【作者】 张鹏；

【导师】 文峰；

【作者基本信息】 电子科技大学 ， 电子信息（专业学位）， 2023， 硕士

【摘要】 少模光纤中的模分复用技术能成倍增加空间信道数,成为空分复用光传输技术的研究热点。理想情况下,少模光纤信道传输的各模式会不受干扰地并行传输。然而在实际环境中,少模光纤往往会受到芯偏移等因素的影响,这些扰动因素会破坏模式传输的正交性,从而导致模式串扰的发生。为了进一步研究少模光纤信道的模式串扰及其抑制技术,本文从光纤真实、精确的矢量本征模出发,开展少模光纤信道的串扰分析和噪声抑制技术方面的工作,主要的研究内容与创新如下:1.在理想少模光纤中矢量模与标量模的模场叠加关系基础上,针对实际非理想情况下少模光纤受到的芯偏移致模式串扰问题,提出一种以少模光纤横向电场分布为基础的芯偏移矢量模融合分析模型。以标量模LP₁₁模组中的四个矢量模TE₀₁、TM₀₁、HE₂₁^o和HE₂₁^e为例,探索模场融合演化规律。从模式比例一致和不一致两种情况出发,观察类矢量模与标量模的相互转化、偏离纤芯的不对称模场分布、单瓣分布光斑以及拖尾光斑分布等模斑情况。总结芯偏移下矢量模光斑演化分类,为分析少模光纤的模式转换和模式串扰提供了新思路。2.为了模拟少模光纤受到扭转和应力扰动因素后的模场光斑分布,开展了基于少模偏振控制器和基于少模光纤膨胀器的横向模场光斑演化实验分析。利用两种扰动方案观察到不对称分布光斑、双瓣向单瓣演化的拖尾光斑、三瓣分布光斑及双瓣拖尾光斑等现象。通过矢量模融合下的少模光纤芯偏移分析模型得到了实验扰动方案中出现的几类模场光斑,进一步验证了所提出的矢量模叠加下少模光纤芯偏移分析模型的正确性。3.针对少模光纤信道中矢量模复用传输过程中模式串扰噪声问题,提出面向矢量模解调的频域自适应最小均方均衡方法。利用矢量模耦合系数C_vμ得到的耦合矩阵开展了 HE₁₁^o、HE₁₁^e、TE₀₁、TM₀₁、HE₂₁^o及HE₂₁^e共六个矢量模复用的传输链路仿真,通过10km、30km及50km传输后各模式的平均信噪比（SNR）分别下降为11.48dB、3.59dB以及1.44dB。在利用频域自适应最小均方算法对三种距离下的模式信号进行补偿前,进行了步长因子μ和均衡器抽头数k的参数优化,经优化后在这三种传输距离下得到最佳的平均SNR分别为24.97dB、24.93dB和24.88dB,补偿后的信号质量与输入SNR接近,验证了本文所用均衡方法对矢量模复用通信的有效性。更多还原

【Abstract】 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.更多还原