Patent ID: 7164838

Claim:
A planar optical waveguide, comprising: a substantially planar waveguide substrate; a lower waveguide core layer; an upper waveguide core layer; a waveguide core between the upper and lower waveguide core layers; lower cladding between the substrate and the lower waveguide core layer; upper cladding above the upper waveguide core layer; and middle cladding between the upper and lower waveguide core layers substantially surrounding the waveguide core, wherein: the lower cladding has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core; the middle cladding has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core; the upper cladding has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core; a width of the waveguide core is substantially larger than a thickness thereof; an upper surface of the waveguide core is substantially flat; and the upper and lower waveguide core layers extend bilaterally substantially beyond the lateral extent of a propagating optical mode supported by the planar optical waveguide, the supported optical mode being substantially confined bilaterally by the waveguide core, further comprising a second waveguide core between the upper and lower waveguide core layers, wherein: a refractive index of the second waveguide core is greater than the refractive indices of the upper cladding, the middle cladding, and the lower cladding; a width of the second waveguide core is substantially larger than a thickness thereof; an upper surface of the second waveguide core is substantially flat; the first waveguide core and the second waveguide core are arranged one-above-the-other within the middle cladding; index contrast between the upper core layer and the upper and middle claddings is less than about 5%; index contrast between the lower core layer and the lower and middle claddings is less than about 5%; index contrast between the first waveguide core and the middle cladding is less than about 5%; index contrast between the second waveguide core and the middle cladding is greater than about 5%; the second waveguide core tapers in at least one transverse dimension in a longitudinal direction along a portion of the waveguide; the tapered portion of the second waveguide core and the first waveguide 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 second waveguide core; and the second waveguide core tapers sufficiently gradually so as to substantially avoid optical coupling between the selected propagating optical mode and other optical modes.