Patent ID: 7184643

Claim:
An optical waveguide, comprising: a substantially planar waveguide substrate; a lower waveguide core; an upper waveguide core, at least a portion the upper waveguide core being positioned above and substantially parallel to at least a portion of the lower waveguide core; lower cladding between the substrate and the lower waveguide core having a refractive index less than refractive indices of the lower and upper waveguide cores; and upper cladding above the upper waveguide core having a refractive index less than refractive indices of the lower and upper waveguide cores, wherein: a width of the lower waveguide core is substantially larger than a thickness thereof along at least a portion thereof, and is substantially flat along that portion of the length thereof, thereby yielding a substantially flat surface for forming at least a portion of the upper waveguide core; the upper and lower waveguide cores are in contact with one another over at least a portion of their respective lengths, and the flat surface comprises a portion of an upper surface of the lower waveguide core; one of the upper and lower waveguide cores comprises a low-index-contrast core, and the other of the upper and lower waveguide cores comprises a high-index-contrast core; the high-index-contrast waveguide core tapers in at least one transverse dimension in a longitudinal direction along the flat portion of the lower waveguide core; the tapered portion of the high-index-contrast core and the portion of the low-index-contrast core in contact with the high-index-contrast core are arranged so as to guide a selected propagating optical mode having a transverse spatial profile that evolves along the length of the tapered portion of the high-index-contrast core; the high-index-contrast waveguide core tapers sufficiently gradually so as to substantially avoid optical coupling between the selected propagating optical mode and other optical modes; the high-index-contrast core comprises silicon nitride or silicon oxynitride less than about 200 nm thick and less than about 2 μm wide along a non-tapered portion thereof; the low-index-contrast core comprises silica or doped silica between about 0.4 μm thick and about 1 μm thick and between about 4 μm wide and about 10 μm wide along at least the flat portion of the lower waveguide; and the upper and lower cladding comprise silica or doped silica.