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 (112, 111); 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 (112, 111) 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; index contrast between the upper core layer and the upper middle claddings (120c, 120b) is less than about 5%; index contrast between the lower core layer and the lower and middle claddings (120a, 120b) is less than about 5%; index contrast between the waveguide core and the middle cladding is less than about 5%; the upper and lower waveguide core layers (112, 111) extend bilaterally substantially beyond the transverse extent of a propagating optical mode supported by the waveguide; said propagating optical mode is confined bilaterally by the waveguide core, characterised in that said propagating optical mode is confined vertically by the upper and lower waveguide core layers (111, 112). 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 (112, 111); 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 (112, 111) 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; index contrast between the upper core layer and the upper middle claddings (120c, 120b) is less than about 5%; index contrast between the lower core layer and the lower and middle claddings (120a, 120b) is less than about 5%; index contrast between the waveguide core and the middle cladding is less than about 5%; the upper and lower waveguide core layers (112, 111) extend bilaterally substantially beyond the transverse extent of a propagating optical mode supported by the waveguide; said propagating optical mode is confined bilaterally by the waveguide core, characterised in that said propagating optical mode is confined vertically by the upper and lower waveguide core layers (111, 112). 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; index contrast between the upper core layer and the upper middle claddings (120c, 120b) is less than about 5%; index contrast between the lower core layer and the lower and middle claddings (120a, 120b) is less than about 5%; index contrast between the waveguide core and the middle cladding is less than about 5%; the upper and lower waveguide core layers (112, 111) extend bilaterally substantially beyond the transverse extent of a propagating optical mode supported by the waveguide; said propagating optical mode is confined bilaterally by the waveguide core, characterised in that said propagating optical mode is confined vertically by the upper and lower waveguide core layers (111, 112).