沙尘天气下激光信号的传输特性

以小角度近似为条件,利用逐级递归的方法推导了激光信号在沙尘天气下的辐射传输方程,得到了多次散射下的光强分布函数,以及波长和不对称因子对光强的影响。同时,通过比较不同散射相位函数及沙尘粒子的散射特性,采用了修正的TTHG(Two Term Henyey-Greenstein)散射相位函数,更加全面地反映了沙粒散射后光强的变化规律。研究结果表明,随着光学厚度的增加,散射光强呈现出先增大后减小的趋势,且多次散射的比重相比于单次散射而言逐渐增大。当散射次数超过3次以上时,接收光强的变化可以忽略不计。相对于Mie理论下的结果而言,采用小角度近似理论,从辐射传输的角度分析沙粒的散射特性误差更小,实现了准确描述沙尘天气下激光信号传输特性的目的。     

沙尘气溶胶粒子群的散射和偏振特性

根据Mie散射理论,以对数正态分布函数描述沙尘气溶胶粒子群的粒径尺度分布,计算了沙尘气溶胶粒子群在0.2～40μm波段间对太阳短波辐射和地球大气长波辐射的单次散射反照率、散射相矩阵函数,揭示了不同相对湿度时,沙尘粒子群对入射辐射的散射和偏振的特征。结果表明,沙尘粒子群的单次散射反照率随着入射波长的增加有较大起伏,不同相对湿度条件下,变化趋势基本一致;在可见光、近红外波段单次散射反照率随湿度增加而变大,湿度95%时非常接近于1;大于10μm的热红外波段单次散射反照率随相对湿度增加而减小,具有较强的吸收辐射能力。散射辐射强度受湿度影响较小,随散射角的增加呈现先减小后增大的趋势,且增大的趋势随着波长的增加而减弱;不同波段上,线偏振和圆偏振随散射角和相对湿度变化存在差异;在前向和后向仅对入射辐射为圆偏振辐射产生圆偏振散射;散射光的偏振特性及其湿度差异主要表现在后向散射区,多以拱形形式体现。拱顶峰值散射角位置存在差异,且峰值散射角随相对湿度的降低向后向漂移。     

Monitoring of pathogen carrying air-borne tea dust particles by light scattering

In order to investigate whether tea (Camellia sinensis (L.) Kuntze) dust particles could be a possible carrier of the pathogen contaminated Mycobacterium a biotechnical procedure was used, and to verify the possibility of monitoring this dust, a laser based light scattering setup was designed and fabricated. Experiments were carried out using the strain Mycobacterium smegmatis mc² 155 as a model organism to study the effect on tea dust particles. Light scattering investigations on both M. smegmatis contaminated and uncontaminated tea dust particle samples were carried out as a function of scattering angle at 543.5, 594.5 and 632.5 nm wavelengths. The results have shown that the behavior of tea dust samples both with and without Mycobacterium varies significantly for all the three different incident laser wavelengths.     

超短激光照射下颗粒的散射效应

对两个纳米颗粒受超短激光照射时表面吸收光强的分布情况进行了研究。基于米散射理论和蒙特卡罗方法建立程序来模拟和追踪光子的运动轨迹,米散射理论主要用于确定光子的散射方向,运用兰贝特定律和解析解对模拟结果进行验证。通过对比材料为金和黑体时双颗粒的光强分布发现,颗粒的散射和激光照射宽度的增加会使颗粒相邻处的光强增大,颗粒底部的光强主要取决于颗粒的反照率大小以及散射光强的分布情况。     

Particle formation in opposed-flow diffusion flames of ethylene: An experimental and numerical study

An experimental and numerical study on particles inception and growth is performed in opposed-flow diffusion flames of ethylene and air characterized by different sooting tendencies. Spectrally resolved UV-visible laser induced fluorescence, laser induced incandescence and laser light scattering measurements are used to characterize different classes of combustion-generated compounds. A detailed kinetic model accounting for both gas-phase and particle formation is used. Comparison between experimental results and numerical predictions gives a qualitative view of the mechanism of particle formation in opposed-flow flames.Particle inception is the result of both chemical growth and coagulation of aromatic compounds. In the region close to the flame front where the temperature is relatively high and radicals are abundant, the particle inception is due to a chemical growth mechanism by which aromatic molecules add aromatic radicals leading to the formation of biphenyl-like structures. The growth process continues as high-molecular mass aromatics are moved away from the flame zone towards the stagnation plane by the addition of acetylene and other aromatics forming particles of increasing sizes. Graphitization of these particles and thermal annealing lead to the formation of soot particles. At relatively lower temperatures, found across the stagnation plane, particles growth still occurs and it is mainly due to a process of physical coagulation of PAHs.The experimental and numerical results obtained in this work demonstrate and explain the sensibility of inception and growth of particles to radical concentration and temperature in opposed-flow flame configurations.     

用全固态激光器和一维CCD测定微粒粒径

建立了一套利用线阵CCD探测散射光强从而测定粒径的实验装置.该装置采用线阵CCD取代传统的同心环探测器,并采用一台小尺寸全固态绿光激光器取代传统的He-Ne激光器.理论上采用全Mie氏散射理论,自行编写了实验数据的计算机拟合程序,可以由实验测得的散射光强角分布反演求出粒径分布信息.对粒径分别为4.91 μm和9.88 μm的聚苯乙烯小球均取得了良好的粒径测量结果.该装置被证实可用于测定的粒径范围为0.7 μm～44.0 μm.     

群体雾粒子散射模型及散射角谱研究

 如何提高制导精度是研制制导系统所要关心的问题,在有雾的天气情况下,群体雾粒子的散射是影响制导精度的重要因素之一。在Mie理论的基础上,从单个球形粒子的散射效率因子出发,得到了1～6μm群体粒子的散射系数。根据布格尔定律导出了群体粒子对激光束的散射光强表达式,然后在接收器视场角范围内,模拟了散射光强的分布情况。结果表明,沿着激光束的传播方向,散射光强存在比较大的起伏,并且越靠近接收器的视场中轴线,散射光强有减少的趋势。     

水雾包裹沙尘颗粒核壳结构的散射特性研究

基于离散偶极子近似法(DDA),对水雾包裹沙尘颗粒的核壳结构光学特性进行研究,计算了长短轴比例为2∶1的椭球形粒子的核壳结构内、外层厚度及散射角度变化对光散射特性的影响。结果表明,内核大小不变,外层厚度由1.2 μm增大到4.8 μm,核壳双层颗粒散射系数和消光系数由3.4和3.43降低到2.543和2.545,且散射相对强度也明显增大。外层厚度不变,内核厚度由0.6 μm增加到2.4 μm,散射系数和消光系数由3.105和3.111变化为2.76和2.9;可见外层厚度对核壳双层颗粒散射特性的影响更大,这是由于散射光主要与外层物质相互作用引起的。散射相对强度随波长的增加而降低,随核壳结构尺度的增加呈现递增的规律。该结果对大气中气溶胶和水雾共同作用时的散射特性,激光在其中的传输特性等研究有参考价值。     

非球形大气粒子对任意波束的电磁散射特性

研究大气中变形球状的气象粒子对电磁波以及激光的散射特性,采用广义米氏(Mie)理论,精确地求解在任意波束中大气粒子的散射强度,数值计算激光束腰位置对散射强度分布的影响,分析彩虹强度和散射角与变形球状粒子偏心率的关系,研究随着长短轴比例增加,彩虹强度峰值的偏移。计算结果表明雨滴在下降过程中,它在各个方向的光散射强度逐步减弱,即随着雨滴偏心率的增大,散射强度减少,彩虹角变大。对于激光入射,当粒子距离波束中心位置越远,粒子的散射就越弱,并且随尺寸参量增加,后向散射振荡的频率要大于前向散射,当束腰位置矢径的大小增加时,后向散射和前向散射强度均变小。     

沙尘大气电磁波多重散射及衰减

为了使干旱沙漠地区的电子系统能够全天候的工作, 必须开展沙尘大气的电磁波多重散射及衰减特性研究. 根据Mie理论、沙尘大气粒子尺寸分布和能见度的关系得到了电磁波沙尘大气传播衰减的计算方法, 计算了不同沙尘大气能见度的37 GHz电磁波的衰减, 与其他经验公式及文献中的实验结果进行比较, 文中方法得到的结果更接近于测量结果. 为了研究较低能见度沙尘暴中电磁波的传播特性, 需研究沙尘大气的多重散射效应. 应用Monte Carlo模拟方法, 在沙尘粒子为干燥和5%水含量时, 模拟了37 GHz和93 GHz电磁波在沙尘大气中传播时考虑多重散射效应的衰减, 并与基于Mie理论的计算结果进行比较, 结果显示, 在37 GHz时, 沙尘大气的多重散射对衰减的影响小, 在93 GHz时多重散射显著, 沙尘大气能见度越低, 多重散射的影响越显著. 粒子水含量增加使电磁波的衰减显著增大, 对多重散射的影响不明显. 因此, 在相同大气能见度下, 沙尘天气越干燥, 多重散射影响越大, 电磁波衰减减小越显著.     

From superradiant Rayleigh scattering to Bragg scattering

We present the results of an experiment on light scattering from an elongated Bose-Einstein condensate interacting with far off resonant laser light. Due to superradiant Rayleigh scattering a coherent superposition of two atomic wavepackets with different momenta forms in the presence of a single laser beam. Varying the intensity of a weak counterpropagating laser beam we observe the transition from the pure superradiant regime to the Bragg scattering regime, where Rabi oscillations in a two-level system are observed. The process is limited by the decoherence between the two atomic wavepackets.     

Studies of laser-induced removal mechanisms for tokamak-like particles

The removal of metallic and carbon particles is a great issue in the framework of the ITER (International Thermonuclear Experimental Reactor) project. Indeed, the presence of these particles in the vessel of a tokamak leads to safety risks.The laser process seems to be a very promising solution for this cleaning. However, the process optimization requires a good knowledge of the removal mechanisms. For this purpose, we investigated the influence of beam parameters, such as laser pulse duration and wavelength, on the cleaning efficiency. In this paper, two kinds of particles are chosen to be studied, carbon aggregates and tungsten droplets, because they are typical of dust collected in tokamak.The results show an influence of beam parameters on the tungsten particles removal efficiency (PRE), whereas this influence is not significant for carbon particles.To help the understanding of the removal mechanisms, substrates and particles were inspected by scanning electron microscopy, before and after the irradiation.We will see on this paper that even if carbon and tungsten particles strongly absorb the different laser wavelength used, the removal mechanisms of these particles are very different.     

Influence of wake bubbles on optical impulse scattering echo

The model of the optical impulse scattering echo based on Mie theory and backscattering properties of bubble populations is presented. The experimental results of optical impulse scattering echo for a ship in the ocean is in agreement with the simulated experiment. The wake bubbles significantly influence the laser scattering echo process in the ocean. When the testing system is outside the wake, the distinct scattering echoes of interface between wake bubbles and water are received. When the testing system is inside the wake, the intensity of the echo attenuates with the increased scattering of wake bubbles. These phenomena of laser echoes in different periods are analyzed.     

气溶胶凝聚粒子散射特性的数值计算

利用Cluster-cluster aggregation (CCA)模型,模拟了由相同数目球形原始微粒凝聚而成的四种随机取向气溶胶凝聚粒子．根据物质的电结构,将气溶胶凝聚粒子离散为一系列偶极子,结合离散偶极子近似方法,在获得每一个偶极子的电偶极矩之后,数值计算了气溶胶凝聚粒子散射强度的角分布,并分析了散射强度随入射光入射角度和气溶胶凝聚粒子尺寸参数变化的规律．结果显示：当散射角较小时,气溶胶凝聚粒子取向和入射光的入射角度对散射强度影响不大,当散射角增大时,散射强度则明显依赖于气溶胶凝聚粒子取向和入射光的入射角度;对于不同尺寸参数的气溶胶凝聚粒子,在同一角度入射情况下,随尺寸参数的增加,气溶胶凝聚粒子的散射主要集中于前向散射．     

基于MATLAB与LabVIEW的 Mie散射参数分析系统

针对激光散射法测量0.1 m~10 m微小球形颗粒过程中,Mie散射理论各参数的分析、计算、最优化选取比较复杂和繁琐,通过收敛速度、折射率和尺度参数测量范围、叠加次数的比较,采用球形粒子Mie散射参量的MATLAB改进算法, 结合MATLAB较强的计算能力和LabVIEW良好的图形化界面,进行LabVIEW和MATLAB混合编程。设计了Mie散射理论参数分析系统,系统实现散射系数的计算,并分别对散射系数与颗粒直径关系曲线、散射强度分布曲线、入射光波长与散射光强关系曲线、颗粒直径与散射光强关系曲线、入射光偏振角与散射光强关系曲线进行分析。通过实验结果和结论对比验证系统可行,且散射系数、消光系数和散射光强的计算结果可精确到小数点后17位,从而达到减少大量计算和编程工作的目的。     

降雨粒子的无线紫外光散射特性

紫外光与降雨粒子相互作用发生散射,散射光特性改变能够反映降雨粒子的相关物理特性（如粒子尺寸参数、浓度、形态）,因此研究粒子的物理参数对散射光特性的影响对有效提高光谱法定量探测降水的精度有很大意义。由于雨滴在非球形降水粒子中具有代表性,以群雨滴粒子为例,采用T矩阵理论,利用紫外光直视和非直视单次散射模型,分析了入射光波长、群雨滴粒子形态、降雨强度、粒径大小与散射光强之间的关系。并用蒙特卡洛方法仿真分析了非球形群雨滴粒子在不同降雨强度和粒径下散射角与散射光强之间的关系,以及降雨环境中的风切变对紫外光散射特性的影响。通过理论及仿真分析,得到了不同群雨滴粒子形态下的路径损耗,不同降雨强度、风切变率和粒径下的散射光强分布。仿真结果表明：在紫外光直视与非直视通信方式下,降雨环境中的通信质量比晴天条件下的通信质量差,即路径损耗增大。当粒径分布已知时,随着降雨强度的增大,衰减系数增大,路径损耗增加,且直视通信方式的路径损耗比非直视降低7 dB左右。随着降雨强度、风切变率和粒子粒径的增大,散射光强曲线整体呈下降趋势,其中,降雨强度的变化对散射光强分布影响程度最大。相同通信距离时,不同降雨强度下的紫外光散射光强分布均随着散射角的增大而减小,当散射角继续增大到90°时,有效散射体体积逐渐减小,接收到的光子能量减小,暴雨中的散射光强衰减程度最大。相同降雨强度下考虑风切变时,相比较无风时的路径损耗增大5 dB左右。除此之外,还研究了椭球形和切比雪夫形粒子对紫外光散射光强的影响,结果表明当粒子粒径分布相同时,椭球形粒子的散射光强衰减较广义切比雪夫形粒子大。根据散射粒子的散射光强分布以及路径损耗能够区分雨滴粒子是否由相同粒径及形态组成,为粒子测量提供理论基础。分析降水中群雨滴粒子的光散射特性,为提高光谱法评估降水衰减的数值模拟方面提供理论依据,为光学技术在探测识别降水现象等气象领域的广泛应用提供了设计参考。     

炸药爆炸作用下液体破碎后颗粒尺寸分布的研究

 对液体抛撒的液滴尺寸进行研究在军事和民用上是很重要的,国内刚开始使用激光散射仪开展此项研究工作。利用R. A. Dobbins等人的液体颗粒测量技术,研制了一套既简单又实用的测量液体抛撒过程中液滴尺寸的实验装置——激光散射仪。对于激光与液体微粒的相互作用,当微粒的反射与折射和吸收效应可被忽略时,可导出液体微粒对激光散射的光强公式。只要测量激光被微粒散射的光强,就可推算出微粒的Sauter平均直径。在使用激光散射仪测量液体抛撒液滴尺寸的实验中,用水代替爆炸抛撒液体,测量结果表明：液体抛撒二次破碎中,在固定位置测量到的云雾区液滴Sauter平均直径随测量时间的增加呈现出减小的趋势;而云雾区的宽度则随着与抛撒中心距离的增大而呈现出增加的趋势;云雾区前沿的液滴Sauter平均直径随着与抛撒中心距离的增加而呈现出先逐渐增大然后迅速减小的趋势。为便于比较,对燃料抛撒二次破碎进行了回收法测量和数值模拟计算,其测量与计算结果与用激光散射仪测量的结果有较好的一致性。     

高功率激光传输过程中散射点对光束质量的影响

为了研究高功率激光传输过程中不透明颗粒引起的光束调制,通过分析这些不透明颗粒的形状和分布特点,建立了位置呈随机分布的高斯状散射点模型,从光束的衍射理论和干涉叠加理论出发,得到该模型下散射点对传输光束质量影响的解析式。数值分析了高斯状散射点的大小、密度、散射面积比及其传输距离对输出光束的近场分布、位相分布和光束透过率的影响,结果显示亚毫米量级散射点的衍射效应引起最大调制可达1．4,光学元件散射面积比小于0．003时才能满足元件透过率大于99．5％的需求。该结果可用于评价高功率激光装置光学元件的加工状况,并对光学元件加工要求和激光装置的洁净度要求有指导意义。     

大气颗粒物多参数激光自混合传感的数值模拟与特性分析

基于颗粒物米氏散射特性、激光自混合三镜腔理论和激光器稳态条件,推导出在大气颗粒物光反馈下激光器频率、功率、线宽的理论表达式,建立了大气颗粒物光反馈下的激光自混合理论模型。同时,数值模拟和分析了大气颗粒物物理参数对激光自混合干涉信号的影响。结果表明,在一定粒径范围内,激光自混合反馈强度随大气颗粒物粒径增大先增大后减小,且反馈强度峰值出现的位置随颗粒物折射率实部的增大、虚部的减小向粒径较大处移动;自混合系统的外腔长度影响自混合干涉信号的波动深度,激光器输出光信号的幅值随外腔长度增大呈指数衰减;自混合干涉信号波动频率与大气颗粒物运动速度呈线性关系。分析结果对基于激光自混合效应的大气颗粒物多物理参数传感具有重要作用。     

Resonant scattering of ions on charged dust particles

Resonant scattering ("scattering through waves") of ions on dust particles in plasmas is considered in the framework of the kinetic theory which consistently takes into account ion-dust collisions and dust charge fluctuations. Resonance with low frequency dusty plasma modes can enhance the ion-dust scattering cross section and the ion drag force on dust particles.