Surface deformation mechanisms in laser smoothing and micromachining of optical glasses

Abstract

The thesis addresses important issues in laser processing of optical glasses, such as fused silica (HPFS®7980 Corning), Borofloat®33, and some selected lead-silicate glasses, when treated by a CO2 laser beam at a 10.6 μm wavelength, using beam diameters of either 1 mm or 50 μm. The investigations were carried out in the melting and vaporization regimes to study laser-induced surface deformations and stresses, and laser smoothing of fused silica etched structures. Novel applications for CO2 laser polishing have been found and some preliminary results are presented. With regard to the surface deformations and stresses, it has been discovered that fused silica behaves differently than other glasses. Deformations in fused silica are observed to be in the form of shallow depressions, as a result of glass densification driven by a fictive temperature increase. These deformations are completely removed by annealing. In raster scanning, the depressions merge to generate surface stress in the range of 10 - 30 MPa, which is largely reduced by annealing. In contrast, CO2 laser radiation of Borofloat®33 produces surface bumps, now driven by both the fictive temperature and an irreversible Marangoni effect. In this case, the bumps are only partially removed by annealing. However, laser machining and polishing conditions for non-cracking treatment of Borofloat®33 have been successfully established, now opening the possibility of using the CO2 laser-based processes for manufacturing micro-optical components. Lead-silicate glasses were found to have strong bumping and complex Marangoni shaping, limiting the prospects for CO2 laser machining of micro-optics. Application of CO2 laser smoothing for surface relaxation of binary gratings and multi-level etched structures has shown that sharp step edges can be relaxed over a distance from submicron to about 30 μm. An optical method based on analysis of light scatter from binary gratings provides an excellent calibration method for CO2 laser polishing. The submicron resolution in smoothing may be applied for fine relaxation of diffractive optics and nanostructures fabricated in fused silica. On the other hand, large scale relaxation of the etched steps provides a promising result to be used for the fabrication of micro-optics, as a viable alternative to the thermal reflow process. A pioneering approach for the rapid prototyping of silica toroidal mirrors has given a high ratio of principle radii of curvature, successfully applied in mode-selective resonator configurations to improve the laser beam quality of planar waveguide lasers.