基于Rugate薄膜的高功率激光非聚焦型空间低通滤波技术研究

The traditional focusing pin-hole spatial filter has lots of disadvantages to improve the beam quality of high power laser beam. So the project analyzes the validity of Rugate coating for non-focusing low-pass spatial filtering of high-power laser beam profoundly. Based on finite-difference time-domain method, the relationship between the angular spectrum response characteristics and the light spectrum response characteristics is investigated. And by the equation of the light spectrum response characteristics with the coating's refractive index, the angular spectrum response characteristics with the change of the refractive index are formulated. And the formulation can be used to conduct the design of Rugate coating for different filtering requirements. The Rugate coating which has narrow angular spectrum bandwidth and wide light spectrum bandwidth simultaneously is designed. So it is proved theoretically that Rugate coating can be used for the spatial filtering of short and ultra short laser pulse. And the configurations for the spatial filtering of short and ultra short laser pulse are fabricated, respectively. Using rigorous coupled wave theory and power spectrum density in spatial spectrum domain, the characteristics of Rugate coating for spatial filtering are evaluated. Based on temperature field distribution theory, the mechanism that Rugate coating has more laser-induced damage threshold than traditional dielectric thin film with the same deposition substances is analyzed. Rugate coating is fabricated by glancing angle deposition method. And it is used to spatially filter laser beam experimentally. Compared with the results of the pin-hole spatial filter, its performance is judged. And the feasibility of substituting Rugate coating for pin-hole spatial filter in high-power laser field is evaluated.

针对传统聚焦型针孔滤波技术在改善高功率激光光束质量方面的缺陷，本项目深入探索Rugate薄膜应用于高功率激光非聚焦型空间低通滤波的可行性。采用时域有限差分理论，分析Rugate薄膜的角谱响应与光谱响应的相互关系，并基于其光谱响应特性与其折射率分布的关系表达式，得出其角谱响应特性随其折射率分布的变化规律，以指导符合特定空间低通滤波需求的薄膜的结构设计。重点解决具有窄角谱选择性，且宽光谱选择性Rugate薄膜设计问题，为脉冲或短脉冲激光空间低通滤波提供理论依据，并分别针对连续、脉冲和超短脉冲激光设计空间低通滤波装置。采用严格耦合波理论和空频域功率谱密度等指标，对其空间低通滤波特性进行评价。利用温度场设计理论，分析在同种镀膜材料的前提下，Rugate薄膜较多层介质薄膜具有更高抗光损伤能力的机理。实验验证采用斜向沉积法所制作薄膜的空间低通滤波效果，并和针孔滤波进行对比，分析其取代针孔滤波的可行性。

空间低通滤波技术是提高激光光束空域质量的重要手段，针对传统的聚焦型针孔滤波技术在改善高功率激光光束质量方面的不足，本课题深入探索Rugate薄膜应用于高功率激光空间低通滤波的可行性。 .本课题的特色和创新点如下：.1、分别分析了普通Rugate薄膜和相移Rugate薄膜的角谱选择性带宽与光谱选择性带宽之间的关系，利用布拉格衍射公式和法布里-珀罗透射公式，以及二阶泰勒级数展开法，分别得出它们二者之间的关系表达式，指出当光束正入射或接近正入射时，这两种薄膜的光谱选择性带宽与角谱选择性带宽均近似成平方正比关系；而当光束倾斜入射时，光谱选择性带宽与角谱选择性带宽近似成正比关系。.2、当光束倾斜入射时，相移Rugate薄膜具有一维空间低通滤波特性，通过计算膜内场强分布，相移处的光强的值最大，且其随着折射率调制度的减小而降低，理论计算结果表明：当折射率调制度达到0.001，厚度达到0.4mm的Rugate薄膜才能使得膜内的场强分布与入射光束在同一量级，然而受限于当前的镀膜工艺，这样的薄膜还很难镀制，由于反射型体光栅在折射率分布上与Rugate薄膜类似，因此可以采用以反射型体光栅构成相移Rugate薄膜的结构，用来实现相移Rugate薄膜所能达到的效果。.3、提出只要是具有像截止滤光片类似的较窄的高低透射率过渡带的Rugate薄膜或多层介质薄膜结构，均可用以实现光束的空间低通滤波，并给出了滤波光路的设置。.4、提出利用薄膜的角谱函数作为目标函数来进行空间低通滤波薄膜的设计，构造了具有截止滤波器透射反射阶跃特性的目标函数，考虑到应用于高功率激光的实际，将膜内场强分布作为限制条件之一，构造了评价函数和罚函数，利用有条件的非线性优化算法和针尖算法，来优化设计薄膜的折射率分布。.5、利用干涉滤光片，实验验证薄膜对空域畸变光束的空间低通滤波效果，得出干涉滤光片可实现光束的一维空间低通滤波特性，间接验证了相移Rugate薄膜亦具有这样的特性。.6、基于严格耦合波理论和二维傅里叶变化，研究了透射型体光栅对连续光束衍射的衍射特性，首次得出了衍射光束中心较入射光束中心在透射型体光栅前后表面的位置变化，指出：在平行于光栅矢量方向上，它们之间的相对位置没有变化，而在垂直于光栅矢量方向上，衍射光束中心较入射光束中心有侧向位移，其大小与光束在均匀介质中的传播位移基本一致，其大小可由几何光学计算得知。