The coated component becomes the damage - threshold limiting element in a laser system. AR and other coatings for high energy lasers, such as CO2 @ 10.6um and YAG @ 1.064um, require high damage thresholds.
Production of damage resistant coatings requires that the substrate surface have a high polish with low surface defects and that the coating layers be deposited with full stoichiometry, low structural defects, and small and low particular spatter.
Any of these imperfections can become sites that cause damage. Low optical absorption and absence of water bands are also important.
The deposition technique, as well as the material preparation, is critical to the fabrication of damage resistant coatings. For example, a sputtered layers coating contains fewer particulates than electron beam evaporated coatings. Since they are denser, any water absorption bands present are shallower, reducing the possibility of thermal disruption.
The damage mechanism is different for CW vs. pulsed laser operation, continuous irradiation or pulses in the micro sec duration time domain appear to cause by heating through film absorption. Melting and puckering is observed. Pulses in the Nano sec range because eruptions associated with contaminations and coatings (or substrate) defects. Localized plasma is formed at the coating surface or internal to the layer which reacts and increases with sub sequent pulse arrival.
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