利用振幅光栅生成拉盖尔、高斯光束的实验研究

拉盖尔高斯(Laguerre-Gaussian，LG)光束由于其独特的相位结构和轨道角动量特性在微粒旋转操纵和信息处理等方面得到了越来越多的重视.对利用振幅型叉状衍射光栅产生LG光束进行了理论分析，采用计算全息图的方法制作了振幅型叉状衍射光栅，实验上获得了角量子数为±1的LG光束，并对光束变换过程中影响光束特性的主要因素进行了讨论.     

拉盖尔高斯光的衍射和轨道角动量的弥散

根据菲涅耳衍射积分和拉盖尔高斯光束场强分布,对拉盖高斯光束中的圆孔衍射、单缝衍射和方孔衍射进行了研究,并分析了拉盖高斯光束的相位结构对光束衍射后场分布的影响。拉盖高斯光束的相位奇点落在衍射孔中心时,由螺旋谱计算出拉盖高斯光束通过单缝和方孔衍射后的轨道角动量的弥散程度,从理论上证明了拉盖尔高斯光束通过圆孔衍射后,轨道角动量不发生弥散。     

近场拉盖尔-高斯光束传输参数的双缝干涉测量实验

拉盖尔-高斯(Laguerre-Gaussian,LG)光束具有轨道角动量,分析近场条件下LG光束通过含光阑光学系统传输的解析公式,对不同阶LG光束通过双缝时双缝间的相位差变化进行了计算。利用计算机生成叉形衍射光栅显示在空间光调制器上,基模高斯光束产生衍射,得到不同阶次LG涡旋光束,通过CCD采集LG空心光束的光斑及双缝干涉后的图样,实现对LG光束传输轨道角动量特性的实验测量。在确定光束束径下,分析了叉形衍射光栅密度、空心光束宽度半径比、双缝宽度等参数对双缝干涉条纹的影响,在距离激光器1m处的SLM上显示4mm×5mm叉形衍射光栅,光栅密度约为16lines/mm,可产生暗斑尺寸在0.5mm~0.9mm之间、宽度半径比为0.2的LG空心光束。LG光束通过双缝宽度0.2mm,双缝间距0.5mm的光栅,可以得到清晰的干涉条纹。研究结果为近场光通信中利用光学涡旋轨道角动量编码信息提供了理论依据。     

扭转对称光束的产生及其变换过程中的轨道角动量传递

研究了采用扭转柱面镜光学系统将厄米-高斯光束变换成为具有轨道角动量的拉盖尔-高斯扭转对称光束. 采用本征模式分解的方法分析了扭转柱面镜光学变换系统实现光束变换的原理.利用光束传输矩阵和二阶矩理论分析计算了光束经过扭转柱面透镜变换过程中的轨道角动量传递过程，证明光束与透镜系统的轨道角动量交换发生在第一个柱面透镜处，光束经过第一个柱面透镜后，具有的轨道角动量保持不变. 关键词： 轨道角动量 光束变换 扭转对称光束 轨道角动量传递     

在大气湍流斜程传输中拉盖高斯光束的轨道角动量的研究

大气湍流引起大气折射率随机变化,导致空间不均匀性.拉盖高斯光束在大气湍流中传输时,空间不均匀性会使光子波函数改变,引起轨道角动量的变化.本文讨论了拉盖尔高斯光束在大气斜程传输时,湍流介质改变光子轨道角动量而形成不同的光子态.计算了螺旋谐波各分量所占光束总能量的权重,分析了拉盖高斯光束的轨道角动量的变化规律.     

高阶椭圆厄密–高斯光束的轨道角动量研究

从傍轴条件下光束轨道角动量的基本理论出发,根据高阶椭圆厄密–高斯光束的光场分布,运用张量方法,对高阶椭圆厄密-高斯光束轨道角动量的密度分布进行了理论分析,得到了求解该密度分布的计算公式,并在给定参量条件下作了数值模拟.进一步对光束中每个光子携带的平均轨道角动量进行了计算,发现其值随着椭圆厄密-高斯光束阶次的增大而增大,表明高阶椭圆厄密-高斯光束能够比椭圆高斯光束或拉盖尔-高斯光束提供高得多的轨道角动量.     

产生拉盖尔-高斯模的全息光栅实验研究

拉盖尔-高斯（Laguerre-Gaussian,LG）模是光子的轨道角动量本征态,它在未来的光镊、量子信息和计算等领域有潜在的应用价值。分别用两种记录介质-全息干板和喷墨透明胶片-制作了全息光栅。通过这种全息光栅在实验上产生了拓扑数从一阶到高阶的拉盖尔-高斯模,同时观察到高阶模的空间不稳定特性。并且详细讨论了光栅制作过程中的各种控制参量如曝光时间及强度对光栅衍射效率的影响。     

涡旋光束与相位全息光栅不对准时的衍射特性研究

研究了涡旋光束与相位全息光栅不对准时衍射光束的解析特性.利用理论推导的方法得出涡旋光束经相位全息光栅接收后一阶衍射光束的解析表达式.然后通过仿真分析分别得出在发生正常对准、横向偏移、角向倾斜及横向偏移和角向倾斜两者同时出现时衍射光束的质心偏移特性和中心强度变化特性.研究表明: 拉盖尔-高斯光束经相位全息光栅衍射后得到的光场表达式为合流超几何函数形式.光束与相位全息光栅间的不对准会引起衍射光束质心的偏移,而且光束质心的偏移量随入射光束偏移距离和偏离角的增加而增加,与入射光束的偏移方向和方位角无关.角向倾斜时 关键词： 拉盖尔-高斯光束 相位全息光栅 横向偏移 角向倾斜     

利用液晶空间光调制器产生高阶拉盖尔高斯光束

本文采用反射式纯相位液晶空间光调制器产生了实验需要的高阶拉盖尔高斯光束。理论上详细计算了生成高阶拉盖尔高斯(Laguerre-Gaussian)光束的最优参数选择,用MATLAB编写程序,制作产生高阶LG光束的相息图。通过CCD采集图像数据,观测了以平面波入射生成拉盖尔高斯光束的强度分布,并利用干涉法验证了产生高阶拉盖尔高斯光束的螺旋相位分布。在此基础上,对实验测得一维强度轮廓进行拟合分析并估算了实验产生LG光束的纯度。     

具有轨道角动量光束入射下的单球粒子散射研究

根据广义Mie理论,研究了具有轨道角动量拉盖尔-高斯光束（LGB）的空间传输特性以及单粒子散射特性.在单体球粒子对高斯光束散射研究的基础上,分析了在自由空间不同传输距离LGB光束的光强分布情况,在不考虑散射和波束相移关系的情况下,将LGB作为入射高斯光束,通过对波束入射时的散射衰减截面求解得到波束因子,利用矢量球谐函数对入射高斯波束进行展开,从而研究了单球粒子在在轴条件下对具轨道角动量高斯波束入射的散射问题.通过数值计算,讨论了散射强度及角分布在不同波束宽度情况下对其散射特性的影响,并与平面波的情况做了对比.结果表明,当波束束腰半径较小时,束腰半径对衰减率的影响较大,而当束腰半径远大于粒子半径时,接近于平面波的情况. 关键词： 广义Mie理论 轨道角动量 Laguerre-Gauss光束 单球粒子     

Effect of Comatic Aberration on the Propagation Characteristics of Laguerre--Gaussian Beams

Seidel comatic aberration is an important cause of deformation for a Laguerre--Gaussian (LG) beam. In addition, mono-axial comatic aberration, whose phase modulation depends only on one transverse coordinate, is also an important cause of beam deformation. Deformation of an LG beam by such aberrations is analyzed through numerical simulation based on the angular spectrum method. It is also shown that for holographically generated LG beams quadratic spatial variation of grating pitch can produce seidel and mono-axial comatic aberrations. An example of an experimentally generated LG beam with mono-axial comatic aberration is reported.     

Generation and detection of linearly polarized axially symmetric beam by an amplitude computer-generated hologram

A novel beam combination scheme was proposed to generate high order linearly polarized axially symmetric beam (LPASB) by superposing two Laguerre–Gaussian (LG) beams with opposite azimuthal quantum numbers. An amplitude computer-generated hologram (CGH) was not only used to generate two LG beams but also used in the combination scheme. Amplitude CGH was also used to detect the order of the LPASB.     

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.     

Quadrant detector calibration for vortex beams

This Letter reports an experimental and theoretical study of the response of a quadrant detector (QD) to an incident vortex beam, specifically a Laguerre-Gaussian (LG) beam. We have found that the LG beam response depends on the vorticity index ?. We compare LG beams with hard-ringed beams and find that at higher ? values, the QD response to LG beams can be approximated by its response to hard-ringed beams. Our findings are important in view of the increasing interest in optical vortex beams.     

Field properties and vacuum electron acceleration in a laser beam of high-order Laguerre–Gaussian mode

The electric field intensity distribution and the phase velocity distribution of high-order Laguerre–Gaussian (LGρ?) mode laser beams are analyzed. Using three-dimensional test particle simulation, the numerical results of electrons accelerated by LG00, LG40 and LG41 mode laser beams are presented. Compared with the LG00 mode (the fundamental mode) laser beam, low-energy injection electrons can be more favorably accelerated in a high-order LG mode laser beam. Contrary to anticipation, a high-order LG mode laser beam with intense axial electric field distribution is inferior to the LG00 mode in capture acceleration for electrons with high injection energy.     

Four wave mixing spectroscopy and additional boundary conditions: A proposal

A nonlinear spectroscopic method for the direct measurement of amplitude ratios among various polariton modes in spatially dispersive media is proposed and theoretically analyzed. In this method four wave mixing among three laser beams is used. Two beams have the same frequency higher than the longitudanal exciton, but different angles of incidence, and excite various polariton modes in the crystal. The third is the probe beam whose frequency can be chosen in the transparent region of the crystal. The probe beam is elastically diffracted from the grating generated by the interference between some appropriate pair of excited modes. The phase matching condition and hence the exit angle of the diffracted beam depend sensitively on the wave-vector of the grating. Thus the contribution from each polariton branch can be measured separately, by selectively tuning the angle of incidence of the probe beam; from the measured ratio the amplitude ratio, and hence the ABC can be obtained.     

Partially coherent Lorentz Gaussian beam and its scintillations

We study the scintillation aspects of partially coherent Lorentz Gaussian (LG) beams via numerically integrating the average and average squared intensity expressions. Within the examined range of input and propagation medium parameters, the LG beams are generally found to offer less and less scintillations against the pure Gaussian beam, particularly when the Lorentzian feature of the beam is emphasized more. This lower scintillation property is exhibited for collimated coherent beams with different Lorentz widths and at on-axis and off-axis positions of the receiver plane. When focusing is introduced, at shorter propagation distances the ordering of the beams remains as described above, but at longer propagations distances a complete reversing of the beam order is observed. Raising the turbulence levels by increasing the structure constant inevitably causes rises in scintillations, while preserving the advantage of LG beams over the pure Gaussian beam. Partial coherence reduces scintillations as expected, at the same time nearly eliminating the scintillation differences between the beam types.     

Displacements and deformations of a vortex light beam produced by the diffraction grating with embedded phase singularity

Properties of vortex light beams produced by a diffraction grating with groove bifurcation (“fork” structure) are studied in the case of small diffraction angles. Analytical expressions are derived for the amplitude distribution of a diffracted beam generated from an incident Gaussian beam with arbitrary radius and wavefront curvature, transversely shifted and inclined with respect to the nominal axis (normal to the grating crossing it at the bifurcation point). In such situations, the output beam becomes asymmetric; the optical vortex core and the intensity maximum displace orthogonally to the incident beam shift direction. The nearest vicinity of the vortex core preserves its circular symmetry and the optical vortex remains locally isotropic. The effects of misalignment depend on the incident beam characteristics, the diffraction order and the propagation distance behind the grating. Experimental measurements support the results of calculations.     

Airy beam laser

A method to design lasers that emit an arbitrary beam profile is studied. In these lasers, output-coupling is performed by a diffraction grating that imposes a phase and amplitude distribution onto the diffracted light. A solid-state laser emitting beams with a two-dimensional Airy intensity profile is demonstrated both theoretically and experimentally. In this case, the diffraction grating adds a transverse cubic phase to the diffracted light. An Airy beam is obtained by performing optical Fourier transform of the out-coupled light. The laser beam profile and power characteristics are shown to agree with theory.