a demultiplexing apparatus for wavelength division multiplexed optical communications makes use of an interleaver to separate alternating wavelengths into separate wavelength groups . these separate wavelength groups may then be demultiplexed using conventional demultiplexers . an optical coupler such as an optical circulator or a curved grating is used to direct the reflected wavelengths to a separate output path . the interleaver preferably has a square wave reflectivity response in the wavelength domain , and reflects every other wavelength to a separate output path than those wavelengths that are transmitted past the interleaver . the interleaver may be a diffraction grating that may be apodized or chirped . the grating may also be a sampled grating structure . alternatively , the interleaver may be a planar waveguide that has a periodic change in the refractive index conditions of a section of the waveguide .