Polarization correction and extension of the Kennaugh-Cosgrifftarget-ramp response equation to the bistatic case and applications toelectromagnetic inverse scattering

An analytical time-domain expression derived by Kennaugh (1967) for the early time impulse response for smooth, convex, perfectly conducting scatterers under the physical optics approximation for the bistatic case is reinterpreted. The physical optics bistatic early time impulse responses can be interpreted as cross-sectional areas of the scatterer. A crude polarization correction to the leading edge of the physical optics impulse response is obtained for the bistatic case, leading to a simple asymptotic relation between the specular principal curvature difference and certain co-polarized phase terms in the bistatic scattering matrix. Applications to direct scattering are discussed. Profile reconstruction from bistatic data with a priori knowledge of the validity range of physical optics in the time domain is proposed and tested with the sphere     

Calculations of Infrared Laser Beam Scattering from Rough Dielectric Objects

The coherent and incoherent scattering cross sections of Infrared Laser Gaussian beam scattering from arbitrarily shaped convex dielectric objects with rough surfaces are investigated by using plane wave spectrum method and physical optics approximation. In the paper, the infrared laser scattering cross sections of rough sphere are calculated at 10.6 μm , and the influence of the beam size, permittivity, and polarization as well as roughness parameters is analyzed numerically. When the beam size is much larger than the size of object, the results in the paper can reduce to those of an incident plane wave. On the other hand, for the case of roughness statistical parameter close to zero, only the forward scattering has a parent difference compared with the result of gaussian beam scattering from smooth sphere.     

A modified geometrical optics method for scattering by dielectric bodies

A method based on ray optics is developed for calculating the scattering from dielectric bodies. The fields of geometrical optics are used except for two types of rays where the fields must be corrected from physical optics solutions. The customary advantages of ray techniques are realized, namely, a simplicity in the resulting formulas, a ready interpretation of the scattering mechanism and the possibility of extension to a wider class of problems through the inclusion of additional rays. The method has been applied to several lossless dielectric shapes: the circular cylinder, the sphere, the prolate spheroid and to a lossy dielectric shell. The relative dielectric constants considered range from 0.25 to 1.80, except in the case of the shell. The calculated results are compared with those obtained from boundary value solutions, with the exception of the spheroid where measured values are used. Good results are obtained for all sizes considered except those which are very small and behave as Rayleigh scatterers. The failure in the region of Rayleigh scattering is to be expected. Thus, for the class of dielectric scatterers treated here there is no region of scattering resonance corresponding to that of similar metallic shapes where the geometrical optics solution is no longer valid.     

涂覆球体的散射矩阵及极化分析

利用物理光学法及平面板块元法，通过场的Ｆｒａｎｚ表达式，对涂覆 散射阵进行了推导和计算，然后通过散射阵分析目标散场场的极化特性参数和最优极化，并给出了具体算体。     

粗糙球体和锥体目标激光散射非相干分量比

为了研究激光散斑对目标探测的影响,采用物理光学近似方法,进行了平面波激光照射在粗糙球体和圆锥体目标时对散射场统计特性的理论分析,推导了粗糙体目标散射场量的二阶统计矩, 数值计算了粗糙球体和锥体的非相干散射分量比随粗糙度、散射角、半径及目标材料的变化情况。结果表明,散射角的变化对粗糙球体散射非相干分量比有影响,粗糙度变大,目标的非相干分量占总散射分量的比重越大;随着粗糙球体半径变小,球体表面越粗糙;圆锥体目标散射非相干分量比的峰值位置随粗糙度变化而不同,但其峰值均位于镜反射方向上;金属类材料比非金属抛光铝材料的非相干分量比小,且半径变化与非相干分量比成正比。该研究结果可为更复杂目标激光散射特性和激光散斑探测、识别的研究提供一定的参考价值。     

Hybridization of curvilinear time-domain integral equation andtime-domain optical methods for electromagnetic scattering analysis

Full-field solutions for scattering and similar problems become prohibitively expensive for electrically large bodies. Fortunately, broadly “optical” methods become accurate as larger bodies are considered. Often, however, large bodies have significant features that are not electrically large and here hybrid approaches are appropriate. We present a novel hybridization of time-domain integral equation methods with time-domain physical optics (PO). For both methods, an isoparametric curvilinear treatment is adopted. The application of the approach is demonstrated by investigating the convergence of the solution for a pulse incident on a large target with a small feature (a 16-pulsewidth plate with a ~1/3-pulsewidth sphere placed centrally just in front of it). It is demonstrated that a full-field solution for the sphere and a fairly small region around the sphere, coupled with the PO solution of the remainder of the plate, produces a converged prediction of the time-dependent fields     

High-frequency analysis of EM scattering from a conducting spherecoated with a composite material

The asymptotic high-frequency solution for the scattered field produced by a plane wave incident on a perfectly conducting sphere coated with a thin composite material is considered. In the shadow region, the high-frequency scattered field is entirely associated with the usual creeping-wave diffraction. In the lit region, the scattered field can be expressed as a sum of geometrical optics (GO) and creeping-wave diffraction terms. The field in the caustic regions, where the ray solution fails, is excluded. The appropriate formula for each term is derived, and the result is presented in a form suitable for computation. Numerical results for the bistatic scattering patterns of the coated sphere show excellent agreement with the rigorous eigenfunction solutions     

时域物理光学法计算目标的微波短脉冲散射

目标的短脉冲散射问题本质上是其宽带散射特性,从时域上获取短脉冲散射问题更为直接。时域物理光学方法具有计算速度快、物理近似意义清晰明确等特点,可直接计算电大尺寸目标的微波短脉冲散射的时域波形。介绍了时域物理光学的理论公式,通过三角型网格剖分建立目标模型,引入Radon 变换计算目标的“冲击响应”,利用卷积计算获得目标的微波短脉冲散射时域波形。通过仿真算例进行验证,计算双导体球模型散射回波验证了该方法的可行性;计算大飞机的微波短脉冲散射波形展示了该方法处理电大尺寸问题的能力。用该方法计算的目标短脉冲散射回波波形可直接作为信号处理研究的输入。     

可见光波段非球形沙尘气溶胶散射和辐射特性的理论模拟

将非球形沙尘气溶胶视为具有一定的尺度谱和形状分布的随机取向椭球粒子群,利用T矩阵和改进几何光学方法（IGOM）模拟了非球形沙尘气溶胶粒子在可见光波段（0.47 ）的散射特性,并与实验室测量结果和等体积球形粒子的计算结果进行了比较。结果表明：利用具有一定的尺度谱和形状分布的随机取向椭球粒子模拟自然界中的非球形沙尘粒子散射特性是行之有效的;用等体积球形粒子得到的单散射相矩阵（特别是单散射相函数）与椭球粒子相比,具有明显的差异,而粒子形状对单散射反照率、不对称因子和消光效率的影响明显偏小。通过比较椭球和球形沙尘气溶胶在可见光波段的辐射特性说明在计算非球形沙尘气溶胶辐射特性过程中应考虑粒子形状对其单散射特性的影响。     

Time-domain electromagnetic computations using a modified EFIE kernel and field-source equilibrium relations

A form for the electric-field dyadic Green's function for free space is derived that allows explicit time evolution of the modified electric-field integral equation (EFIE) applied to surface scattering. The modified EFIE kernel, here called a "source function," has an integrable singularity in the source region, and is shown to be equivalent, in the frequency domain, to the standard dyadic Green's function. With definitions of "local" and "non-local" fields at a conductor surface, both electric and magnetic versions of the relations between non-local fields and equilibrium surface sources (currents and charges) are derived. These field-source equilibrium (FSE) relations are exact if all the non-local fields are included: the interaction fields, as well as the usual incident fields from distant sources. When the interaction fields are neglected, the magnetic-field version of the FSE relations becomes the usual physical optics approximation. Source functions and the FSE relations were used in two three-dimensional, time-domain numerical simulations to compute radiation patterns from a conical helical antenna driven at a fixed frequency, and scattering of a CW plane wave by a perfectly conducting sphere. This surface-scattering simulation was explicit but remained stable. Excellent agreement between the computed and known results validated the approach.     

The geometric optics contribution to the scattering from a large dense dielectric sphere

An analysis is presented for calculating the backscattered fields of an electromagnetic plane wave by lossless dielectric spheres of arbitrary density. This method involves the Watson transformation which serves to split the exact Mie solution, given as an infinite series, into the geometrical optics fields and the diffracted fields. The former comes from the illuminated region of the sphere and may be obtained from the geometrical optics method. The latter comes from the shadow region and consists of two different types of surface waves. One is a "creeping wave" analogous to that of perfectly conducting spheres. The other is a wave which enters the sphere and emerges as a surface wave in the shadow region. This wave is unique to dielectric spheres and is the stronger of the two surface waves. In the widely used geometric optics methods it is assumed that the optics fields are the dominant contributors even though stationary rays which are not in the direction of backscatter must be added in to give a degree of agreement with the exact Mie series results. In this paper we derive the optics fields and show that they differ in some respects from those obtained by the geometric optics method. They are smaller than heretofore assumed and contribute negligibly to the backscatter in this particular range ofka(4-20). Using our rigorous approach we can show the diffracted fields to be the major contributors to the total backscatter. Numerical results for the backscattering cross sections using diffracted and optics fields, and optics fields alone will be presented for relative index of refraction of 1.6. The agreement between our results (diffracted and optics) and exact results from the Mie series is excellent. A subsequent paper will be concerned with the diffracted fields.     

光镊对血红细胞横向光阱力的研究

在强聚焦激光光场中,以射线光学(ray-optics)计算模型为基础,对几何尺寸远大于光波长的米氏球状粒子所受激光微束横向(沿y轴方向)光阱力进行了计算。并且在给定参数条件下,进行了数值仿真,根据仿真结果,讨论了光束束腰半径、激光波长与光阱品质特性的关系。并对血红细胞进行了实验研究,得出了横向光阱力与微粒中心偏离光轴距离的关系曲线,实验结果和理论相一致。     

Skin optics

Quantitative dosimetry in the treatment of skin disorders with (laser) light requires information on propagation of light in the skin related to the optical properties of the individual skin layers. This involves the solution of the integro-differential equation of radiative transfer in a model representing skin geometry, as well as experimental methods to determine the optical properties of each skin layer. These activities are unified under the name skin optics. This paper first reviews the current status of tissue optics, distinguishing between the cases of: dominant absorption, dominant scattering, and scattering about equal to absorption. Then, previously published data as well as some current unpublished data on (human) stratum corneum, epidermis and dermis, have been collected and/or (re)analyzed in terms of absorption coefficient, scattering coefficient, and anisotropy factor of scattering. The results are that the individual skin layers show strongly forward scattering (anisotropy factors between 0.7 and 0.9). The absorption and scattering data show that for all wavelengths considered scattering is much more important than absorption. Under such circumstances, solutions to the transport equation for a multilayer skin model and finite beam laser irradiation are currently not yet available. Hence, any quantitative dosimetry for skin treated with (laser) light is currently lacking.     

太赫兹波在粗糙金属球体目标上的散射特性

太赫兹波在粗糙金属球面散射中相干散射与非相干散射同时存在.在低频端散射主要为相干散射,而相干部分随球表面粗糙度增大迅速递减;在高频端散射主要为非相干散射,且散射结果与辐射的分布方式及分布规律有关,结果通常不是唯一的.高端与低端的散射结果通常相差很大,但当粗糙金属球表面的粗糙度服从高斯分布时,高频端雷达散射截面的结果与低频端雷达散射截面的结果近似一致.     

复杂雷达体的二次电磁散射

讨论了用几何光学和物理光学方法计算两面元对上的二次电磁散射。建立了二次散射理论模型，并利用快速射线追踪搜索算法计算了典型形体及复杂目标的二次散射。计算表明该方法结果与理论值非常接近，而且处理计算速度非常快，在复杂目标可视化预估系统中可提高预估速度和可靠性。     

平面上方小球的光散射计算

本文基于扩展的Mie理论方法求解平面上方小球的散射问题。通过建立小球和平面的模型,解决小球和平面的边界条件问题,并利用矢量波函数展开的方法求得了散射场。强调了小球与表面的相互作用。散射总场是小球本身的散射场,平面反射场,和小球与平面之间的相互作用场三者的叠加。结论给出了均匀介质小球的微分散射截面图并进行了详细讨论。     

A fast physical optics (FPO) algorithm for double-bounce scattering

The fast physical optics (FPO) method for computing back-scattered fields over ranges of aspect angles and frequencies is extended to encompass double-bounce scattering. Computations are performed within the framework of the physical optics approximation appropriate in the high-frequency regime. The proposed algorithm is directly applicable to fixed angle bistatic configurations and a variety of double scattering settings. The method comprises two steps: 1) decomposition of the scatterer into subscatterers and computation of the scattering amplitudes of all pairs of subscatterers and 2) interpolation, phase correction, and aggregation of the scattering amplitude patterns of all subscatterer pairs into the pattern of the entire scatterer. The proposed method is especially suited for generating synthetic data for radar imaging simulations.     

Incremental length diffraction coefficients for the shadow boundaryof a convex cylinder

Incremental length diffraction coefficients (ILDCs) are obtained for the shadow boundaries of perfectly electrically conducting (PEC) convex cylinders of general cross section. A two-step procedure is used. First, the nonuniform (NU) current in the vicinity of the shadow boundary is approximated using Fock (1965) functions. The product of the approximated current and the free-space Green's function is then integrated on a differential strip of the cylinder surface transverse to the shadow boundary to obtain the ILDCs. This integration is performed in closed form by employing quadratic polynomial approximations for the amplitude and unwrapped phase of the integrand. Examples are given of both the current approximations and the integration procedure. Finally, as an example, the scattered far field of a PEC sphere is obtained by adding the integral of the NU ILDCs of a circular cylinder along the shadow boundary of the sphere to the physical optics (PO) far field of the sphere. This correction to the PO field is shown to significantly improve upon the accuracy of the PO far-field approximation to the total scattered field of the sphere     

复杂目标多次散射计算的高频混合方法研究

在物理光学方法基础上,结合几何光学方法和射线跟踪方法,导出一种能有效计算复杂目标多次散射的高频混合方法一区域投影物理光学方法（AP／PO）。为验证方法的有效性,应用AP／PO方法计算了角反射器和相互垂直的两个导体板的雷达截面,计算结果与测量结果有很好的一致性。通过对某飞机模型RCS的计算及与实验数据对比,进一步证明AP／PO方法适用于复杂目标多次散射的计算。     

Physical optics polarization scattering matrix for a top hatreflector

The polarization scattering matrix for a cylinder on a circular disk is derived subject to the physical optics (PO) approximation. The multiple scattering between the cylinder and the circular disk ground plane is obtained by invoking image theory, and is expressed as a bistatic return from the cylinder and its image, due to the image field. Results for a particular case are evaluated and compared with measurements