Incoherent radiation of amplifying random media

Incoherent radiation of amplifying random media was investigated using Monte Carlo simulation and the characteristics of random laser were observed The entire emission spectra of the random media would become narrow abruptly when pumping energies exceeded certain threshold values, and as the pumping energies further increased, distinct sharp peaks would emerge on the spectrum background. The intensity of a certain spatial point within the scattering media was contributed from emissions of many frequencies. The intensity of a single frequency of the entire spectrum was contributed with emissions from wide spatial location and angle ranges. It has been pointed out that the incoherent radiation of amplifying random media is essentially different from amplified spontaneous emission without any feedback as well as conventional resonant-feedback laser. The explanation on distinct sharp peaks is that rare photons have experienced rather more scattering events and walked longer paths in amplifying random media, which accumulates more gain for these photons.     

增益随机散射介质中的非相干辐射

用蒙特卡罗方法仿真了增益随机散射体中的非相干辐射,观察了非相干随机激光的特性。当抽运能量超过一定阈值时,散射体的整体辐射谱突然变窄;随着抽运能量继续增大,在光滑谱背景上会出现分离尖峰;散射体内空间某位置处频率组成不是单一的;辐射谱中某单个频率的空间方向分布和位置分布比较广。增益随机散射体中产生的非相干随机激光本质上既不同于无反馈的普通放大自发辐射,又不同于相干反馈形成的常规激光。解释了非相干随机激光辐射谱上出现分离尖峰的原因,出现这种现象是由于少数光子在增益散射体中经历较多次数散射后得到了相对充分的放大。     

Monte Carlo simulation for millimeter wave propagation and scattering in rain medium

As the operating frequencies of communications systems more higher into the millimeter wave range, the effects of multiple scattering in precipitation media become more significant. This paper treats the problems of electromagnetic multiple scattering in rain medium by the Monte Carlo method. The em wave is regarded as a Markov chain of photon collisions in a medium in which it is scattered and absorbed. For the sake of simplicity, the polarization is not taken into account, the above mentioned problems are described by the scale integro-diffierential equation of transfer. When the plane wave through a random medium with particle size distribution, the technique of weighted average is used to characterize the radiation intensity, including average scattering, absorption coefficients and phase function. The Monte Carlo simulation algorithms are done for the rain attenuation and reflectance at millimeter wavelength region. Our computational results are in good agreement with experimental data of rain attenuation.     

无序介质的非相干背散射

讨论两种散射,导出相应的分布函数,运用蒙特卡罗法模拟光子在无序介质中的随机行走过程,得到非相干背散射空间分布及反射率。计算结果表明：1、散射不同,非相干背散射空间分布和反射率不同,非相干背散射反射率随光入射角增大而增大;2、非相干背散射反射率随无序介质吸收系数增大而减小,通过测量非相干背散射反射率可得吸收系数,进一步可得每次散射光子被微粒吸收的概率;3、非相干背散射反射率随光子平均自由程增大而减小,通过测量非相干背散射反射率可得平均自由程,这比检测相干背散射来得到平均自由程要容易。最后,讨论了背散射对无序激光器的影响。     

光在不同浓度介质中散射传输的偏振特性分析

文中以典型偏振蒙特卡罗模型为基础,引入了一种介质浓度系数的表征方法,设计实现了一个光在不同浓度介质中散射传输的三维仿真系统,分析了引入浓度系数方法的可行性,并讨论了光在不同浓度介质中散射传输的偏振特性影响.仿真实验结果表明,文中方法实现了散射介质浓度的表征.散射介质浓度对斯托克斯矢量各分量强度和偏振度有着直接影响;斯托克斯矢量各分量的强度随散射介质浓度增加呈近似指数衰减;光在散射介质中传输后,偏振度随浓度系数变化总体趋势呈"倒N"型.     

Finite-difference time-domain simulation of scattering from objectsin continuous random media

A three-dimensional (3D) finite-difference time-domain (FDTD) scheme is introduced to model the scattering from objects in continuous random media. FDTD techniques have been previously applied to scattering from random rough surfaces and randomly placed objects in a homogeneous background, but little has been done to simulate continuous random media with embedded objects where volumetric scattering effects are important. In this work, Monte Carlo analysis is used in conjunction with FDTD to study the scattering from perfectly electrically conducting (PEC) objects embedded in continuous random media. The random medium models under consideration are chosen to be inhomogeneous soils with a spatially fluctuating random permittivities and prescribed correlation functions. The ability of frequency averaging techniques to discriminate objects in this scenarion is also briefly investigated. The simulation scheme described in this work can be adapted and used to help in interpreting the scattered field data from targets in random environments such as geophysical media, biological media, or atmospheric turbulence     

A Monte Carlo method for radar backscatter from a half-space randommedium

The problem of electromagnetic multiple scattering in a random medium is treated by a Monte Carlo method, in which an incident beam of photons is progressively scattered by scattering centers in the medium. The theory characterizes each scattering by functions describing the probability of the photon being scattered or absorbed, and the probability of its being scattered into certain directions. This process is tracked until the photon is finally absorbed or backscattered into the receiver. Variance reduction techniques are introduced to reduce the computation time required for acceptable ensemble averages of the backscattering cross sections. Ellipsoidal dielectric scatterers are used to model circular disk-shaped leaves, elliptical disk-shaped leaves, and needle-shaped leaves, which are randomly distributed in a half-space medium. The Monte Carlo simulations give good comparison with experimental data of backscattering cross sections from fields of wheat, corn, and milo     

A Monte Carlo study of the rough-sea-surface influence on the radarscattering from two-dimensional ships

The generalized forward-backward (GFB) method was introduced in Pino et al. (1999) for computing the electromagnetic scattering from two-dimensional targets on a rough surface. The GFB method is used in this article to generate numerical data for a Monte Carlo simulation of the horizontally polarized radar cross section (RCS) of two-dimensional ship-like targets on random rough sea surfaces. The RCS is computed as a function of the incidence angle and wind speed for a large number of surface realizations. It is found that the mean RCS of a given target on a rough surface is generally lower than or equal to the RCS of the same target on a flat surface, while the maximum RCS is usually greater than or equal to the flat-surface case. It is also observed that the variations in the RCS introduced by the rough surface become less significant as the elevation angle approaches grazing     

Alloy scattering and high field transport in ternary and quaternary III–V semiconductors

A technique is described for the estimation of the influence of random potential alloy scattering on the high field transport properties of quaternary III–V semiconductors obtained by Monte Carlo simulation. The approach is based on an extension of a theoretical model for scattering in the ternary alloys. The magnitude of the scattering potential is an important parameter in alloy scattering, and three proposed models for calculating this potential are discussed. These are the energy bandgap difference, the electron affinity difference, and the heteropolar energy difference for the appropriate binary compounds.The technique is used in the Monte Carlo method to study the influence of alloy scattering on the transport properties of III–V quaternary alloys. The results of this study are used in a device model to estimate device parameters for FETs.     

Mueller matrix elements that characterize scattering from coatedrandom rough surfaces

The Mueller matrix completely characterizes scattered electromagnetic waves. It relates the incident to the scattered Stokes vectors. The Mueller matrix, which contains intensity and relative phase data, is very useful for remote sensing. The Mueller matrix characterizing scattering from coated two-dimensional (2-D) random rough surfaces is obtained from full-wave solutions for the scattered fields considered in the companion paper. The general bistatic scattering case is considered in the analysis. However, for the numerical simulations presented here, the backscatter case is considered in particular, since backscatter is usually measured in remote sensing. The uniformly coated 2-D random rough surfaces are assumed here to be homogeneous and isotropic, with a Gaussian surface-height joint probability-density function. The diffuse incoherent and coherent contributions to the Mueller matrix elements are evaluated. The computer simulations of realistic models of relevant physical problems related to remote sensing of irregular stratified media can be used to determine the optimum modes of detection involving the selection of polarization, frequency, backscatter angle, and the specific Mueller matrix elements most sensitive to changes in medium parameters     

水泥路面激光散射特性研究

为了通过激光散射特性识别不同粗糙程度水泥路面,设计了路面粗糙度测量系统.使用千分表测量水泥路面高度分布,计算高度均方根与相关长度.根据以上参量采用功率谱频域滤波生成随机粗糙表面以模拟水泥路面,通过切平面近似将粗糙面离散为大量微面元,由菲涅耳公式计算本地场,利用蒙特卡罗方法获取不同偏振光入射条件下粗糙面双向与后向散射光强...     

Multiple Scattering Solution of Millimeter Wave Propagation in Strong Sandstorm

As the operating frequencies of communication systems more higher into the millimeter wave range, and the density of particles in medium is more denser, the effects of multiple scattering in sandstorm become more significant. This paper treats the problems of electromagnetic multiple scattering in strong sandstorm by the Monte Carlo method. Based on the analytical theory of multiple scattering, the millimeter wave propagation and scattering in discrete random media are investigated by means of the particle-tracking technique. The millimeter wave is regarded as a Markov chain of wave particle collisions in a medium in which it is scattered and absorbed. Considering the effect of multiple scattering, millimeter wave attenuation induced by strong sandstorm is simulated numerically. The values of theoretical calculation are in good agreement with the measured results of simulated experiment at 34 and 93 GHz.     

基于主动式圆偏振光照射的探测建模与分析

圆偏振光有良好的“偏振记忆”效应,为了验证在 强散射介质中通过圆偏振光的旋性 差异可以有效抑制散射光的影响,实现复杂背景下的目标探测功能。从经典的米氏散射 理论和菲涅尔反射理论出发,结合偏振蒙特卡罗方法,构建了主动式圆偏振光在散射介质中 探测目标的模型,通过追踪每个光子的偏振态变化,统计分析了圆偏振光经过介质散射和目 标反射后的Stokes矢量信息。仿真结果表明,文中建模方法可以明显区分出散射介质中是否 存在被探测目标;不同的介质环境和不同的探测距离都会对目标的偏振成像造成不同程度的 衰减,而圆偏振差分成像及圆偏振度成像均可实现偏振成像的增强处理。文中建模方法可以 为全偏振探测的理论研究及实际应用提供借鉴。     

Polarimetric channel characterization of foliage for performanceassessment of GPS receivers under tree canopies

The attenuation, depolarization, and fluctuation of a microwave signal going through a tree canopy are investigated by developing a Monte Carlo based coherent scattering model. In particular, the model is used to analyze the performance of Global Positioning System (GPS) receivers under tree canopies. Also the frequency and time-domain channel characteristics of a forest are investigated when a transmitter is outside and a receiver is inside a forest. A fractal algorithm (Lindenmayer system) is used to generate the structure of coniferous or deciduous trees whose basic building blocks are arbitrarily oriented finite cylinders, thin dielectric needles, and thin dielectric disks. Attenuation and phase change of the mean field through foliage is accounted for using Foldy's approximation. Scattering of the mean field from individual tree components and their images in the underlying ground plane are computed analytically and added coherently. Since tree trunks and some branches are large compared to the wavelength and may be in the close proximity of the receiver, a closed-form and uniform expression for the scattered near-field from dielectric cylinders is also developed. Monte Carlo simulation of field calculation is applied to a cluster of trees in order to estimate the statistics of the channel parameters, such as the probability density function (pdf) of the polarization state of the transmitted field, path loss, and the incoherent scattered power (the second moment of the scattered field), as a function of the observation point above the ground     

长波长红外光大气信道传输

采用光子追踪法,模拟光子在大气信道中的随机迁移路径及散射后随机迁移方向。引入光束发散角和接收视场角等参数,建立了包含散射作用的长波长红外大气信道传输模型。运用蒙特卡洛方法进行仿真,分析了雾环境下长波长红外光的大气传输特性。与朗伯-比尔定律进行对比,发现在能见度较低、通信距离较近时接收机接收的散射能量不能被忽略。分析了通信距离、能见度、光束发散角、接收视场角对链路损耗的影响,分析了不同阶次散射对接收机接收能量的影响。发现在给定参数条件下,四阶及以上高阶次散射对接收机接收能量几乎可以忽略。     

基于柱面扫描近场成像的RCS 测量方法研究

该文提出一种基于柱面扫描近场成像的RCS(Radar Cross Section)测量新方法:以理想的各向同性点散射中心模型为核心假设,通过详细的理论推导给出了一种具有通用性的基于柱面扫描近场成像的RCS 测量方法。该方法先得到目标的3 维雷达散射图像,再通过这些等效理想散射中心的散射场叠加获得远处散射场进而给出目标的远场RCS 值。该方法不仅能得到被测目标的3 维雷达散射图像,还能获得一定立体角域的目标远场RCS。相比只能得到2 维雷达散射图以及2 维平面角域RCS 结果的圆迹扫描测试相比,该文所提的柱面扫描测试能得到更多的目标散射信息,具有较强的实用性。仿真结果验证了新方法的可靠性。      

A numerical study of scattering from an object above a rough surface

A numerical model is applied in a Monte Carlo study of scattering from a three-dimensional penetrable object above a lossy dielectric rough interface. The model is based on an iterative method of moments solution for equivalent electric and magnetic surface current densities on the rough interface and equivalent volumetric electric currents in the penetrable object. Both time-and frequency-domain results are investigated to illustrate the relative importance of coherent and incoherent scattering effects in the sample problem considered. Results show that a four-path model using a reduced-reflection coefficient can be reasonable for coherent scattering predictions and that incoherent object/surface interaction effects can make significant contributions to received cross sections.     

A method based on the average granulometric size of speckle for characterizing the scattering media

A method based on morphological granulometric analysis for the speckle image is presented to estimate the scattering coefficient of the scattering media. It is experimentally demonstrated that the average granulometric size, derived from the successive opening operations, is approximately in inverse proportion to the scattering coefficient of the sample in a broad illumination. Furthermore, a theoretical model based on the spatial correlation function of the back scattered intensity for the experimental phenomenon is reported. The analysis demonstrates that the explicit theoretical relationship between average granulometric size of speckle and scattering coefficient is established, and the theoretical prediction is consistent with the experimental result.     

Analysis of circuit tolerance based on random set theory

Monte Carlo Analysis has been an accepted method for circuit tolerance analysis, but the heavy computational complexity has always prevented its applications. Based on random set theory, this paper presents a simple and flexible tolerance analysis method to estimate circuit yield. It is the alternative to Monte Carlo analysis, but reduces the number of calculations dramatically.     

激光探测尾流微气泡的偏振特性研究

为了研究尾流中激光照明气泡幕的散射光强度和偏振的特性,利用基于偏振光传输的蒙特卡洛模型,对偏振激光入射含气泡群水体的三维空间分布模式进行仿真计算。研究了气泡和气泡群在不同气泡尺度,不同散射角条件下的散射光强和偏振状态;分析了气泡幕的气泡数密度,厚度对于散射光强度和偏振状态的影响。研究表明,散射光的强度和偏振度对气泡尺度和散射角较为敏感,气泡尺度参数越大,散射光强和偏振特征越趋向于集中在传输方向的小角度散射;气泡幕的数密度和厚度越大,散射光的强度随散射角度变化的敏感度下降,退偏振效果增强。