【作者】 李慧；

【导师】 张晓光；

【作者基本信息】 北京邮电大学 ， 电子科学与技术， 2020， 博士

【摘要】 移动互联网、大数据、云计算、5G等信息化浪潮的兴起带来了极大的网络容量需求,光纤通信容量短缺的问题越来越严重。空分复用(SDM)被认为是目前增加光纤通信系统容量最有前景的方案之一。轨道角动量(OAM)模式复用作为SDM的一种,因其可以避免接收端复杂的多输入多输出数字信号处理技术,引起了研究者们的广泛关注。而基于OAM模式的SDM系统的实现最关键的因素就是光纤,它直接决定了信息的传输距离以及接收端信号处理的复杂度,因此找到一种拥有良好特性的OAM模式传输光纤(简称OAM光纤)并研究光纤中的OAM模式传输特性至关重要。本文围绕OAM光纤这一主题,针对OAM光纤设计以及OAM模式耦合机理展开深入研究,所取得的主要研究成果为:1.首次提出了支持OAM模式传输的环形光子晶体光纤的设计工具。给出了适合环形光子晶体光纤系列的有效归一化参数的定义,以此为基础设计了各阶矢量模式截止条件随所定义的有效归一化参数变化构成的模式导图,同时设计了模式有效折射率差,限制损耗,有效模场面积随有效归一化参数变化所定义的等值颜色图,给出了自旋轨道耦合与归一化参数之间的关系图。针对设计OAM模式传输光纤考虑因素众多、设计复杂的问题,给出了方便易用的设计指导解决方案。最后,利用提出的设计工具,给出了一些OAM光纤设计的实例,证明了设计工具的有效性与实用性。2.椭圆OAM光纤中模式耦合机理的分析。针对OAM光纤的椭圆形变,基于耦合模理论与数值仿真,深入分析了椭圆形变OAM光纤中同一 OAM模式群内的模式耦合效应。发现了椭圆形变不仅在高阶矢量模式的奇偶模之间引入了时延差,而且破坏了光纤中本征模结构的现象,且椭圆形变大小不同,造成的本征模的结构破坏也不同。当椭圆形变比较小时,本征模的结构破坏可以忽略不计,只有同一矢量模式组成的两个OAM模式之间存在耦合;当椭圆形变比较大时,本征模的结构破坏不可忽略,此时同一模式群内的四个OAM模式之间都存在耦合,且同一矢量模式组成的两个OAM模式之间的耦合比较严重。OAM光纤中模式耦合机制的研究,为更好地模拟OAM光纤中的模式传输铺平了道路。3.OAM光纤双折射模式色散的研究。当内在缺陷和外在扰动比较小,或者同一模群中HE与EH模之间的有效折射率差足够大时,可合理假设HE与EH模组成的OAM模式彼此独立,此时OAM光纤中的模式耦合类似于单模光纤中的偏振模色散。本论文借鉴单模光纤偏振模色散的分析方法,给出了 OAM光纤中模态的琼斯空间,斯托克斯空间以及庞加莱球表示,利用琼斯矩阵传输法建立了 OAM光纤双折射模式色散的数学模型。与偏振态相比,模态分布与空间方位角有关,使OAM光纤中模态的表示以及双折射模式色散建模都包含了空间方位角这一参量。更多还原

【Abstract】 The rise of mobile internet,big date,cloud computing,5G and other information wave has brought great demand of network capacity,and the capacity crunch of optical fiber communication is more and more serious.Space division multiplexing(SDM)is considered to be one of the most promising ways to increase the capacity of optical fiber communication.Orbital angular momentum(OAM)mode multiplexing,as a kind of SDM,has attracted the attention of researchers because it can avoid the complex multi input and multi output(MIMO)digital signal processing technology at the receivers.The most critical part of OAM multiplexing system is optical fiber,which directly determines the transmission distance and the complexity of digital signal processing,thus it is of great importance to find an OAM fiber owning good properties and study the mode transmission characteristics.This paper focuses on the design of OAM fibers and the analysis of mode coupling mechanism in OAM fibers.The main research achievements are as follows1.The design tool for the family of circular photonic crystal fiber(C-PCF)supporting orbital angular momentum(OAM)modes is proposed.The calibrated effective normalized parameters for the C-PCF family are presented.Based on the proposed effective normalized parameters,the modal map defined by the cutoff conditions of vector modes as a function of effective normalized parameters,the color map defined by the effective index difference,confinement loss and effective mode field area as a function of effective normalized parameters,and the spin orbital coupling as a function of effective normalized parameters are also proposed.Aiming at the problem of designing OAM fibers with many factors and complicated design,a convenient and easy-to-use design guidance solution is given.At the end of paper,we also give some fiber design examples by using the design tool,which proves the effectiveness of the design.2.Analysis of mode coupling mechanism in elliptical OAM fibers.For the elliptical deformation of OAM fibers,based on the coupling mode theory and numerical simulation,the coupling in the same OAM mode group is analyzed in depth.It is found that the elliptical deformation does not simply introduce a time delay between even and odd modes,but also change the structure of eigenmodes,and the structural damage of the eigenmodes are different under different strength of elliptical deformation.When the elliptical deformation is relatively small,the structural damage of the eigenmode can be neglected,coupling occurs between the two OAM modes composed of the same vector mode.When the elliptical deformation is relatively large,the structural damage of the eigenmode cannot be ignored,coupling occurs among the four OAM modes in the same mode group,and the coupling between the two OAM modes composed of the same vector mode is serious.The study of mode coupling mechanism in OAM fibers paves the way for better simulation of mode transmission in OAM fibers.3.Analysis of OAM fiber birefringence modal dispersion.When the internal defects and external disturbances are relatively small,or the effective refractive index difference between HE and EH modes in the same mode group is large enough,it can be reasonably assumed that the OAM modes composed of HE and EH modes are independent of each other.At this time,mode coupling in OAM fibers is similar to polarization mode dispersion(PMD)in single-mode fiber.This paper draws on the analysis method of PMD in single-mode fiber,gives the Jones space,Stokes space and Poincare sphere representation of the modes in OAM fibers,and uses the Jones matrix transmission method to establish the mathematical model of OAM fiber birefringence modal dispersion.Compared with the polarization,the mode is related to the spatial azimuth angle,so that the representation of the mode in the OAM fiber and the birefringence mode dispersion modeling include the parameter of the spatial azimuth angle.更多还原