One aspect of the invention provides a mode size converter having a first end and a second end. The mode size converter includes: a silicon waveguide having an inverse taper from the first end; and a silicon nitride waveguide having an inverse taper relative to the first end. The silicon nitride waveguide is adjacent and substantially parallel to the silicon waveguide. Another aspect of the invention provides an optical assembly including: a mode size converter as described herein; and a fiber optic optically coupled to the silicon nitride waveguide at the second end of the mode size converter.
