division multiplexing system.Reduced transmission distance.Among them,the differential mode group delay is caused by the different transmission speeds of different spatial modes in the few-mode fiber,which will lead to delay expansion and greatly increase the complexity of the multi-input multi-output digital processing equalization algorithm in the mode division multiplexing system..Measuring the differential mode group delay of few-mode fibers can provide a theoretical basis for signal equalization and improve the equalization performance.Therefore,measuring the differential mode group delay of few-mode fibers is a very meaningful work.This paper studies the differential mode group delay measurement of few-mode fibers from both theoretical and experimental aspects.The main work of the paper is as follows:First,the characteristics of the transmitted few-mode fiber has been analyzed.The mode orthogonality of few-mode fibers and the mode field distribution of each mode are theoretically analyzed;the causes of Fresnel reflection and back Rayleigh scattering and their effects on optical signals in fiber links are analyzed.The generation mechanism and measurement principle of differential mode group delay of few-mode fiber are studied.Secondly,the differential mode group delay measurement method based on time-of-flight is studied.Aiming at the problem that the differential mode group delay of different spatial modes needs to be measured in the few-mode fiber,the principle of differential mode group delay measurement based on time-of-flight is studied,and a time-of-flight-based differential mode group delay measurement system is built.The influence of its own introduction and the setting of pulse parameters are analyzed;the differential mode group delay of 9.7 km three-mode fiber and 5 km six-mode fiber is experimentally tested;it is in good agreement with the calibration parameters of fiber manufacturers.Thirdly,a differential mode group delay measurement technique of few-mode fiber based on correlation coding is proposed.Aiming at the problem that the accuracy of the current differential mode group delay single-ended measurement is not ideal,the relevant coding technology is used to eliminate the noise introduced by the few-mode fiber during the transmission process,increase the received signal energy,and improve the accuracy of the measurement results.A differential mode group delay measurement system of few-mode fiber based on correlation coding is built.Using this test system,the differential mode group delay of 9.7 km three-mode fiber and 5km six-mode fiber is experimentally measured;Compared with the time-of-flight method,it has a good consistency and achieves the same measurement accuracy as the time-of-flight method;it has higher accuracy than the back Rayleigh scattering method that can perform single-ended measurements;Single-ended measurement of differential mode group delay in few-mode fibers.Lastly,the essential summation of the complete text is presented,the differential mode group delay measurement method based on time-of-flight is studied,and a differential mode group delay measurement technique based on correlation coding is proposed,which solves the problem of differential mode group delay.Single-ended measurement accuracy is not ideal.In the future,the integration of the experimental measurement system,a further improvement of the technique of differential mode group delay measurement,and the realization of simultaneous measurement of various fault damages in few-mode fibers are prospected.