Research On Mode Coupling And Equalization Technology Of Mode Division Multiplexing System

Into the 21 st century,due to the rapid development of the Internet,cloud computing and the rise of large data and other technologies,the demand for information data is explosive growth.The traditional single-mode fiber transmission capacity by utilizing time and frequency,and modulation coding degrees of freedom has been close to the limit.In optical communication system under the precondition of existing infrastructure,how to reduce cost,improve the transmission capacity of optical fiber communication system as far as possible has become a key research topic.In this context,as a kind of capacity expanding method with great potential for application,mode division multiplexing(MDM)technology has attracted the attention of researchers both inland and abroad.Utilizing the spacial modes as a new degree of freedom,combined with few-mode fiber(FMF)suitable for long distance signal transmission,MDM technology can multiply the capacity of long-haul communication systems.However,linear impairments such as differential mode group delay(DMGD)and mode coupling(MC)severely impact the transmission performance.The channel condition in MDM system is far more complex,which makes mode demultiplexing one of the most critical problems in MDM technology.Therefore,the main objective of this paper is to develop an effective digital signal processing algorithm with low computational complexity to implement mode demultiplexing through theoretical analysis on FMF channel characteristics.The details of this paper are as follows:First,FMF and the transmission characteristics in the FMF are introduced.The main sources of crosstalk in FMF are MC(strong and weak)and DMGD.The mathematical model of the multi-mode fiber link is established on the basis of the above theory.Second,the time-domain equalization technique of demultiplexing is studied.The basic concept of adaptive channel estimation and least mean square algorithm(LMS)are introduced.Based on the LMS algorithm,the principles of single channel constant modulus algorithm(CMA)and multimodulus blind equalization algorithm(MMA)are deduced.Then,the single channel adaptive equalization algorithm is extended to obtain the structural model and calculation formula of MIMO equalization algorithm.Third,the frequency-domain equalization technique of demultiplexing is studied.frequency-domain MMA(FD-MMA)maintains all the advantages over the time-domain MMA(TD-MMA),meanwhile,the using of block processing and fast Fourier transform(FFT)makes computational complexity lower enough to satisfy the requirements of the MDM system.Finally,based on the theoretical analysis of MDM system link,a 3×3 few-mode fiber simulation system including LP01,LP11a and LP11b is established.Coherent detection is performed on the receiver side,and the demodulated signal is processed by the DSP module.FD-MMA and TD-MMA equalization algorithm is used demultiplexing.The performances of the algorithms are evaluated and compared by constellation diagram,bit error rate(BER)and computational complexity.As for QPSK format,the average OSNR by FD-MMA equalization algorithm is improved 2dB at a BER of 10-3.