| With the rapid development of emerging technologies such as artificial intelligence and high-definition video,global data traffic has shown an exponential growth trend.In addition,traditional technologies for increasing the capacity of single-mode optical fibers such as time division multiplexing and coding have reached the limit.Space-division multiplexing technology can exponentially increase the transmission capacity of optical fiber communication systems by the new dimension of space,mainly including two types of multi-core fiber and mode division multiplexing(MDM).The MDM technology transmits a limited number of orthogonal modes used as independent channel transmission information in a few mode fiber(FMF),which can exponentially increase the transmission capacity of the optical communication system.However,due to the manufacturing process defects and the unfavorable factors of the external environment in actual few mode fibers,coupling occurs between the modes,which will result in channel crosstalk.At the same time,mode dispersion and polarization-mode dispersion will seriously affect the transmission performance of the system.Therefore,how to compensate the damage of optical fiber link and the demultiplexing of received signal is the research focus of MDM system.This paper focuses on the demultiplexing problem of MDM system.Firstly,the mode and link damage of few mode fiber are introduced.Based on the matrix transmission theory,the transmission model of MDM system is deduced.Secondly,we introduce the blind equalization technology which includes frequency domain constant modulus algoritlhm(FD-CMA)and frequency domain multi-modulus algorithm(FD-MMA).The main work of this article is summarized as follows:(1)The link damage in FMF is analyzed,including mode coupling,mode group delay and polarization mode dispersion.The strong coupling matrix and weak coupling matrix of the 6X6 polarization mode multiplexing system are deduced,and the simulation model of the few-mode fiber link is established based on this,while achieving the simulation process of the fiber link.The frequency domain equalization algorithm in digital signal processing module at the receiving end is studied.The FD-CMA algorithm and the FD-MMA algorithm in the 2×2 mode multiplexing system are introduced and derived in detail.Finally,the two algorithms are extended to 6X6 system.(2)The balance performance of FD-CMA and FD-MMA algorithms is compared.Based on the 6×6 polarization mode multiplexing system including LPoi mode,LP11a mode,LP11b mode,and polarization mode of each spatial mode,the FD-CMA and FD-MMA algorithms are compared according to the bit error rate and the equalized constellation.Through simulation and verification,in 6x6 PDM-MDM system,when the transmitted signal is QPSK and the bit error rate reaches 10-3,the optical signal noise ratio(OSNR)of FD-MMA algorithm is improved by 1dB compared with the FD-CMA algorithm;when the transmitted signal is 16QAM,the OSNR of FD-MMA algorithm is improved by 3.5dB.In the 2×2 MDM system,when the transmitted signal is QPSK and the error rate reaches 10-3,the OSNR required by FD-MMA and FD-CMA algorithm is basically the same;when the transmitted signal is 16QAM,the OSNR of FD-MMA algorithm is improved by 2dB compared with the FD-CMA algorithm.Therefore,FD-MMA algorithm performs better when the transmitter has a non-constant mode signal.(3)Based on 6×6 PDM-MDM system,the performance of the two algorithms is simulated with different rates and different modulation formats coexist.It can be seen from the equalized constellation diagram that the FD-MMA equalization effect is better while different rates coexist. |
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