In order to study the dependence of the focus on the quality of the femtosecond laser beam, the tighten focusing of the femtosecond laser beam reflected by a 90° off-axis parabolic mirror was studied experimentally and theoretically trough the classical geometrical optical model. The obtained theoretical and experimental results indicate that the peak power density of the laser spot will reduce by half when the declination reaches 3mrad. Simultaneously the optical path difference resulted from incident angle causes time broadening of the spot on the target surface and impairs the focal spot intensity to some degrees. A 3TW Ti: sapphire laser system was adopted in the present work. The best focal spot obtained in the present work with aaf/ off-axis parabolic mirror is 5. 6μm ×5. 4μm and the peak power density is 3. 83×1017W/cm2 and 1. 23 ×1019W/cm2 for 3mJ and 96mJ ultra-short pulse laser separately. The study of the tighten focusing for ultra-short laser pulse reflected by 90° off-axis parabolic mirror. This research provides important information for the study on the interactions between the femtosecond laser pulses and solid, gas, clusters and so on.%为了研究超短激光脉冲光束质量对焦斑的影响,采用经典的几何模型进行了理论模拟分析,并实验研究了离轴90°抛物面镜的飞秒激光光束的紧聚焦特性.由理论分析和实验结果可知,当入射失准角为3 mrad时,将从空间上使焦斑峰值功率密度减半,失准角的存在将引起到达靶面的激光脉冲的时间展宽从而降低焦斑峰值功率密度;利用3TW钛宝石飞秒激光系统,通过f/1离轴90°抛物面镜对激光束进行紧聚焦,得到最佳焦斑尺寸为5.6μm×5.4μm,对应于3mJ和96mJ的飞秒脉冲激光光束的焦斑峰值功率密度分别为3.83×1017W/cm2和1.23×1019W/cm2.结果表明,该研究为开展激光与固体、气体、团簇等物质相互作用的相关实验提供了重要的参考.
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