Silicon nanotaper couplers and mode-matching devices

1. An arrangement for providing optical coupling between a free-space propagating optical signal and an ultrathin silicon waveguide formed in an upper silicon layer of a silicon-on-insulator (SOI) structure, the SOI structure further comprising:

a silicon nanotaper structure formed in the upper silicon layer (SOI layer) of the SOI structure, the silicon nanotaper having a first, narrow endface and a second, larger endface with a laterally extending tapered structure therebetween, the second, larger endface coupled to the ultrathin silicon waveguide to provide optical coupling therebetween;

a dielectric waveguide coupling layer having a top major surface, with a refractive index greater than the index of the dielectric insulating layer but less than the refractive index of silicon, disposed so as to overly a portion of the dielectric insulating layer in a region where an associated portion of the SOI layer has been removed and optical coupling into the remaining portion of the SOI layer is desired, with an end portion of the dielectric waveguide coupling layer overlapping an end section of the silicon nanotaper including the first, narrow endface to form a mode conversion region between the free-space propagating optical signal and the ultrathin silicon waveguide; and

a free-space optical coupling arrangement disposed over the top major surface of the dielectric waveguide coupling layer so as to couple the propagating optical signal between free space and the dielectric waveguide coupling layer and thereafter into the ultrathin silicon waveguide.

2. An arrangement as defined in claim 1 wherein the free-space optical coupling arrangement includes an evanescent coupling layer disposed over at least a portion of the dielectric waveguide coupling layer.

3. An arrangement as defined in claim 2 wherein the free-space optical coupling arrangement further comprises an optical prism coupler disposed over the evanescent coupling layer so as to couple a free space signal into and out of the underlying dielectric waveguide coupling layer.

4. An arrangement as defined in claim 3 wherein the optical prism coupler comprises silicon.

5. An arrangement as defined in claim 3 wherein the optical prism coupler comprises a dielectric material with a refractive index at least equal to the refractive index of the dielectric waveguide coupling layer.

6. An arrangement as defined in claim 1 wherein the dielectric waveguide coupling layer further comprises focusing optics to more narrowly confine the lateral boundaries of a propagating optical signal as the propagating optical signal enters the mode conversion region.

7. An arrangement as defined in claim 6 wherein the focusing optics comprises at least one waveguide mirror formed by disposing properly patterned regions of silicon dioxide into the dielectric waveguide coupling layer.

8. An arrangement as defined in claim 1 wherein the dielectric waveguide coupling layer further comprises at least one passive optical component disposed at a location separate from the mode conversion region.

9. An arrangement as defined in claim 1 wherein the free-space optical coupling arrangement comprises a coupling grating disposed on the surface of the SOI structure.

10. An arrangement as defined in claim 9 wherein the arrangement further comprises an evanescent coupling layer formed over a portion of the dielectric waveguide coupling layer.

11. An arrangement as defined in claim 1 wherein the dielectric waveguide coupling layer comprises a layer of material selected from the group consisting of doped silicon dioxide, silicon nitride, silicon carbide and silicon oxynitride.

12. An arrangement as defined in claim 1 wherein the dielectric waveguide coupling layer exhibits a tapered lateral profile so as to taper inward toward the mode conversion region and reduce optical reflections.