Fiber coupler for silicon photonics

A fiber-to-waveguide mode converter comprising:

• a silicon substrate member;
• a first dielectric material having an elongated body laid in a lengthwise direction from a back end to a front end with a portion proximate to the back end being supported by the silicon substrate member and a remaining portion proximate to the front end being suspended from the silicon substrate member or supported by a second dielectric material thereof;
• a first waveguide segment formed from the back end along the lengthwise direction to a tail end on the first dielectric member, the tail end being a distance short to reach the front end of the first dielectric material;
• and
• a first cladding material overlying both the first waveguide segment and the first dielectric material, the first cladding material having a refractive index smaller than the first waveguide segment and includes an index-graded structure formed at least over the first waveguide segment with decreasing index from the tail end toward the back end, wherein the index-graded structure comprises a plurality of patterned slots having a depth partially into the first cladding material and having increasing widths and increasing slot-slot spacing from a location proximate to the tail end of the first waveguide segment toward the back end.