KrF laser-induced absorption in synthetic fused silica

In order to estimate the long-term behavior of synthetic fused silica under 248 nm irradiation, KrF laser beam transmission has been measured on 40 cm long samples. Due to this long optical path length, the measuring sensitivity for induced absorption is high enough to apply relatively low pulse energy densities in the range from 100 mJ/cm2 down to 15 mJ/cm2. The maximum number of pulses was 750 million with a repetition rate of 1000 Hz. The damage behaviors of the two synthetic fused silica types Suprasil 2 and Suprasil 312 differ markedly. Suprasil 2 exhibits a rapid damage process at the beginning of irradiation followed by a slower linear absorption increase afterwards and the 'strong absorption transition'. Induced absorption in Suprasil 312 increases s-shape like during irradiation reaching a plateau level for higher number of pulses. We describe both damage behaviors with phenomenological model equations. These are used to extrapolate to parameter ranges, that can not be investigated easily.