Absorption in a paraboloid of revolution-shaped welding or drilling cavity irradiated by a polarized laser beam

Abstract

Energy flux absorbed by a paraboloid of revolution-shaped cavity subject to a focused polarized laser beam is systematically and quantitatively investigated. The incident flux is considered to be TEM00 mode of a Gaussian distribution specified by the wavelength, spot size, and focal location of the laser beam. By neglecting absorption and scattering within the plasma in the cavity and accounting for diffuse and Fresnel specular reflection of the wall, the predicted results show that in contrast to cutting or etching, energy absorbed near the critical radius for a deep welding or drilling cavity is higher for s-polarization than that for p-polarization. On the other hand, absorption for p-polarization is higher than that for s-polarization in a shallow cavity. The critical radius indicating the jump of energy flux absorbed is independent of polarization and optical properties of the workpiece. The use of constant specular reflectivity instead of more elaborate c-polarized Fresnel reflectivities to predict absorption is accurate. A decrease in the wavelength of the laser beam and increase in cavity depth reduce the critical radius and enhance the peak of energy flux absorbed. As the focal distance and spot size decrease, both the critical radius and peak of energy flux absorbed increase. The predicted energy flux absorbed from the Monte Carlo method agrees well with an asymptotic solution.