Scattering of a Gaussian Beam by Stepped Index Luneberg Lens

The scattering and focusing characteristics of a Gaussian beam with phase distribution is studied by a stepped index Luneberg lens. A model experiment was performed using a six-layer spherical lens. Two kinds of beams, that is, broad and narrow beam are examined for the incident Gaussian beam. We find that focusing effect of a narrow beam is wholly lower than that of a broad beam. The focal point is also shifted toward outer of sphere because of phase lag for the beam source. The spot size equals almost the wavelength for the various beam parameters.     

椭圆高斯波束中小粒子的一阶散射特性

研究电磁波束的小粒子散射特性在环境监测等领域有着广泛的应用。将电磁波束做级数展开,给出了各项对应电磁波的物理意义及粒子内电场的解析式。以椭圆高斯波束为例,对粒子内电场以及后项与前项电磁波束中粒子内电场的大小的比值随照射距离、频率的变化做了仿真。研究了椭圆高斯波束中小粒子的散射特性,通过仿真分析了波束腰宽和频率对散射特性的影响,验证了算法的有效性。研究表明,波束腰宽可以改善粒子的识别性能,粒子对高阶项的散射场远小于对次阶项的散射场,瑞利粒子内场表达式中坐标的幂次与外场波束表达式中坐标的幂次相同。所提方法简单,为研究电磁波束的粒子散射特性探索了新的途径。     

Novel Gaussian beam method for the rapid analysis of largereflector antennas

A relatively fast and simple method utilizing Gaussian beams (GBs) is developed which requires only a few seconds on a workstation to compute the near/far fields of electrically large reflector antennas when they are illuminated by a feed with a known radiation pattern. This GB technique is fast, because it completely avoids any numerical integration on the large reflector surface which is required in the conventional physical optics (PO) analysis of such antennas and which could take several hours on a workstation. Specifically, the known feed radiation field is represented by a set of relatively few, rotationally symmetric GBs that are launched radially out from the feed plane and with almost identical interbeam angular spacing. These GBs strike the reflector surface from where they are reflected, and also diffracted by the reflector edge; the expressions for the fields reflected and diffracted by the reflector illuminated with a general astigmatic incident GB from an arbitrary direction (but not close to grazing on the reflector) have been developed in Chou and Pathak (1997) and utilized in this work. Numerical results are presented to illustrate the versatility, accuracy, and efficiency of this GB method when it is used for analyzing general offset parabolic reflectors with a single feed or an array feed, as well as for analyzing nonparabolic reflectors such as those described by ellipsoidal and even general shaped surfaces     

Influence of Gaussian Beam on Terahertz Radar Cross Section of a Conducting Sphere

In RCS estimation, usually a plane wave is assumed; while in real measurements at terahertz frequencies, generally a Gaussian beam or a similar beam source is adopted. In this paper, the RCS of a conducting sphere is discussed under the condition that the incident wave is a Gaussian beam. In the estimation, the influence of 2.52 THz collimated laser beam on RCS is discussed and the RCS changing with scattering angle and some other factors is obtained; at the same time, the comparisons between different incident beams, plane wave and Gaussian beam, are also given. The estimation results show that choosing a beam width of 40 mm is appropriate when the sphere radius is 10 mm and the distance between the sphere and the beam source is 1 m.     

On the use of a Gaussian beam to isolate the edge scattering from a plate of finite size

Free-space focused beam measurement systems are sometimes used to illuminate an edge of a sample under investigation while observing the scattering response. Typically, these kinds of measurements are performed to try to categorize the scattering from an edge of a material. Therefore, in this paper, a study is performed to obtain guidelines for the conditions under which the scattering of the beam is almost completely determined by the illuminated edge; that is, there is negligible scattering due to other physical structure immediately outside of the spot of the beam. The guidelines are obtained by comparing edge scattering due to a Gaussian beam incident on an infinitesimally thin, perfectly conducting plate with the edge scattering due to the same beam incident on an infinitesimally thin, perfectly conducting half-plane. The half-plane can be viewed as the ideal object for studying edge scattering because it contains only one edge. So the comparison between the plate and the half-plane determines the effect of the additional edges of the plate on the scattered field.     

A hybrid FEM-based procedure for the scattering from photoniccrystals illuminated by a Gaussian beam

We provide an efficient numerical procedure for evaluating the field scattered by two-dimensional (2-D) photonic crystals when they are illuminated by Gaussian beams. In particular, the incident Gaussian beam is interpreted as a spectrum of both homogeneous and inhomogeneous plane waves. The scattering of each plane wave is analyzed by resorting to a hybrid technique combining the finite-element method (FEM) with a Floquet modal expansion. Moreover, by applying the standard saddle point method, the evaluation of the field at a specific point of the exterior medium is reduced to the contribution of the fundamental Floquet mode of a single plane wave belonging to the incident spectrum, strongly enhancing the numerical efficiency     

空心高斯光束通过光阑透镜系统的传输特性

为了研究空心光束通过硬边光阑透镜傍轴ABCD光学系统后的传输特性,利用柯林斯衍射积分公式,推导出了空心高斯光束通过受圆孔硬边光阑限制的傍轴ABCD光学系统的传输公式,所得公式可用来描述空心高斯光束通过任意傍轴ABCD光学系统的传输.研究了光阑、透镜以及光阑透镜系统对空心高斯光束传输特性的影响,并用数值例做了详细说明.结果表明,光阑和透镜均会使光束的光场分布向源平面前移,光阑会破坏空心高斯光束的空心性,而单纯的透镜只改变光场分布,不会破坏光束的空心性.这一结果对于空心光束的产生和应用有理论指导意义.     

离轴球形粒子对高斯波束的散射

本文根据广义米氏理论，将入射高斯波束用矢量球谐函数展开，研究离轴球形粒子的电磁（光）散射。获得波束因子／ｇ表达式和它的局域近似结果，以及散射系数ａｎ和ｂｎ的计算公式。讨论了波束宽度和粒子离轴位置对散射系数、消光系数、散射强度角分布，以及对粒子的内场和表面场分布的影响     

高斯光束经波长级圆孔衍射的轴上光强特性

基于横截面上精确表述的光强和精确的衍射场公式 ,对高斯光束经波长级圆孔衍射的轴上光强特性进行了研究。结果表明 ,高斯衍射光束的轴上光强特性取决于初始高斯半宽度w0 和波长级圆孔的孔径R。对于w0 /R≥ 1的高斯衍射光束 ,轴上光强存在的极值个数和出现的位置仅由比值m=2R/λ决定 ,最大的轴上光强均出现在N =R2 / (λz) =1的地方 ;至于轴上光强极值的峰和谷明显与否 ,取决于w0 /R的比值 ,比值越大 ,轴上光强极值的峰和谷就越明显。当w0 /R的比值足够大时 ,就趋向于平面波入射时的情形。而对于w0 /R<1的这一类高斯衍射光束 ,轴上光强存在特定的演化规律 :随着初始高斯半宽度的减小 ,轴上光强极值个数逐步减少直至全部消失。     

多层球对高斯波束的散射

本文根据广义米氏理论，将入射高斯波束用矢量球谐函数展开，研究多层介质球的电磁（光）散射，提出波束因子ｇｎ的改进算法以及散射系数ａｎ和ｂｎ的迭代公式和算法，并进行了数值计算与讨论。     

Analysis of EM penetration into and scattering by electricallylarge open waveguide cavities using Gaussian beam shooting

Gaussian beam (GB) representation methods are used to analyze the electromagnetic coupling into and the scattering by a large nonuniform cavity. The aperture field in the cavity is decomposed into beams using the Gabor expansion, and shooting techniques are then employed. The method is illustrated only for the two-dimensional (2-D) case. The GBs are tracked axially using the rules of beam optics which ignore any beam distortion upon reflection at the walls. The effects of beam distortion are not significant for relatively slowly varying waveguide cavities. The field scattered into the exterior by the termination within the cavity is found using a reciprocity integral formulation which requires a knowledge of the beam fields near the termination. Numerical results based on this GB approach are presented and compared with results based on an independent reference solution     

高斯光束通过非线性抛物形梯度折射率棒透镜的变换

本文导出了高斯光束通过非线性抛物形梯度折射率棒透镜的变换公式。结果表明,它的变换特性依赖于光功率、入射光束的束腰、物距以及透镜参数。     

Elliptic Gaussian Optical Beam in Nonlinear Conical Graded-index Rod Lens

The propagation and transformation of an elliptic Gaussian optical beam(EGB) passing through a Kerr-law nonlinear conical graded-index rod lens are presented. In the rod lens, the EGB is treated as two dependent optical beams. The dimensionless beam-width parameters and the inverses of the curvature radii for the wavefront of the two beams are given semi-analytically,and the transformations of the EGB with the rod lens are derived by use of the ABCD law of Gaussian optical beam.     

洛伦兹光束通过一阶轴对称光学系统的传输变换特性

从广义惠更斯-菲涅耳衍射积分出发,导出了洛伦兹光束通过一阶轴对称光学系统传输变换的解析公式.给出了洛伦兹光束的束宽、瑞利长度和发散角的表达式,讨论了这些参量通过自由空间的变换规律.结果表明,洛伦兹光束的瑞利长度大于高斯光束的瑞利长度,洛伦兹光束光斑半径的扩展速度慢于高斯光束,洛伦兹光束的远场发散角小于高斯光束的远场发散角.结果还表明,自由空间中洛伦兹光束的束宽、瑞利长度和发散角的变化规律与高斯光束相应参量的变化规律相同.     

结构参量对脉冲高斯光束布拉格衍射特性的影响

利用二维耦合波理论,研究了体光栅对时间和空间都为高斯型分布的脉冲激光光束的衍射性质,得到了衍射和透射光振幅的解析表达式.讨论了体光栅的结构参量对衍射光束的空间和频谱强度分布以及衍射效率的影响.结果表明,体光栅的波长选择带宽和衍射光束带宽随着结构参量的增大而增大,衍射效率随着入射脉冲时间宽度的增大而增大,并趋于定值,结构参量值越小,达到饱和的速度越慢,饱和值越大.     

萨那芒特棱镜对单模高斯光束光强分布影响的分析

根据光在萨那芒特棱镜胶合介质层中的干涉效应,分析了萨那忙特棱镜对单模高斯光束光强分布的影响;结果表明：对于给定的入射单模高斯光束,若光在胶合层界面上的入射角、胶合层的厚度和胶合剂的折射率,三者确定其二,棱镜对透射光束光强分布的影响将随另一参量的变化作周期性振荡,且透射高斯光束的形状也会随之改变;相比较而言,棱镜对透射 光的影响要大于 光,但从总体上看,棱镜无论对 光还是对 光的影响均小于3％,所以在要求不是特别严格的应用中,可以忽略萨那忙特棱镜对单模高斯光束光强分布的影响。     

高斯光束通过非线性梯度折射率透镜的传输特性

基于无像差自聚焦理论,使用ABCD定律导出了高斯光束在非线性梯度折射率介质中的解析传输公式。采用矩阵光学方法研究了高斯光束通过非线性梯度折射率透镜的传输特性。结果表明,介质的非线性性和非均匀性、激光功率及入射光束参数影响其传输特性。     

Reflection matrix for Gaussian light beams in FEL oscillators

The transformations of Gaussian radiation beams caused by reflection off mirrors is an important issue for the operation of free-electron lasers as oscillators. The reflected radiation from a single incident Gaussian mode will contain other modes due to the finite size, the deflection of the beam, and mismatches in the curvature. A method for analytic computation of the reflection matrix is developed. The mirror surface that reflects spherical incoming wavefronts into spherical outgoing is determined analytically. The cross coupling among vector components of the radiation field, caused by the curvature of the mirror surface, is included     

Narrow-waisted Gaussian beam discretization for short-pulseradiation from one-dimensional large apertures

We develop a Gabor-based Gaussian beam (GB) algorithm for representing two-dimensional (2-D) radiation from finite aperture distributions with short-pulse excitation in the time domain (TD). The work extends previous results using 2-D frequency-domain (FD) narrow-waisted Gaussian beams. The FD algorithm evolves from the rigorous Kirchhoff integration over the aperture distribution, which is then parameterized via the discrete Gabor basis and evaluated asymptotically for high frequencies to furnish the GB propagators to the observer. The TD results are obtained by Fourier inversion from the FD and yield pulsed beams (PB). We describe the resulting TD algorithm for several aperture distributions, ranging from simple linearly phased (linear delay) to arbitrary time delay profiles; the latter accommodate the important case of focusing TD aperture fields. For modulated pulses with Gaussian envelopes, we compute accurate closed form analytic solutions, which have been calibrated against numerical reference data. Our results confirm that the previously established utility of the Gabor-based narrow-waisted FD-GB algorithm for radiation from distributed apertures remains intact in the TD     

椭圆高斯光束的衍射和象差

从惠更斯－菲涅耳原理出发,采用数值积分方法分析了会聚椭圆高斯光束焦点附近的三维光分布状态,讨论了按照均匀光波球差最佳校正形式对椭圆高斯光束进行象差平衡时的中心点亮度变化,研究表明,在工程实际中,采用普通光学系统的象差平衡理论对半导体激光光学系统进行设计是完全可行的。