Filtering of unpolarized light by gratings

The filtering effect we are dealing with in the present paper is obtained by using a diffraction grating made by perturbing a planar waveguide. A phenomenological theory allows us to deduce important propel-ties of the sharp filtering phenomena generated by this kind of structure when the incident light excites guided waves. It is shown that the resonance phenomenon occurring when a single guided mode is excited acts on a single eigenvalue of the Hermitian reflection matrix. As a consequence, we demonstrate that a high efficiency filtering of unpolarized light requires the simultaneous excitation of two uncoupled guided waves. This criterion is reached when these guided waves propagate in orthogonal directions. Numerical examples are given.