Research On Equalization Technology Of Optical Fiber Communication System Based On PAM4 Signal

Optical fiber,as an important basic communication facility in the information society,has been driving the rapid development of the Internet.In recent years,global Internet Protocol(IP)traffic has exploded,driven by newly launched Internet services such as the Internet of Things,4K/8K video applications,and virtual reality(VR).To meet the needs of large-scale and low-cost deployments,short-haul optical links based on direct detection/intensity modulation(IM/DD)have been widely studied for data center optical interconnects as well as fiber access networks.With increasing traffic demands,many advanced modulation formats are used to increase spectral efficiency and reduce the bandwidth limitation of electronic and optical components,such as four-level pulse amplitude modulation(PAM4),carrierless amplitude phase modulation(CAP),discrete multitone modulation(DMT).Considering the system power consumption and implementation complexity,the most attractive modulation format in short-haul optical links is PAM4,which is considered as a very promising candidate format for next-generation passive optical networks.However,compared with non-return-to-zero(NRZ)coding,PAM4 is more sensitive to nonlinear damage and inter-symbol crosstalk caused by system optoelectronic devices.In order to solve this problem,the equalization technology based on PAM4 signal has become an important research field.This paper aims to study the equalization technology required by PAM4 signal in IM/DD optical fiber communication system.Through the IM/DD simulation system based on PAM4 built by VPI and Matlab software,the feasibility of the proposed algorithm is verified,and the advantages and disadvantages of different algorithms are compared.The main research content of this paper consists of the following three parts.(1)The framework and important components of the IM/DD optical fiber communication system are introduced,and the main sources of damage to the PAM4 signal in the IM/DD system are analyzed.Among them,the nonlinear distortion of PAM4 signal caused by insufficient linearity of Mach Zender Modulator(MZM)and the intersymbol interference of PAM4 signal caused by fiber dispersion are mainly studied.On this basis,it is found that the nonlinear distortion of the signal caused by MZM will combine with the intersymbol crosstalk to cause more serious nonlinear damage.A 50Gbits/s IM/DD simulation system using MZM modulation based on PAM4 signal is built using Matlab and VPI.(2)Using Volterra Series-based Nonlinear Equalizer(VNLE)and Artificial Neural Network-based Nonlinear Equalizer(ANN-NLE)to damage system nonlinearity to compensate.A VNLE based on Least Squares(LS)is proposed,which can further improve the bit error rate performance compared with the traditional VNLE based on Least Mean Square(LMS).The simulation results show that when the transmission distance is 10 km,the LS-based VNLE can obtain better bit error rate performance than the LMS-based VNLE.Compared with LMS-based VNLE,the minimum received optical power is reduced by about 1d Bm,and VNLE-LMS is reduced by about 1.5d Bm compared with linear equalization.The BER performance of VNLE-LS and ANN-NLE after equalization is basically the same,and the bit error rate performance of ANN-NLE is obviously better than that of VNLE based on LMS.To reduce the bit error rate below 10-2,the minimum received optical power required by ANN-NLE is about 1.5d Bm lower than that of VNLE based on LMS.(3)A joint scheme combining Digital Predistortion(DPD)and Linear Equalizer(LE)based on memoryless polynomial is proposed.This scheme pre-compensates the nonlinear distortion caused by MZM to the signal at the transmitting end,and compensates the intersymbol interference caused by the dispersion of the fiber at the receiving end.The combination of the two kinds of damage is avoided to produce more serious nonlinear damage.The simulation results show that the BER performance of this joint scheme is better than that of the complexity of the latter.When the transmission distance is 10 km,if the bit error rate is to reach 2×10-3,the minimum received power required by the joint scheme is 1.5d Bm lower than that of the LMS-based VNLE.