Study On Vector Mode Crosstalk Analysis And Noise Suppression Technology Of Few-Mode Fiber Channel

The mode-division-multiplexing technology based on few-mode fibers（FMFs）can multiply increase the number of spatial channels,and has become a research hotspot in spatial-division-multiplexing（SDM）optical transmission system.Ideally,the optical modes propagating over a FMF channel will be transmitted in parallel without interference.However,in practical implementation,FMFs are often affected by factors such as core eccentricity（CE）,which can disrupt the orthogonality of optical modes,leading to the mode crosstalk（MC）.In order to further study the MC and its suppression technology in FMF channels,this thesis starts from the true and accurate vector eigenmodes of optical fibers,and carries out works on crosstalk analysis and noise suppression technology in FMF channels.The main research contents and innovations are as follows:1.According to the superposition relationship between vector modes（VMs）and scalar modes（SMs）in ideal FMF,a VM fusion analysis model in the CE-FMF channel is proposed based on the transverse electric-field distribution of FMF,which could be used to address the mode crosstalk issue encountered in the real non-ideal FMF.Taking the four VMs,i.e.,TE₀₁,TM₀₁,HE₂₁^o,and HE₂₁^e in the same SM group of LPii as an example,the evolution of the mode-field fusion is thoughtfully explored.For the two cases of the consistent or inconsistent mode-proportions,the four types of light spot,i.e.,the mutual conversion between VM and SM,the asymmetric mode-field distribution deviating from the fiber core,the light spot with the single-lobe distribution,and the light spot with a trailing tail have been observed.Summarizing the evolving classification of VM light spots under CE provides a new idea to analyze the mode conversion and crosstalk happened in FMF.2.In order to simulate the mode-field light-spot distribution for a FMF subjected to twisting and stress disturbances,the experimental analysis of the transverse mode-field evolution in a few-mode polarization controller and a FMF expander was carried out.Using the two experimental schemes,multiple phenomena such as the light spot with asymmetric distribution,the trailing light-spot evolving from double lobes to single lobe,the triple-lobes distribution light spot,and the trailing light-spot with double lobes were observed.The similar transverse mode-field distributions have also been obtained by using the VM fusion model in the CE-FMF channel,further verifying the correctness of the proposed analysis model.3.A frequency-domain adaptive Least-Mean-Square（LMS）equalization method for VM demodulation is proposed to solve the problem of the mode crosstalk in VMmultiplexing transmission over FMF channels.Using the coupling matrix obtained from the VM coupling coefficient,six-VMs multiplexing transmission links(HE₁₁^o,HE₁₁^e,TE₀₁,TM₀₁,HE₂₁^o,and HE₂₁^e)were simulated.After 10,30,and 50km transmission,the average SNR of each mode decreased to 11.48dB,3.59dB,and 1.44dB,respectively.The step-size factor μ and the equalizer tap-number k were optimized,which could be using the frequency-domain adaptive LMS equalization algorithm to compensate for the VMmultiplexing signals at three distances.After optimization,the optimum average Signalto-Noise Ratio（SNR）obtained under these three transmission distances is 24.97dB,24.93dB,and 24.88dB,respectively.The signal quality after compensation is close to the input SNR,verifying the effectiveness of the equalization method used in this thesis for the VM-multiplexing communication.