摘要
在高功率激光驱动的惯性约束聚变的大型光学系统中,光学元件的负载能力是限制激光器高通量输出的“瓶颈”,提高光学元件的抗激光损伤能力,延长光学元件的使用寿命是目前面临的重要问题之一。
本文根据亚表面划痕诱导激光损伤物理图像即损伤开始和增长,以及减缓工艺三个方面展开。讨论目前普遍接受的几种损伤机理;测试亚表面不同形貌特征划痕的激光损伤阈值;利用有限元方法模拟计算亚表面径向和赫兹锥形划痕对入射光场的调制;利用紫外、二氧化碳激光预处理光学元件,剔除表面杂质和修复亚表面缺陷,有效消除损伤的产生,减缓损伤增长。
本文主要工作和得到的结论如下:
1)实验用康宁、肖特公司生产的熔石英基片,用光学显微镜观测熔石英表面划痕的形貌,用原子力显微镜测量不同形貌划痕的宽度和深度分布特征。根据划痕的形貌特征将其分为径向划痕(Boussinesq-point-force crack,BPFC)、赫兹锥形划痕(Hertzian-conical scratch,HCS)和塑性压痕(Plastic indent,PI)三类,用Nd:YAG激光器(SAGA)以n-on-1的方式测试损伤阈值。结果表明,锐度较大的BPFC损伤阈值不超过2.0J/cm~2;深度小于1μm的HCS阈值可达2.6J/cm~2;形变较大的PI阈值至2.8J/cm~2,形变较小的PI激光损伤阈值与无缺陷材料3.3J/cm~2相当。
2)采用有限元方法对熔石英亚表面缺陷(平面和锥形划痕)周围的光强分布进行了数值模拟。研究表明,划痕形状、几何尺寸、方位角、入射角等是影响划痕周围光强分布的主要因素;位于光入射表面的划痕对光强的增强效果比位于出射表面的弱;在理想形状的划痕截面和表面同时发生内全反射时,平面划痕周围的光强增强效果明显。锥形划痕周围的光强分布为正确解释交叉划痕夹角对角线附近的损伤提供了理论依据。
3)结合紫外和二氧化碳激光预处理工艺,采用光栅式扫描方式对元件表面进行辐照,通过紫外激光辐照,表面的部分杂质和水份被离化分解,隐藏于基片亚表面层的缺陷如划痕等充分暴露;经CO_2激光预处理,使得表面熔融蒸发,从而使表面杂质和缺陷得到有效清除和修复,预处理后激光损伤阈值平均提高30%。
Load capacity of optical components of high power laser system,during process of the ICF(inertial confinement fusion) drive by high power laser,is the key problem of limit output of high power laser system,one of the chief issues facing at present is to enhance the resistance to optical components and to prolong the using life-span.
The paper presents a physical image of laser induced damage of sub-surface crack about initiation and damage growth,as well as mitigation of laser induced damage in fused silica sub-surface.Some well accepted damage mechanisms are discussed;and laser induced damage threshold of different morphology cracks are tested;the modulation of incidence light of planar radial cracks and hertzian conical cracks are simulated with finite element method;UV and CO_2 lasers are used to irradiation and pretreat the surface of fused silica to eliminate impurity and rehabilitate defect on the subsurface,so that can remove availably the initiation and mitigate damage increase rate validly.
This is what we did mostly and received conclusions:
1) The Coming and Scott fused silica are used in the experiences,the micro sub-surface cracks of fused silica are observed by optical microscope,which width and depth are measured by AFM.The cracks are sorted into three types according to the morphology characters:Boussinesq-point-force crack(BPFC),Hertzian-conical scratch (HCS) and plastic indent(PI).The laser induced damage threshold of cracks is measured by the SAGA-YAG solid-state laser by means of n-on-1.The results show that the damage threshold of sharp tip BPFC is less than 2.0J/cm~2,that of HCS extends to 2.6 J/cm~2,and that of deep deformation PI is 2.8 J/cm~2.The laser induced damage threshold of the shallow deformation indent is equal to that of bulk material 3.3J/cm~2.
2) The finite element method is used to simulate the light intensity distribution in the vicinity of defect(planar and conical crack)on the fused silica subsurface.The results show light intensity distribution round the crack is resulted from several factors, including crack morphology,geometry dimension,azimuth,and incidence angle etc.. The enhancement effect of crack on the output surface is stronger than input surface, when total internal reflection occurs at both the ideal morphology planar crack and the surface,higher light intensification can occur.The light distribution in the vicinity of conical crack provides a theory seed to explain exactly why the plot adjacent diagonal of cross-crack-comer is firstly damaged.
3) UV and CO_2 lasers are used to irradiation the surface of fused silica by means of raster-reticle-scan.By irradiated with UV laser,part of impurity and moisture content on the surface is ionized and decomposed,defects hided under the substrate sub-surface are exposed entirely,CO2 pretreat subsurface to make the surface of fused silica fusing and evaporating.Thereby inclusions are eliminated and defects are rehabilitated,laser induced damage threshold average enhance 30%.
引文
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