Catastrophic versus Microscopic Damage: Applicability of Laboratory Measurements to Real Systems

At ultraviolet wavelengths, damage to both coatings and bare surfaces is dominated by the presence of discrete localized defects. During multiple-shot irradition, the overwhelming majority of these defects are damaged by the first or first few shots. Initially, damage morphology is that of a crater of approximately 10 microns in diameter; however, upon continued irradiation, one of two events can occur; either the crater grows to catastrophic dimensions or it remains unchanged. In the latter case, the damage is only observable under a microscope, it may be indistinguishable from cosmetic defects before irradiation, and it is likely that any related degradation in optical performance is unmeasurable. In view of the generally accepted definition of laser damage (i.e. any visible change in the surface), it is important to consider the implications for real systems. These are discussed in the context of ultraviolet test results for both coatings and surfaces.