短脉冲激光对硅基探测器的热作用模型选择

从傅里叶模型和非傅里叶模型的基本方程出发,通过有限差分方法对方程进行数值求解。分别分析了10,1．0,0．1,0．01 ns这4种脉宽的脉冲激光作用于硅材料时两种传热模型温度曲线的相对变化;讨论了热弛豫时间对非傅里叶模型数值结果的影响。结果表明:脉宽小于或等于100 ps的激光作用于硅材料时,表层温度上升缓慢,会发生载流子效应,非傅里叶模型可以合理地反映这种现象;对于一般材料,载流子效应发生的条件是脉宽小于或等于材料热弛豫时间,此时应当用非傅里叶模型描述加热过程。     

Thermal Stress Analysis of a Continuous and Pulsed End-Pumped Nd:YAG Rod Crystal Using Non-Classic Conduction Heat Transfer Theory

In this paper, temperature distribution in the continuous and pulsed end-pumped Nd:YAG rod crystal is determined using nonclassical and classical heat conduction theories. In order to find the temperature distribution in crystal, heat transfer differential equations of crystal with consideration of boundary conditions are derived based on non-Fourier’s model and temperature distribution of the crystal is achieved by an analytical method. Then, by transferring non-Fourier differential equations to matrix equations, using finite element method, temperature and stress of every point of crystal are calculated in the time domain. According to the results, a comparison between classical and nonclassical theories is represented to investigate rupture power values. In continuous end pumping with equal input powers, non-Fourier theory predicts greater temperature and stress compared to Fourier theory. It also shows that with an increase in relaxation time, crystal rupture power decreases. Despite of these results, in single rectangular pulsed end-pumping condition, with an equal input power, Fourier theory indicates higher temperature and stress rather than non-Fourier theory. It is also observed that, when the relaxation time increases, maximum amounts of temperature and stress decrease.     

基于双相延迟模型的飞秒激光烧蚀金属模型

为了分析飞秒激光烧蚀过程,在双相延迟模型的基础上建立了双曲型热传导模型.模型中考虑了靶材的加热、蒸发和相爆炸,还考虑了等离子体羽流的形成和膨胀及其与入射激光的相互作用,以及光学和热物性参数随温度的变化.研究结果表明:等离子体屏蔽对飞秒激光烧蚀过程有重要的影响,特别是在激光能量密度较高时;两个延迟时间的比值对飞秒激光烧蚀过程中靶材的温度特性和烧蚀深度有较大的影响;飞秒激光烧蚀机制主要以相爆炸为主.飞秒激光烧蚀的热影响区域较小,而且热影响区域的大小受激光能量密度的影响较小.计算结果与文献中实验结果的对比表明基于双相延迟模型的飞秒激光烧蚀模型能有效对飞秒激光烧蚀过程进行模拟.     

Thermoelastic wave induced by pulsed laser heating

In this work, a generalized solution for the thermoelastic plane wave in a semi-infinite solid induced by pulsed laser heating is developed. The solution takes into account the non-Fourier effect in heat conduction and the coupling effect between temperature and strain rate, which play significant roles in ultrashort pulsed laser heating. Based on this solution, calculations are conducted to study stress waves induced by nano-, pico-, and femtosecond laser pulses. It is found that with the same maximum surface temperature increase, a shorter pulsed laser induces a much stronger stress wave. The non-Fourier effect causes a higher surface temperature increase, but a weaker stress wave. Also, for the first time, it is found that a second stress wave is formed and propagates with the same speed as the thermal wave. The surface displacement accompanying thermal expansion shows a substantial time delay to the femtosecond laser pulse. On the contrary, surface displacement and heating occur simultaneously in nano- and picosecond laser heating. In femtosecond laser heating, results show that the coupling effect strongly attenuates the stress wave and extends the duration of the stress wave. This may explain the minimal damage in ultrashort laser materials processing. Received: 23 May 2000 / Accepted: 26 May 2000 / Published online: 20 September 2000     

飞秒激光烧蚀金属靶的冲击温度

在明确飞秒激光与物质相互作用过程冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝靶和铜靶过程中的冲击温度与其他物理量的关系, 利用飞秒激光烧蚀金属的双温模型提取了冲击温度的绝对值, 基于非傅里叶热传导模型计算了冲击温度的分布. 此项研究结果对飞秒激光安全加工含能材料有借鉴意义. 关键词： 飞秒激光 含能材料 烧蚀 冲击温度     

平板在任意周期表面热扰动作用下的非Fourier热传导的求解与分析

分析了平板前表面遭受任意周期热扰动这类非Fourier传热情形下的温度响应. 采用双曲型热传导方程描述平板表面温度急速变化时的热传导问题. 为求解此类方程, 首先 利用分离变量法和Duhamel积分原理, 得到了平板前表面遭受突变热流和简谐热流两种情况下的解析解.然后, 在此基础上应用Fourier级数展开法和叠加原理, 获得了平板前表面热流任意周期变化时非Fourier热传导下温度场的解析表达式. 利用得到的解析表达式进行数值模拟, 分析了不同热松弛时间、 不同时刻和不同位置对温度响应的影响, 讨论了非Fourier热传导模型所给出的温度响应与Fourier热传导模型的差别. 这种方法能够处理许多在生产实际中具有周期边界条件的非Fourier热传导问题. 关键词： 非Fourier 热传导 周期变化 温度响应 平板     

多光束纳秒紫外激光制作硅表面微结构

使用波长351 nm的半导体泵浦全固态脉冲激光器（DPSSL）作为光源,经过位相光栅分束,形成干涉光场,在硅表面直接刻蚀微结构,制作了周期为0.55 µm,槽深可达55 nm的一维微光栅和周期为1.25 µm,刻蚀深度45nm的正交微光栅结构。给出了微光栅形貌结构的扫描电子显微镜(SEM)和原子力显微镜(AFM)的测量结果。正交微光栅的一级衍射效率在1.8%到6.3%之间。该研究是改变硅表面微结构,优化硅材料特性的一种新方法,并扩展了大功率激光刻蚀在表面微加工领域的应用。     

高功率双包层光纤激光器温度场动态分析

基于动态热传导方程,对高功率双包层光纤激光器的热效应进行了数值模拟,研究了高功率双包层光纤激光器内部温度场的空间及时间特性.计算表明,对于芯径为15 μm的光纤,开启泵浦光后约20 s光纤温度达到稳定值,关闭泵浦光后约20 s光纤冷却到室温,如采用脉冲泵浦,温度分布将随时间变化,波动频率与泵浦频率相同,泵浦频率越高波动越小,越接近相同平均功率的连续泵浦.     

Numerical simulation of a CW-pumped Cr:YAG passively Q-switched Yb:YAG pulsed laser

A general model of a CW-pumped passively Q-switched laser is presented which considers the pump power and excitation state of the absorber. The recovery time of the absorber is affected by the Q-switched laser pulse length and repetition rate, which results in changes of the output power, peak power and pulse duration of a CW-pumped passively Q-switched solid-state laser. At low pump power, the final inversion population density alters slowly by a small amount and the effect of the absorber's finite recovery time on laser output energy can be ignored. At high pump power, the final inversion population density increases quickly with pump power increase and the output energy decreases greatly. Simulation results of a CW-pumped, Cr:YAG Q-switched, Yb:YAG laser are reported. The numerical simulation results agree with the theoretical analytical analysis.     

考虑非傅立叶效应的固液相变分子动力学模拟

采用耦合双温度模型的分子动力学方法对飞秒激光照射金箔的固液相变过程进行了模拟研究,利用序参数法对固液原子进行判定从而确定了金箔发生相变时的固液界面位置和温度,对基于傅立叶定律的抛物线模型和考虑非傅立叶效应的双曲线模型模拟得到的结果进行对比分析,在此基础上采用耦合双曲线模型的分子动力学方法研究了激光能量密度和脉冲宽度对金箔相变过程的影响.结果表明,当激光作用于金箔时,金箔上表面首先熔化,固液界面随时间不断向金箔底部移动,并且在相同条件下,双曲线模型下的金箔熔化深度和固液界面温度均大于抛物线模型的结果.当考虑非傅里叶效应时,激光能量密度越大,固液界面温度越高,金箔熔化时间越短;激光脉冲宽度越小,固液界面温度越大,金箔熔化速度越快.     

数值模拟抽运分布对端泵激光器晶体热透镜球差的影响

为了研究端面抽运情况下,激光晶体在不同分布的抽运光抽运时热透镜球差的变化,通过对稳态热传导方程和Zernike多项式的求解,建立了热透镜球差与抽运光强度分布的模型,对模型进行了理论分析和仿真研究,并对仿真结果做了进一步理论和仿真分析.结果表明:在相同的抽运功率下,二阶超高斯分布抽运光抽运时球差最大,且随着抽运分布系数k的增大(除高斯分布外)球差逐渐减小;随着抽运功率的增加,抽运分布系数k对球差的影响逐渐加重,且不同分布系数k所产生的球差差距逐渐增大;并对二阶超高斯分布抽运光抽运得到最强激光功率的照射范围进行了理论分析和仿真分析,得知在相同抽运功率下,二阶超高斯分布抽运光得到最强激光功率的范围最宽为0.30—0.63倍高斯半径.     

高热流密度激光作用下含湿多孔材料的传热传质分析

建立了快速高热流密度激光作用下含湿多孔材料传热传质数学模型,并采用恰当的数值处理方法进行了数值计算.结果表明,快速高热流密度激光作用下含湿多孔材料的传热传质具有经典热质传递理论难以解释的超常效应-非傅立叶与非费克效应,且非费克效应比非傅立叶效应更明显,较高含湿量的材料在高热流密度激光作用下的非费克效应应予以足够重视.     

LD端面泵浦变热导率圆片Yb∶YAG激光器的热效应

针对激光二极管端面泵浦圆片Yb∶YAG晶体产生的热效应问题,以实际工作特点为基础,通过热传导理论分析了热效应。分析了不同泵浦功率、超高斯阶次、光斑半径、晶体尺寸因素对变热导率圆片Yb∶YAG晶体温度场的影响。研究结果表明,使用泵浦功率为60W、超高斯阶次为5、光斑半径为400μm的泵浦光对含质量分数为8%、晶体半径为4mm、厚度为0.5mm的圆片Yb∶YAG晶体进行泵浦,在将晶体的热导率分别视为常量和变量时,泵浦端面获得的最大温升分别为52.15℃和59.51℃。根据计算结果,设计了适合的激光器热稳腔,能充分抑制激光器产生的热效应问题。     

Non-Fourier heat conduction with radiation in an absorbing, emitting, and isotropically scattering medium

This article numerically analyses the combined conductive and radiative heat transfer in an absorbing, emitting, and isotropically scattering medium. The non-Fourier heat conduction equation, which includes the time lag between heat flux and the temperature gradient, is used to model the conductive heat transfer in the medium. It predicts that a temperature disturbance will propagate as a wave at finite speed. The radiative heat transfer is solved using the P₃ approximation method. In addition, the MacCormack's explicit predictor-corrector scheme is used to solve the non-Fourier problem. The effects of radiation including single scattering albedo, conduction-to-radiation parameter, and optical thickness of the medium on the transient and steady state temperature distributions are investigated in detail. Analysis results indicate that the internal radiation in the medium significantly influences the wave nature. The thermal wave nature in the combined non-Fourier heat conduction with radiation is more obvious for large values of conduction-to-radiation parameter, small values of optical thickness and higher scattering medium. The results from non-Fourier-effect equation are also compared to those obtained from the Fourier equation. Non-Fourier effect becomes insignificant as either time increases or the effect of radiation increases.     

超短激光打孔中快速相变的格子玻尔兹曼模拟

采用双重分布函数的格子玻尔兹曼模型，对单脉冲激光金属打孔过程中的快速相变传热进行研究.模型考虑了金属材料熔化后熔体的流动换热，并采用浸没移动边界方案对过程中的固液界面进行追踪.采用纯导热模型和考虑对流的换热模型计算，将结果和试验进行对比，结果表明：在激光打孔过程中熔体的流动对相变传热产生较大影响，采用考虑流动换热模型的结果与实验更接近.进而对熔化速度、熔化深度以及温度场的变化进行分析，并探讨不同激光工艺参数对相变过程的影响.模拟发现一个脉冲结束后，激光的脉宽越大，孔深越小，孔径越大，且最高温度较短脉冲激光越低.     

微型电阻加热推进器内的传热问题

分析了微型电阻加热推进器中沸腾的特征以及过热极限温度,分别采用经典的抛物型热传导方程和非傅立叶导热的双曲型模型(C-V模型)、双元相滞后模型(DPL模型)来描述导热过程,采用有限差分法对微型电阻加热推进器中导热过程的初始阶段进行了数值求解.发现非傅立叶模型较傅立叶模型有-定的时间延迟,DPL模型与C-V模型的延迟差别不太大;在加热过程中,加热电阻及其临近的水很快会升至300℃,而大部分水的温度升得较慢.     

脉冲激光二极管泵浦调Q激光器泵浦时间特性研究

从调Q激光器的速率方程出发,对脉冲激光二极管泵浦的调Q激光器在不同泵浦功率及不同透过率情况下的最佳泵浦时间进行了理论分析。计算结果表明,在透过率为20%,腔内损耗为0.05的条件下,1—3倍阈值泵浦功率时,最高效率对应的最佳泵浦时间为1—1.5倍Nd∶YAG荧光寿命;7倍阈值泵浦功率及以上时,最佳泵浦时间约为荧光寿命的一半或者更短。在脉冲泵浦能量相同时,采用较短的泵浦时间(即泵浦功率较高),调Q输出的能量转换效率较高。     

Diode-pumped Actively Q-switched Nd:GdVO<Subscript>4</Subscript> Green Laser with Periodically Poled KTP

A diode-pumped actively Q-switched Nd:GdVO₄ green laser with periodically poled KTP (PPKTP) and Acoustic-optic (AO) modulator has been realized. The dependences of pulse energy, pulse width, and peak power on incident pump power are measured. At the pump power of 4.1 W and repetition rate of 10 kHZ, we can obtain a 19.2 ns pulse with pulse energy of 0.11 mJ and peak power of 5.8 kW, which are almost three times of that in the conventional KTP green laser, respectively. A rate equation model is introduced to theoretically analyze the results obtained in the experiment, in which the Gaussian spatial distribution of the intracavity photon density is taken into account. The numerical solutions of the rate equations agree with the experimental results.     

Numerical modeling of the thermal lensing effect in a grazing-incidence laser

The numerical modeling of thermal lensing effect is investigated in a grazing-incidence laser. The deformation of the bounce face is introduced into the modeling for the first time, and the Gaussian distribution of the pump light and the anisotropic heat conduction are considered. The results indicate that the proportion of the deformation on the bounce face to the thermal lensing effect is as high as 80% for small grazing-incident angle of 5°. The thermal lensing effect sensitively depends on the pump power, grazing-incident angle and the pump distribution in a grazing-incidence bounce geometry laser.     

多孔介质高强度传热传质的理论研究

提出了高强加热下含湿多孔介质传热传质新模型,模型包括的水分种类齐全、水分迁移机制全面,假设条件相对较少,考虑了非Fourier传热效应和非Fick传质效应,模型通过具体的含湿量分区分析得到简化。