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

Fused-silica microstructural changes associated with localized 10.6 microm CO(2) laser heating are reported. Spatially resolved shifts in the high-frequency asymmetric stretch transverse-optic phonon mode of SiO(2) 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.