Numerical evaluation of optical single-scattering properties using multiple-scattering inverse transport methods

Five inverse methods for determining the single-scattering properties from monochromatic optical experiments in a homogeneous multiple-scattering slab geometry are presented and compared using analytical calculations. The methods require only measurements of the specific intensities entering and leaving the target.One method requires the pulsed irradiation of the target by a non-normally incident beam, followed by measurements for all times of the time-dependent specific intensities over both the polar and azimuthal angles. Two other methods are based on normally-incident irradiation and require measurements over only the polar angle; one method requires time-dependent measurements for all times following a laser pulse, while the second permits use of a continuous-working laser but requires experiments on targets of different thicknesses. The last two methods are obtained from the Laplace-transform of time dependent specific intensities; one method requires that the Laplace transform variable s be very large, while the other requires using a set of negative s-values tending to the limit for which Laplace transformed specific intensity becomes unbounded.     

A point source analytical model of inverse pulsed laser deposition

A simple analytical model for inverse pulsed laser deposition is proposed. In the model the motion of the evaporated material is assumed to emerge as from a point source located above the surface of evaporation at some distance. The obtained thickness profiles of inverse deposited films agree well with those calculated by the test particle Monte Carlo method. The proposed approach has been applied for analysis of experimental data on inverse pulsed laser deposition of graphite in nitrogen atmosphere with nanosecond pulses of laser fluences between 1 and 7 J/cm². The model describes well the thickness profiles and pressure dependence of film growth rate for inverse deposition.     

激光脉冲宽度对远距离尾流气泡后向检测的影响

为了分析激光脉冲宽度对远距离尾流气泡后向检测的影响,基于Fournier Forand体积散射函数,通过Monte Carlo方法建立了水中激光脉冲后向散射信号时域特征的分析模型.利用该模型研究了初始激光脉冲宽度不同时,水中远距离舰船尾流气泡的激光脉冲后向散射信号变化情况.结果表明:随着初始激光脉冲宽度的增加,后向散射信号中水体散射信号与尾流气泡回波信号的轮廓变得模糊|当脉冲宽度增大到一定程度时,无法从后向散射信号中辨别出回波信号|并且,随着尾流区气泡散射强度的减小以及气泡区与检测器之间距离的减短,这种变化趋势变得更加明显.基于仿真结果,提出一种基于逆卷积运算的尾流气泡回波信号提取方法.     

Advanced Geometrical Modeling of Focused Beam Reflectance Measurements (FBRM)

In the past decade the use of focused beam reflectance measurements (FBRM) has been established as an on line and in situ particle characterization technique. However, a model is required to obtain full information from the FBRM signal and to compare the results to other measurement techniques. Different modeling approaches can be found in the literature. All of these assume a laser focus of zero extension, motionless particles and fully opaque particles. It is shown in this work that these assumptions do not hold even for ideally spherical particles. For large, opaque particles, the particle velocity and a depth dependent laser velocity have to be considered. For highly transparent particles, backscattering only occurs near the edges of a crystal. Consequently, two more refined modeling approaches – the velocity model and the edge scattering model – based on Monte‐Carlo techniques are developed and verified in this work.     

Application of the Monte Carlo method for modeling passage of ultrashort laser pulses through an inhomogeneous medium with moving scatterers

We present the results of numerical modeling of passage of ultrashort laser pulses through an inhomogeneous layered medium with moving scatterers, based on solution of the nonsteady-state radiation transport equation by the Monte Carlo method. We consider the effects of Doppler broadening of the backscattered radiation spectrum in biological tissues. We have analyzed the dynamics of propagation of a short laser pulse within a multilayer model of human skin. We have studied the possibilities for tomography of different layers of biological tissue based on analysis of the spectrum of the scattered radiation pulse.     

非链式脉冲DF化学激光器反应动力学模型

依据非链式脉冲氟化氘(DF)激光器的反应机理, 采用速率方程理论, 综合考虑了基态DF分子、D₂分子、D原子、F原子对激发态DF分子的消激发作用, 建立了非链式脉冲DF激光器反应动力学模型. 运用Runge-Kutta法对该模型进行数值计算, 得到了增益区内各组分粒子数密度随时间的变化关系. 进而运用该模型研究了工作气体配比和输出镜反射率对DF激光器腔内光子数密度、 单脉冲能量、脉冲宽度和输出功率的影响, 得到了最佳气体配比和最佳输出镜反射率参数. 采用放电引发方式对非链式DF激光器进行了实验研究, 实验测得脉冲波形及单脉冲能量与速率方程理论模型计算结果基本一致. 本文的研究结果可为非链式脉冲DF激光器的优化设计提供理论参考. 关键词： 脉冲DF激光器 动力学模型 速率方程 数值计算     

光在随机增益介质中的放大

 结合环形腔理论，运用蒙特卡罗法模拟了光子在介质中的随机行走。研究了倍频Nd:YAG（脉宽6 ns，频率20 Hz）脉冲激光器作为泵浦光，在TiO₂ / 若丹明 B有机增益介质中，散射微粒的颗粒密度和泵浦光面积对随机激光器阈值强度的影响。模拟结果表明：随机激光阈值和光子在增益介质中的随机行走路程长度和光子通过边界返回增益区和非增益区的几率有关。随着泵浦光面积的增加，随机激光器阈值降低；增益介质中散射颗粒密度的增加降低了随机激光器的阈值。     

大气激光通信中的多层空间脉冲位置幅度调制

针对现有光空间脉冲位置调制频谱效率低、激光器利用率不高等问题,将分层技术与空间脉冲位置幅度调制相结合,提出了一种适合于大气激光通信的多层空间脉冲位置幅度调制方案.通过额外增加少量几个激光器构成多层结构,并通过脉冲位置幅度调制中的脉冲位置携带比特信息,不同层通过脉冲幅度得到区分.介绍了系统中层映射、空间脉冲位置幅度映射及其逆映射的原理,并推导出该方案的误码率表达式.利用蒙特卡洛仿真方法进一步验证了该方案的正确性,并与传统空间调制系统的性能进行了对比.结果表明:与传统光空间调制系统相比,所提方案提高了系统的频谱效率,且所用激光器数目更少.在传输比特相同的条件下,相对于(32,4,128)-空间脉冲位置调制系统,(9,4,8,2)-多层空间脉冲位置幅度调制系统的频谱效率提高了16倍,当误码率为10-3时,其信噪比改善了约1dB,且所用激光器数目不到前者的1/3.其中,括号中的参数分别表示激光器数目、探测器数目、采用调制方式的阶数及层数,层数为1时忽略.     

随机阈值冲击模型金属化膜脉冲电容器可靠性分析

基于惯性约束聚变强激光装置中金属化膜脉冲电容器"自愈"的失效机理,提出了更为适合的冲击模型金属化膜脉冲电容器可靠性评估方法。现有的加速退化试验中,失效阈值往往是事先确定的,但考虑可靠性产品个体差异及环境影响的不同,这是不合理的,针对此问题提出了基于随机阈值的冲击模型金属化膜脉冲电容器退化建模的方法。由于模型过于复杂,采用MCMC方法进行参数估计。最后通过对电容器的仿真实验,将随机阈值下的评估结果与固定阈值的情况相对比,说明了该模型和方法的合理性,并进一步分析了不同的阈值分布均值和方差对产品可靠性的影响。     

Variable order spherical harmonic expansion scheme for the radiative transport equation using finite elements

We propose the P_N approximation based on a finite element framework for solving the radiative transport equation with optical tomography as the primary application area. The key idea is to employ a variable order spherical harmonic expansion for angular discretization based on the proximity to the source and the local scattering coefficient. The proposed scheme is shown to be computationally efficient compared to employing homogeneously high orders of expansion everywhere in the domain. In addition the numerical method is shown to accurately describe the void regions encountered in the forward modeling of real-life specimens such as infant brains. The accuracy of the method is demonstrated over three model problems where the P_N approximation is compared against Monte Carlo simulations and other state-of-the-art methods.     

Generalized Pseudospectral Method for Solving the Time-Dependent Schrödinger Equation Involving the Coulomb Potential

We present an accurate and efficient generalized pseudospectral method for solving the time-dependent Schrödinger equation for atomic systems interacting with intense laser fields. In this method, the time propagation of the wave function is calculated using the well-known second-order split-operator method implemented by the numerically exact, fast transform between the grid and spectral representations. In the grid representation, the radial coordinate is discretized using the Coulomb wave discrete variable representation (CWDVR), and the angular dependence of the wave function is expanded in the Gauss-Legendre-Fourier grid. In the spectral representation, the wave function is expanded in terms of the eigenfunctions of the field-free zero-order Hamiltonian. Calculations on the high order harmonic generation and ionization dynamics of hydrogen atom in strong laser pulses are presented to demonstrate the accuracy and efficiency of the present method. This new algorithm will be found more computationally attractive than the close-coupled wave packet method using CWDVR and/or methods based on evenly spaced grids.     

各向异性介质辐射特性参数联合反演

本文基于BP神经网络方法结合蒙特卡洛和BEER定律辐射传输模拟方法建立了联合反演各向异性散射介质的辐射特性参数模型。首先采用半球透射率结合半球反射率反演模型反演了各向同性介质的吸收系数和散射系数,在此基础上增加准直透射率,建立了联合反演各向异性介质的吸收系数、散射系数和散射不对称因子三参数联合反演模型。反演结果表明该模型能准确反演出介质辐射特性参数,具有实用意义。此外,为了检验测量误差对模型的反演准确性的影响,分别在不同程度测量误差情况下进行反演,结果显示测量误差对散射不对称因子反演值影响较大。     

Features of femtosecond laser pulse reflection from a sharp boundary of relativistic laser plasma

The features of femtosecond laser pulse reflection from a plasma target of near-critical density were analytically and numerically studied in a wide range of laser pulse intensities and durations. Based on analytical calculations, it is shown that the pulse reflectance at a constant intensity decreases with decreasing pulse duration. Based of numerical simulation results, it is shown that, as a superintense femtosecond laser pulse is incident on a plasma layer with a concentration close to the critical one, a quasi-periodic electron-density structure is formed in the plasma bulk, which can both increase and decrease the the laser pulse reflectance.     

含时量子蒙特卡罗方法研究两电子原子在强激光作用下电子的动力学行为

通过改进含时量子蒙特卡罗方法研究了一维模型双电子原 子在强激光作用下的电子动力学过程. 与准确的数值积分求解含时薛定谔方程相比, 计算得到的波包对应的量子系综中粒子的动力学变化和含时波包演化结果定性一致, 且大幅度地提高了计算效率. 根据计算得到的经典粒子系综的动力学演化行为, 分析了原子在强激光作用下的激发、电离、重散射等非线性过程.     

An anti-reflective 1D rectangle grating on GaAs solar cell using one-step femtosecond laser fabrication

We report the fabrication of the anti-reflective micro/nano-structure on absorbing layer of GaAs solar cell surface using an efficient approach based on one-step femtosecond laser irradiation. Morphology of the microstructures and reflectance of the cell irradiated are characterized with SEM and spectrometer to analyze the influence of laser processing parameters on the change of microstructures induced and the reflectance. It has been found that the rectangle grating micro/nano-structure with a period of 700 nm and width of 600 nm is obtained neatly with laser pulse energy of 30.5 μJ(pulse duration is 130 fs, center wavelength is 800 nm, scanning speed is 2.2 mm/s and spot diameter is 22 µm). Reflectance has been suppressed to 23.6% with rectangle structure from 33% of planar cell. In addition, simulation using a finite-difference-time domain(FDTD) method results show that the rectangle grating micro/nano-structure can effectively suppress the reflection within large wavelength ranges.     

基于漫反射光谱的组织光学参数测量系统与方法研究

在体组织光学参数测量是生物医学光子学研究重点,不仅为人体成分无创检测、光学成像、光动力疗法等研究提供基础,并且可以快速获取人体光学参数变化,为临床诊断提供依据。研究了利用单一源探距离漫反射光谱在体测量光学参数的测量系统与反构方法。漫反射光谱测量系统由宽谱光源、高分辨光纤光谱仪及光纤探头组成,结构简单,测量方便,可准确快速测量样品漫反射光谱。在光纤探头几何形状基础上,研究了光纤收集及系统传递函数,在此基础上对反构算法进行了校正。光学参数反构算法中正向模型基于Monte Carlo以及神经网络方法,适用光学参数范围大,计算速度快;逆向算法采用主成分分析与非线性建模拟合相结合的方法,可抑制测量噪声影响。在测量系统及反构算法基础上,进行了组织仿体光学参数测量实验,结果表明,利用单一源探距离下漫反射谱,可以较为准确获取吸收系数以及约化散射系数,均方根误差分别达到4.58%以及7.92%。为保证系统测量准确性,测量波长范围应覆盖样品中所含吸收物质吸收峰范围。所研究的在体组织光学参数测量方法为人体成分无创检测及测量条件变化获取提供了基础。     

大气到海洋激光通信信道仿真

详细分析了蓝绿激光穿过大气海洋信道的信道效应和物理特性,利用理论分析和蒙特卡罗模拟方法完成对光信道仿真。介绍了大气海洋激光通信蒙特卡罗模拟方法及计算步骤。研究了不同厚度云层对光束投影面积的展宽,和光脉冲穿过不同深度海水后的空间分布和时域信号波形,讨论了其不同的展宽机理,并对信道的噪声分布进行了分析。发现云层对光脉冲展宽作用在云层厚度500 m时达到饱和,信号能量的随机起伏随海水深度的增加而增大,但能量分布的半峰全宽并不增加,大气海洋的综合信道效应可以用时延滤波器进行建模。     

Hierarchical Bayesian inference for Ill-posed problems via variational method

This paper investigates a novel approximate Bayesian inference procedure for numerically solving inverse problems. A hierarchical formulation which determines automatically the regularization parameter and the noise level together with the inverse solution is adopted. The framework is of variational type, and it can deliver the inverse solution and regularization parameter together with their uncertainties calibrated. It approximates the posteriori probability distribution by separable distributions based on Kullback–Leibler divergence. Two approximations are derived within the framework, and some theoretical properties, e.g. variance estimate and consistency, are also provided. Algorithms for their efficient numerical realization are described, and their convergence properties are also discussed. Extensions to nonquadratic regularization/nonlinear forward models are also briefly studied. Numerical results for linear and nonlinear Cauchy-type problems arising in heat conduction with both smooth and nonsmooth solutions are presented for the proposed method, and compared with that by Markov chain Monte Carlo. The results illustrate that the variational method can faithfully capture the posteriori distribution in a computationally efficient way.     

水下激光通信中脉冲时域展宽的模拟计算方法

针对激光脉冲在水下传输时导致脉冲的时域展宽,严重地影响了水下激光通信的性能。以蒙特卡罗模拟为手段,在一定的水质条件下,对激光脉冲经过不同传输距离后的脉冲宽度进行了计算机模拟,模拟结果与实验测量数据相符,精确度高于其他方法,该方法尤其适用于激光脉冲远距离传输的情况,证明了该方法可准确地计算激光脉冲展宽后的脉宽值,能有效地提高水下激光通信的性能。     

Theory of laser action in scattering gain media

A laser model based on feedback produced by scattering has been developed to explain the narrow linewidth emission and input-output behavior observed in scattering gain media. The model is based on the transient two-level laser equations and includes the detailed spectral properties of the dye gain system. Monte Carlo methods were employed to calculate the threshold gain required for modeling the input-output and linewidth emission characteristics.