Effects of polarizations of a laser on absorption in a paraboloid of revolution-shaped welding or drilling cavity

The effects of polarizations on the apparent absorptivity or integrated energy absorbed by a paraboloid of revolution-shaped cavity subject to a laser beam are systematically and quantitatively investigated. The incident flux is considered to be a TEM00 mode of a Gaussian distribution specified by the wavelength, spot size, and focal location of the laser beam. Using a Monte Carlo method and neglecting the absorption within the plasma in the cavity, the results show that the predicted energy-transfer efficiency versus incident flux and cavity depth agrees well with the experimental data. The apparent absorption of p polarization is found to be always higher than that of s polarization, even though the latter causes higher energy fluxes absorbed than the former near the critical radius. The apparent absorptivity for s polarization as a function of cavity depth can be divided into three parts: an initial decrease, rapid increase, and gradual increase. Interestingly, the polarization of circularly polarized incoming rays is gradually transformed into s polarization in the course of multiple reflections. The apparent absorptivity as a function of polarizations, specular reflectivity, penetration depth, focal location, and absorptive and refractive indices is also presented. (C) 2004 American Institute of Physics.