Research On Impairments And Compensation Of Coherent Optical Transmission Signals Based On Photon Lantern

Few-mode fiber(FMF)has more mode channels and lower nonlinear effects than single-mode fiber(SMF),and lower inter-mode dispersion than multi-mode fiber(MMF).An mode-division multiplexing(MDM)system built with FMF can obtain several times the transmission capacity of a SMF system,and it has the conditions to achieve long-distance transmission.Therefore,MDM system based on FMF has become one of the popular solutions to increase the transmission capacity of traditional SMF systems.However,due to the performance deficiencies of the mode multiplexer/demultiplexer,the uneven refractive index distribution of the FMF,and the uneven gain curve of the fiber amplifier,there are differential mode group delays(DMGD),mode coupling(MC),and mode-dependent loss(MDL)in the FMF channel,which will have a significant impact on the signal.Whether the channel impairments can be controlled in the transmission process and effective compensation can be made at the receiving end has become the key to whether the MDM system can achieve large-capacity,long-distance and high-rate transmission.Relying on vertical scientific research projects,this paper studies the causes of inherent impairments,the control method of impairments,and the performance of compensation algorithms in the MDM system based on FMF,the main work of this paper is as follows:1.Introduce the basic concept of the MDM coherent communication system,and derive the definition of linear polarization mode and the orthogonality of the modes in the FMF.The causes of the three types of inherent impairments in the FMF are analyzed,which provides a theoretical basis for the construction of an MDM coherent communication system.2.Based on the matrix transmission theory,model the FMF channel,analyze the effects of the three kinds of inherent impairments on the signal from the mathematical point of view,and pave the way for the research of the signal compensation algorithm in the future.3.According to the established channel model,a VPI optical simulation software is used to set up an MDM transmission simulation platform,and a program written in Matlab software is used to implement the DSP processing of the signals.In this platform,the equalization effect of LMS algorithm on DMGD and MC in FMF is verified through simulation.The performance of LMS algorithm and MMSE-SIC algorithm under the influence of MDL is compared.The shortcomings and application scenarios of this algorithm provide a reference for the selection of MIMO algorithms in subsequent experiments.4.A three mode graded-index FMF and a mode-selective photonic lantern were used to build an experimental platform for mode division multiplexing transmission.The coupling matrix of the mode-selective photonic lantern was measured and the sensitivity of the device to polarization was found.Subsequently,the 16QAM transmission experiment of 12.5GBaud symbol rate was completed using the non-degenerate mode(LP01+LP11a)channel and the degenerate mode(LP11a+LP11b)channel under the strong/weak coupling of the mode channel respectively.The DSP module and LMS equalization algorithm established in the simulation platform were used to process the signals,and the constellation diagram and bit error rate curve of the received signal under strong/weak coupling conditions were obtained.The experimental results show that the system can achieve better transmission results when the mode coupling strength is controlled in the channel.