A multiple-core planar optical waveguide comprises: a substantially planar waveguide substrate; a lower waveguide core; an upper waveguide core; lower cladding between the substrate and the lower waveguide core; middle cladding between the waveguide cores; and upper cladding above the upper waveguide core. Overlapping portions of the waveguide cores are positioned one above the other and substantially parallel. The lower, middle, and upper claddings have refractive indices less than refractive indices of the lower and upper waveguide cores. The widths of the waveguide cores are substantially larger their thicknesses along their overlapping portions. The overlapping portions of the waveguide cores jointly support a propagating optical mode.