基底对光学薄膜弱吸收测量的影响

 对表面热透镜技术测量光学薄膜弱吸收低频调制时不同基底对测量的影响进行了理论分析。用Lambda—900分光光度计测量了K9和石英基底的Ti₃O₅单层膜的吸收值，将该组样品作为定标片；用表面热透镜装置分别测量了BK7和石英空白基底及HfO₂，ZnO两组不同基底不同厚度单层膜样品的吸收。通过分析比较同一工艺条件下镀制的不同基底薄膜样品用与其同种和不同种基底定标片定标测量的结果，表明在低频测量时需要用与测量样品同种基底的定标片定标；不同厚度样品的测量结果表明，在不能严格满足热薄条件时,测量结果需引入修正值。     

锁相热像方法实验测量Nb2O5薄膜吸收率

利用锁相热像方法定量测量了光学镀膜的吸收率。待测薄膜吸收周期调制的激光能量，在表面形成热波，将红外相机记录的热分布信号进行锁相相关处理，获得信噪比提高的热图像。采用标准吸收样品对系统进行定标，可获得光热信号幅度与样品吸收率之间的定量联系，进而在相同实验条件下测量待测样品，可通过光热信号直接计算获得其绝对吸收率。在1 060 nm波长处开展了实验研究，测量获得了不同厚度Nb₂O₅镀膜的吸收率数值，实测的吸收可达80 ppm。     

表面热透镜薄膜吸收测量灵敏度提高方法

为提高表面热透镜薄膜吸收测量仪的灵敏度，在表面热透镜衍射理论基础上，通过数值模拟给出了探测激光腰斑半径、探测激光腰斑到样品表面距离、样品到探测光纤端面距离等仪器参数的优化方法. 经优化调整后该仪器能达到优于0.1ppm量级的薄膜吸收率测量灵敏度. 关键词： 吸收测量 表面热透镜 光热形变 薄膜     

用共线光热偏转技术测量光学薄膜的微弱吸收

用具有高灵敏的共线光热偏转技术研究SiO_2、ZrO_2、MgF_2、ZnS等单层光学薄膜的吸收特性,测得它们的吸收率.实验结果与激光量热法及横向光热偏转技术符合良好,表明共线光热偏转技术是测量光学薄膜弱吸收的较理想方法.     

应用离子后处理技术提高薄膜激光损伤阈值

利用电子束热蒸发方法在K9玻璃基底上沉积氧化锆薄膜,并对其中一些样品用低能O2+进行了后处理。采用表面热透镜技术测量薄膜样品表面弱吸收,采用显微镜观察样品离子后处理前后的显微缺陷密度。测试结果表明:经离子后处理样品表面的缺陷密度从18.6/mm2降低到6.2/mm2,且其激光损伤阈值从15.9 J/cm2提高到23.1 J/cm2,样品的平均吸收率从处理前的1.147×10-4降低到处理后的9.56×10-5。通过对处理前后样品的表面微缺陷密度、吸收率及损伤形貌等的分析发现:离子后处理可以降低薄膜的显微缺陷和亚显微缺陷,从而降低薄膜的平均吸收率,同时增强了薄膜与基底的结合力,提高了薄膜的激光损伤阈值。     

辐射加热Al等离子体的吸收谱实验

报道了辐射加热Al样品的K壳层辐射吸收谱实验. 在神光Ⅱ激光装置上,将8路主激光注入锥柱型金腔产生高温辐射源,利用该辐射源加热腔内的Al薄膜样品,产生温度达到几十电子伏的热稠密等离子体. 相对主激光延迟一定时间后,利用第9路激光短脉冲聚焦打靶加热金盘,产生短脉冲X光点光源. 通过测量075—085nm波长范围内未经样品衰减以及经过样品衰减后的背光源辐射光谱,得到了Al样品的K壳层吸收谱. 利用细致谱线计算的吸收谱对实验光谱进行拟合,确定了Al样品等离子体的电子温度. 关键词： Al等离子体 吸收谱 不透明度     

辐射加热Al等离子体的吸收谱实验

报道了辐射加热Al样品的K壳层辐射吸收谱实验. 在神光Ⅱ激光装置上，将8路主激光注入锥柱型金腔产生高温辐射源，利用该辐射源加热腔内的Al薄膜样品，产生温度达到几十电子伏的热稠密等离子体. 相对主激光延迟一定时间后，利用第9路激光短脉冲聚焦打靶加热金盘，产生短脉冲X光点光源. 通过测量075—085nm波长范围内未经样品衰减以及经过样品衰减后的背光源辐射光谱，得到了Al样品的K壳层吸收谱. 利用细致谱线计算的吸收谱对实验光谱进行拟合，确定了Al样品等离子体的电子温度.     

10.6μm激光辐照下光学薄膜的微弱吸收测量

 建立了表面热透镜技术测量光学薄膜微弱吸收的实验装置，对10.6μm CO₂激光辐照下镀制在Ge基底上的不同厚度的单层ZnS，YbF₃薄膜，以及镀制在Ge基底上不同膜系的（YbF₃/ZnSe）多层分光膜的弱吸收进行了测量，并对实验结果作了分析和讨论。实验结果表明，利用本实验系统已测得的待测样品的最低吸收为2.87×10^-4，测量系统的灵敏度为10^-5。     

工作于非标准环境下体吸收型激光能量计的研制

研究了能直接工作于非标准环境下的体吸收型激光能量计.由能量计热传导、热对流、热辐射的单位时间热交换方程推导出单位时间内能量计升温过程中热能损失的数学模型,根据数学模型对能量计整个升温过程的热损失进行补偿,使得能量计对于不同脉冲长度入射激光的测量结果重复性由4.7%提高到了0.6%,消除了能量计热损失给测量带来的不利影响.针对环境温度从-40℃变化到70℃测温用铜-康铜热电堆对1℃温差响应电压剧增30%的问题,在环境试验箱进行不同环境温度下的激光能量测量实验,得出了能量计不同环境温度下测量结果的修正系数,并利用最小二乘法建立了修正系数同环境温度之间的函数关系,使得环境温度对测量结果的影响得到了修正.     

太阳能选择性吸收涂层高温光谱吸收率的测量方法

太阳能热发电是高效利用太阳能资源的有效途径之一,对缓解能源危机和环境污染具有重要的推动作用和深远的社会意义。选择性吸收涂层是太阳能真空集热管的重要组成部分,是决定太阳能与热能转换效率的关键因素。针对高温状态下太阳能选择性吸收涂层光学性能的表征问题,提出一种适用于高温金属-陶瓷选择性吸收涂层太阳光谱吸收率的测量方法。基于双探测器的傅里叶光谱仪和具有涂层加热功能的积分球,研制了能够防止高温氧化并模拟涂层工作温度的高真空测量装置,实现0.3～2.5 μm、室温-700 ℃太阳光谱吸收率的测量。选取磁控溅射制备的Mo-SiO₂选择性吸收涂层作为测量样品,该样品具有双吸收层的多层膜结构。对涂层样品不同温度下的太阳光谱吸收率进行了测量实验,室温测量值与理论计算值进行了对比分析,结果表明具有较好的一致性,最大偏差仅为2.9%,验证了涂层太阳光谱吸收率测量方法的可行性。高温光谱吸收率测量对涂层参数设计的优化及吸收性能的提高具有重要的指导意义及推动作用。     

缺陷对HfO2薄膜的激光弱吸收与损伤阈值的影响

激光弱吸收是导致光学薄膜损伤的重要原因。在(5~43)mPa的氧分压下制备并测试了一组HfO2薄膜。实验发现,当氧分压小于20mPa时,薄膜弱吸收越大,损伤阈值越低;当氧分压大于20mPa时,薄膜的损伤阈值与弱吸收并不一一对应,具有较高弱吸收的薄膜可能同时具有较高的损伤阈值。建立了缺陷模型,采用有限元法模拟了缺陷对弱吸收测量和损伤阈值测量的影响,分析了缺陷尺寸、密度、吸收系数对弱吸收和损伤阈值的影响。研究结果显示,吸收系数高于薄膜1 000倍的缺陷可以降低薄膜的损伤阈值1 000倍,却并不影响薄膜的弱吸收。缺陷对HfO2薄膜的激光弱吸收与损伤阈值测试有完全不同的影响,是导致某些薄膜弱吸收与损伤阈值背离的原因。     

Combined laser calorimetry and surface thermal lens technique for optical coating characterization

A combined laser calorimetry (LC) and surface thermal lens (STL) technique with a continuous-wave (cw) modulated excitation is developed for characterizing the absorption properties of optical coatings. LC and STL signals are simultaneously obtained with highly reflective coating samples irradiated at 1064 nm. With a modulated irradiation power of 2.6 W, the limits of absorptance detection for the LC and STL signals are approximately 1.2×10^-6 and 0.5×10^-6, respectively. The influences of different excitation parameters (modulation frequency, excitation-beam power and irradiation time) on the absorptance measurements are experimentally analyzed. The applicability of the combined LC and STL technique to the investigation of the laser-induced absorption changes of optical coatings in an absolute absorptance scale is demonstrated. PACS 42.87.-d; 78.20.-e     

Narrow-band spectral features of structured silver surface with rectangular resonant cavities

This paper is aimed to investigate spectral properties of structured silver surface with periodic rectangular hollow cavities. Numerical computation is conducted to obtain spectral distribution of surface absorptance with different structural parameters using the finite-difference time-domain (FDTD) method. By means of numerical examples, the effects of structural parameters, incident angle and azimuthal angle on the spectral features of the structured surface are discussed. It is found that the structured surface shows the characteristics of the peak absorption in the vicinity of resonant wavelength of rectangular cavity. For some special structure parameters, the peak absorptance of the incident plane wave can reach as high as above 80% due to the excitation of microcavity effect. The optimal narrow-band absorption can be achieved by the rational design of the structural parameters of rectangular cavity. The directional dependence of spectral absorptance is also analyzed and the results reveal that the absorption peak positions are incident-angle-independent. The results show that the microscaled rectangular cavities fabricated on the low-emissivity silver surface are very efficient for selective improvement of the radiative features, which provides guidance for the design of narrow-band infrared thermal emitters.     

利用集总参数法测量材料对激光的吸收率

材料对激光吸收的研究工作对激光加工的发展有着非常重要的意义.而吸收率测量方法的研究是材料对激光吸收研究工作的基础.从量热的角度出发,利用传热学中的集总参数法建立了测量吸收率的方法,设计了相应的试验装置,并测量了几种常用工程材料对激光的吸收率. 关键词： 集总参数法 吸收率 激光     

A New Synthetical Model of High-Power Pulsed Laser Ablation

We develop a new synthetical model of high-power pulsed laser ablation, which considers the dynamic absorptance, vaporization, and plasma shielding. And the corresponding heat conduction equations with the initial and boundary conditions are given. The numerical solutions are obtained under the reasonable technical parameter conditions by taking YBa2CusO7 target for example. The space-dependence and time-dependence of temperature in target at a certain laser fluence are presented, then, the transmitted intensity through plasma plume, space-dependence of temperature and ablation rate for different laser fluences are significantly analyzed. As a result, the satisfactorily good agreement between our numerical results and experimental results indicates that the influences of the dynamic absorptance, vaporization, and plasma shielding cannot be neglected. Taking all the three mechanisms above simultaneously into account for the first time, we cause the present model to be more practical.     

Heating effects induced by a pulsed laser in a semi-infinite target in view of the theory of linear systems

The two-dimensional Laplace integral transform technique is used to solve the problem of heating a semi-infinite target induced by the surface absorption of a laser pulse. Mathematical expressions for the surface temperature and the temperature profile within the target are obtained considering cooling and temperature-dependent absorption coefficient at the front surface. The linear behaviour of the system is found to be established for all cases except for that of temperature dependent surface absorptance. As an illustrative example, computations are carried out on Al, Cu, Ag and Au targets using an empirical formula for the temporal distribution of a measured laser pulse.     

A New Synthetical Model of High-Power Pulsed Laser Ablation

We develop a new synthetical model of high-power pulsed laser ablation, which considers the dynamic absorptance, vaporization, and plasma shielding. And the corresponding heat conduction equations with the initial and boundary conditions are given. The numerical solutions are obtained under the reasonable technical parameter conditions by taking YBa₂Cu₃O₇ target for example. The space-dependence and time-dependence of temperature in target at a certain laser fluence are presented, then, the transmitted intensity through plasma plume, space-dependence of temperature and ablation rate for different laser fluences are significantly analyzed. As a result, the satisfactorily good agreement between our numerical results and experimental results indicates that the influences of the dynamic absorptance, vaporization, and plasma shielding cannot be neglected. Taking all the three mechanisms above simultaneously into account for the first time, we cause the present model to be more practical.     

Fresnel absorption of 1 μm- and 10 μm-laser beams at the keyhole wall during laser beam welding: Comparison between smooth and wavy surfaces

The angle-dependent absorption of laser beams at metal surfaces is described by the Fresnel-equations. During keyhole laser welding the essential interaction takes place at very striping angles of incidence of the order of 1-8 degrees at the front of the vapour capillary, called the keyhole. For a smooth vapour capillary, laser beams with a wavelength of about 1 μm operate in a Fresnel-regime where the absorptance increases with the angle of incidence at the wall, towards the weak Brewster-angle maximum. In contrast, for 10 μm-lasers high absorptance around the more pronounced Brewster-angle peak takes place. From high speed imaging keyhole surface waves were observed. Mathematical modelling of the laser-keyhole interaction demonstrates that already relatively little waviness of the melt surface at the keyhole strongly modulates the angles of incidence and in turn the Fresnel-absorption due to varying angles of incidence, soon also leading to shadow zones. Due to this local variation of the angle of incidence the absorptance tends towards the angle-averaged value, with the consequence that for 1 μm-lasers the direct absorptance and in turn the penetration depth increases, particularly at low welding speed, while for 10 μm-lasers it generally decreases.     

K9基片的亚表面损伤探测及化学腐蚀处理技术研究

 研究了几种类型的腐蚀液对K9基片化学腐蚀的影响。通过腐蚀液对基片纵向腐蚀速度的变化初步判断了K9基片重沉积层的深度。考察了腐蚀前后基片表面参数的变化以及腐蚀对激光损伤阈值的影响。研究表明,特定的腐蚀液能够对K9基片进行平稳可控的腐蚀,并且腐蚀能提高其激光损伤阈值,其主要原因是去除了重沉积层及表面、亚表面缺陷中的污染物,但过多的腐蚀会暴露本来为重沉积层所掩盖的划痕等亚表面缺陷,所以腐蚀并非越深越好。同时,表面各种杂质与缺陷的去除能够提高材料的机械强度,从而也有利于提高材料的激光损伤阈值。