A method includes providing a semiconductor body comprising a surface with a recessed portion therein. The recessed portion includes a bottom surface. Optical waveguide cores in a first array of optical waveguide cores extend side-by-side at the bottom surface. The method further includes providing a second array of optical waveguide cores over the first array of optical waveguide cores. Optical waveguide cores in the second array of optical waveguide cores extend side-by-side. Each optical waveguide core in the second array of optical waveguide cores is in an adiabatic coupling relationship with a corresponding optical waveguide core in the first array of optical waveguide cores. The method also includes applying an optical waveguide cladding material over the second array of optical waveguide cores.