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中性密度滤光片的典型结构是在K9玻璃上镀金属膜,来实现对激光的有效吸收.由于损伤阈值较低,严重限制了其在高能激光系统中的应用.实验研究了较高激光能量密度下滤光片的损伤形貌和损伤机理.损伤形貌的变化特征是:随着激光能量密度的增加,滤光片先出现损伤点,后以损伤点为中心产生裂纹,且裂纹长度逐渐变长,最终连接成线状和块状,导致大面积的薄膜脱落.建立了缺陷吸收激光能量升温致中性密度滤光片表面薄膜损伤的模型,计算了薄膜表面的温度和应力分布,讨论了薄膜表面不均匀温升造成的径向、环向和轴向热应力分布.理论分析显示:环向应力是造成薄膜沿径向产生裂纹的主要原因.当激光能量密度大于约2.2 J/cm~2,杂质粒子半径大于140 nm且相邻杂质粒子之间的距离小于10μm时,裂纹才能大量连接起来引起薄膜的大面积脱落.
The typical structure of neutral density filter is to metal film coated on K9 glass to realize the effective absorption of laser.As the damage threshold is lower, its application in high energy laser system is severely restricted.Experimental study of higher laser Energy density of the damage morphology and damage mechanism of the damage morphology changes are as follows: With the laser energy density increases, the filter first damage point, the damage point as the center of the crack, and the crack length Gradually become longer and longer, and finally become linear and massive, resulting in the detachment of a large area of the film. A model of damage to the surface of the neutral density filter caused by laser ablation is established, and the temperature and stress distribution on the surface of the film are calculated, The radial, circumferential and axial thermal stress distribution caused by the non-uniform temperature rise of the film surface are discussed.The theoretical analysis shows that the circumferential stress is the main reason for the film to crack in the radial direction.When the laser energy density is more than about 2.2 J / cm ~ 2. When the radius of the impurity particles is more than 140 nm and the distance between adjacent impurity particles is less than 10μm, the cracks can be connected in a large number to cause the large area of the film to fall off.