高能激光光束质量评价参数适用性探讨

光束质量是决定高能激光系统综合性能的重要指标,针对不同应用目的,国内外研究人员提出了多种参数来评价激光的光束质量。本文对常用的光束质量评价参数在评价高能激光光束质量时存在的局限性进行了分析,重点介绍了光束传输因子BPF,对其在高能激光领域中的适用性进行了探讨,该参数的评价只需探测高能激光的功率即可,测量过程简单可行,最后对BPF测量不确定来源进行了定性分析。     

霍尔推力器内放电等离子体数值仿真分析

为研究霍尔推力器通道内放电过程和等离子体的分布情况,对霍尔推力器建立二维仿真模型。得到推力器通道中Xe~+和Xe~(++)数密度分布、速度分布、电流密度分布、电势分布和电子温度分布等,通过不同时刻Xe~+和Xe~(++)离子数密度的变化,观察到推力器内放电达到稳定运行的过程。由结果可知此推力器工作电压350V,电流4.2A,工质气体流量42SCCM时的比冲约为1300S。与文献中报道的实验结果和数值模拟结果对比,具有很好的一致性。一方面验证了该模型用于霍尔推力器数值仿真的有效性,另一方面得到了稳态霍尔推力器内较详细的放电等离子体参数分布。     

光束质量β因子对热晕的影响

本文研究衍射效应、湍流、束抖动、光束质量对聚焦束热晕的影响,更侧重于光束质量β因子对热晕的影响,得到了稳态、长脉冲和短脉冲热晕与β的四个关系式f_(c1)、f_(D1)、f_(lc1)、f_((?)c1)。这对以后的激光大气传输的热晕实验具有重要的参考价值。     

基于微透镜阵的外腔谱组束系统光束质量分析

针对基于微透镜阵的光纤激光外腔谱组束系统的光束质量评价问题,利用高斯光束的传输变换理论,建立了评价谱组束激光光束质量的理论模型.通过数值模拟,详细分析了各相关参数对谱组束光束质量的影响,结果表明:在基于微透镜阵的谱组束系统中,离焦量、微透镜焦距及模场半径是影响谱组束激光光束质量的主要因素;阵列宽度对谱组束激光光束质量影...     

激光通道传输热效应仿真

采用数值计算方法分析了高能激光在光束控制器内部传输所产生的热效应及其对激光远场光斑的影响。结果表明,对于较高激光能量,光速控制传输通道中的激光热效应不容忽视,它会明显影响光束控制系统的发射性能,降低远场目标处的激光能量集中度,并使光斑产生较严重的畸变。     

人字形波纹板式蒸发器数值模拟

本文根据两相流动换热理论,提出了一个板式换热器的稳态分布参数模型.在此模型基础上,利用分布参数法求解各控制方程,得出了制冷剂的温度、压力、气液相速度以及冷冻水温度、压力沿通道方向上的分布情况.此模型可用于分析换热器的整体性能,为板式换热器的优化设计、制冷系统的匹配提供依据.     

纳米尺度通道中气体粘度的分子动力学模拟

利用非平衡分子动力学方法模拟了气体在纳米尺度通道中的运动特性,统计获得通道中流动的速度剖面和剪切应力分布,并利用牛顿粘性定律首次获得了纳米尺度通道中的等效粘度分布。结果表明,纳米尺度通道中的粘度不是一个常数。在壁面附近,由于壁面原子和气体分子的相互作用,存在壁面效应,气体的粘度较小;而通道中心区域的粘度与实验结果符合较好,壁面对粘度的影响范围为20 nm左右。通道高度对中心区域粘度的影响很小,而温度对其的影响较大,粘度值随温度的增加而增大。不同通道高度下,壁面附近粘度的分布几乎一致;不同温度时,壁面附近粘度的分布随温度的增加而增加。     

飞行器周边气体折射率变化对激光测量影响的模拟研究

在利用激光测量飞行器姿态参数的实验中,当测量光线穿过飞行器所产生的射流场时,场内气体光折射率不同而使光路发生弯曲,从而对测量结果造成影响.本文根据空气射流动力学原理,对飞行器周围射流场内气体的光折射率分布规律进行分析,建立简化模型,并以此得出影响测量误差的相应因素,估算出测量光束弯曲的角度偏差,从而对测量结果进行修正;同时可以提高系统的测量精确性,对测量系统的设计有所帮助.模拟试验结果表明,该计算方法可以有效的计算出测量结果的误差.     

几何因素对微通道脉管制冷性能影响的分子动力学分析

运用分子动力学模拟方法,研究微通道脉管内部微观的制冷效应,讨论了微通道脉管几何因素对制冷性能的影响。建立了矩形截面微通道脉管模型,模拟微通道脉管内工质He气体的充放气过程,获得通道内部轴向压力、速度、温度的分布。分析了脉管固有尺寸对微通道脉管冷热端温度的影响。研究表明:模型1B-2L中随着过程进行,压力梯度逐步减小,最终达到平衡,同时会出现略微的逆向梯度现象;原子轴向速度在200 ps时最大为434 m/s,随后轴向速度峰值向管内移动并递减。维持模型宽度恒定增加长度或者维持模型长度恒定增加宽度,热端温度均有所上升,冷端温度有所下降。长度宽度的增加对微通道脉管性能都有一定促进作用;保持长径比,同比例放大模型,在一定程度上优化了脉管的工作性能,存在一个最优脉管容积。     

采用液晶光阀的氦氖激光准直控制

激光光束的漂移,功率稳定性和激光光斑的强度分布对称性是影响准直精度的主要参数。通过分析液晶光阀的特性,提出了一种智能型光束准直控制方法,可同时控制激光功率的稳定性和光斑的强度分布对称性。利用热平衡技术,设计了一套温度实时反馈控制的准直装置。实验结果证实了该方法的可行性和有效性。     

Calculation of temperature distribution and thermo-optical effects in double-end-pumped slab laser

The temperature distribution and thermo-optical effects in a double-end-pumped slab laser are investigated analytically. The theoretical model is given by considering heat generation on both sides of an active medium due to pumping. With account for the pump beam divergence and the heat load, the heat conduction equation is solved, and the temperature distribution and thermal effects, such as thermal lensing and thermal stress, are obtained. The results are applied to a typical Nd:YVO₄ laser crystal slab and discussed.     

Three-dimensional calculation of high-power, annularly distributed, laser-beam-induced thermal effects on reflectors and windows

Based on the three-dimensional transient heat conduction equation and the elastic stress-strain equation, the temperature rise, distortion, and equivalent stress distributions of a high-reflectivity silicon reflector and a white bijou window irradiated by a high-power sloped annularly distributed laser beam are simulated using a three-dimensional finite element model (FEM). The effects of laser intensity, output duration, beam obscure ratio, and laser intensity spatial gradient on the results are especially investigated. The effects of mirror and window thermal distortion on laser beam phase aberrations are also evaluated. This noncylindrosymmetric three-dimensional FEM can be used to evaluate high-power, high-energy, laser beam-induced thermal effects on optical components.     

Analytical approach to thermal lensing in end-pumped Yb:YAG thin-disk laser

Thermal lensing in the thin-disk laser influences the output beam quality and optical efficiency significantly. In this paper, an analytical approach is taken to study the production mechanisms, features, and influences of thermal lensing in the end-pumped thin-disk laser. We calculate the distributions of temperature, stress, strain, and expansion in the disk and the curvature of the crystal using an analytic method. The expressions of the thermal lens focal length depending on the radius are presented. The optical path difference, a major cause of thermal lensing, is induced by the thermo-optical effect, the photoelastic effect, and inhomogeneous distribution of thermal expansion and the excited population. Thermal lensing is found to be aspheric with undesired aberrations and birefringence effects. Furthermore, a convex mirror due to the axial temperature gradient occurs in a free disk, and the convex mirror is found to be spherical in the center region of the disk. Based on the results of our analysis, the aspect ratio and size of the laser mode of the gain region may be adjusted to limit the damaging effects of thermal lensing.     

钛合金表面激光熔覆TiC/NiCrBSi涂层温度场有限元模拟

为在钛合金表面获得优质激光熔覆涂层,用有限元方法研究了激光熔覆工艺对熔池温度场分布和凝固后熔覆层组织的影响,考虑相变潜热、辐射对流散热以及温度对热物理性能的影响等因素,建立三维有限元模型模拟了Ti6Al4V合金表面激光熔覆TiC/NiCrBSi复合涂层过程中的温度场,并结合熔覆过程的温度场分布,对涂层的形貌、结合区、基...     

Simulation of temperature distribution in single metallic powder layer for laser micro-sintering

Simulation of temperature distribution in single metallic powder layer for laser micro-sintering (LMS) using finite element analysis (FEA) has been proposed, taking into account the adoption of ANSYS μMKS system of units, the transition from powder to solid and the utilization of moving laser beam power with a Gaussian distribution. By exploiting these characteristics a more accurate model could be achieved. The effects of the process parameters, such as laser beam diameter, laser power and laser scan speed on the temperature distribution and molten pool dimensions have been preliminarily investigated. It is shown that temperature increases with the laser power and decreases with the scan speed monotonously. For the laser beam diameter during single-track, the maximum temperature of the powder bed increases with the decrease in the laser beam diameter, but far from the center of the laser beam area, the temperature increases with the laser beam diameter. The molten pool dimensions in LMS are much less than that in classical selective laser sintering (SLS) process. Both molten pool length and width decrease with the laser beam diameter and the laser scan speed, but increase with the laser power. The molten pool length is always larger than the molten pool width. Furthermore, the center of molten pool is slightly shifted for the laser multi-track.     

Influence of thermal deformations of the output windows of high-power laser systems on beam characteristics

By using the well-known Green's function methods, we study the three-dimensional temperature distributions and thermal deformations of the output windows of unstable optical resonators induced by an incident annular laser beam. Some expressions and theoretical profiles of the temperature distributions and thermal deformations as functions of the radius and of the thickness of optical windows are obtained. Moreover, the influence of the thermal deformations of sapphire, silica, and silicon windows within unstable optical resonators on the Strehl ratio and on the far-field laser intensity distribution is also discussed. Under conditions of 50-kW intense laser irradiation during 5 s, the maximum thermal deformation in sapphire, silica, and silicon substrates is 1.993, 0.393, and 6.251 microm, respectively. Under the same conditions the Strehl ratio of sapphire is higher than that of silica.     

Laser heat treatments driven by integrated beams: role of irradiation nonuniformities

An analysis is given of how nonuniformities in the laser beam intensity translate into variations on the induced temperature distribution on an irradiated sample. The study involves materials with different thermal conductivities. By use of a reshaped irradiating beam obtained with a multifaceted integrating mirror, a three-dimensional numerical calculation allows us to establish both surface and in-depth temperature distributions. The results show that in the case of materials such as glass (i.e., with low thermal conductivity) large thermal gradients occur both on the surface and in depth during irradiation. However, the lateral heat flow is high enough to strongly reduce the surface gradients as soon as the laser irradiation ends. Conversely, in good thermal conductors such as nickel, the laser intensity nonuniformities induce a thermal peaking of the surface with lateral thermal gradients that are by no means negligible. Experimental evidence during laser glass polishing that confirms the numerical assessments are also provided.     

Thermal effects in a hollow waveguide beam launch for CO(2) laser power delivery

Thermal effects caused by launching conditions in a CO(2) laser beam delivery that uses metallic hollow waveguides is investigated. It is found that front-end clipping is the main cause of thermal loading and generates a steep temperature gradient at the fiber front end while the continuous beam attenuation produces a temperature distribution declining slowly along the waveguide.     

Study of an Optical Device Used to Homogenize a Laser Beam. Application to Emissivity Measurements on Semitransparent Materials at High Temperature

On an experimental apparatus designed to measure the directional spectral emissivity of semitransparent materials at high temperature (2000°C), a specific optical device (kaleidoscope) is mounted in order to homogenize the energy distribution of a CO₂ laser beam, which is used as a heating source. An objective of this work, for operating in the best conditions, is to obtain a uniform temperature of the tested sample. This study, based on Fourier optics, focuses on a square aperture kaleidoscope. A model was developed to simulate the energy distribution of the laser beam, at any transverse plane on the optical path. The final objective is to simulate the energy distribution on the sample surface to optimize the homogenization device taking into account the surface temperature gradient induced by the local energy distribution. To validate this model, quantitative comparisons of theoretical simulations and experimental thermal spots are performed.