Impact of contaminant size and concentration on continuous-wave laser damage threshold

Undetected contamination can cause catastrophic damage to optical components in high average power laser systems with potential for additional damage to optical mounts and contamination of nearby optics, causing unexpected downtime and requiring costly repairs. Such particle-induced damage depends strongly on the characteristics of both the contaminant material and the underlying coating or substrate. This work analyzes the effects of carbon-based contaminant particles on the high average power laser-induced damage threshold (LIDT) of optics, specifically the effects of particle size and concentration. To develop a theoretical model that can predict particle-induced LIDT, experimental measurements of CW laser damage were taken for contaminant particle sizes ranging from 0.5-40μm with concentrations ranging from ∼0.6mm^-2 to ∼400mm^-2. Simple theoretical arguments can explain many of the qualitative results, and a quantitative free-carrier absorption model that incorporates contaminant material evaporation, diffusion, redeposition, and oxidation closely fits the experimental data. This model has been used to draw general conclusions about safe contamination levels, specifically that systems using Ta₂O₅/SiO₂ reflectors at an irradiance greater than 100kW/cm² should strictly avoid carbon-based contaminants with diameters greater than approximately ∼12.3μm. Local contaminant concentrations with a fill factor greater than ∼0.5% are also likely to cause damage, regardless of contaminant particle size.