An optical wavelength selective device is provided that is particularly useful as a demultiplexer. The device is made from a planar slab optical waveguide material transmissive to light. Perturbations in the form of holes are etched into the slab waveguide cladding or preferably into the core of the waveguide thereby changing the effective refractive index of regions within the core. The holes are very closely spaced and form a transmissive optical grating, having dimensions suitable for operating in the Raman-Nath regime. Mirrors or lenses are formed within the device for guiding an input beam into the waveguide as a parallel light source.