利用柱透镜调控涡旋光束的拓扑结构

提出了一种利用柱透镜调控涡旋光束拓扑结构的方法.利用计算全息法制作的叉形光栅掩模板,实验获得了具有不同拓扑荷的涡旋光束,分析了涡旋光束通过柱透镜变换后的强度和相位分布.结果表明,涡旋光束经柱透镜变换后,其拓扑荷符号将发生改变,并且高阶涡旋光束退化为多个分离的一阶涡旋光束.利用高阶激光模式的线性叠加特性以及古依相移对实验结果进行了理论解释,并通过数值模拟对实验结果进行了验证.     

平板式螺旋相位板的设计与应用

传统的螺旋相位板是一种利用沿方位角方向介质材料高度递增实现对光束相位调控产生涡旋光束的光学器件,由于这种特殊的几何结构特征使其不能通过相位板的叠加而调控出射光束所携带的角量子数.本文基于坐标变换方法将介质材料沿方位角方向折射率不变而高度递增的传统螺旋相位板变换为一种介质材料沿方位角方向高度不变而折射率递增的平板式螺旋相位板.通过理论分析与数值模拟,发现本文所设计的平板式螺旋相位板不仅与传统螺旋相位板一样能够产生高质量的涡旋光束,而且平板式螺旋相位板的高度和涡旋光束携带的角量子数可以根据介质材料的折射率选取而任意调节.为了实际应用的需要,可以通过叠加多层平板式螺旋相位板以获得不同角量子数的涡旋光束.这种平板式螺旋相位板在光传输、光通信等领域具有广阔的潜在应用价值.     

用四台阶相位板产生涡旋光束

涡旋光束的产生与应用是当前光学领域的研究热点. 利用傅里叶级数展开法分析了四台阶相位板的相位结构, 发现四台阶相位板可看作是由一系列不同拓扑荷数的螺旋相位板所组成, 用线偏振光直接照射相位板时, 将产生多级衍射光波, 各级衍射光均为不同拓扑荷数的涡旋光波, 由于多级衍射光波间的干涉导致光强分布偏离轴对称分布, 因而与涡旋光波有一定差距. 在此基础上, 提出了用四台阶相位板产生涡旋光束的新方案, 借助于Mach-Zehnder 干涉仪光路, 两块四台阶相位板产生的衍射光干涉叠加, 通过调节干涉仪光路的相位差, 使一部分衍射级干涉相消, 另一部分衍射级干涉相长, 相互加强, 从而把线偏振光转换为涡旋光束. 数值模拟计算了几种周期数不同的四台阶相位板衍射光强和角动量分布, 并与螺旋相位板进行比较, 证明用简单的四台阶相位板不仅能够获得与用螺旋相位板相同的涡旋光束, 而且可以用周期数较小的四台阶相位板产生具有大拓扑荷数的涡旋光束, 降低了制作相位板的难度.     

激光直写制作高阶螺旋相位板及其性能

将拓扑荷为4的螺旋相位光束与平面光干涉的计算全息图输入到空间光调制器中,得到含有多个衍射级次的高阶涡旋光束。为提高衍射效率,利用激光直写技术制作拓扑荷为4的高阶螺旋相位板,经测定,相位板深度理论数值为1.073 m,测量数值为1.082 m,相位板制作误差在0.83%以内。平行光束通过此相位板时,在夫琅和费衍射场获得一个高质量的高阶光学涡旋,光强分布与理论数值基本吻合,衍射效率达到86%。     

部分相干幂指数相位涡旋光束的传输特性研究

本文针对部分相干幂指数相位涡旋光束的传输特性开展研究,首先建立了部分相干幂指数相位涡旋光束的传输理论模型;然后,仿真研究了部分相干幂指数相位涡旋光束在自由空间和ABCD光学系统中的传输特性,研究结果表明部分相干幂指数相位涡旋光束在自由空间传输时,拓扑荷数、幂指数和相干长度都对光强的分布有着一定的影响,而随着传播距离的增大光斑的面积逐渐增大;当光束在聚焦系统中传输时,只有拓扑荷数和幂指数的变化会影响光强的分布,而相干长度对光束整体强度的分布没有太大的影响,只影响了光斑的质量.本文研究成果揭示了部分相干幂指数相位涡旋光束的传输特性,为其在光学捕获等领域的应用打下了理论基础,对促进新型光场调控理论及应用研究具有重要的意义.     

对称艾里涡旋光束的传播特性

提出了一种对称艾里涡旋光束.在对称立方相位的基础上引入螺旋相位因子形成新的相位图,加载该相位的高斯光束经傅里叶变换后产生对称艾里涡旋光束.研究表明:对称艾里涡旋光束出现自聚焦现象,且由于轴上涡旋的存在,其中心呈现中空现象,涡旋结构携带的轨道角动量使光束在初始平面处发生旋转,拓扑荷数越大,中空结构越大,旋转角度也越大,而拓扑荷数的正负仅改变光束的旋向.理论分析表明,离轴涡旋在初始平面处,因与光轴中心距离较远,并不会影响对称艾里光束主瓣的能量,即中心并无光强为零区域,但随着传播距离的增加,离轴涡旋逐渐出现在对称艾里光束其中的一个主瓣上.此外,对称艾里光束还可以加载多个离轴涡旋,能同时无损伤地捕获多个微粒.该光束不仅具有自聚焦特性,并且其中心呈中空结构,同时还携带轨道角动量,在光学微操纵与生物医学等方面有潜在应用.     

部分相干涡旋光束研究进展

涡旋光束具有螺旋波前、携带相位奇点和轨道角动量等物理特性,在粒子操控、量子信息、超分辨成像、光通信等领域具有重要的应用,并已成为学术界的研究热点。得益于相干光学理论的快速发展,将相干性作为新的自由度引入涡旋光束中,提出新型涡旋光束即部分相干涡旋光束。相较于完全相干涡旋光束,部分相干涡旋光束具有独特的物理内涵和光学特性,尤其是对其相干性和拓扑荷的联合调控会引发一系列奇特的新物理效应(如相干奇点、光束整形、偏振态转换、自修复等)。回顾了部分相干涡旋光束的基本理论及发展历程,着重对部分相干涡旋光束的理论模型、传输特性、实验产生、实验测量和应用基础研究进行了阐述。     

高阶涡旋光束生成技术研究

应用液晶空间光调制器加载螺旋相位片的方法可以高效地生成涡旋光束,但由于液晶空间光调制器分辨率有限,在加载高阶螺旋相位片时其中心会出现相位失真,造成涡旋光束质量不高。通过在螺线相位片中心区域引入闪耀光栅的方法,得到了光束质量较高的高阶单模涡旋光束和多模涡旋光束,为生成高阶涡旋光束提供了一种很好的方法。     

数字全息术测定涡旋光束拓扑电荷数

提出了一种基于数字全息技术测定涡旋光束拓扑电荷数的方法.该方法通过数字全息技术获取涡旋光束和参考光的全息图并重构出涡旋光束的波前相位,判定相位围绕相位奇点的周期性分布来测定涡旋光束的拓扑电荷数.在拓扑电荷数取值分别为整数和分数的情况下,通过对数值模拟和实验结果的比较,表明该方法能够较准确地测定出拓扑电荷数.     

利用周期型正交二元相位板产生正方阵列涡旋和螺旋光场

设计了一种周期型正交二元相位板,其具有周期型矩形调制单元,各单元的相位调制量为0或π。该相位板的空间频谱具有中心直流分量为0的特点,分别对该频谱中心区域的4个一级频谱点和8个二级频谱点进行相位调制,可得到正方阵列光斑和正方阵列涡旋。阵列涡旋中各涡旋光束的拓扑荷数l=±1,在空间交错分布。由于两个正方阵列光场在垂直传播方向的横向周期恰好重合,且两个光场沿光轴方向的波数不同,因此叠加之后可以形成强度分布随传输距离旋转的正方阵列螺旋,阵列中存在两种具有相反螺旋方向的螺旋光束,同样在空间呈交错分布。此外,讨论了产生最佳螺旋光束的相位板设计条件,并给出理想情况下系统的能量利用率,所得实验结果验证了该方法的可行性。     

Selective generation of high-order optical vortices from a single phase wedge

We demonstrate that it is possible to generate high-order optical vortices from a single phase wedge by applying an incident beam with an annular intensity distribution. Various topological charges of optical vortices are realized by a static phase wedge when the position and radius of the annular illumination are changed accordingly.     

Optical vortices and topological effects in coiled fibers with combined anisotropy

The propagation of optical vortices in coiled anisotropic elliptical fibers is studied. The structure of high-order modes and their propagation constants are determined. The possibility of the generation of TE and TM modes from optical vortices with a unit topological charge in weakly anisotropic coiled fibers is demonstrated. The topological effects that arise upon the propagation of radiation along strongly anisotropic elliptical coiled fibers are investigated. It is established that topological effects are suppressed by anisotropy in the case of the propagation of l = 1 modes; the effects are manifested in these fibers in the form of the rotation of the field intensity distribution by a solid angle spanned by the light trajectory only for certain combinations of l > 1 modes.     

Dynamic behaviors of optical vortices in dual-core photonic crystal fibers

Novel evolution dynamics of optical vortices propagating in a dual-core photonic crystal fiber (PCF) is investigated, which can be explained well by using the equivalent dipole mode superposition principle. Thanks to the coupling between the two cores of PCF, exciting vorticity splitting and topological charge-flipping are achieved by inducing a vortex beam into one of the two cores of the PCF. What is more, the evolutions of two vortices located in each core separately can be controlled by means of modulating the initial phase difference of them. Our results may offer possibilities for applications of optical vortices, orbital angular momentum modulations, as well as optical communications.     

有倾斜透镜存在时两个刃型位错的相互作用

对存在倾斜透镜时两个刃型位错的相互作用进行了研究.研究表明, 两个离轴刃型位错在一定条件下由于相互作用会消失, 并有一个或两个非正则光涡旋产生, 一个共轴刃型位错和一个离轴刃型位错相互作用时产生一个非正则光涡旋. 当初始场中两个刃型位错相互垂直或者平行时, 出射场中会有一个或者两个刃型位错出现. 改变透镜的倾斜因子不影响出射场中位相奇点的类型和数量, 但位相奇点的横向位置与倾斜因子有线性关系. 两个刃型位错相互作用产生的光涡旋对的三维轨迹是非线性的, 但光涡旋对的中心沿直线传输. 关键词： 位相奇点 刃型位错 非正则光涡旋 倾斜透镜     

Revealing optical vortices with a small number of photons

Detecting the optical vortices of darkness hidden in an ultra‐weak background is a difficult task. Here we report an experiment demonstrating that the optical vortices can be directly visualized and identified with a smaller number of photons. Our method is based on the extension of the spiral phase contrast technique to incorporate vortex phase plates (VPP) of high‐order topological charges. In our experiment, we prepare optical vortex arrays of interesting structures such as Arabic numerals and the wings carrying various topological charges. By placing various VPP filters in the Fourier plane of a 4f imaging system, the embedded vortices of an incident ultra‐weak light field can be visualized, revealing both their positions and topological charges. It is found that a higher order vortex generally requires a smaller number of photons to be detected. Our method may find potential application in the fields of astronomical optics and biosensing in an ultra‐weak light environment.

     

Propagation dynamics of off-axis noncanonical vortices in a collimated Gaussian beam

It is widely accepted that an off-axis noncanonical vortex moves across the free-space diffracting Gaussian beam without rotation. But our analysis shows that the vortex swirls a while before it approaches infinite. By neglecting the divergence of the host beam, we focus on this rotation characteristics of the vortices in linear homogeneous media. For the symmetrical host beam, it is found that the vortex moves along an elliptical trajectory, while the topological charge and the angular momentum of the vortex core relative to the beam axis are conserved. For the asymmetrical host beam, the vortex trajectory is rather complicated, since the noncanonical parameter varies as the light propagates, resulting in topological charge inversion. But we find that the vortices are always confined in a rectangular area, and the rotation direction is determined by the topological charge.     

利用全息法在偶氮聚合物薄膜中记录涡旋光场

利用全息技术在偶氮聚合物薄膜中记录了拓扑荷数q=–1,1,2,4的涡旋光场,并将记录的原始叉形光栅与计算全息光栅进行对比,对不同拓扑荷数涡旋光的记录速率和偶氮材料的可重复擦写性能进行了测试;记录完成后,将复现涡旋光与高斯光束干涉,并与原始涡旋光和原始叉形光栅对比,分析了记录质量.实验结果表明:高阶涡旋光场的全息叉形光栅会在记录过程中发生劈裂,轻微劈裂的涡旋光束仍维持一个稳定的环状结构;全息记录过程中不同拓扑荷数的涡旋光束记录速率较为统一,偶氮材料可经过上百次的擦写而不出现疲劳;再现涡旋光与原始涡旋光在光强分布结构上保持高度一致,再现涡旋光的干涉条纹与原始涡旋全息光栅保持高度一致,涡旋光及其携带的拓扑荷信息可被有效记录和读取.     

Topology of Knotted Optical Vortices

Optical vortices as topological objects exist ubiquitously in nature. In this paper, by making use of the Duan's topological current theory, we investigate the topology in the closed and knotted optical vortices. The topological inner structure of the optical vortices are obtained, and the linking of the knotted optical vortices is also given.     

Experimental observation of tangent-azimuthal non-spiral optical vortex

Optical vortices have attracted much attention recently due to their novel properties and widening applications. And lots of optical vortices can be obtained though most of them turn on spiral pattern on increasing azimuthal angle. In this paper, one kind of non-spiral optical vortex was proposed whose front phase distribution is tangent function of azimuthal angle. And this kind of optical vortices were also observed experimentally by computer-generated hologram method. It was found that when topological charge is smaller than unit one, vortex beam shape changes considerably on increasing topological charge, from hollow pattern to curve shape. When topological charge is bigger than unit and is times of 0.5, vortex beam turns symmetrical polygonal pattern though there is crack between adjacent sides, and the side number is twice of topological charge.