Mode control in heterogeneously integrated photonics

A device comprising:

• first, second and third elements fabricated on a common substrate;
• wherein the first element comprises an active waveguide structure characterized by a mesa, supporting a first optical mode, the second element comprises a passive waveguide structure supporting a second optical mode, and the third element, at least partly butt-coupled to the first element, comprises an intermediate waveguide structure characterized by an intermediate waveguide core cross section including a shallow step, the intermediate waveguide structure supporting at least one intermediate optical mode;
• wherein a tapered waveguide structure in at least one of the second and third elements facilitates efficient adiabatic transformation between the second optical mode and one of the intermediate optical modes;
• wherein no adiabatic transformation occurs between any of the intermediate optical mode 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.