广义旋光纤结构中光轨道角动量的产生和传输

With increasing recognitions of OAM (Orbital angular momentum) of light in science and technology, the generation and propagation of OAM beams has been a key challenge. In fibers, OAM and SAM (Spin angular momentum) of an individual mode are dependent, and the total angular is determined by the angular order of the mode. It means, once SAM is determined and maintained, the corresponding OAM will be generated and maintained. Based on the fact, we will try to solve the new problems of generating and maintaining OAM beams in fibers from the new perspective of generating and maintaining circular-polarized fiber modes with the corresponding angular orders in circular polarization fiber devices. Taking advantage of our solid research foundation on the coupled-mode analysis for the circular polarization generation and maintaining structures based on spun fiber and chiral fiber gratings (spun fibers with shorter spun pitches), as well as the mathematical conciseness and physical intuition of our unified coupled-mode analysis model, we propose a generalized spun fiber structure with N-order rotation symmetry in its cross section and flexible spun pitch suitable for realizing both fiber and grating, and will develop and improve the coupled-mode analysis model for the generalized structure. In chiral fiber gratings, based on the linear and angular momentum conservation, we will investigate frequency-selectivity, angular order selection and polarization-selectivity of couplings between circular-polarized modes with different angular orders, and reveal the rules of generating and converting the corresponding OAM beam, as well as the interactions of OAM and SAM. In addition, we will give a physical picture of the interactions of OAM and SAM with chiral fiber gratings by an analogical model of acousto-optic interactions. Based on the theoretical analysis, we will study and demonstrate two novel devices, that is, a dual OAM generator and a broadband OAM generator. On the other hand, from viewpoint of mode coupling, to demonstrate the feasibility of maintaining certain OAM beams in spun fibers and explore the ideal structures, we will examine how the spun pitch and index profile in the cross section affect the stable propagations of circular polarization modes, and then suggest the design rules for maintaining the circular polarization modes with certain angular orders. Further, we will focus on the influence of ellipse polarization to evaluate the effectiveness of generating and maintaining OAM beams in the generalized spun fiber structures. Meanwhile, we will do experiments on the generation and propagation of OAM beam with charge- 2 to demonstrate the theoretical analysis. This project will lay foundations of further understanding and wide applying OAM beams.

光轨道角动量(OAM)的科学与应用意义日益显著，如何产生和传输OAM成为研究与应用中的一个关键问题。研究表明，在光纤中，模式的OAM和SAM(自旋角动量)相关联，且总角动量的大小由模式的角向阶数决定。据此，我们拟通过选择和保持一定角向阶数模式的SAM，实现对相应OAM的产生和保持。本项目提出广义旋光纤结构，以产生和保持对应于一定OAM的高阶圆偏振模；该结构在横截面上，具有N阶旋转对称的折射率分布；其旋距在范围选择上，适于实现传统意义的旋光纤和更小旋距的旋光栅。拟基于耦合模理论，建立广义旋光纤结构的系统分析体系。通过旋光栅中不同角向阶数圆偏振模式之间耦合的相位和角动量匹配关系，研究耦合的频率选择、偏振选择，以及角向阶数选择机制，揭示对应OAM的产生及其与SAM相互作用的规律；通过可类比的物理模型，描述对应的物理图像；设计多功能和宽带OAM发生器。从模式耦合的角度，研究旋光纤中具有一定角向阶数的圆偏振模稳定传输的充要条件，探求保持相应OAM的结构设计原则。研究椭圆偏振对产生和保持OAM的影响。实验验证OAM量子数为2的光束的产生和传输。以新视角和方案应对光纤中光OAM产生和保持这一新挑战。

光轨道角动量（OAM）的产生与传输是认识和应用光OAM的基础。由于光纤模式的OAM和SAM(自旋角动量)相关联，且总角动量量子数由模式角向阶数决定；本项目提出广义旋光纤结构，通过选择和保持一定角向阶数模式的SAM，实现相应OAM在光纤中的产生和保持。该结构截面折射率分布具N阶旋转对称性，旋距可调节以实现传统意义的旋光纤和更小旋距的旋光栅（手征光纤光栅）。基于耦合模理论与圆偏振模展开，面向截面具有N阶旋转对称性的广义旋光纤结构，建立了理论分析模型；为该结构的分析和应用奠定了基础。利用旋光栅中不同角向阶数圆偏振模耦合的相位和角动量匹配关系，确定了模式耦合的频率、偏振及角动量选择机制，揭示了OAM产生及其与SAM相互作用的规律，为研究该结构中光OAM产生与转换提供了简明直观的理论依据。基于此，分别利用模式耦合的多重选择机制与渐变旋转结构中的绝热耦合效应，设计了结构紧凑的多量子数和宽带OAM光纤发生器。另一方面，光OAM在广义旋光纤结构中的稳定传输取决于结构对高阶圆偏振模式的保偏特性，取决于相应直光纤结构中高阶模式的双折射特性。从耦合模的解析分析出发，建立截面N阶旋转对称几何型光纤的分析模型，借助于模式耦合的角向匹配条件，得到对应直光纤中各阶模式是否存在双折射的判断依据，并借助于波导模式的数值求解做了验证；为研究广义旋光纤结构中保持圆偏振态和传输光OAM奠定了基础。为实现旋光栅结构的灵活制备，构建基于熔融旋转的制备平台；并提出了两种无需熔融旋转的偏光双折射光纤旋光栅的制备方案。开展了OAM产生与传输的验证性实验。利用该实验平台的设计思路，设计并模拟了基于光OAM的光纤应变传感系统；模拟表明该传感系统可实现高达10^-9的应变灵敏度。利用偏振选择耦合，提出实现基于旋光栅的多参量传感的新方案。研究了基于数字微镜阵列的OAM产生、传输与检测，为基于光OAM的自由空间通信提出了新方案。