Multiple scattering in random media

The paper is an application of a general microscopic approach to the theory of the average scattering matrix for a particle interacting with random scatterers. We present a detailed treatment for the case of uncorrelated positions of the scatterers. First, the general two-body additive approximation is used to truncate the hierarchy of correlation functions for fluctuations. It is shown that the self-energy is accurate through the fourth power of the individual scattering amplitude. Second, the hierarchy is terminated at the next stage. The self-energy is correct to the sixth power of the scattering amplitude.Work supported in part by the National Science Foundation under Contract No. NSF DMR 79-23213.     

Analysis of random laser scattering pulse signals with lognormal distribution

The statistical distribution of natural phenomena is of great significance in studying the laws of nature. In order to study the statistical characteristics of a random pulse signal, a random process model is proposed theoretically for better studying of the random law of measured results. Moreover, a simple random pulse signal generation and testing system is designed for studying the counting distributions of three typical objects including particles suspended in the air, standard particles, and background noise. Both normal and lognormal distribution fittings are used for analyzing the experimental results and testified by chi-square distribution fit test and correlation coefficient for comparison. In addition, the statistical laws of three typical objects and the relations between them are discussed in detail. The relation is also the non-integral dimension fractal relation of statistical distributions of different random laser scattering pulse signal groups.     

Multiple scattering in random media I. Restricted two-body additive approximation

We present a microscopic theory of the problem of finding the properties of a particle interacting with potentials located at random sites. The sites are governed by a general probability distribution. The starting point is the multiple scattering equations for the amplitude k ₁|T |k ₂ in terms of the individual scattering amplitudes k ₁|T |k ₂. We work with quantitiesA defined by k ₁|T |k ₂=k ₁|T |k ₂exp[i(k ₁–k ₂)R ]. The theory is based on a splitting of the fundamental equation forA into equations for the mean A and the fluctuationsAA . Neglect of the fluctuations yields the quasicrystalline approximation. We rearrange the equation forAA to isolate the collective part of the fluctuations. We then make the simplest microscopic truncation which is thatAA is a restricted two-body additive function of the site positions. With the contribution of the collective fluctuations, this yields results forA that are accurate to ordert ⁴.Work supported in part by the National Science Foundation under Contract No. NSF DMRWork supported in part by the National Science Foundation under Contract No. NSF DMR     

Introduction to light scattering by Gaussian random particles

Background, current status, and future prospects are offered for “Light scattering by Gaussian random particles: Ray-optics approximation” [1]. The stochastic geometry of the random particle is called the Gaussian random sphere. The radial distance of the Gaussian sphere is lognormally distributed. Two logarithmic radial distances at a given great-circle angle apart relate to one another according to the covariance function. Sample Gaussian particles can be conveniently generated using a Legendre polynomial expansion for the covariance function and a spherical harmonics expansion for the logarithmic radial distance. The ray-optics approximation consists of the geometric-optics and forward-diffraction parts fully accounting for polarization. It is valid for particles much larger than the wavelength of incident light and with central phase differences much larger than unity. The numerical ray-tracing algorithms are general and, in principle, applicable computationally to arbitrarily shaped non-spherical particles.     

Light scattering by nanosized systems with different spatial organizations

The integrodifferential equation method is used to study the spectrum of a nanoparticle colloid for the example of interaction of three arbitrarily arranged dielectric particles made up of nonresonant atoms (the eigenfrequency of the transition is far from the emission frequency) with incorporated barium atoms in an external optical radiation field. The effect on the light-scattering properties of a nanosphere in the ensemble of its distant “neighbors” is studied; an additional peak associated with them is observed as a frequency close to the resonance for an isolated nanosphere, which under certain conditions has higher intensity than the main peak corresponding to optical near-field resonance in a two-particle system. The dependence of the spectrum of the nanosized system on the geometric structure is studied, and it is shown that very precise tuning of the resonance frequency is possible by varying the angular distribution of the particles. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 641–646, September–October, 2006.     

Two-frequency mutual coherence function and its applications to pulse scattering by random rough surface

An analytic expression of the two-frequency mutual coherence function (MCF) was derived for a two-dimensional random rough surface. The scattered field was calculated by the Kirchhoff approximation, which is valid in the case that the radius of curvature of the surface is much larger than the incident wave length. The scattering problem of narrowband pulse was investigated to simplify the analytic expression of the two-frequency MCF. Numerical simulations show that the two-frequency MCF is greatly dependent on the root-mean-square (RMS) height, while less dependent on the correlation length. The analytic solutions were compared with the results of Monte Carlo simulation to assess the accuracy and computational efficiency. Supported by the National Natural Science Foundation of China (Grant No. 60571058) and the National Defense Foundation of China (Grant No. 51403020505DZ0111)     

Light scattering by Gaussian random ellipsoid particles: First results with discrete-dipole approximation

We introduce the stochastic geometry of a Gaussian random ellipsoid (GE) and, with the discrete-dipole approximation, carry out preliminary computations for light scattering by wavelength-scale GE particles. In the GE geometry, we describe the base ellipsoid by the three semiaxes a?b?c. The axial ratios b:a and c:a appear as two shape parameters additional to those of the Gaussian random sphere geometry (GS). We compare the scattering characteristics of GE particles to those of ellipsoids. Introducing irregularities on ellipsoids smoothens the angular scattering characteristics, in a way analogous to the smoothening of spherical particle characteristics in the case of GS particles.     

Comparison of principal modes and spatial eigenmodes in multimode optical fibre

Optical waveguide theory is an established part of optical physics. Yet only recently have fundamental phenomena such as spatial eigenmodes and principal modes been demonstrated experimentally. This work leverages recently developed techniques enabling detailed spatiotemporal characterisation of multimode fibre to provide new insights into the fundamentals of fibre propagation. This paper presents detailed analysis of all 420 of a fibre's principal modes and spatial eigenmodes and compares the similarity and differences between these two phenomena. It was found that even over very short lengths, the principal modes can not only significantly suppress modal dispersion but are also a more physically meaningful basis than spatial eigenmodes.

     

一维随机激光器中准态模的饱和特性研究

针对随机激光器的半经典理论是应用电磁场时域有限差分法数值求解半经典随机激光模型方程组，基于半经典理论数值研究了一维随机激光器中准态模式的饱和特性，获得了不同准态模的空间分布和频谱分布图。计算结果分析显示：准态模式的某些特征参数如局域程度将会影响系统的饱和特性。利用这一结果，可以通过单频抽运方法选择具有低阈值、高饱和强度的准态模式作为整个激光系统的稳态输出模式。     

超短THz脉冲在随机散射介质中的传播

用Monte-Carlo方法模拟了超短THz脉冲在随机散射介质中的传播.根据Mie理论计算出随机散射介质的散射系数和各向异性因子，研究了随机散射颗粒及介质厚度的大小对透射脉冲的影响.结果表明在Mie散射范围内，在相同的浓度下，颗粒尺寸越小，散射介质越厚，THz散射越严重，对透射脉冲的影响越大.散射同时降低了THz脉冲在随机散射介质中的成像分辨率. 关键词： 超短THz脉冲 随机散射介质 Mie理论 Monte-Carlo方法     

二维随机介质中准态模的频谱时间演化特性

采用时域有限差分法数值求解Maxwell方程组，分析了平面随机介质中光波模式的频谱时间演化特性.随机介质的特征可以用散射颗粒的随机构形、介质参数（如颗粒的尺寸和填充率等）以及介质形态等因素来描述.这些因素决定了随机介质在准稳态下模式的频率特性与数量，但具有不同因素的随机介质中模式的产生、选择与演化，具有大致相同的特征与速度.平面随机介质的这些冷腔特性，与传统光腔中模式的产生、选择和演化的特征非常相似. 关键词： 激光物理 随机激光器 无序介质中的光学特性     

基于增强瑞利反馈的单模窄线宽随机激光器

仿真说明了单模光纤(SMF)中瑞利散射(RS)的机理, 指出纤芯掺杂的不均匀性以及拉丝过程引起的光纤几何尺寸的随机变化是光纤中RS产生的主要原因, 并以此为基础制作了损耗为0.54 dB/km的散射光纤. 在通信波段, 5 km该散射光纤的瑞利背向散射(RBS)强度高于相同长度的SMF-28近5 dB. 在基于RBS单模随机激光器的数值模拟中, 大量的具有随机幅度和相位的纵模在经历不平坦增益的多次放大之后, 只有在增益最大点附近的模式能够克服损耗成为输出模式. 实验中以掺铒光纤作为增益介质, 500 m散射光纤提供随机反馈, 窄带布拉格光纤光栅(FBG)作为波长选择器件, 得到线宽约3.5 kHz、对比度近50 dB的单模激光输出. 与采用相同长度SMF-28的随机激光器相比, 其阈值电流降低了80 mA, 相同抽运条件下的最大输出功率提高了3 dBm. 该单模窄线宽随机激光器的输出波长的调谐特性仅由FBG的中心波长决定.     

随机激光器中准态腔的阈值与其局域化程度的关系

基于光波在有限随机介质中的局域化理论，利用有限时域差分法数值求解Maxwell方程 组，研究了随机介质中的激光现象，分析了准态模的放大与其 空间局域性的关系. 通过研究二维非增益随机介质中光波的局域化，确定了准态模的空间分 布和频谱特征. 通过引入增益，研究了准态模的放大过程和阈值特性. 结果表明空间局域化 强的准态模在增益介质中被优先放大，且有较低的阈值. 关键词： 随机激光器 准态模 随机介质中的光学特性     

任意截面介质波导导模的叠代矩量法分析

本文提出用以计算任意截面介质波导色散关系的叠代矩量法,并给出用叠代矩量法计算椭圆介质波导、三角形波导、平切圆波导、蛋形截面波导等的传播特性的实例.计算结果与有限元法、点匹配法等精确数值计算结果符合很好.本方法简便易行,计算工作量较小.     

Ti扩散LiNbO_3光波导的喇曼散射

本文报道Ti扩散LiNbO_3光波导中四种直角配置的喇曼散射光谱。实验中,我们观测到了在这些配置下异常的喇曼光谱现象。这一现象可归结为光波导中入射光TE/TM模式的变换行为。     

Anomalous neutron scattering and ferroelectric soft modes

It is suggested that anomalous neutron scattering could prove a powerful experimental tool in studying ferroelectric phase transition, the sublattice displacements of the soft modes as well as their symmetry characteristics.     

圆介质参考波导特征方程的近似式及其应用

本文将作为参考波导的圆介质波导模式的特征方程进行了有效的近似简化,使之成为独立的四个简单特征方程组。利用这些方程组作为参考波导的特征方程来计算各种复杂波导的传输参数时,能够大大简化计算工作。     

二维无序介质中横磁模的谱线宽度随抽运强度的变化特性

基于随机激光时域理论,数值研究了二维随机激光器中横磁模式的输出光谱线宽度,获得了谱线宽度与抽运强度的关系曲线,由此曲线可以计算横磁模式的很多特征参数.从谱线宽度的角度来看,横磁模式的输出特性结果与已有的横电模式结果相比,横磁模式具有较好的输出特性. 关键词： 随机激光器 偏振 谱线宽度 无序介质中的光学效应     

The heat capacity of small metallic grains studied by the random matrices theory

The random matrices theory is applied to a study of the heat capacity of small metallic grains. The numerical calculations indicate that the level distribution and the difference between the particles respectively with an even and an odd numbers of electrons are important for the heat capacity of the small metallic grains at a low temperature and the level correlation mainly affects the heat capacity at a high temperature.     

高斯型弱散射屏产生的像面散斑场的分布特性研究

弱散射屏产生的远场散斑由一个中央亮斑和一个分布于亮斑周围而与正态散斑类似的散斑结构, 根据弱散射屏远场的散斑图样, 人们假设弱散射屏产生的像面散斑为均匀背景与正态散斑两者相干叠加的结果, 但这种假设与实际像面散斑存在歧异, 基于上述情况, 本文利用4f高通滤波光学成像系统, 研究了高斯型弱散射屏产生的像面散斑场的统计特性, 得出只有表面均方根粗糙度与入射光波的波长相差不多时上述假设才是可行的结论. 关键词： 弱散射屏 f光学成像系统')" href="#">4f光学成像系统 像面散斑