| Mode group diversity multiplexing (MGDM) uses the idea of Multiple InputMultiple Output (MIMO) in wireless telecommunication system for reference, makingfull use of several groups of modes in the multimode fiber to create independent,parallel communication channels, which can transmit signals of different types ofbusiness respectively and improve the capacity of multi-mode fiber significantly. Inthe MGDM system, each output signal of the system is the weighted sum of all sourcesignals because of the effect of mode coupling between the mode groups, thus thecrosstalk between the channels appears; Besides, several groups of modes carry onesignal together, and each output signal suffers pulse spreading due to the modedispersion resulting in inter-symbol interference (ISI). When the signal rate is low,inter-channel crosstalk is the main problem affect the system performance, easyMIMO algorithm can be proposed to overcome the problem, and successfully realizedthe separation of low rate baseband signal. But when the signal rate is high, modedispersion will influence result the de-multiplexing badly, limit the transmissionperformance of MGDM system. How to eliminate the mode dispersion and recoverthe source signals under the condition of mode dispersion is the chief problem to beresolved, which is also the main task in this thesis.This paper has done a lot of work on the further development of this technologyon the basis of previous research. a system model of MGDM is built, and a2×2MGDM simulation and experimental system based on1km-long silica GI-MMF isdesigned and realized, which achieve communications successfully, especially go intothe method how to eliminate the mode dispersion and improve the transmissionperformance of MGDM system. Based on the principles and theories of MGDMtechnology, analysis the key factors of mode dispersion in system, we propose amethod that by choosing the optimal incident condition of the signal light to weakenthe mode dispersion, and improve the transmission performance of MGDM system.The works in this paper are as follows:Firstly, go into the key technology of the MGDM system. The expression of theelectric and magnetic field plane wave and optical field distribution of linearpolarization mode is derived from Maxwell’s equations, and the theory of modegroups is introduced; a mathematical model of the IM-DD M×N MGDM system isdeveloped, and the relationship between N input signals and M output signals is described with matrix, while discussing the MGDM system capacity, analyzing thenoise in the system; At the same time, the principle of selective mode excitation isstudied in-depth; under different incident condition of the signal light, the optical fielddistribution at the exit surface of multimode fiber is investigated experimentally; theFastICA algorithm based on negative entropy is introduced and used as thede-multiplexing method, also its feasibility is analyzed.Secondly, the basic idea and principle of system optimization are introduced. Thereasons and determinants of the mode dispersion which affect the transmissionperformance of system are analyzed, also find out the solution. We investigate therelationship about the welding condition between SMF and MMF with the numbers ofthe excited mode group, choose the optimal size of Gaussian optical spot and theoptimal core-offset distance by simulating calculation and comparing the eye diagramopening of the received signal, and weaken the mode dispersion as much as possible,which can improve the performance of system greatly.Thirdly, the2×2IM-DD MGDM simulation system is built, In the simulationsystem, the transmission experiment of different rates of digital signal is carried out.then under the condition of off-line, de-multiplexing of the real data which aregathered from the MGDM experimental system are realized with the improvedFastICA algorithm, and get the system bit error.discuss the influence factors of thesystem performance, including the incident condition of the signal light, nonlinearfunction of the FastICA algorithm, center signal to offset signal power rate, Theresults show that after the optimization for the incident conditions of the signal lightincluding the choice for the optimum core-offset distance and the optimum size ofGaussian optical spot, the simulation realized that two10Gb/s signals are transmittedover550m of multi-mode fiber, and without any electronic dispersion compensation,which can meet the requirements of the short distance access network.Finally, the2×2IM-DD MGDM experimental system is built. In the realexperimental system, we have successful completed the transmission of two digitalsignals over1km-long silica GI-MMF, including rate155Mb/s and622Mb/s,622Mb/sand1.25Gb/s,1.25Gb/s and1.25Gb/s, the eye diagram opening is improved bychoose the suitable offset, detector location, and the center signal light to offset signallight power rate, then under the condition of off-line, de-multiplexing of the real datawhich are gathered from the MGDM experimental system are realized with theimproved FastICA algorithm, successful restore the original signal, by comparingexperimental results of the optimization system and without optimization, show theeffectiveness of the proposed method.Applying the method of choosing the optimal the incident condition of the signal light to weaken the mode dispersion, instead of electronic equalization at thereceiving end, and adjusting the power ratio between center signals and offset signalto extend the transmission distance effectively, which greatly improve thetransmission performance of the system, These studies have an important significancein promoting the practical course of MGDM technology. |
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