无序激光器激光出射的空间分布

讨论一种散射,导出相应的分布函数。分析增益区几何形状,运用蒙特卡罗法模拟光子在无序介质中的随机行走,得到激光出射的空间分布。结果表明：在界面法线方向,激光强度最强,随角度增大强度逐渐减弱,并且光强关于法线旋转对称。激光出射的空间分布与无序介质中光子平均自由程有关,据此可通过测量激光出射的空间分布来反推光子平均自由程。出射的激光光子绝大多数（一般95%以上）都是在离界面3个平均自由程距离内散射出来的。     

超短脉冲在散射介质中传播的时间和空间特性研究

采用蒙特卡罗方法,研究了散射介质各向异性因子等参数对超短脉冲在散射介质中传播的时间和空间分布以及各散射阶次光子强度比例分布的影响。研究结果表明,当各向异性因子或吸收系数减小、散射系数或传播深度增大时,脉冲的时间展宽和空间扩散都随之增强,高散射阶次光子强度比例也随之增大。     

部分空间相干光经准均匀介质的散射

基于一阶波恩近似,研究了部分空间相干平面光经准均匀介质的散射特性,得到了远场散射光谱强度和光谱相干度的解析表达式。讨论了入射光的空间相干性和介质特性对散射场光谱强度和光谱相干度的影响。比较了入射光是完全相干光和部分相干光时,散射场光谱强度和相干度的区别。研究结果表明:入射光的空间相干性对散射场光谱强度的分布有重要影响;随着入射光空间相干长度的增大,光谱宽度减小;光谱相干度随着入射光束腰宽度或介质有效半径的增大而减小。当准均匀介质的有效半径和相干长度、入射光的空间相干长度满足一定条件时,散射光是完全相干光。     

半无限空间中光子密度波扩散方程的求解

改进了原无限大模型，求解了无限大空间中光子密度波的扩散方程，找出了相应条件下的Green函数，演算出了与入射面相对应的光子通量和介质中的吸收系数改变量之间的关系，所用模型更接近于实验研究情况，所得结论能直接用于多散射介质中成像问题的实验研究。即通过对光子通量的检测，反演出吸收系数改变量的分布，从而实现用光子密度波理论在多散射介质中的成像问题的解决。     

光子增加相干叠加态的Wigner函数

利用算符作用在参考态上产生新的量子态的方法 ,将产生算符作用在相干叠加态 上,构造了光子增加相干叠加态。给出了其归一化系数和Wigner函数的解析表达式。通过 数值计算,讨论了相干态平均光子数和两相干态的相位差对Wigner函数的影响。研究结果 表明,随两相干态的相位差增大,Wigner函数的负值深度和负值区域时而增大时而减小, 它的负部 体积随相位差的演化呈现出准周期性演化,表明选择合适的相位差值对增强量子态的量子特 性有利;另一方面,随相干态平均光子数逐渐增大,Wigner函 数的负部体积增大,表明光子增加相干叠加态的量子特性增强。     

多散射介质中光子密度波扩散方程的求解

将光源作为一个复数点光源,根据实验研究模型的要求,求解了多散射介质中光子密度波扩散方程,找出了与入射面相对应面的光子通量和介质中吸收系数改变量之间的关系。所得结论能直接用于多散射介质中成像问题的实验研究之中,即通过对光子通量的检测,反演出吸收系数改变量的分布,从而达到多散射介质中的成像问题的解决。     

无序增益介质中光的相干背散射的放大

十几年前已证实光的相干背散射的增强是光的一种弱定域现象。这种增强归因于沿着某多次散射路径传播的光波和在其时间反演路径上传播的光波之间的干涉．最近Lawandy等发现弥散着强散射体的激光染料的辐射呈现出多模激光振荡器辐射的光谱和时域方面的特性，尽管该系统并不具有外腔。上述实验开辟了一个新的领域，即不仅可以研究无序损耗介质中的散射液的相干，而且可以研究无序有增益的体系中的散射波的相干效应。本文报道在R-640染料和TiO2散射微粒的厚样品中，对背散射及其放大随不同实验条件的变化的进一步研究．我们的样品是采用Rhodam…     

无序一维三元光子晶体的能带特性研究

运用光学传输矩阵理论，研究了具有无序结构的一维三元光子晶体的能带特性。结果表明，与周期结构相比，无序结构可以显著地拓宽光子晶体的光子带隙；随取无序膜层数目的增加，带隙逐渐变宽，3种折射率介质均取无序的情况下，带隙拓宽到550～1800nm的区间范围，是周期结构光子晶体带隙宽度的2倍多。讨论了无序度、不同折射率分布对带隙的影响，随着无序度和高低折射率的差别的增大，带隙变宽。     

石墨烯声子平均自由程随温度变化规律研究

考虑原子的短程相互作用和原子的非简谐振动,推导出石墨烯的声子平均自由程随温度变化的关系式,探讨了非简谐振动对它们的影响。结果表明:石墨烯的声子平均自由程随温度升高而非线性减小,温度低于20 K时,自由程随温度升高而急剧减小;温度高于300 K时,自由程随温度升高而减小的速度逐渐减慢;声子平均自由程随温度的变化率dl/dt均为负值,且随着温度升高而减小。在极低温条件下(T<20 K),自由程的温度变化率随温度升高而变化很大,此后,变化速度逐渐减慢;与简谐近似的值相比,非简谐项引起的声子平均自由程及其变化率的差值均随温度升高而增大。     

相干背散射法测量微结构光纤中的光子局域化

光子局域化是光子晶体的重要性质,提出采用相干背散射法对微结构光纤(MOFs)进行测量,为了便于理解并解释光子局域化的理论,在可见光波段分别测量了石英晶体、集束式微结构光纤和双芯型微结构光纤,应用光波经典扩散理论,搭建了实验平台。实验结果表明,对石英晶体的探测没有发现相干现象,而在575 nm左右微结构光纤中产生了很强的干涉。分析认为是由于二维微结构光纤的端面中分布着从节区到脉区变化的周期结构,尺寸约为500~600 nm,相当于一些不连续的散射粒子,当入射575 nm光波长和这些粒子的尺寸相接近时,满足了光子局域化的条件,便产生了强烈的背相散射相干现象。     

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     

Electromagnetic modeling of multipath scattering from breakingwater waves with rough faces

The low-grazing-angle (LGA) backscattering from one-dimensionally rough surface profiles approximating breaking water waves with roughened front faces has been numerically examined. The added front-face roughness approximates that expected from wind generation. The reference "exact" backscattering was found using a numerical technique based on the moment method. A model-based approach to predict the backscattering was also implemented. In this, the crest scattering was found directly using the moment method, the multipath scattering was modeled using physical optics, and the distributed-surface scattering from the small-scale roughness was found from the two-scale model. The calculations show that the roughness adds incoherent components to both the vertically (VV) and horizontally (HH) polarized scattering cross sections. At VV, this is due to the random scattering from the small-scale roughness, while at HH it results from random changes in the multipath interference due to the large-scale roughness. As the mechanisms for the incoherent scattering are independent, it is difficult to predict the magnitude of the HH-to-VV backscattering ratio that will occur with specific realizations of the roughness from the underlying breaking-wave shape alone, particularly with large rms roughness added. Overall, the model-based calculations give a good prediction of both the coherent and incoherent scattering coefficients     

半导体双光子吸收的阈值光强研究

研究非共振条件下光脉冲在半导体中传输引起的非线性吸收动力学过程，在薄样品近似下推导了描术双光子吸收及其诱导自由载流子吸收相对强弱的阈值光强公式，分析了样品厚度对阈值光强的影响，解释ＧａＡｓ双光子吸收系数的测量值在很大范围内变化的主要原因。     

浑浊水体激光后向散射特性仿真与实验研究

由于水体的吸收和散射作用,光束能量在传播的过程中会产生衰减,激光脉冲会被展宽,制约着水下激光雷达的探测范围和探测精度。文中以浑浊水体环境下水下弱小目标探测为应用背景,建立了水下光子传播的蒙特卡洛仿真模型,模拟了不同衰减系数和散射率的水体后向散射回波信号,并对相应的水体后向散射回波信号变化趋势进行了分析。仿真结果表明：随水体衰减系数的增加,近场水体激光回波信号接收光子数逐渐增多;随水体散射率的增加,回波信号光子消亡速度逐渐降低。开展了不同浊度下的激光雷达回波信号的测试实验,实验结果表明：随水体衰减系数的增加,水体激光后向散射回波幅度逐渐增高,脉冲宽度逐渐展宽。在进行浑浊水体水下弱小目标探测时,随水体衰减系数的增加,应通过逐渐减小激光器能量或接收系统增益来增强水体回波与目标回波之间的差异,以此提高浑浊水体水下弱小目标探测的信噪比。实验验证了理论与仿真结果,为浑浊水体环境下水下弱小目标激光探测系统在不同水质下的激光能量选取、接收系统增益设计等提供理论支撑。     

双光子光折变材料中的刚性光伏孤子

为了研究不同参量对双光子光折变材料中的刚性光伏空间孤子的影响,推导出了刚性光伏空间孤子的演化方程,根据实际的物理条件,采取一定的数学近似,对演化方程进行化简,从而得出演化方程的亮暗孤子的解析解,并讨论了不同参量,如光伏电场强度、背景暗辐射光强度、启动光强度、吸收系数和光波长对孤子的影响。结果表明,除了光伏电场强度,亮孤子的宽度随其增加而减小之外,亮孤子的宽度都随其它参量的增加而变大,只是变大趋势的速度不同。研究结果可为空间光孤子理论发展提供理论依据。     

NUMERICAL SIMULATIONS OF POLARIZED SCATTERING FROM RANDOM CLUSTERS OF SPATIALLY-ORENTED, NON-SPHERICAL SCATTERS

Employing multiple scattering formulation of T-matrix method, numerical simulations are developed and applied to polarized scattering from random clusters of spatially-oriented, non-spherical particles. Polarized scattering is numerically presented for the functional dependence on particle shape, size, spatial distribution and orientation, and other physical parameters. Numerical calculations of backscattering from randomly clustered particles are well compared with that from independent particles and clusters. It can be seen that spatial distribution and orientation of non-spherical particles can have significant effect on scattering.     

Effect of multiple scattering on line-of-sight propagation

This paper deals with the effect of small random variations in the atmosphere's refractive index upon electromagnetic waves propagated along a line-of-sight path. These fluctuations are considered to be due only to the turbulent character of the medium, and the effect of the earth and other bodies is taken to be negligible. Previous analyses of this problem have utilized the first Born or single-scattering approximation. This method, however, assumes that each part of the scattering region is illuminated by the incident wave, i.e., that any incident wave is scattered at most once. The applicability of this approximation in the microwave region is questionable since the mean free path for scattering at high frequencies decreases as the square of the frequency. If at microwave frequencies the transmission distance is large compared to the mean free path length multiple-scattering effects are of importance. It is these effects which are considered here. Multiple-scattering analysis is used to determine expressions for the mean value, mean-square value, and variance of the field at a single receiver. Also determined is the correlation function for zero time difference for the fields at two spaced receivers, as well as the mean value, mean square, and variance of the difference of the fields at two spaced receivers.     

稀疏矩阵规范网格结合物理双网格分析介质海面散射特性与试验验证

采用传统数值方法计算介质粗糙海面后向电磁散射时,波束照射面积随擦地角减小急剧增大,需要消耗大量的内存和计算时间。稀疏矩阵规范网格法用快速傅里叶变换计算远区相互作用,极大地提高了计算速度,同时基于物理的双网格法,依据格林函数在介质和自由空间中随作用距离的衰减特性,采用不同分区的网格划分技术,有效减少了介质粗糙面计算所需的内存量。该文基于Monte-Carlo方法产生不同海情PM谱海面,将上述两种方法结合,数值研究了S和Ku波段后向散射特性随擦地角的变化,并与造波池海面后向电磁散射试验进行对比。试验采用连续波扫频方法测量了造波池模拟的中低海情和缩比高海情1维PM谱海面后向散射系数。计算结果与测量数据相吻合,证明了方法具有较高的效率和可行性。结果分析表明,不同海情下海表面相关长度和散射特性存在明显差异。     

Narrowband acoustic Doppler volume backscattering signal .I. Evolutionary spectral analysis

This paper examines the statistical properties of the narrowband Doppler volume backscattering process and analyzes its evolutionary spectrum. After clarifying the mechanism of both the finite duration Doppler effect and the continuously space-shifted integration process, the first two order time-varying statistics under a more general assumption, i.e., von Mises distribution, of random phase are derived. The generalization permits nonuniform phase tendency, which occurs in layered medium scattering. Based on the locally stationary process model, the evolutionary spectrum of the signal is derived. It is shown that the variation of the backscattering strength enters the spectrum as an amplitude modulation, whereas the variation of the random phase distribution acts as both the amplitude modulation and the frequency modulation. Finally, the observability of the average flow speed using the spectral centroid estimate is discussed.