LDA侧面泵浦Nd:YAG激光器的热效应分析

 在高斯光强近似下对泵浦LD光强分布模型进行修正，建立了LDA侧面泵浦固体激光介质内热源分布的数值模型。用有限元法计算模拟了三角均匀分布侧面泵浦结构激光棒瞬态温升过程及稳态温度分布情况。讨论比较了泵浦源的高斯强度近似和均匀强度近似下激光棒内温度分布情况，并对激光棒的类热透镜的焦距进行实验测量。实验和数值计算说明了LDA泵浦结构和冷却场的非均匀分布使实际温度场偏离均匀泵浦时的二次曲线分布模型，激光晶体热效应产生的类透镜会聚作用的不对称导致了激光器输出光束质量在x，y方向上的不同。     

侧面抽运板状激光介质的热汇冷却

高热负荷固体激光介质的热效应已经成为制约激光器功率进一步提高的严重障碍,只有对激光介质进行有效的冷却才能保证其安全运行。以不均匀换热系数模型为基础,研究了具有非均匀内热源的侧面双向抽运板状激光介质在狭窄通道强制对流冷却情况下的耦合换热问题,对热汇冷却方案下介质的温度分布和热应力分布进行了数值模拟和分析,并对复合介质、蓝宝石和金刚石三种热汇材料进行对比。结果表明,忽视换热系数的非均匀性将导致应力计算结果偏低。对于侧面抽运、侧面冷却的激光介质,金刚石热汇冷却方案最佳,蓝宝石热汇方案次之,而复合介质方案不宜采用。     

浑浊介质中激光侧向散射实验与仿真研究

通过对Mie散射相位函数的累积概率密度插值获得散射角,建立了激光在浑浊介质中传输的蒙特卡罗仿真模型。借助模型研究了激光在均匀单分散聚苯乙烯浑浊介质中的多次散射现象,探究了不同光学厚度和散射相位函数的浑浊介质对激光多次散射的影响。在研究中控制5μm和10μm两种粒径聚苯乙烯颗粒的浓度,改变浑浊介质的光学厚度分别为2、5和8,进行了理论仿真和实验的二维侧向散射图像比对,发现相同介质条件下仿真与实验得到的散射光强衰减百分比差异均小于16%,散射光强曲线走势基本一致。仿真还能够提供不同散射阶次光强大小的分布情况,能准确分析多次散射的影响。     

激光、激光技术 激光工作物质

TN244 2005053303 侧面抽运板状激光介质的热汇冷却=Cooling of a side- pumped laser slab by heat sink[刊,中]／王军荣(清华大学 航天航空学院,传热强化与过程节能教育部重点实验室． 北京(100084)),闵敬春…∥光学学报．-2005,25(6)．- 829-934 以不均匀换热系数模型为基础,研究了具有非均匀内 热源的侧面双向抽运板状激光介质在狭窄通道强制对流 冷却情况下的耦合换热问题．对热汇冷却方案下介质的温 度分布和热应力分布进行了数值模拟和分析,并对复合介 质、蓝宝石和金刚石3种热汇材料进行对比。结果表明, 忽视换热系数的非均匀性将导致应力计算结果偏低。对 于侧面抽运、侧面冷却的激光介质,金刚石热汇冷却方案 最佳,蓝宝石热汇方案次之,而复合介质方案不宜采用。 图5表2参14(于晓光)     

瞬态激光脉冲在吸收、发射性介质内引起的温度响应

本文导出了平行光入射辐射穿过半透明界面,在吸收、发射性介质内产生的辐射热源的表达式。数值模拟了瞬间激光脉冲入射下：（1）半透明介质与不透明介质在非人射面上的过余温度响应;（2）入的激光的波长对温度响应的影响;（3）界面光学特性（两侧均为不透明界面,两侧均为半透明界面,一侧为半透明、另一侧为不透明界面）对温度响应的影响;（4）采用Planck、Rosseland平均吸收系数处理非灰介质时,对辐射与导热瞬态耦合换热的影响。     

板状激光振荡介质温度场和应力场的数值模拟

以非均匀内热源模型为基础,并考虑材料的导热系数和热膨胀系数的温度相关性,利用有限元方法,对板状激光振荡介质在不同功率和冷却强度下的温度和热应力分布进行了数值模拟及分析。根据计算结果提出了最大有效换热系数的概念,指出换热系数超过最大有效换热系数后,继续强化传热对减小高功率激光器的热效应已无明显效果。以此得出采用常规冷却技术所允许的最大泵浦加热功率。     

利用瞬态辐射信号峰值重构半透明介质内部特性

本文采用有限体积法模拟超短脉冲激光在一维参与性介质内的传输,得到瞬态辐射信号,即时域反射信号与透信号随时间变化关系,利用透射和反射信号的极值并结合微粒群算法(PSO)反演估算介质内部物性。本文研究了均匀介质时信号极值计算的目标函数随吸收、散射系数变化关系,结果表明采用极值法能够准确地反演均匀介质物性。此外,研究了两层非均匀介质时目标函数随散射系数变化关系。在吸收系数已知时,采用单面照射激光所得信号极值能够反演出两层介质各自的散射系数。最后,本文研究了同时反演两层介质的吸收和散射系数,采用综合反演的方法能够准确地反演出各层介质的吸收和散射系数。     

基于频域辐射传输模型的光学成像研究

本文以超短脉冲激光照射参与性介质的光学成像为研究背景,分别构建了短脉冲激光在参与性介质内的频域辐射传输正问题模型和根据边界探测所得频域信号重建介质内部光学参数的逆问题模型。在瞬态辐射传输方程的基础上,利用傅里叶变换得到频域辐射传输方程,采用有限体积法求解频域传输方程,模拟超短脉冲激光在二维参与性介质内传输的过程,得到介质边界的出射频域辐射信号。选取共轭梯度法作为反演算法,采用伴随差分模型求解目标函数梯度,重建了二维非均匀参与性介质内不同位置内含物的光学参数分布。结果表明,基于频域辐射传输方程的伴随差分模型能够较为准确地反演多维参与性介质内的光学参数。     

超短脉冲激光在二维非均匀介质内的瞬态辐射传输

随着超短脉冲激光的快速发展,吸收散射性介质内的瞬态辐射传输引起了广泛的关注.本文采用最小二乘有限元法模拟了超短脉冲激光局部入射条件下,具有高散射核的二维非均匀介质内的瞬态辐射传输.研究了不同边界位置上反射和透射信号随时间的变化情况.结果表明,对于具有高散射核的非均匀介质,能够揭示散射核位置的双峰现象可能在早期的反射信号中发生,早期的反射信号比后期的、经过介质衰减后信号更重要.因此,在利用短脉冲激光进行光谱分析和成像等技术中,人们应该重视早期反射信号的测量.     

二极管侧面泵浦薄片激光器泵浦均匀性分析

 建立了二极管侧面泵浦复合薄片激光介质Nd:YAG/YAG的数值模型：二极管阵列的快轴垂直于薄片激光介质表面的排布,二极管对称排列在增益介质的周围,从侧面进行泵浦,通过微柱透镜对二极管的快轴进行准直。模拟并实验研究了激光二极管慢轴方向的远场分布特性,结果发现在近距离时激光二极管慢轴方向上的远场分布近似为高斯分布。对于二极管参量,研究发现泵浦二极管越多,增益介质内泵浦光分布就越理想;增益介质吸收系数越小,泵浦的均匀性就越好,但总的吸收效率下降;二极管与工作物质的距离越近,工作物质靠近中心的区域对泵浦光的吸收就越多,但泵浦的均匀性就越差。选用增益介质为Nd:YAG/YAG的复合薄片介质,当掺杂原子分数以及增益介质的吸收系数不同时,发现0.6%掺杂的增益介质(吸收系数为0.24 cm^-1)的泵浦均匀性比1%掺杂(吸收系数为0.51 mm^-1))有明显改善,实验结果与模拟结果一致。     

传导冷却端面泵浦板条放大器波前畸变数值研究

 利用数值模拟的方法分析了均匀冷却与非均匀冷却对传导冷却端面泵浦板条放大器波前畸变的影响,结果发现：信号光的波前畸变强烈依赖于zig-zag周期数,一定的光路可以将波前畸变减小到λ/10量级;板条长度方向散热不均匀及两块热沉之间散热能力的差异对波前畸变的影响很小,而宽度方向散热不均匀对波前畸变的影响较大;当宽度方向散热不均匀时,波前畸变主要为倾斜像差,输出光束的传输方向将发生偏折。因此,在非均匀冷却条件下,为了减小宽度方向散热不均匀性的影响,应选择反射次数较少的之字型光路。     

波面热畸变自校正型板条增益介质的瞬态温度分布

提出一种新的激光增益介质板条抽运结构,这种结构能对由激光增益介质板条温度分布不均所造成的波面热畸变实现自校正,并建立了这种抽运结构下激光增益介质板条的瞬态温度分布理论模型,通过解热传导方程,推导出板条增益介质的瞬态温度分布的解析表达式。同时对抽运参量分别为抽运脉冲能量为5.8×104J、抽运重复频率为2Hz和抽运脉冲能量为3.2×104J、重复频率为10Hz两种情况下两种下N-31型钕玻璃板条的瞬态温度分布分别进行了数值计算,分别给出两种情况下10s内和60s时钕玻璃板条的温度分布图并对结果进行了分析和对比,表明采用这种抽运结构的增益介质板条激光器可以以类似热容激光器的方式在短时间内实现高平均功率、高光束质量运转。     

A modification of the cumulative wavenumber method to compute the radiative heat flux in non-uniform media

This paper presents a modification of the cumulative wavenumber (CW) method to determine the radiative heat flux in non-uniform participating gases. Previous works in the literature have shown that the CW method renders accurate estimates of the radiative volumetric heat source in the medium. However, as will be shown in this work, the radiative heat flux can present considerable deviation of the correct solution, which results from the radiative energy balance not being satisfied by the CW method. A modification of the method is devised in this work to satisfy the radiative energy balance while keeping the same value of the radiative volumetric heat source. The proposed methodology is applied together with the discrete ordinates method to solve the radiation heat transfer in a one-dimensional slab containing a non-isothermal layer of carbon dioxide. The results are compared to the benchmark line-by-line (LBL) integration, and show that the modified CW method can satisfy the radiative energy balance, improving the estimation of the radiative heat flux in the medium.     

有矩形脉冲内热源的平板中非傅里叶导热研究

 对矩形激光短脉冲能量以内热源形式在有限厚绝热平板内释放所导致的非傅里叶导热过程进行了理论研究，利用格林函数与有限积分变换方法求得了绝热平板内温度分布的解析解,分析了平板内热波传播与反射行为以及温度场的特征。研究结果表明，温度波动频率取决于平板厚度和材料的热弛豫时间，而其幅值则与内热源释放区域的大小和脉冲宽度的长短有关。     

Power dissipation and temperature distribution in piezoelectric ceramic slabs

A method is presented to determine power dissipation in one-dimensional piezoelectric slabs with internal losses and the resulting temperature distribution. The length of the slab is much greater than the lateral dimensions. Losses are represented using complex piezoelectric coefficients. It is shown that the spatially non-uniform power dissipation density in the slab can be determined by considering either hysteresis loops or the Poynting vector. The total power dissipated in the slab is obtained by integrating the power dissipation density over the slab and is shown to be equal to the power input to the slab for special cases of mechanically and electrically excited slabs. The one-dimensional heat equation that includes the effect of conduction and convection, and the boundary conditions, are then used to determine the temperature distribution. When the analytical expression for the power dissipation density is simple, direct integration is used. It is shown that a modified Fourier series approach yields the same results. For other cases, the temperature distribution is determined using only the latter approach. Numerical results are presented to illustrate the effects of internal losses, heat conduction and convection coefficients, and boundary conditions on the temperature distribution.     

玻璃板条激光器的热应力

建立在介质内部均匀发热基础上的板条激光器热效应理论与器件实际相差甚大,本文根据介质对光的吸收规律建立介质内部非均匀发热模型,计算与实验结果表明,新的理论模型更接近于实际激光器件。 关键词：     

Dimensional scaling of flame propagation in discrete particulate clouds

The critical dimension necessary for a flame to propagate in suspensions of fuel particles in oxidiser is studied analytically and numerically. Two types of models are considered: First, a continuum model, wherein the individual particulate sources are not resolved and the heat release is assumed spatially uniform, is solved via conventional finite difference techniques. Second, a discrete source model, wherein the heat diffusion from individual sources is modelled via superposition of the Green's function of each source, is employed to examine the influence of the random, discrete nature of the media. Heat transfer to cold, isothermal walls and to a layer of inert gas surrounding the reactive medium are considered as the loss mechanisms. Both cylindrical and rectangular (slab) geometries of the reactive medium are considered, and the flame speed is measured as a function of the diameter and thickness of the domains, respectively. In the continuum model with inert gas confinement, a universal scaling of critical diameter to critical thickness near 2:1 is found. In the discrete source model, as the time scale of heat release of the sources is made small compared to the interparticle diffusion time, the geometric scaling between cylinders and slabs exhibits values greater than 2:1. The ability of the flame in the discrete regime to propagate in thinner slabs than predicted by continuum scaling is attributed to the flame being able to exploit local fluctuations in concentration across the slab to sustain propagation. As the heat release time of the sources is increased, the discrete source model reverts back to results consistent with the continuum model. Implications of these results for experiments are discussed.     

Numerical Simulation of the Non-Fourier Heat Conduction in a Solid-State Laser Medium

A hyperbolic model of non-Fourier heat conduction with non-uniform heat source is used to simulate the transient heat transfer in a high-pulse-pumped solid-state laser medium. The temperature fields are numerically analysed using the finite difference method combined with the TDMA algorithm for different pump power densities, pulse durations, thermal relaxation time and cooling intensities, respectively. The calculated results are compared with those predicted by the parabolic heat conduction model based on the Fourier law. The results indicate that the non-Fourier heat conduction phenomenon in laser media should be considered when the pump power density exceeds 104 W/m^2 or under low pulse duration. In addition, the conditions of non-Fourier effects and their influencing factors are analysed.