Research On Equalization Technology In Few Mode Fiber Mode Division Multiplexing Systems

With the development of the information age,the single mode fiber(SMF)system capacity gradually reach the ceiling of nonlinear Shannon limit.Mode division multiplexing(MDM)of the few mode fiber(FMF)utilizes a small and stable orthogonal mode as an independent channel to transmit information,which improves the system's transmission capacity.At the same time,the polarization division multiplexing(PDM)technology also doubles the transmission capacity of the system without changing the structure of the fiber link.However,damages such as mode dispersion(MD),chromatic dispersion(CD),and polarization mode dispersion(PMD)in the FMF will result in serious crosstalk between the channels of the system,so the equalization algorithm in MDM demultiplexing needs to be studied.The main work of this article include:Firstly,introduce the concept of a FMF and analysis its transmission theory.Starting from the Maxwell wave equation are used to analysis the transmission characteristics of the modes in the optical fiber,and the orthogonality of the linear polarization mode is demonstrated.The damage occurred in the FMF was studied,and the influence of mode coupling and mode group delay on the signal of the FMF was analyzed.Next the theoretical model and key technologies of the PDM-MDM technology are discussed.Also analyzed the demultiplexing equalizer technology used in the MDM system.These theories is used to establish the polarization MDM simulation system.Based on the above theories,the VPI simulation platform was used to model 3LP and 6LP two systems with different number of modes.Secondly,to solve the problem of high complexity and slow convergence rate of least mean square(LMS)algorithm,two improved frequency-domain LMS algorithms are proposed for PDM-MDM system.The two improved frequency-domain LMS algorithms are verified by the demultiplexed constellation diagram.The symbol error rate(SER)of demultiplexed signals of the two improved frequency-domain LMS algorithms was observed under the conditions of high-order modulation,transmission distance,and dispersion parameter variation.The results show that the two improved frequency-domain LMS algorithm has the advantages in convergence speed and complexity in the similar SER performance.Finally,the recursive least squares algorithm in frequency domain is applied to polarization mode demultiplexing,the complexity and convergence speed of RLS algorithm and the LMS algorithm are compared by using the MATLAB simulation.Then,by following the same way that the frequency-domain LMS algorithm is improved,frequency-domain RLS algorithm is simulated and analyzed by PDM-MDM system,and its complexity is compared to frequency-domain LMS algorithm.The results show that the frequency-domain RLS algorithm has better SER performance than the frequency-domain LMS algorithm,which demonstrate the feasibility and development potential of the frequency-domain RLS algorithm.