The absorption of incident beams during laser drilling of metals

Laser produced plasma plays an important role in the laser drilling of sheet metals as it can partially block and absorb the incident laser beam. A previous study of the transient properties of charged particles in the plasma plume has shown that, at low electron densities with high electron temperatures, laser drilling improves. This suggests that measurement of the absorption of the plasma plume is essential. The present study covers measurement of the absorption of a HeNe beam passing transversely through the plasma plume. The measurement was carried out using two fast response photodiodes and was repeated for sub-atmospheric pressures of air. The results obtained show that drilling is best at a pressure of 200 torr (2.7 x 10⁴ Pa) and rapid expansion of the flares is favourable at 2 mm above the surface. Coupling of absorption and heating is also best at this pressure.