基于Fokker-Planck方程的电介质材料短脉冲激光破坏机制分析

基于Pokker-Planck方程,建立一个描述短脉冲激光作用下电介质材料中导带电子能量分布随时间变化的模型,用数值方法计算电子能量分布与电子数密度随时间的演化过程,根据临界等离子体密度准则得到了不同激光脉冲宽度和波长下电介质材料（以SiO2为例）的破坏阈值。结果发现,尽管激光波长通过3种途径对材料破坏阈值的确定产生影响,但三者的共同作用导致在激光波长分别为1 060,800,532 nm时材料破坏阈值随脉宽变化曲线十分接近。讨论了碰撞电离、光致电离两种机制对材料激光破坏分别所起的作用：当激光脉宽大于1 ps时,碰撞电离对材料的破坏起主要作用;当激光脉宽小于1 ps时,光致电离对破坏阈值的影响越来越明显,但是碰撞电离仍然不可忽略,碰撞电离与光致电离同时起重要作用。     

基于Fokker-Planck方程的电介质材料短脉冲激光破坏机制分析

基于Pokker-Planck方程,建立一个描述短脉冲激光作用下电介质材料中导带电子能量分布随时间变化的模型,用数值方法计算电子能量分布与电子数密度随时间的演化过程,根据临界等离子体密度准则得到了不同激光脉冲宽度和波长下电介质材料(以SiO<,2>为例)的破坏阈值.结果发现,尽管激光波长通过3种途径对材料破坏阈值的确定...     

物理参数变化对短脉冲激光激励温度场的影响

 为研究多物理参数（耦合系数、电子热导率、电子热容、晶格热容）同时随温度变化对短脉冲激光辐照金属材料产生温度场分布的影响,基于双温耦合理论,建立了短脉冲激光辐照金属材料金的加热过程的有限元求解模型。在同时考虑脉冲激光的空间、时间分布和多参数同时随温度变化的情况下,得到短脉冲激光辐照金属材料金激励产生的温度场二维瞬态分布,并进一步比较了多物理参数同时随温度变化和采用室温物理参数两种情况下温度场分布的区别。数值结果表明：多物理参数同时随温度变化使电子温度和晶格温度的上升变快,最大值变大,而且使得材料中激光穿透直接辐照到的区域温度变高。     

短、超短脉冲激光时间分布特性的几种测量方法

本文介绍几种短、超短激光脉冲的测试方法。     

短脉冲激光泵浦的光热偏折光谱理论

普遍应用的光热偏折光谱通常所采用对连续激光调制变为ｍｓ级脉冲光作泵浦光，这里我们首次讨论了当泵浦东为δ函数时，光热偏折光谱在平行检测情况下的理论效应，结果表明，后者具有更高的灵敏度和更为简单的表达式，文中还讨论了把短脉冲视为δ函数的条件。     

激光脉冲波形对烧蚀Si靶表面温度的影响

利用双温方程对激光烧蚀Si靶的过程进行了数值模拟,并结合合适的初始条件和边界条件,研究了在飞秒、皮秒激光作用下,脉冲波形（矩形、梯形、三角形和高斯形）对Si靶表面载流子和晶格温度分布的影响。结果表明：激光功率密度是影响载流子温升的主要因素,矩形脉冲激光烧蚀Si靶表面载流子的峰值温度最高,而高斯分布的脉冲引起靶面载流子峰值温度最低。可见,激光脉冲波形对Si靶表面载流子的温度分布具有重要影响。所得结果可为制备高质量的薄膜提供理论依据。     

激光脉冲波形对烧蚀Si靶表面温度的影响

利用双温方程对激光烧蚀Si靶的过程进行了数值模拟,并结合合适的初始条件和边界条件,研究了在飞秒、皮秒激光作用下,脉冲波形(矩形、梯形、三角形和高斯形)对Si靶表面载流子和晶格温度分布的影响。结果表明:激光功率密度是影响载流子温升的主要因素,矩形脉冲激光烧蚀Si靶表面载流子的峰值温度最高,而高斯分布的脉冲引起靶面载流子峰值温度最低。可见,激光脉冲波形对Si靶表面载流子的温度分布具有重要影响。所得结果可为制备高质量的薄膜提供理论依据。     

短脉冲激光加热模型的遴选原则

 利用推导出的1维半空间光致热弹性应力响应的精确解,对激光与材料相互作用的加热模型的选取进行了研究。理论分析和计算结果表明：常用的表面加热模型只有在热穿透深度远大于光吸收长度时有效,若将其应用于超短脉冲激光（ps以下量级）加热的情况,可能会导致较大的误差;皮秒和飞秒激光辐照的热效应须用体加热模型计算。     

飞秒双脉冲激光照射金属薄膜的热行为

通过双温方程对飞秒单脉冲与双脉冲照射金薄膜进行了计算模拟分析,得到了金靶的电子温度和晶格温度随着时间空间的变化。在同样激光能量密度下,单脉冲与双脉冲使得金膜温度的变化表明双脉冲使得更多的激光能量渗透到靶材内部,这些能量可以使得烧蚀深度更深,有利于提高激光烧蚀靶材的效率。计算结果显示随着激光能量密度的增加熔化面深度逐渐增加,单脉冲与双脉冲熔化面深度的变化明显不同。在激光能量密度高于损伤阈值附近,单脉冲的烧蚀深度大于双脉冲的烧蚀深度,随着激光能量密度增加,双脉冲的烧蚀深度将大于单脉冲的烧蚀深度。     

皮秒激光烧蚀Al膜的理论分析与实验验证

为研究Al膜受短脉冲激光作用时的能量传输过程,建立了一维半经典双温热传导模型。根据材料加工过程的熔化现象,对模型的相变区域进行有效设置。通过有限元法求解,得到晶格温度随时间分布的规律。根据自由电子气理论优化了模型,得到Al膜表面反射率和热吸收系数随时间的分布图。得到了激光辐照所产生热电场的分布规律,并分析了电子温度梯度对其影响。描绘出电子漂移运动速度的分布规律,证实激光作用产生的热电场是电子漂移运动的主导因素,发现最大漂移速度位置随时间的延长而加深。对激光作用后的晶格温度进行了区域性分析,定义了过热加热区,并得到激光烧蚀深度随时间的变化关系。实验结果表明,不同功率皮秒激光烧蚀Al膜的深度接近于理论计算结果。     

LDA侧面泵浦薄片激光器的泵浦光和温度分布

建立了激光二极管阵列(LDA)环绕侧面泵浦复合薄片激光器的数值模型,LDA的快轴垂直于薄片表面并被压缩。依据LDA的输出光束特性,考虑到介质与空气的对流热交换和材料热导率的温度相关性,根据经典热传导方程,运用有限单元法,得出了薄片内的泵浦光和温度分布。分析了LDA个数、泵浦距离、吸收系数和光束发散角对薄片内泵浦光分布和吸收效率的影响规律,讨论了温度与泵浦功率、换热系数、冷却液温度和时间的变化规律,所得的有关规律与相关实验相符合。计算结果可为LDA泵浦固体激光器的结构优化设计和实验研究提供理论参考。     

Near-Field Analysis of Micro-Aperture Surface Emitting Laser for High Density Optical Data Storage

We present the design of a novel metal micro-aperture surface emitting laser for tera byte optical data storage. The field distribution near micro-aperture is calculated by 2-dimensional finite element method, and it is shown that a spot size as narrow as 100 nm beyond a diffraction limit is obtainable.     

环形激光二极管抽运棒状激光器中瞬态温度和热应力分析

直接从激光二极管发光强度的角分布出发,采用光线追迹方法获得激光棒内的热沉积分布,在此基础上采用热传导模型和热力模型,比较了不同抽运功率、不同棒半径下达到稳态温度分布的时间,并且对稳态和瞬态热应力进行了详细模拟计算。结果表明,采用环形激光二极管阵列侧面抽运的棒状激光器中的热效应问题十分严重,不同的抽运结构参量下,温度分布不同;达到稳态所需时间随棒半径增大而增加,而不受抽运功率的影响;抽运功率越大,棒内温差增大,热应力也越大;热破坏主要集中于激光棒中心区域和表面区域。     

Diagnostics of Tin Plasma Produced by Visible and IR Nanosecond Laser Ablation

We present the optical emission spectroscopic studies of the Tin (Sn) plasma, produced by the fundamental (1064 nm) and second (532 nm) harmonics of a Q switched Nd: YAG pulsed laser having pulse duration of 5 ns and 10 Hz repetition rate which is capable of delivering 400 mJ at 1064 nm, and 200 mJ at 532 nm using Laser Induced Breakdown Spectroscopy (LIBS). The laser beam was focused on target material by placing it in air at atmospheric pressure. The experimentally observed line profiles of four neutral tin (Sn I) lines at 231.72, 248.34, 257.15 and 266.12 nm were used to extract the electron temperature (T_e) using the Boltzmann plot method and determined its value 6360 and 5970 K respectively for fundamental and second harmonics of the laser. Whereas, the electron number density (N_e) has been determined from the Stark broadening profile of neutral tin (Sn I) line at 286.33 nm and determined its value 5.85 x 10¹⁶ and 6.80 x 10¹⁶cm^–3 for fundamental and second harmonics of the laser respectively. Both plasma parameters (T_e and N_e) have also been calculated by varying distance from the target surface along the line of propagation of plasma plume and also by varying the laser irradiance. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A coupled edge-/face-based smoothed finite element method for structural-acoustic problems

The edge-based smoothed finite element method (ES-FEM) and the face-based smoothed finite element method (FS-FEM) developed recently have shown great efficiency in solving solid mechanics problems with triangular and tetrahedral meshes. In this paper, a coupled ES-/FS-FEM model is extended to solve the structural-acoustic problems consisting of a plate structure interacting with the fluid medium. Three-node triangular elements and four-node tetrahedral elements are used to discretize the two-dimensional (2D) plate and three-dimensional (3D) fluid, respectively, as they can be generated easily and even automatically for complicated geometries. The field variable in each element is approximated using the linear shape functions, which is exactly the same as that in the standard FEM. The gradient field of the problem is obtained particularly using the gradient smoothing operation over the edge-based and face-based smoothing domains in 2D and 3D, respectively. The gradient smoothing technique can provide a proper softening effect to the model, effectively solve the problems caused by the well-known “overly-stiff” phenomenon existing in the standard FEM, and hence significantly improve the accuracy of the solution for the coupled systems. Intensive numerical studies have been conducted to verify the effectiveness of the coupled ES-/FS-FEM for structural-acoustic problems.     

Suppressing the mechanical quadrature error of a quartz double-H gyroscope through laser trimming

In this paper,we introduce a z-axis quartz gyroscope using a double-H tuning fork,which has a high sensitivity.However,it also causes a large mechanical quadrature error.The laser trimming method is used to suppress this error at quartz level.The trimming law is obtained through the finite element method(FEM).A femtosecond laser processing system is used to trim the gold balancing masses on the beams,and experimental results are basically consistent with the simulated ones.The mechanical quadrature error is suppressed by 96%,from 26.3°s-1 to 1.1°s-1.Nonlinearity changes from 1.48%to 0.30%,angular random walk(ARW)is reduced from 2.19°h-1/2 to 1.42°h-1/2,and bias instability is improved by a factor of 7.7,from 197.6°h-1 to 25.4°h-1.     

Characteristics of Semi-conductor Laser Diode with Nonlinearly Tapered Waveguide

A novel semiconductor laser diode with nonlinearly tapered waveguide is proposed, and analyzed by a finite element beam propagation method (FM-BPM). The results show that the coupling efficiency is increased, and the laser's far field divergence is decreased effectively.     

一种抗单元逆转的软组织有限元模型

由于Green应变具有单元翻转不变性,在大变形的仿真中有可能会出现单元的逆转,破坏模型网格的拓扑结构,导致仿真失败,因此提出一个新的有限单元模型,在采用Green应变度量变形同时把单元体积变化率和单元内压强关系引入到本构方程中,解决单元逆转问题;同时,模型导出的单元弹性势能是单元结点位移的多项式函数,有效地简化了共轭梯度法的求解,效率显著提高.实验结果表明,该模型可以应用于软组织大变形的实时仿真中.