Research On Demultiplexing Technology For Mode Division Multiplexing System With MDL

In recent years,with the increasing demand for network bandwidth,the problem of "bandwidth depletion" has become increasingly prominent.One of the most important medium of transmission network,single-mode optical fiber,closes to the‘Shannon limit' as a result of its inherent nonlinear effects and the spontaneous emission noise of amplifiers.In order to solve this problem,the technology of the mode division multiplexing(MDM)based on the few mode fiber emerges.The MDM technology uses the orthogonality of each mode in the FMF to transmit information independently,thus expanding the transmission capacity of the communication system linearly.However,its specific damage,such as mode coupling(MC),differential mode group delay(DMGD)and mode dependent loss(MDL),will cause serious crosstalk and inter-symbol interference.Most of the previous studies only consider the effects of mode coupling and differential mode group delay.But in real MDM systems,especially for long distance transmission systems,mode dependent loss is inevitable.Unlike other damage,MDL will destroy the orthogonality of transmission matrix and limit the capacity of transmission system.Therefore,we need to study the demultiplexing technology to compensate MDL.This paper is mainly based on the study of the demultiplexing technology of the long range MDM system over FMF under the condition of MDL.We analyse the mode orthogonality and the specific damage of MDM system,and we focus on the the affect mechanism of MDL.On the basis of this,the matrix transmission of long distance MDM system is modeled.We explore the performance of demultiplexing algorithms and the tolerance for MDL on the condition of existing MDL.The MDL-compensation technology based on optical devices and equalization algorithms is proposed.The work of this article is as follows:Firstly,the theory of optical waveguide is used to deduce the mode orthogonality of FMF,which lays a theoretical foundation for the realization of the mode division multiplexing.We focus on the characteristic of the specific impairments of the MDM system,such as MC,DMGD and MDL.Secondly,a transmission model of FMF link is established based on the matrix-transmission-model and the influence of MC,DMGD and MDL.And a 6×6long distance MDM simulation system is built based on the above model on VPI Transmission Maker 8.7 combined with MATLAB.Thirdly,the specific impairment of long distance MDM system,MDL,is analyzed.And the relationship between MDL and bit error ratio(BER)is explored.Then the influence of MDL is simulated by 6×6 long distance MDM simulation system.Fourthly,the demultiplexing algorithm is studied under the condition of existing MDL.The FD-LMS algorithm,FD-ICA algorithm and FD-IVA algorithm are used for demultiplexing in MDM system under the condition of MDL.The MDL tolerance of different algorithms are acquired by comparing the BER performance of two cases(exiting MDL and not)at different values of coupling coefficient.The results show that FD-IVA algorithm can enhance the tolerance of MDL,and when the MDL is low,the multiplexing performance is good.Finally,a MDL-compensation technique based on the combination of optical devices and equalization algorithm is proposed.MDL could be reduced by optical devices,and the redundant MDL is compensated by the equalization algorithm.We combine the mode scrambler and the gain equalization amplifier with the FD-LMS and FD-IVA algorithms respectively,and the demultiplexing performance of two methods are simulated.Compared with the application of equalization algorithm,the simulation results show that the demultiplexing performance of the proposed method is better,which makes it more suitable for long distance MDM system.