Degradation of optical components in laser machines for manufacturing

Coated ZnSe optical components are irradiated with high-power pulsed CO2 laser radiation (lambda=10.6 micrometer, pulse length about 100 ns) at fluences up to 250 J/sqcm. The components are characterized at various stages of irradiation by optical microscopy, interferometric microscopy, profilometry, surface chemical analysis (x-ray photoemission and Auger electron spectroscopy), and surface structural analysis (micro-Raman spectroscopy). Two types of coating damage occur within the iradiated area of the component: a breaking apart of the ZnSe overlayer of the coating system over relatively large areas resulting in a network structure, and the formation of isolated craters of diameter about 30-50 micrometer extending in depth about 5 mirometer through the coating system down to the ZnSe substrate. Chemically, the irradiated area is characterized by an oxidation of both Zn and Se and an increase in the stoichiometric ratio of Zn to Se. These effects are especially pronounced at the crat er defects, and are atributed to localized optical absorption, leading to thermal stress and chemical reactions of Zn and Se with atmospheric or adsorbed water and/or oxygen. Structurally, the coatings exhibit a polycrystalline structure with no orientation of the in idiviual grains. During irradiation the grain size diminishes giving, in addition, indication for built - in stress and partial melting at higer laser fluences.