强激光诱导光学元件损伤的研究

【作者】 邱荣；

【导师】 王俊波；

【作者基本信息】 中国工程物理研究院 ， 光学工程， 2013， 博士

【摘要】 强激光诱导光学元件的损伤是制约包括ICF激光驱动器在内的强激光系统向更高更强发展的最大瓶颈,受经济效益和技术发展的驱使,这也将是一个持续的挑战。损伤形貌是损伤机制的外在体现,激光辐照过程中等离子体和冲击波的产生与发展是损伤及损伤增长的主要原因,研究强激光诱导光学元件损伤的形貌,研究损伤过程的超快动力学特征,分析强激光诱导光学元件损伤的规律,有助于我们理解激光诱导元件损伤的物理内涵,掌握损伤的物理机制和物理规律,对改进光学元件的加工工艺,增加光学元件的使用寿命,降低高能激光系统的运行成本和提高负载能力有现实意义。本论文涉及对多个波长的纳秒激光诱导光学元件损伤的实验研究。主要思路是从实验定性研究1064nm、532nm和355nm三波长的纳秒激光诱导光学元件的初始损伤及损伤增长出发,结合超快时间分辨阴影成像技术,获取了熔石英光学元件前后表面和体内的等离子体、冲击波的产生与发展过程的时间分辨图像,分析了纳秒激光诱导光学元件损伤的规律和机制。本论文简述了高功率激光与物质相互作用理论,综述了强激光诱导光学元件损伤的物理机制,实验分析了初始和损伤增长的规律和特点,据此分析了提高光学元件抗激光损伤能力的方法和途径。纳秒激光作用下,光学元件使用和加工过程中残留于表面和亚表面的各种缺陷是导致强激光辐照下元件损伤的直接原因。强激光诱导光学元件损伤是一个涉及光热、光化学、光声、光电、等离子体和冲击波、激光参数和材料性质等众多物理效应和参数的复杂过程,主要的损伤机制有：划痕、裂纹、杂质等缺陷吸收引起的热力学损伤,裂纹对激光场的调制导致局部场强增强引起的光学击穿,受激布里渊散射激发超声波引起的损伤,非线性自聚焦引起的损伤,任何一次损伤实际都是多种机制共同作用并相互促进的结果,这些作用过程都发生在很短的时间内,这增加了我们研究和掌握损伤规律的难度。实验研究了1064nm、532nm和355nm三个波长的激光分别对光学元件前、后表面和体内的初始损伤和损伤增长,研究了三个波长两两同时辐照下光学元件的损伤行为,比较和分析了各波长激光对光学元件的初始损伤和损伤增长规律和机制。研究结果表明：表面和亚表面缺陷是纳秒激光辐照下光学元件损伤的最主要原因,等离子体和冲击波在材料体内和空气中的产生与发展的不同导致了前后表面的损伤规律的巨大差异；对应于不同的激光波长,裂纹和以及损伤坑内材料对激光的吸收差异是损伤增长差异的主因,实验结果表明可见光和紫外激光辐照下的熔石英玻璃的体内成丝损伤也与点缺陷有关。对应于不同的激光波长,吸收杂质的种类和数密度差异巨大,恰当的激光辐照预处理能够提高元件对后续激光辐照的的损伤阈值；但在紫外激光辐照下,多脉冲的损伤累积效应明显。在两个波长的激光同时辐照下,杂质和缺陷先被短波长激光电离,产生少量的自由电子,这些自由电子对后续激光无选择的强吸收,极大的增强了总的吸收效率,降低了损伤阈值,增大了损伤程度。采用超快时间分辨阴影成像技术,本文研究了纳秒激光辐照损伤熔石英光学玻璃前、后表面和体内的动力学过程,实验观察了前后表面的空气和材料中等离子体和冲击波的差异,对比分析了前后表面和体内的损伤差异及损伤机制。在前表面,由杂质吸收产生的初始等离子体位于空气中,等离子体对激光能量的强吸收对后续激光形成屏蔽效应,使前表面空气等离子体中积累了大量的能量,元件表面损伤以高温等离子体的表面烧蚀为主,材料破坏不严重。在后表面,由于初始等离子体产生于元件表面的材料体内,等离子体的屏蔽效应增加了激光能量在材料中的沉积,材料中等离子体聚集的能量通过爆炸释放,爆炸产生的冲击波和等离子体高温烧蚀是后表面损伤的主要机制,同时伴随着烧蚀物质的喷发去除。在材料内部,通过对损伤区域等离子体和冲击波的观察和分析表明体内损伤主要自聚焦和点缺陷吸收两种机制主导,而且点缺陷吸收诱导材料体内损伤从焦点开始沿着激光入射逆方向的点燃时间存在先后顺序。另外,等离子体膨胀、冲击波传播及其与界面的相互作用、物质去除等过程在材料中产生了复杂的应力波,这些应力波虽然没有对材料产生明显的破坏,但对其产生机理和特点的分析,将有助于更深入的理解强激光与材料相互作用机制。更多还原

【Abstract】 High power laser-induced damage of optical elements is one of the main limiting factors for ICF laser driver development to higher and stronger, driven by the economic benefits and technical development, it will be a continuing challenge. Damage morphology is the external manifestation of the mechanism of injury, plasma and shock wave generation and development is the main reason of the damage and damage growth. Study of the morphology of the strong laser induced damage and the ultrafast kinetic characteristics of the damage process, analysis the law of the strong laser induced damage, helps us to understand the physical meaning of the laser-induced material damage, helps us to understand the physical meaning of the laser-induced material damage, master the physical mechanism and law of damage, has practical significance to improve the processing of optical components, to increase the useful life, to reducing the running costs of the high-energy laser systems and increase load capacity.In this paper, the Nd:YAG nanosecond laser induced damage of the optical components are studied. The main idea is based on the1064nm,532nm and355nm three wavelength nanosecond laser induced optical element initial damage and damage growth morphology, combining with ultrafast time-resolved method of the shadow map, obtaining the time-resolved images of the process of generation and development of shock waves and plasma on the input-surface, output-surface and in material in fused silica, and analysis the laws of physics and the mechanism of optical component damage induced by nanosecond laser.First of all, a brief description of the pulsed laser and material interaction theory and the physical mechanism of optical element damage induced by high power laser, analyzed the methods and ways of improving the laser damage resistance of optical components. Under the nanosecond laser irradiation, the surface and sub-surface defects remains by the processing and use of the process is the direct cause component damage. Laser induced damage of the optical component is a complex process involving photothermal, photoacoustic, optoelectronics, laser parameters, material properties, linear and nonlinear effects, plasma and shock wave and many other physical effects. The major damage mechanism are:heating by the absorption of scratches、cracks、impurities and other defects, ultrasonic induced damage by Stimulated Brillouin Scattering, avalanche ionization and multiphoton ionization damage, the damage caused by the nonlinear self-focusing. In the general case, typically damage is the results of many of these mechanisms together and promotes each other. Experimental study of1064nm,532nm and355nm three-wavelength laser induced initial damage and growth damage of surface and material in fused silica, to study behavior which under multiple wavelengths combination on laser-induced damage, comparison and analysis of each wavelength laser initial damage and damage growth laws and mechanisms of optical components. The results show that:the surface impurities and defect is the main cause of optical components surface damage induced by nanosecond laser, the development and nature of plasma and shockwave in material and air led to the huge difference of the front and back surfaces of the initial damage and damage growth. Corresponding to different laser wavelengths, the cracks and materials in damage crater will effect damage growth, self-focusing filamentation damage is according to point defects too. For different laser wavelength, the type and number density of damage pioneer has huge difference. Laser preparation condition can improve the damage threshold, damage accumulation effect are more obvious under the ultraviolet laser irradiation. Under the two wavelengths laser simultaneously irradiation, at first, impurities and defects by the short wavelength laser ionization, and then, the free electrons generated by the ionizing greatly enhance the absorption of long wavelength laser.The dynamic process of nanosecond laser induced damage of fused silica’s input-surface, exit-surface and bulk in air was investigated by ultrafast time-resolved shadowgraphs. The comparison and analysis of the damage mechanism among the input-surface, exit-surface and bulk were performed. In the input-surface, generation and development process of plasma and shock wave were observed in air and materials. Three stress waves were observed in material under the sub-nanosecond laser, and the bulk damage was observed near the input-surface. In the output-surface, in addition to the shock wave formation and development process is observed, but also the process of material ablation and explosive phase ejections were obtained. In the bulk, the results show that both the self-focusing and defect’s absorption answerable for the damage, damage induced by the defect’s absorption maybe have temporal difference. The present research is instructive to understand the laser-induced damage mechanism of the fused silica.更多还原