具轨道角动量光束用于光通信编码及解码研究

无衍射光是在自由空间传播过程中一定范围内横向强度分布保持不变的光场,具有方向性好以及传输距离远等特点,某些类型的无衍射光具有轨道角动量.分析了具有轨道角动量光束以及无衍射光在空间中的传输特点,以计算全息图作为反射式相位调制的空间光调制器(SLM)实现光学变换,并通过由锥形透镜构成的模式转换器,将携带信息的拉盖尔-高斯(L-G)光束转换成为具有相同拓扑荷的高阶Bessel无衍射光束,提出了一种通过具有轨道角动量的高阶Bessel无衍射光束来进行空间光信息传输的编码解码方法,在保证激光大气通信的安全保密性的同时,提高了信息传输的密度和准确率.     

聚焦高阶Bessel-Gauss光束重建的理论和实验研究

提出利用薄透镜元件对聚焦后的高阶Bessel-Gauss光束进行重建. 基于衍射理论, 分析了高阶Bessel-Gauss光束聚焦后的重建行为, 数值模拟高阶Bessel-Gauss光束经薄透镜聚焦再通过另一薄透镜重建的三维光场分布和截面光强分布图. 结果表明, 高阶Bessel-Gauss光束经单个薄透镜后产生中空的局域空心光束, 在焦点位置处为圆环, 尔后迅速发散; 在焦点后合适位置处放入另一薄透镜可矫正焦点后发散的光束, 使得其后光场不变, 仍满足Bessel分布; 实验结果与理论分析相符合. 研究结果对光镊、粒子捕获与操控具有一定的指导意义. 关键词： 衍射理论 传输矩阵 高阶Bessel-Gauss光束 重建     

周期局域空心光束自重建的理论模拟与实验

利用衍射积分理论和干涉理论分析了轴棱锥对无衍射贝塞尔(Bessel)光束进行线聚焦产生局域空心光束遇障碍发生自重建的全过程。数值模拟了周期局域空心光束的传输情况及轴上放入圆形障碍物后不同位置的截面光强分布图,并计算了障碍物后最小自重建距离。研究结果表明周期局域空心光束遇到障碍物后,会绕过障碍物继续向前传输,并且传输一段距离后恢复原来的周期局域空心光束的特性。设计实验光路图对理论模拟进行了验证,通过显微镜和照相机系统拍摄得到轴上圆形障碍物和方形障碍物前后光束的截面光强分布图,实验与理论模拟相吻合。研究结果使得周期局域空心光束的应用得到了扩展。     

轴棱锥透镜面形偏差对无衍射光束传播特性的影响

为了探究轴棱锥透镜面形偏差对无衍射光束传播特性的影响,建立了三种相对理想锥镜母线的偏离模型,即母线向内侧凹陷、外侧凸出、顶点呈双曲线分布的轴棱锥透镜.推导了各偏离锥镜的振幅透过率函数,并基于菲涅耳衍射理论对平面波经三种母线偏离锥镜的出射光场及光强分布进行了理论分析与数值模拟.结果表明:锥镜母线内凹时,光场强度变小,焦深延长,纵截面光场逐渐发散,沿轴向传输时中心光斑直径逐渐变大;锥镜母线外凸时,光场强度激增,焦深变短,纵截面光场逐渐汇聚,沿轴向传输时中心光斑直径逐渐变小;锥镜顶点呈双曲线分布时,光场振荡加剧,场强和焦深几乎不变,纵截面光场发散后逐渐恢复至均匀分布状.研究结果可为分析轴棱锥透镜锥面加工误差对无衍射光束传播特性的影响、进一步完善无衍射光束衍射场理论,以及拓宽其工程应用范围等提供参考.     

径向偏振高阶贝塞尔-高斯涡旋光束的传输及其偏振特性

理论和实验研究了径向偏振高阶贝塞尔-高斯涡旋光束在自由空间传输的光强分布和偏振特性.数值计算表明,在无衍射区域,随着传输距离的增加,光强分布基本保持不变;对于零阶光束,其偏振态一直保持径向偏振不变;对于高阶光束,其偏振态发生变化,由初始的径向偏振逐渐变为椭圆偏振和线偏振的杂化状态.实验上,利用径向偏振转换器、螺旋相位板和轴棱锥元件产生了径向偏振高阶贝塞尔-高斯涡旋光束,探测了其通过不同偏振片后的光强分布.实验结果与理论结果基本相符合.该研究也提供了一种产生杂化偏振矢量光束的方法.     

角向偏振无衍射光束的传输特性及其偏振态研究

从理论和实验两个方面研究角向偏振无衍射光束在自由空间和障碍物空间中的传输特性. 理论上模拟了角向偏振无衍射光束在自由空间以及障碍物空间传播过程中的光强分布与偏振态变化情况. 实验中利用高斯光束先通过偏振转换仪, 再入射到轴棱锥上产生角向偏振无衍射光束. 进而研究其在空间中的传输情况, 并通过分析其经过扇形障碍物后光强分布变化情况讨论障碍物对偏振态的影响和光束的自重建特性. 结果表明: 障碍物遮挡区域光强沿z轴方向逐渐恢复, 且阴影区域沿障碍物所在位置的相反方向移动; 光束的偏振态局部发生变化, 该变化与障碍物所在位置有关. 理论仿真和实验结果一致.     

双级无衍射光束的生成理论与实验研究

为进一步完善多级无衍射光束的传输与变换理论,拓展双级无衍射光束的应用范围,根据轴锥镜法和菲涅耳衍射理论对双级无衍射光束的光场分布做了详细理论分析;基于轴锥镜法搭建生成双级无衍射光束的实验平台,获取了沿光轴传输至不同距离处的截面光斑及其光强分布,探究了平行光束不同入射倾角对无衍射光束传输变换的影响.实验结果表明:双极无衍射出射光场由四个区域组成,在干涉重合区其光强为一、二级轴棱锥出射场的耦合叠加,其余区域仍保持单级无衍射光束的传播特性,其光强大小受轴棱锥透镜的入射光阑与透镜底角影响,光斑破裂程度与平行光束的入射倾角呈正相关.本文对双极无衍射光束的空间传播特性、光强分布特性、光强影响因素等做了详细分析,对拓展多级无衍射光束的应用具有指导意义.     

体视显微镜与CCD成像系统在高阶贝塞尔光束光斑测量中的应用

主要对光学测量仪器体视显微镜与CCD成像系统在高阶贝塞尔光束光斑测量中的应用进行分析,通过以非相干LED绿光经过螺旋相位板和轴棱锥产生的高阶贝塞尔(Bessel)光束为例,模拟出沿轴向不同距离处的截面光强分布图,并与相应的实验参数(如:最大无衍射距离、中心暗斑直径等)进行比较。实验结果表明,体视显微镜与CCD成像系统的测量结果相吻合,但是体视显微镜测量误差较大会影响测量结果的准确度。     

经光阑衍射的高阶贝塞耳光束位相奇点演化特性

推导出了高阶贝塞耳光束通过有光阑近轴ABCD光学系统传输的解析公式,用以研究了高阶贝塞耳光束被光阑衍射位相奇点的演化特性.结果表明:高阶贝塞耳光束经光阑系统衍射后,中心光涡旋始终存在,拓扑电荷守恒,但涡旋核大小会随光阑半径、传输距离和光束阶数而变化;随光阑半径和传输距离变化,圆刃型位错会消失或产生.     

经光阑衍射的高阶贝塞耳光束位相奇点演化特性

推导出了高阶贝塞耳光束通过有光阑近轴ABCD光学系统传输的解析公式,用以研究了高阶贝塞耳光束被光阑衍射位相奇点的演化特性.结果表明:高阶贝塞耳光束经光阑系统衍射后,中心光涡旋始终存在,拓扑电荷守恒,但涡旋核大小会随光阑半径、传输距离和光束阶数而变化;随光阑半径和传输距离变化,圆刃型位错会消失或产生.     

Generation of a large orbital angular momentum beam via an optical fiber winding around a curved path and its application

Many papers have been published on the study of orbital angular momentum(OAM) of the laser modes based on the Laguerre–Gaussian(LG) beam and helical phase plate or rotating cylindrical lens, and the principal ray of the LG beam remains straight line. These ways are difficult to get a large OAM modes. In this paper, we propose a novel method to prepare a large OAM modes when the light propagates through a optical fiber winding around a curved path, and establish a theoretical framework based on the principal ray path changing. Firstly, we investigate three kinds of winding structure.Secondly, based on the analytical solutions and numerical calculations, we can find that the spiral body can achieve a large OAM temporal pulse. At the same time, based on the zero OAM diffraction diagram, we can obtain an improvement of resolving power beyond Rayleigh's criterion. Finally, applying a large OAM diffraction pattern to realize 12-bitencodes,we can obtain a high-security optical information transfer system.     

Rotation of laser beams with zero of the orbital angular momentum

We show that among the multitude of rotating light beams whose complex amplitude can be represented as a linear superposition of the Laguerre-Gaussian (LG) modes with definite numbers there are light beams with zero orbital angular momentum (OAM) and vice versa, multi-mode LG beams that show no rotation and are lacking the radially symmetric intensity distribution can possess the non-zero OAM. Also, we give examples of the rotating light beams with zero OAM, represented as a superposition of the Bessel and new hypergeometric modes. Using an SLM, we generate a rotating Bessel beam with zero OAM for the first time.     

Relation between asymmetrical orbital angular momentum and irradiance-profile rotation for partially coherent beams

On the basis of the irradiance-moment formalism for describing general partially coherent beams, we investigate the relation between the spatial orientation of the transverse beam profile upon propagation and the asymmetrical part of its orbital angular momentum (OAM). More specifically, a necessary and sufficient condition (one-to-one correspondence) is shown between freely propagating non-rotating beams and vanishing asymmetrical OAM. As a corollary, it is obtained that any beam emerging from optical systems that transform a rotating field into a non-rotating beam exhibits vortex OAM only.     

Efficient sorting of orbital angular momentum states of light

We present a method to efficiently sort orbital angular momentum (OAM) states of light using two static optical elements. The optical elements perform a Cartesian to log-polar coordinate transformation, converting the helically phased light beam corresponding to OAM states into a beam with a transverse phase gradient. A subsequent lens then focuses each input OAM state to a different lateral position. We demonstrate the concept experimentally by using two spatial light modulators to create the desired optical elements, applying it to the separation of eleven OAM states.     

Ultrasonic diffraction of Bessel light beams in uniaxial gyrotropic crystals

Specific features of the acousto-optic diffraction of Bessel light beams propagating in the vicinity of the optical axis of a uniaxial gyrotropic crystal have been investigated. The dependences of the diffraction efficiency on the acousto-optic interaction length, ultrasound power, and polarization state of the incident Bessel light beam have been analyzed using the coupled-wave equations and the overlap-integral method. It is shown that polarization-independent diffraction of Bessel light beams is observed in paratellurite crystals, when the Bragg diffraction efficiency is independent of the polarization state of the incident beam. The physical reason for this diffraction has been established (both theoretically and experimentally) to be simultaneous implementation of two processes of anisotropic scattering, at which the Bragg synchronism conditions are satisfied for orthogonal polarized Bessel beams with elliptical polarization.     

Comparison of three kinds of polarized Bessel vortex beams propagating through uniaxial anisotropic media

A comparison of differently polarized Bessel vortex beams propagating through a uniaxial anisotropic slab is discussed in terms of the vector wave function expansions.The magnitude profiles of electric field components, the transformation of polarization modes, and the distributions of orbital angular momentum(OAM) states of the reflected and transmitted beams for different incident angles are numerically simulated.The results indicate that the magnitude profiles of electric field components for different polarization modes are distinct from each other and have a great dependence on the incident angle,thus the transformation of polarization modes which reflects the change of energy can be affected largely.As compared to the x and circular polarization incidences, the reflected and transmitted beams for the radial polarization incidence suffer the fewest transformation of polarization modes, showing a better energy invariance.The distributions of OAM states of the reflected and transmitted beams for different polarization modes are diverse as well, and the derived OAM states of the transmitted beam for radial polarization present a focusing effect, concentrating on the state between two predominant OAM states.     

非相干光源无衍射光的自重建

基于Hankel 波理论分析了非相干光源产生Bessel光束的自重建特性, 利用光学设计软件ZEMAX模拟了Bessel光束经过轴上圆形障碍物后的截面光强分布. 由于发光二级管(LED) 具有一定的频谱宽度且不像激光具有很高的相干度, 因此我们采用一定频宽范围的连续谱来描述. 从模拟结果可以直观地看出Bessel光束被轴上圆形障碍物遮挡后逐步完成自重建, 说明用LED非相干光作为光源具有自重建特性.实验上采用LED和轴棱锥元件产生Bessel光束, 然后通过轴上圆形障碍物、轴上方形障碍物, 并拍摄了不同位置处的截面光强分布图, 证实了非相干光源产生Bessel光束的自重建特性.实验结果和模拟结果相符合. 关键词： 无衍射光束 LED光源 轴棱锥 自重建     

Geometrical phase shift of the extrinsic orbital angular momentum density of light propagating in a helically wound optical fiber

We calculate the intrinsic spin and extrinsic orbital angular momentum densities of an electromagnetic plane wave propagating in a helically wound optical fiber. The geometrical phase shift of the extrinsic angular momentum density of light traveling in such a fiber is derived analytically and discussed in comparison with that of the intrinsic angular momentum density.     

Propagation characteristics of orbital angular momentum modes at 810 nm in step-index few-mode fibers

In free-space or in optical fibers, orbital angular momentum(OAM) multiplexing for information transmission has been greatly developed. The light sources used were well coherent communication bands, and the fibers used were customized. Here, we use an 810 nm femtosecond laser to generate optical vortices carrying OAM and then feed them into two kinds of commercial step-index few-mode fibers to explore the transmission characteristics of OAM modes. We also propose a method without multiple-input multiple-output digital signal processing to identify the input OAMs. It is of great guiding significance for high-dimensional quantum information experiments via the OAMs as a degree of freedom, using the light generated by the spontaneous parametric down-conversion as the source and the commercial fibers for information transmission.