| With the rapid development of the communication industry to the 5G,VR?cloud mobile phone?live-streaming?cloud storage and new data services are emerging in an endless stream,the demand of the network bandwidth is becoming greater than ever before.At present,the technologies in optical network to increase the capacity mainly include wavelength division multiplexing,polarization multiplexing,code division multiplexing,orthogonal frequency division multiplexing and high-dimensional modulation,etc.However the technologies are all based on single mode fiber?SMF?transmission medium.And the nonliner effects in SMF,the transmission capacity is reaching its limits.Under the background,space division multiplexing?SDM?based on spatial dimension has become a research hotspot,especially the mode division multiplexing?MDM?based on few-mode fiber?FMF?.The MDM technology is applying the several independent orthogonal modes or mode groups in FMF as transmission channels,and it's considered as the most feasible optical expansion technology.In this paper,we have researched the MDM technology deeply.We build a mode division multiplexing communication experimental system with the mode MUX/DEMUX based on mode selective photonic lanterns?PLs?.The communication performance of MDM system is studied experimentally in two communication modes:intensity modulation&direct detection?IM-DD?and phase modulation&coherent detection.The main work of this paper is as follows:Firstly,the basic theory of mode transmission is introduced and analyzed.Start with the wave equations,the theory of linear polarization mode and mode orthogonal transmission is derived.And then,we analyze the damage factors during mode transmission,especially the mode cross-talk which caused by mode coupling.Secondly,the structure,working principle and performance characteristics of a variety of mode multiplexers/demultiplexers,especially the photonic lantern is analyzed.By experiments,the performance of photonic lanterns is tested,we get the modal diagram of each mode.In addition,the insertion loss and the mode cross-talk in dB of the PLs are given.Then,by using the photonic lantern as the mode multiplexer,we build the MDM communication system with IM-DD.At a transmission rate of 5Gb/s,the transmission experiments of 3×3 and 6×6 MDM systems with different transmission distances are demonstrated.The waveforms,eye-diagram and BER have been measured.The result of experiment indicates when signal received power is higher than-19.4dBm?-18.5dBm?-18.8dBm?-18.2dBm?-17.9dBm?-16.8dBm,the BER of six channel all can attain the order of 10-3.Following,by using the IQ modulators and coherent receiver,we build the MDM system with QPSK modulation format.The signal waveforms of the 3×3 and 6×6 MDM systems at 5Gb/s and 10Gb/s transmission rates are measured respectively.By off-line processing,the constellation and bit error rate curves of each channel are obtained.On the basis,we analyze the communication performance of the MDM system.For example,when each channel received power is higher than-15.5dBm,the MDM system can realize 6×10Gb/s QPSK signal transmission over 10km few-mode fiber.Finally,we have summarized the main work of this paper.We analyze the feasibility of using the mode selective photonic lantern as a mode multiplexer/demultiplexer,and then evaluate the performance of MDM system based on photonic lantern.At last,we look forward to the performance optimization and achieve higher rates of MDM communication system. |
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