A device has first, second and third elements fabricated on a common substrate. The first element supports a first optical mode in a first active waveguide, the second element supports a second optical mode in a second passive waveguide, and the third element, at least partly butt-coupled to the first element, has a bottom waveguide, supporting a bottom intermediate mode, overlying a top waveguide, supporting a top intermediate mode. A tapered waveguide structure in the top waveguide facilitates efficient adiabatic transformation between the top intermediate mode and the bottom intermediate mode. Tapering in at least one of the bottom waveguide and the second waveguide facilitates efficient adiabatic transformation between the second optical mode and the bottom intermediate optical mode. No adiabatic transformation occurs between either of the intermediate optical modes and the first optical mode. Lithographic alignment marks facilitate precise alignment of the three elements during device fabrication.