Calculation and measurement of scattering for investigation of microroughness in optical coatings

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Abstract
Shows how the spatial distribution of scattered light depends on the coating design, on the interface roughness and on cross-correlation laws between interfaces. In order to reduce the high number of parameters involved in the calculation, it is important to know whether the roughness in the stack can be deduced from the one of the substrate (causal model), or if they are essentially due to the grain size of material in thin film form. For this, the substrate is accurately characterized before coating (roughness, autocorrelation length and isotropy degree). Each material is then associated with a cut-off frequency which characterizes its action at each interface, and therefore scattering can be predicted in whole space for any coating deposited on this substrate. Consequently calculation and measurement of scattering isotropy are compared for each defect spatial frequency, which clearly shows the origin of microroughness. In a more general way, measurement of substrate before coating is enough to have an accurate prediction of angular scattering after coating.