【作者】 陈宽能；

【导师】 陈明君；

【作者基本信息】 哈尔滨工业大学 ， 机械制造及其自动化， 2010， 硕士

【摘要】 KDP晶体是一种性能优良的非线性光学晶体材料,因其具有较大的非线性光学系数、较高的激光损伤阈值、较宽的透光波段、优良的光学均匀性、易于实现相位匹配、易于生长等特点,而被广泛地用作光学频率转换器件以及光电开关元件。但是,KDP晶体生长以及后续的超精密加工过程中,会形成各种缺陷和表面特征,严重地影响着晶体的激光损伤阈值等光学性能。例如,生长过程中形成的各类包裹体缺陷以及单点金刚石铣削后表面形成的小尺度波纹等。因此,研究这些缺陷对其激光损伤阈值的影响规律和程度大小,不仅可以进一步解释KDP晶体的激光损伤机理,还可为提高光学性能提供理论基础和参数依据,进一步指导快速生长工艺的改进以及加工参数的合理选取,以满足目前激光核聚变装置对KDP晶体光学元件所提出的苛刻要求,对惯性约束核聚变(Inertial Confinement Fusion,ICF)工程至关重要。本文首先对采用时域有限差分方法(Finite Difference Time Domain,FDTD)建模过程中所涉及到的空间和时间步长选择、吸收边界条件和周期性边界条件设置等相关内容进行分析,并建立平面电磁波垂直投射到光滑的和镀膜的KDP晶体表面的分析模型,采用FDTD方法求解晶体内部的光强,与公式计算结果以及薄膜增透理论进行对比验证,为后续的分析工作做了铺垫。然后阐述小尺度波纹的产生和提取,分析小尺度波纹、凹坑和包裹体对KDP晶体内部光强分布的影响规律和程度大小。结果表明:小尺度波纹、凹坑和包裹体会使晶体内部的衍射场发生畸变,导致晶体内部光强呈现强弱分布;且畸变程度与不同特征参数(小尺度波纹幅值和周期、凹坑大小、包裹体大小和折射率)有关。最后对KDP晶体激光损伤阈值实验进行分析,结果表明:晶体的前、后表面损伤和体损伤多以炸裂为主,且后表面炸裂的程度明显比前表面严重;粗糙度不能作为评价表面形貌对KDP激光损伤阈值影响的唯一标准;小尺度波纹对激光的小尺度扰动是激光发生小尺度自聚焦的重要原因。更多还原

【Abstract】 crystal is widely used as frequency converter and photoectric switch in optical project. Howerever, the defects on the surface or inside KDP crystal as a result from the process of growth and cutting will greatly affect the optical quality of KDP crystal such as laser damage threshold. For example, the different inclusions inside KDP crystal formed in the process of rapid growth and the micro-waviness on the surface of KDP crystal left by single point diamond turning. Therefore, making an in-depth study on the laser damage mechanism and degree affected by those defects not only can explain the causes of damage and provide the theoretical basis for increasing laser damage threshold, but also can guide technics of rapid growth improving and working parameters choosing. Thus, it is significant to reach the rigorous standard for devices of Inertial Confinement Fusion.In this paper, firstly presented the fundamental theory of Finite Difference Time Domain method that we used to analyze the subsequent problems; built the model of uniform plane electromagnetic wave irradiating on the surface of smooth and film-coated KDP crystal by 2-D FDTD method and campared the results with theoretical results; made preparatory work for subsequent analysis.Then, presented the formation and extraction of micro-waviness and built the physical model of defects such as micro-waviness, concave pit and inclusion. analyzed the law of how the defects impacting on near-filed light intensity distribution inside and on the surface of KDP crystal by FDTD. Results indicate: defects will distort the incident wave and lead to distribution of strength and weakness of light intensity inside KDP crystal; the degree of distortion has relation with feature parameters of micro-waviness, concave pits and inclusions.At last, carried out experiments of variable parameter processing using single point diamond turning (SPDT) and laser damage threshold detection of KDP crystal. Results indicate: major damage of KDP crystal is break and the degree of break on back surface is worse than front surface; roughness can not used to be the only criterion of assessment for effect of surface topography on laser damage threshold of KDP crystal; micro-waviness is a crucial influence factor of self-focusing damage.更多还原