Stress- and Defect Damage of CO2 - Laseroptics: Time of Damage Analysis

摘要

Laser damage experiments in the long-pulse regime show for coated C02-Iaser optics two distinct damage mechanisms. These damage mechanisms can be distinguished by their time of damage behaviour. The time of damage is measured with a previously presented setup, which was refined to improve the accuracy of the measurements. The two mechanisms are interpreted as induced by either defects or stresses in the coating. For the defect induced case, damage occurs at or before the peak fluence of the laser pulse, while for the stress induced case, the damage is observed later.nFor both mechanisms, analytical transient heat flow calculations are discussed. While for the defect induced damage, a good thermal contact of the small defects to the host explains the observed behaviour, for the stress induced damage a one-dimensional heat-flow model of a film-substrate system is used taking into account the actual temporal profile of the laser pulse. The experimental data of coated metal mirrors as well as ZnSe- and Germanium optics are very well described by this model even if bulk material parameters are used for the film. Some samples show both damage mechanisms. In this case the defect induced LIDT is lower than the stress induced one. The separation of these mechanisms is useful for an effective im¬provement of optical coatings.