重复脉冲激光辐照光学材料的热力效应

在建立高斯型重复脉冲激光辐照光学材料模型的基础上,得到了圆柱型光学材料的二维温度场和热应力的解析解.以K9玻璃为例,通过数值计算得到同条件下重复脉冲激光对光学材料的损伤阈值.研究表明：在高斯型重复脉冲激光辐照下,损伤阈值受到脉冲数目、宽度、重复频率以及脉冲激光光斑半径的影响,多数情况下K9玻璃会发生热应力损伤.     

用于激光推进的高功率激光器的选择

 从激光推进的要求出发，阐述了用于激光推进的高功率激光器的选择原则，即激光器必须满足：（1）高的平均功率和峰值功率；（2）高的单脉冲能量；（3）高的重复频率；（4）优良的大气传输特性。主要分析了目前YAG固体激光器、自由电子激光器和TEA脉冲CO₂激光器的特点，通过上述4个方面性能的比较，认为在目前水平下，TEA脉冲CO₂激光器是进行激光推进的首选强激光源，其优点表现在：功率可达10kW量级，单脉冲能量可达0.5~1kJ，重复频率为20~40Hz；激光波长处于大气传输窗口，对大气变化不敏感；工作物质快速流动，不存在热透镜效应和破坏阈值；相关光学元件易于制造；光束质量较好；运行成本低。     

多脉冲激光对K9玻璃的表面损伤实验研究

 实验研究了波长为1 064 nm、脉宽为10 ns、重复频率为1 Hz的激光脉冲对K9玻璃的表面损伤特点,给出了脉冲透过能量随激光脉冲作用次数变化的规律。采用3维立体显微镜对损伤形貌进行观察,发现K9玻璃的损伤表面呈环状分布,分为烧蚀区、微裂纹区和断裂区。随着激光脉冲个数的增加,损伤由点状破坏演变为损伤区,微裂纹逐渐增长,损伤面积逐渐增大。基于激光支持的爆轰波理论分析,激光与脆性材料的相互作用可引起微裂纹的大量增长。在多脉冲激光的作用下,K9玻璃损伤的累积效应明显,表面损伤阈值明显降低,表面裂纹增长明显,损伤面积逐渐增大;但随着激光脉冲的继续增加,这种损伤趋于稳定。     

纳秒激光在K9玻璃中聚焦的损伤形貌研究

详细研究了纳秒激光脉冲在K9玻璃内部产生损伤的形貌特点.整个损伤形貌是前端较大、后端逐渐减小,呈纺锤形.损伤区可分为四种类型的损伤形貌：损伤点轴向的丝状等离子通道、熔化区域、裂纹区域和裂纹末端的折射率变化区域.给出了激光脉冲能量在空间的沉积函数和冲击波膨胀压强的表达式,并根据压强的空间分布特点对相应的损伤形貌进行了分析,理论分析与实验结果相符. 关键词： 激光损伤形貌 移动损伤模型 冲击波 激光能量沉积     

电光调Q脉冲Ho:YLF激光器实验研究

2μm低重复频率高峰值功率高光束质量激光在中长波光参量非线性频率变换等领域具有较为广阔的应用前景。采用L型谐振腔结构,使用42 W的掺Tm光纤激光器泵浦Ho:YLF晶体。基于磷酸钛氧铷（RTP）电光调Q技术,实现了重复频率50 Hz、脉冲宽度18 ns、脉冲能量13.5 mJ、峰值功率0.75 MW的2.05μm Ho:YLF固体激光输出。光束的水平方向和竖直方向M2因子分别为1.4和1.1。该Ho:YLF固体激光器采用光纤激光器泵浦,具有结构紧凑的特点,为更高能量的Ho激光输出奠定了基础。     

短脉冲激光诱导薄膜损伤的等离子体爆炸过程分析

短脉冲激光诱导薄膜材料损伤过程的研究通常止于薄膜材料发生喷溅.超热喷溅物质吸收剩余激光脉冲能量将形成剧烈的等离子体爆炸过程.采用两步数值计算方法处理等离子体微滴的爆炸过程,即在每一个数值计算时间步长内,将爆炸过程分为两步,第一步处理微滴的绝热膨胀及裂解过程;第二步处理微滴对激光脉冲能量的吸收过程.有效地将微滴吸收激光能量的物理学过程与爆炸动力学过程耦合到一起.分析了喷溅物质微滴在剩余激光脉冲作用下,其半径、膨胀（加）速度、裂解（加）速度、电子及离子的密度与温度等参量随时间变化的演化情况.结果表明：材料喷溅 关键词： 光学薄膜 激光损伤 等离子体 爆炸     

高功率陶瓷板条激光器

高功率固体激光在先进制造、能源、信息、医疗及国防等领域有广泛应用。激光陶瓷材料具有掺杂浓度均匀可控、抗激光热损伤能力强、易于制备大尺寸和复合结构等优点，被认为是高功率固体激光的重要增益材料。报道了本课题组近年来在高功率陶瓷板条激光方面的主要研究进展。采用中国科学院上海硅酸盐所制备的大尺寸Nd:YAG陶瓷板条，实现了平均功率4.35 kW的1 064 nm脉冲激光输出，脉冲宽度为160μs，重复频率为400 Hz。此外，采用中科院上海硅酸盐所制备的大尺寸Yb:YAG陶瓷板条，实现了平均功率9.8 kW的1 030 nm脉冲激光输出，脉冲宽度为560μs，重复频率为160 Hz，光光转换效率为60%。这些研究表明，激光陶瓷材料具备实现高功率、高光束质量与高效率激光的潜力。     

红外光电探测器的激光损伤分析

分析了国内外有关红外光电探测器激光损伤阈值的大量测量数据。讨论了激光损伤探测器的机理以及目前尚未解决的问题。比较了高重复频率脉冲激光和连续波激光对探测器的损伤效果。分析表明 ,在相同平均功率的条件下 ,高重复频率脉冲激光与连续波激光相比 ,探测器损伤效果前者要比后者大得多     

激光烧蚀硅表面的发射光谱分析

研究了不同能量的纳秒激光脉冲聚焦到单晶硅片上时,激光等离子体的自由电子密度和温度以及损伤形貌随激光脉冲能量增加的变化规律。研究结果表明：激光脉冲击穿硅介质产生的激光等离子体的体积以及自由电子密度和温度大小,决定了硅表面损伤的形貌特点和大小。自由电子密度和温度的变化特点是：随着激光脉冲能量的增加,激光等离子体的体积不断增大,自由电子温度缓慢增加而密度基本不变。又由于激光等离子体的自由电子密度和温度呈现从中心到边缘由大到小的变化趋势,所以损伤形貌总的特点是内部区域的熔化非常充分,形成明显的周期性排列的规则条纹,且条纹的排列趋势呈现环状;中部区域熔化不充分,形成条纹不很规则;边缘区域处分界明显,有时出现等离子体产生喷溅变色痕迹。     

单光束扩束扫描激光周视探测系统参数对探测能力的影响

针对现有单光束激光同步扫描周视探测对脉冲重复频率要求较高,难以实际应用的问题,提出单光束扩束扫描激光周视探测方法.基于单光束扩束扫描激光周视探测工作原理,推导了最低扫描频率和脉冲频率解析式;分析了圆柱目标回波特性及关键参数截面衰减系数,建立了脉冲扩束激光圆柱目标回波功率数学模型,讨论了系统参数对截面衰减系数的影响,得到最大相邻脉冲光束夹角表达式;重点分析了脉冲频率、光束角和光束入射角对不同直径目标的探测能力的影响;得到了探测系统对典型条件下最大光束角、最低脉冲频率的计算方法.结果表明,对扫描光束稍加扩束可有效降低脉冲重复频率要求.研究结果可为单光束脉冲激光周视探测系统设计、优化提供理论依据.     

切向气流作用下玻璃纤维复合材料的激光辐照效应

实验研究了样品表面有切向空气气流、切向氮气气流和无气流时,976 nm连续激光对玻璃纤维增强E-51环氧树脂复合材料的辐照效应。结果表明:无气流时,喷出的热分解产物会对入射激光产生屏蔽作用;有气流时,激光对玻璃纤维的破坏方式是其升温熔化后再被切向气流带走;当激光功率密度较低时,切向空气气流以加强样品表面的对流冷却作用为主,不利于激光对玻璃纤维复合材料的破坏;当激光功率密度较高时,切向空气气流以降低屏蔽作用和提供氧气助燃为主,有利于激光对玻璃纤维复合材料的破坏。三种气流状态下,质量损失随功率密度呈现单调增加趋势,当入射激光功率密度在100~600 W/cm2范围内,随着功率密度的增大,激光能量的利用效率逐渐增大并趋于稳定。     

Surface damage morphology investigations of silicon under millisecond laser irradiation

The surface damage morphologies of single crystal silicon induced by 1064 nm millisecond Nd:YAG laser are investigated. After irradiation, the damage morphologies of silicon are inspected by optical microscope (OM) and atomic force microscope (AFM). The plasma emission spectra of the damaged region are detected by the spectrometer. It is shown that surface oxidation and nitridation have occurred during the interaction of millisecond laser with silicon. In addition, the damage morphologies induced by 2 ms and 10 ns pulse width laser are compared. The damage morphology obtained by 2 ms laser is an evident crater. Three types of damage morphologies are formed at different laser energy densities. The circular concentric ripples are found surrounding the rim of the crater. The spacing of the ripples is 15 ± 5 μm. Two types of cracks are observed: linear crack and circular crack. The linear crack is observed in the center of the damaged region which propagates to the periphery of the damaged region. The circular crack is located at the rim of the crater. The damage morphology induced by 10 ns laser is surface layer damage. The periodic linear waves are generated due to the interference between the incident beam and the scattered beam. The spacing of the ripples is 1.54 μm which is close to the incident laser wavelength 1.064 μm. The linear crack is located at the center of the damaged region. Furthermore, for the same laser energy density, the dimension of the damaged region and the crater depth induced by 2 ms laser are greater than that of 10 ns laser. It indicates that the damage mechanism under millisecond pulse laser irradiation is strongly different from the case of nanosecond pulse laser.     

Applied multi-pulsed laser in surface treatment and numerical-experimental analysis

This paper presents a comparison between simulation and experimental results of the melting process of metallic material by a pulsed laser source Nd-YAG. The simulations of temperature and velocity fields of melted material were done by solving the transient heat transfer and fluid-flow equations. Variations of the thermophysical properties were considered. Furthermore, the model included the effects of the surface-tension gradient on the fluid surface and the buoyancy force. The simulation was useful in improving our understanding of the phenomena occurring in the treated material. Using a laser triangulation sensor, an experimental study was also conducted on the surface profile of the melted zones to seek a relationship between the so-called keyhole effect and the laser triangulation measurements. The keyhole effect induced strong surface deformations and often formed cavities, which were undesirable in the surface treatment process. The laser power, energy density, and treatment duration could be optimized to prevent the keyhole effect. The predicted laser melted zone (LMZ) morphology was in good agreement with the corresponding experimental measurements for various irradiation conditions, as long as the keyhole effect did not occur.     

Measurement of sensitivities for the thermal recording of laser beams

A simple technique for determining the energy sensitivities for the thermographic recording of laser beams is described. The principle behind this technique is that, if a laser beam with a known spatial distribution such as a Gaussian profile is used for imaging, the radius of the thermal image formed depends uniquely on the intensity of the impinging beam. Thus by measuring the radii of the images produced for different incident beam intensities the minimum intensity necessary (that is, the threshold) for thermographic imaging is found. The diameter of the laser beam can also be found from this measurement. A simple analysis based on the temperature distribution in the laser heated material shows that there is an inverse square root dependence on pulse duration or period of exposure for the energy fluence of the laser beam required, both for the threshold and the subsequent increase in the size of the recording. It has also been shown that except for low intensity, long duration exposure on very low conductivity materials, heat losses are not very significant.     

Radiation transfer in metallic powder beds used in laser processing

Two-dimensional radiation transfer in a powder layer backed with a substrate of the same material and normally irradiated with a narrow axially symmetric bell-like or the flat-top laser beam is numerically calculated. This corresponds to physical experiments with the powder layer of 50 μm thickness and the laser beam diameters 60–120 μm. The powder bed is treated as an equivalent homogeneous absorbing scattering medium, the radiative properties of which are estimated from the optical properties of the solid phase and the morphological parameters of the powder bed. The theoretical analysis shows that the absorptance of a semi-infinite powder bed of opaque particles is a universal function of the absorptivity of the solid phase being independent of the specific surface and the porosity. This is confirmed by literature experimental data. The radial transport of the radiative energy due to scattering of the incident laser beam in the powder layer can considerably reduce the deposited energy at the centre of the beam but the widening of the radial profile of the deposited energy is not pronounced. The fraction of laser energy deposited within the projection of the incident laser beam is evaluated. The efficiencies of laser heating the whole powder/substrate system and the substrate decrease with increasing the reflectivity of the material. More uniform heating of the powder layer can be attained at higher reflectivity.     

248 nm高反膜抗激光损伤性能

用电子束蒸发法在熔融石英基底上沉积了适用于248nm的HfO2/SiO2高反膜,为提高其抗激光损伤能力,设计并制备了两种保护层,一种是在常规高反膜系的基础上镀制二分之一波长厚度的SiO2保护层,另一种是用Al2O3/MgF2做保护层。测试了3种高反膜样品的激光损伤情况,通过损伤形貌的变化分析了两种保护层使抗激光损伤能力提高的原因以及存在的问题。     

Laser surface melting: an analytical approach to calculate heat and mass flows in liquid phase

Summary The paper describes a three-dimensional analytical model to calculate the thermal and velocity fields induced in a melted pool during high-power laser irradiation. The dependence of the thermophysical properties of matter on temperature is accounted for. The thermal field in solid phase is also discussed. The beam power accounting for the melting process is calculated by means of an energy balance involving the total energy in liquid phase plus the energy flowing through the liquid-solid interface.     

Laser printing of enamels on tiles

A Nd:YAG laser beam is used as a tool to print patterns of coloured enamels on tile substrates. For this, the laser beam is scanned over a layer of raw enamel previously sprayed on the tile surface. The possibility to focus the laser energy to heat a small zone without affecting the rest of the piece presents some advantages in front of traditional furnace techniques in which the whole piece has to be heated; among them, energy saving and the possibility to apply enamels with higher melting temperatures than those of the substrate. In this work, we study the effects of laser irradiation of a green enamel, based in chromium oxide pigment and lead frit, deposited on a white tile substrate. Lines obtained with different combinations of laser beam power and scan speeds were investigated with the aim to optimize the process from the point of view of the quality of the patterns. For this purpose, the morphology of the lines and their cross-sections is studied. The results show that lines with good visual properties can be printed with the laser. The characteristics of the marked lines were found to be directly related with the accumulated energy density delivered. Moreover, there is a linear relationship between the accumulated energy density and the volume of melted material. A minimum accumulated energy density is required to melt a shallow zone of the glazed substrate to allow the adhesion of the enamelled lines.     

紫外准分子激光损伤典型光学材料的特性分析

根据已经建立的紫外准分子激光损伤典型光学材料的理论模型,研究了准分子激光对透明光学材料（石英玻璃）和非透明光学材料（K9玻璃）的损伤特性,并结合实验结果证实了理论模型的有效性。研究表明：在准分子激光对非透明光学材料辐照下,激光光斑半径越大,产生的热应力和温度越小;脉宽越小,产生的热应力越大;随着脉冲数的增加,温度和热应力都逐渐增大。值得注意的是,当重频增加至45 Hz以上时,熔融损伤阈值开始低于应力损伤阈值,这说明当重频增加到一定程度时,非透明光学材料将首先产生熔融损伤,而不再是应力损伤。在准分子激光对透明光学材料辐照下,杂质微粒的半径和掩埋深度对光学材料温度场分布有着重要影响。但当杂质半径和掩埋深度超过一定的数值时,杂质粒子的存在与表面温度并无联系。理论模型能够较好地解释石英玻璃前/后表面相同的初始损伤形貌特征。