Determination of the leaky modes of a corrugated waveguide: application to the study of anomalies of dielectric coated gratings

The presence of a protective dielectric layer on diffraction gratings may product anomalies which are characteristic of the layer. Their existence is related to the generation of leaky modes, for the two classical cases of polarization. For the TM (S) case, where anomalies already exist for a bare grating, the perturbations due to the presence of the coating are discussed. Diagrams are given which make it possible to predict the evolution of these anomalies when an arbitrary parameter varies. For the TE (P) case of polarization, a phenomenon of total absorption by the coated grating, which is analogous to the previous observed one for a bare grating and TM polarization, is observed for particular values of the parameters.