Study of Pulsed-Laser Drilling Process Using Transparent Materials: Time-Resolved Observation in Drilled Holes

当高功率激光脉冲作用于透明材料表面或内部时，会引起材料对激光辐射的非线性吸收，从而引起材料的损伤。该工艺用于透明材料的激光打标。我们用具有几纳秒时间分辨率的频闪成像系统对这一过程进行了动态观测。从Q开关Nd：YAG激光器输出的基波辐射（1064nm）聚焦在PMMA块的表面或内部。使用来自相同激光器的二次谐波辐射（532nm）作为照明光，并且通过具有532nm带通滤波器的CCD相机拍摄图像。利用光学延迟线控制基波光和二次谐波光之间的不同间隔，拍摄一系列图像。在间隔大于50ns的观测中，用另一束激光作为照明光源。损坏同时发生在几个点上，并且清楚地观察到从这些点开始的冲击波的传播。激光引起的损伤（裂纹）持续发展，直到激光脉冲后的几微秒。研究了脉冲能量和靶面粗糙度对激光能量的影响。

When we focus high power laser pulses on a surface or in inside of transparent materials, non-linear absorption of the laser radiation occurs and some laser-induced damages are introduced. This process is used in laser marking of transparent materials. We have made dynamical observations of the process by the stroboscopic imaging system which has a few nanoseconds time resolution. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at the surface or inside of PMMA blocks. Second harmonic radiation (532nm) form the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter of 532 nm. Series of images were taken at different intervals between fundamental and second harmonic light, which was controlled by optical delay line. In observation at longer interval than 50ns, another laser was used as illuminating source. Damages occur at several points simultaneously and propagation of shock waves started from these points is clearly observed. Laser induced damages (cracks) continue to develop until some microseconds after laser pulse. Effects of pulse energy and roughness of the target surface are studied.