Contribution of spherical aberrations to the vestige structure induced by laser damage

Abstract

The contribution made to the propagation of a Gaussian beam by the spherical aberrations introduced by the lens used to focus the beam into the test sample is considered as a possible explanation of the multivestige structure observed in laser-induced damage of transparent materials. The paraxial approximation for the wave equation is used to determine the intensity of the beam on the beam axis when the aberrations are present. The inclusion of spherical aberrations modifies the form expected for a diffraction-limited Gaussian beam, shifting the main peak in intensity away from the focus and suppressing it. More importantly, oscillations are introduced in the intensity prior to (past) the focus when the spherical aberrations focus off-axis rays prior to (past) the geometrical focus. Although the spatial arrangement of the peaks in the intensity appears consistent with some of the experimental results for the vestige structure, the spacing between peaks does not correspond to the observed spacing between damage sites. These findings and other possible explanations of the vestige structure are considered critically.