沙丘粗糙面的二次极化电磁散射

研究了新月形沙丘粗糙面的二次极化电磁散射. 结合射线追踪理论, 由一次散射面元的反射场照射到二次散射面元, 采用基尔霍夫近似推导了二次散射面元的二次极化散射场. 计算结果表明二次极化散射结果在特定的角度和类型范围内有显著影响. 在电磁波射向背风坡时可以发现其同极化散射截面在入射角较大时大于其他入射方向的结果, 入射角在休止角附近时的交叉极化散射截面出现峰值, 以及前后狭长沙丘之间的二次极化散射特别突出. 本文结果可用于反演分析沙漠地区的风场信息. 关键词： 新月形沙丘 二次极化散射 射线追踪 休止角     

Fractal characteristics investigation on electromagnetic scattering from 2-D Weierstrass fractal dielectric rough surface

A normalized two-dimensional band-limited Weierstrass fractal function is used for modelling the dielectric rough surface. An analytic solution of the scattered field is derived based on the Kirchhoff approximation. The variance of scattering intensity is presented to study the fractal characteristics through theoretical analysis and numerical calculations. The important conclusion is obtained that the diffracted envelope slopes of scattering pattern can be approximated as a slope of linear equation. This conclusion will be applicable for solving the inverse problem of reconstructing rough surface and remote sensing.     

Determination of surface profile statistics from electromagnetic scattering data

A new reverse Monte Carlo method for the determination of the surface profile statistics from differential reflection data for the scattering of electromagnetic radiation from rough surfaces is presented. The method is used to extract the power spectrum of the surface profile from scattering data recently measured by West and O'Donnell [J. Opt. Soc. Am. A 12, 390 (1995)]. Excellent agreement with the power spectrum of the surface profile measured by contact profilometry is obtained.     

Grazing-angle scattering of electromagnetic waves in periodic Bragg arrays

A new powerful approximate approach for the theoretical analysis of Bragg scattering in oblique strip-like periodic arrays with the scattered wave propagating almost parallel to the array boundaries – grazing-angle scattering (GAS) – is introduced and justified. This approach is based on allowance for the diffractional divergence of the scattered wave by means of the parabolic equation of diffraction and Fourier analysis. The divergence is demonstrated to be an intrinsic physical cause of GAS. Detailed theoretical analysis of steady-state GAS is carried out for bulk and guided optical modes. It is demonstrated that the most interesting feature of GAS in arrays of width that is greater than a critical width is a unique combination of two strong simultaneous resonances with respect to frequency and angle of scattering. In such wide arrays, GAS is demonstrated to be not only unusually sensitive to angle of scattering, but also to small variations of array width and grating amplitude. Entire concentration of the resonantly strong scattered wave inside the array is shown to be possible. A relationship between GAS, conventional Bragg scattering, and extremely asymmetrical scattering (i.e. where the scattered wave propagates parallel to the array boundaries) is analysed. Applicability conditions for the used approximations and obtained results are derived and discussed.     

Scattering of an electromagnetic wave from a slightly random dielectric surface: Yoneda peak and Brewster angle in incoherent scattering

Abstract

The scattering of an electromagnetic wave from a two-dimensional, slightly rough dielectric surface is studied based on the stochastic functional approach. It is shown that in the case of TM(p)-polarized incidence there exists a zero in the incoherent scattering at the angle we call the ‘Brewster scattering angle’, which depends on the incident angle in contrast to the Brewster angle of coherent reflection which is independent of the incident angle, that a ‘quasi-anomalous scattering’ can generally occur in the optically denser medium at the critical angle of total reflection in both TE(s)- and TM(p)-polarized incidence, regardless of which side of the random surface is illuminated, and that the Yoneda peak in the x-ray scattering can be interpreted as a special case of the quasi-anomalous scattering which becomes sharper when the relative refractive index becomes closer to unity as in the x-ray region. Cross-polarized scattering and enhanced backscattering due to the second-order effect are also calculated.     

Numerical study of electromagnetic scattering from one-dimensional nonlinear fractal sea surface

In recent years, linear fractal sea surface models have been developed for the sea surface in order to establish an electromagnetic backscattering model. Unfortunately, the sea surface is always nonlinear, particularly at high sea states. We present a nonlinear fractal sea surface model and derive an electromagnetic backscattering model. Using this model, we numerically calculate the normalized radar cross section (NRCS) of a nonlinear sea surface. Comparing the averaged NRCS between linear and nonlinear fractal models, we show that the NRCS of a linear fractal sea surface underestimates the NRCS of the real sea surface, especially for sea states with high fractal dimensions, and for dominant ocean surface gravity waves that are either very short or extremely long.     

An efficient and stable spectral method for electromagnetic scattering from a layered periodic structure

The scattering of acoustic and electromagnetic waves by periodic structures plays an important role in a wide range of problems of scientific and technological interest. This contribution focuses upon the stable and high-order numerical simulation of the interaction of time-harmonic electromagnetic waves incident upon a periodic doubly layered dielectric media with sharp, irregular interface. We describe a boundary perturbation method for this problem which avoids not only the need for specialized quadrature rules but also the dense linear systems characteristic of boundary integral/element methods. Additionally, it is a provably stable algorithm as opposed to other boundary perturbation approaches such as Bruno and Reitich’s “method of field expansions” or Milder’s “method of operator expansions”. Our spectrally accurate approach is a natural extension of the “method of transformed field expansions” originally described by Nicholls and Reitich (and later refined to other geometries by the authors) in the single-layer case.     

A high accuracy algorithm for 3D periodic electromagnetic scattering

We develop a highly accurate numerical method for scattering of 3D electromagnetic waves by doubly periodic structures. We approximate scattered fields using the Müller boundary integral formulation of Maxwell’s equations. The accuracy is achieved as singularities are isolated through the use of partitions of unity, leaving smooth, periodic integrands that can be evaluated with high accuracy using trapezoid sums. The removed singularities are resolved through a transformation to polar coordinates. The method relies on the ideas used in the free space scattering algorithm of Bruno and Kunyansky.     

Observation of phase-matched electromagnetic scattering in an active dielectric cylinder

A theoretical investigation of oblique plane-wave electromagnetic scattering in an active dielectric cylinder predicted the existence of anomalous resonances at discrete plane-wave angles of incidence. These resonances may be understood as being due to a leaky-wave phase-matching boundary condition. Experiments were performed with active dielectric cylinders to confirm the existence of discrete resonances. Cross coupling between TE and TM modes was clearly detected for both active and passive scattering. Enhancement of active scattered field intensities was observed in experiments with finite-diameter pump and probe laser beams. Optical pumping of a dye solution was used to provide the gain.     

Effects of aggregation on the electromagnetic resonance scattering of dielectric spherical objects

Summary The effects of aggregation on the electromagnetic resonance scattering by small, strongly reflecting spheres are studied. Two or more spheres are allowed to cluster together to form rather complicated anisotropic scatterers whose extinction cross-section is calculated as a function of the wave number of the incident field in the neighbourhood of the lowest resonance of the individual spheres. The macroscopic absorption coefficient of a low-density random dispersion of clusters is also calculated in the same range of wave number. The results for two-, three- and four-sphere clusters are discussed. Work supported in part by C.N.R. through the G.N.S.M. and in part by the U.S. Army European Research Office through grant DAJA45-84-C-0005 and contract DAJA37-81-C-0895.     

The scattering of waves by a periodic surface

The paper derives a general formula for the scattering of electromagnetic (or sound) waves from a periodic, perfectly conducting (or perfectly rigid) surface. No restrictions are imposed on the angle of incidence, the size of the surface or the degree of roughness. Except for the basic Kirchhoff approximation, the method is exact. The results confirm, generalise or correct special cases obtained by more complicated methods.

---

, ( ) . , . . , , .

     

Double-resonant extremely asymmetrical scattering of electromagnetic waves in non-uniform periodic arrays

A new type of Bragg scattering – double-resonant extremely asymmetrical scattering (DEAS) of optical waves in oblique, non-uniform, periodic Bragg arrays is analysed theoretically and numerically. Steady-state DEAS is demonstrated to occur in the extremely asymmetrical geometry where the scattered wave propagates parallel to the front array boundary. The non-uniform array is represented by two joint uniform, strip-like, periodic arrays with different phases (and amplitudes) of the grating. DEAS is characterised by a unique combination of two simultaneous resonances with respect to frequency and phase variation at the interface between the joint arrays. As a result, a strong resonant increase in the scattered wave amplitude compared with the amplitude of the incident wave is predicted and investigated theoretically. The amplitude of the incident wave inside the array is also shown to increase resonantly in the middle of the array where the step-like variation in the phase of the grating takes place. The effect of different widths of the joint arrays, and magnitudes of the grating amplitudes on DEAS is analysed. Physical explanations of this type of scattering, based on the diffractional divergence of the scattered wave from one of the joint arrays into another, are presented.     

大尺度分层介质粗糙面电磁散射的特性研究

首先建立大尺度分层介质粗糙面散射的物理模型, 基于Stratton-Chu积分方程和Kirchhoff近似导出了粗糙面散射场的计算公式. 采用高斯随机粗糙面来模拟实际的分层介质粗糙面, 通过数值计算得到了正下视单站雷达接收到的后向散射回波. 理论推导了散射场强度与表面粗糙度之间的定量关系, 并从数值仿真的角度分析了表面和次表面的粗糙度对散射回波的影响, 给出了散射场随粗糙度变化的曲线. 最后考察了分层介质的电特性参数(介电常数和电导率)对分层粗糙面散射场的影响, 并对计算结果做出了分析.     

Wave scattering from a periodic random surface generated by a stationary binary sequence

This paper studies the scattering of a TE plane wave from a periodic random surface generated by a stochastic binary sequence using a stochastic functional method. The scattered wave is first expressed as a product of an exponential phase factor and a periodic stationary process. The periodic stationary process is then expressed by a harmonic series representation, that is a 'Fourier series' with 'Fourier coefficients' given by mutually correlated stationary processes. These stationary processes are regarded as stochastic functionals of the binary sequence and they are represented by orthogonal binary functional expansions with band-limited binary kernels. The binary kernels are determined up to the second order from the boundary condition. Then, several statistical properties of the scattering are calculated numerically and illustrated in figures. It is found that, in the binary case, the second-order scattering cross section has a subtractive term and becomes much smaller than the first-order one.     

Diffraction and scattering of TM plane waves from a binary periodic random surface

This paper deals with the diffraction and scattering of a TM plane wave from a binary periodic random surface generated by a stationary binary sequence using the stochastic functional approach. The scattered wave is represented by a product of an exponential phase factor and a periodic stationary process. Such a periodic stationary process is regarded as a stochastic functional of the binary sequence and is expressed by an orthogonal binary functional expansion with band-limited binary kernels. Then, hierarchical equations for the binary kernels are derived from the boundary condition without approximation. We point out that binary kernels obtained by a single scattering approximation diverge unphysically when the periodic random surface is zero on average, thus the effects of multiple scattering should be taken into account. The expressions of such binary kernels are obtained using the multiply renormalizing approximation. Then, statistical properties such as differential scattering cross-section and the optical theorem are numerically calculated with the first two order binary kernels and illustrated in the figures. It is found that the incoherent Wood's anomaly appears in the angular distribution of scattering even when the surface has zero average.     

Analysis of multiple scattering from two-dimensional dielectric sea surface with iterative Kirchhoff approximation

<正>An iterative method in the Kirchhoff approximation is proposed for high frequency multiple electromagnetic scattering from two-dimensional dielectric sea surface.The multiple interaction of the scattering field is characterized with the corrected electromagnetic currents of the wind-driven sea surface.The actual surface currents are approximated with the iterative solution of the corrected currents.A newly developed sea spectrum,Elfouhaily spectrum,is utilized to build the sea surface model.The shadowing correction is improved by the Depth-Buffer algorithm.The validity of the iterative Kirchhoff approximation is verified by the agreement of backscattering coefficients with the measured data.     

Fast iterative approach to the difference scattering from a dielectric target above a rough surface

1 Introduction Electromagnetic scattering from the target above or beneath a rough surface has at- tracted much interest during recent years, because of extensive applications to radar surveillance, target detection, ground radar probing, and so on[1―3]. In order to numeri- cally simulate scattering from composite model of the target and underlying rough sur- face, some fast numerical methods, such as general forward backward method and spec- trum acceleration algorithm (GFBM/SAA)[4,5], fin…