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

当将高功率激光脉冲聚焦在透明材料表面或内部时，会产生激光辐射的非线性吸收，并引起激光损伤。该工艺用于透明材料的激光打标。我们利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察。调q Nd:YAG激光器输出的1064nm基频辐射聚焦在PMMA块的表面或内部。采用同一激光器的二次谐波辐射（532nm）作为照明光，采用532nm带通滤波器的CCD相机拍摄图像。在光延迟线的控制下，以基频光和次谐波光的不同间隔拍摄一系列图像。在间隔大于50ns的观测中，采用另一种激光作为光源。损伤同时发生在几个点上，从这些点开始的冲击波传播可以清楚地观察到。激光损伤（裂纹）在激光脉冲后几微秒内仍会继续发展。研究了脉冲能量和目标表面粗糙度的影响。

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.