Light scattering to detect imperfections relevant for laser-induced damage

Laser-induced damage of optical surfaces, thin film coatings, and materials is greatly influenced by imperfections such as surface and interface roughness and surface or subsurface defects. All these imperfections give also rise to light scattering. Light scattering techniques are thus well suited to identify and characterize damage-relevant features. Additionally, they are non-contact, highly sensitive, and enable large sample areas to be investigated. Conventional characterization techniques are usually confined to small sample areas. A light scattering method will be presented that provides roughness and defect maps even of large and curved surfaces. Subsurface defects also play a critical role as damage precursors. Many detection methods are still based on wedging and/or etching the sample surface. A new non-contact approach to detect subsurface damage using polarized light scattering will be presented. In addition, a method will be discussed that provides information about the structural and optical properties of multilayer coatings by analyzing the scattered light distribution. Even small deviations of the illumination and the design wavelengths or angles can lead to substantial field enhancements inside the coating which can be clearly observed as resonant scattering wings.