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; and upper cladding above the upper waveguide core. At least a portion the upper waveguide core is positioned above and substantially parallel to at least a portion of the lower waveguide core. The lower and upper claddings have refractive indices less than refractive indices of the lower and upper waveguide cores. The width of the lower waveguide core is substantially larger than its thickness along at least a portion of its length, and is substantially flat along that portion of its length, thereby yielding a substantially flat surface for forming at least a portion of the upper waveguide core.