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

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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.