Multilayer-coated optics: guided-wave coupling and scattering by means of interface random roughness

For plane-wave light incident upon multilayer optical components, we use first-order perturbation theory to calculate two effects of interface roughness: (1) angle-resolved scattering and (2) guided-mode coupling. The interface roughness, which is assumed to be a random variable having root-mean-square roughness much less than the incident wavelength, is the perturbation parameter. When guided-wave mode resonances are inherent in optical component design, we show that fractions of incident energy scattered directly and coupled into guided-wave modes are comparable. It follows that, in consideration of multilayer optics, energy coupled into guided-wave modes can be an equally important consideration, as this energy may then end up as an additional contribution to scattering and absorption.