Thermophysical Modeling of CO₂ Laser-Silica Glass Interaction

The thermophysical nature of rapid CO₂ laser heating of silica glass is explored using a numerical simulation that considers the temperature dependence of the glass thermophysical properties. A three dimensional heat transport model is developed to investigate the change in glass fictive temperature that occurs as a result of a CW CO₂ laser processing of silica glass. The model reveals that the laser processing results in an increase in fictive temperature in the local laser affected zone. The fictive temperature is elevated by about 1000K, uniform to within 5% over the laser affected zone, and transitions abruptly to the untreated glass value outside of the laser affected zone. This increase corresponds to a change in the glass physical and chemical properties, which can be revealed through wet chemical etching. The relationship between etch rate and the fictive temperature is determined by etching samples of known fictive temperature. The thermal penetration depth, defined as the maximum depth to which the glass fictive temperature is increased by the laser-silica glass interaction, can be determined from the thermophysical model and compared with the results from wet chemical etching.