An optical filter or multiplexer/demultiplexer, including a plurality of optical waveguides forming a planar structure. Each optical waveguide has a total length including one or more first segments with a first width and at least one or more second segments with a second width, the first width being larger than the second width. The sum of lengths of the one or more first segments in each optical waveguide is larger than half the total length of the waveguide.