激光在圆锥形小孔孔壁上的多次反射吸收研究

在假设小孔为圆锥形的基础上，按照几何光学理论，通过跟踪光线在小孔内的多次反射轨迹，研究了激光在圆锥形小孔孔壁上的多次反射吸收，计算了小孔孔壁通过多次反射吸收的激光功率密度，分析了孔壁的多次反射次数、聚焦透镜的焦距以及圆锥形小孔的顶角等因素对小孔孔壁上的激光功率密度分布的影响．计算结果表明：小孔孔壁上吸收的总激光功率密度主要取决于激光在小孔孔壁上前几次反射；在小孔上部，孔壁吸收的总激光功率密度主要取决于直射到小孔孔壁上的激光功率密度；孔壁的多次反射只对小孔下部孔壁上的激光功率密度分布有影响．小孔的形状、尺寸取决于小孔孔壁上吸收的激光功率密度大小．小孔孔壁上吸收的激光功率密度与小孔的形状、深度和大小之间存在一个自我调节和自我适应的过程．孔壁上吸收的激光功率密度愈大，小孔的深度和直径也愈大，锥顶角愈小，小孔孔壁上吸收的激光功率密度分布还与聚焦透镜的焦距有关．在保证激光焊接所需要的激光功率密度的前提下，采用长焦距的聚焦透镜，有利于把激光能量直接导入工件材料内部。

Study on the Fresnel Absorption and Multiple Reflections in a conical Keyhole

On the assumption of conical keyhole and using geometrical optics theory, a detailed study on Fresnel absorption and multiple reflections on the keyhole wall in deep penetration laser welding of metals was made by tracing a beam light. On the basis of the laser intensity absorbed on the keyhole wall, the effects of such factors as the multiple reflections, the focal length of the lens and the vertex angles of the keyhole on the laser intensity distribution on the keyhole wall were studied. The total laser intensity absorbed on the keyhole wall was mainly determined by the first few reflections. On the top of the keyhole, the total laser intensity absorbed on the keyhole wall was mainly determined by the direct laser incidence. Multiple reflections could only influence the laser intensity absorbed on the bottom of the keyhole wall. The shape and the size of the keyhole were determined by the laser intensity absorbed on the keyhole wall. There is a self-adjusting and self-adapting process between the shape and the size of the keyhole and the laser intensity absorbed on the keyhole wall. The greater the laser intensity absorbed on the keyhole wall, the deeper the keyhole, the bigger the keyhole diameter, and the smaller the vertex angle of the keyhole. Moreover, the laser intensity absorbed on the keyhole wall is closely related to the focal length of the lens. On condition that enough laser intensity can be provided for deep penetration laser welding, the use of lens with long focus helps deliver the laser energy directly to the interior of the keyhole.