Laser transmission through a hole

Since 60’s laser drilling is an industrial application to drill sub-millimeter holes in metallic target. However, it is still not easy to drill hole in targets which have complex geometry of internal cavity. The difficulty is to not impact the opposable wall of the cavity. This paper deals with the study of hole transmission in function of peak power and target thickness. Holes are drilled with a millisecond laser source in percussion regime. For this study the metallic targets thickness drilled is from 1 to 20 mm. The transmission is measured with the help of a photodiode. Results reveal that very simple equations describe the laser transmission through a breaking through hole and a drilling threshold is estimated. The laser transmission is constant until a drilled depth depending on peak power. The transmission decreases because laser make reflexions on hole whole.