An optical frequency filter comprises a support layer having reflective elements formed thereon, the reflective elements defining at least one periodic grid of substantially parallel slits, the period P, the height, and the width of the slits being selected in such a manner that the reflective elements form a wavelength-selective structure for a wavelength lying in a determined range of wavelengths. The support layer material has a refractive index nh and includes inclusions of a material of refractive index nb, where nb is strictly less than nh. The inclusions are flush with the surface of the support layer opposite from its surface on which the reflective elements are formed, and present height hbwherein 0.5hh≦hb≦1hh, hh being the support layer height, and width lbwhere 0.05P≦lb≦0.75P. Each inclusion is situated at least in part between two reflective elements.