Effects of Multiple Reflections on Hole Formation During Short-Pulsed Laser Drilling

Beam guiding effects during laser drilling due to multiple specular reflections inside the hole are analyzed for the case of very short laser pulses (nanosecond range). Specular reflections are valid for materials that retain a smooth surface during laser evaporation (small optical roughness compared to the laser wavelength). The problem is assumed to be two-dimensional axisymmetric (unpolarized laser), with the hole geometry defined by nodal values connected through a cubic spline. The net radiative flux onto a surface node is determined through ray tracing methods. The resulting absorbed laser flux is combined with a simple quasi-one-dimensional conduction model (to assess the minor conduction losses) and an Arrhenius evaporation rate model, to predict hole development as a function of time through iteration. To stabilize this highly nonlinear and thus unstable problem (in numerical analysis as well as in experiments) the laser beam is diffused a small amount from the specular direction (to also account for the limitation that no beam can be focused down to a point), and by periodic slight smoothing of the irradiation levels. Results show that drilling rates are increased dramatically due to beam trapping for highly reflective materials, resulting in a more pointed hole profile.