A (de)multiplexer for use in optical communications systems for multiplexing and demultiplexing an optical signal consisting of optical channel(s) of different wavelength(s) includes a multiplex optical waveguide propagating a plurality of optical channels and a plurality of single channel optical waveguides, each propagating a single channel. Each of the waveguides has a receiving/transmitting end. A diffraction grating is optically coupled between the multiplex optical waveguide and the single channel optical waveguides for diffracting an optical signal between the receiving/transmitting end of the multiplex optical waveguide and the receiving/transmitting end of the single channel optical waveguide. The grooves of the diffraction have spacing which varies according to an algorithm such that a portion of the optical signal diffracted by each groove is offset in a direction of dispersion relative to the portions of the optical signal diffracted by the other surfaces to broaden the transmission band at the receiving/transmitting ends of the single channel and multiplex optical waveguides. A method for fabricating a grating as described above includes etching grooves having spacing which varies according to the algorithm such that a portion of the optical signal diffracted by each surface is offset in a direction of dispersion relative to portions of the optical signal diffracted by each other surface.