Heterogenously integrated short wavelength photonic platform

A device comprising:

• first, second and third elements fabricated on a common substrate;
• wherein the first element comprises an active waveguide structure supporting a first optical mode in a first waveguide, the second element comprises a passive waveguide structure supporting a second optical mode in a second waveguide, and the third element, at least partly butt-coupled to the first element, comprises a bottom intermediate waveguide structure overlying the second element and a top intermediate waveguide structure overlying the bottom intermediate waveguide structure;
• wherein the bottom intermediate waveguide structure supports at least one bottom intermediate mode in a bottom waveguide, and the top intermediate waveguide structure supports at least one top intermediate mode in a top waveguide;
• wherein a tapered waveguide structure in the top waveguide of the third element facilitates efficient adiabatic transformation between one of the top intermediate modes in the top waveguide and one of the bottom intermediate modes in the bottom waveguide;
• wherein one or more tapered waveguide structures in at least one of the bottom waveguide of the third element and the second element waveguide facilitates efficient adiabatic transformation between the second optical mode and one of the intermediate optical modes of the third element bottom waveguide;
• wherein no adiabatic transformation occurs between any of the intermediate optical modes and the first optical mode;
• and
• wherein mutual alignments of the first, second and third elements are defined using lithographic alignment marks that facilitate precise alignment between layers formed during processing steps of fabricating the first, second and third elements.