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Subsurface damage of optical components and the influence on scattering properties

The influence of sub surface damage under smooth optical surfaces on the scattering properties was investigated. Usually this sub surface damage is filled and covered by a polishing layer. Therefore sub surface damage does not contribute to the micro roughness of the surface. Three glasses, SF3, BK7, and SUPRASIL, with different Knoop hardness and related different sub surface damage density were chosen for the measurements. Three samples of each glass were polished with increasing polishing time in order to reduce the layer which contains the sub surface damage. Beside the extensive measurements of the scatter behavior the samples were also investigated by means of microscopy (Nomarski, darkfield, cross polarization) and optical profilometry. The stray light was detected in the case of reflection (back scatter), transmission (forward scatter) and total reflection. In the case of totalreflection the scattered light behind the reflection surface was investigated. The detected scatter light was integrated over the measurement range and the resulting value was compared with the polishing time. Additional investigations were carried out to determine the influence on the light polarization.