Analysis of microstructural relaxation phenomena in laser-modified fused silica using confocal Raman microscopy

Fused-silica microstructural changes associated with localized 10.6 mu m CO2 laser heating are reported. Spatially resolved shifts in the high-frequency asymmetric stretch transverse-optic phonon mode of SiO2 were measured using confocal Raman microscopy, allowing construction of axial fictive temperature (T-f) maps for various laser-heating conditions. A Fourier conduction-based finite-element model was employed to compute on-axis temperature-time histories, and, in conjunction with a Tool-Narayanaswamy form for structural relaxation, used to fit T-f(z) profiles to extract relaxation parameters. Good agreement between the calculated and measured T-f was found, yielding reasonable values for relaxation time and activation enthalpy in the laser-modified silica.