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飞秒脉冲激光入射光学薄膜形成瞬态光场分布是一个非稳态过程,该过程不同于纳秒脉冲或连续波入射的情形,不能直接采用求解薄膜特征矩阵的方法进行处理。采用多光束叠加的方法建立了超短脉冲入射单层膜的反射率和内部光强分布的理论模型,并根据ZnS薄膜材料的参数和单层增反膜的特点进行了数值计算。结果表明,对单层增反膜,薄膜反射率与脉宽成正比,并随脉宽增加逐渐趋近于连续波入射时的情形。在同一脉宽下,膜层厚度增加,反射率下降,且反射脉冲形状也发生改变。膜层中的光强分布计算结果也明显不同于连续波辐照情形,且薄膜厚度越大,差异越显著,表现为连续波入射时,膜层内的光强分布呈等振幅的波动,而超短脉冲入射时,波动的振幅逐渐增大,在膜层和玻璃分界处达到最大值。
The transient light field distribution caused by femtosecond pulse laser incident optical film is a non-steady state process, which is different from the case of nanosecond pulse or continuous wave incident and can not be directly processed by solving the film feature matrix. A theoretical model of reflectivity and internal light intensity distribution of ultrashort pulse incident monolayer films was established by multi-beam superposition method. The numerical calculation was carried out according to the parameters of ZnS thin film material and the characteristics of single layer antireflection film. The results show that for the single-layer antireflection film, the reflectance of the film is proportional to the pulse width, and gradually increases with the increase of the pulse width when the continuous wave is incident. In the same pulse width, the film thickness increases, the reflectivity decreases, and the reflected pulse shape also changes. The results of light intensity distribution in the film were also significantly different from those of continuous wave irradiation. The larger the film thickness, the more significant the difference was. The results showed that the light intensity distribution in the film fluctuated in equal amplitude when the continuous wave was incident When the ultrashort pulse is incident, the amplitude of the fluctuation gradually increases and reaches a maximum value at the boundary between the film and the glass.