Optical mode convertor

An apparatus comprising:

• a buried oxide (BOX) layer;
• a silicon (Si) slab positioned on the BOX layer;
• and
• a Si waveguide positioned on the BOX layer;
• wherein the apparatus includes:
• an input stage with an input Si slab height, an input Si waveguide height, and an input height difference between the input Si slab height and the input Si waveguide height;
• an output stage with an output Si slab height that is different from the input Si slab height, an output Si waveguide height that is different from the input Si waveguide height, and an output height difference between the output Si slab height and the output Si waveguide height that is different from the input height difference;
• and
• a transition stage positioned between the input stage and the output stage, wherein the transition stage has a transition Si slab height, a transition Si waveguide height, and a transition height difference between the transition Si slab height and the transition Si waveguide height.