Low index, large mode field diameter optical coupler
View Patent ↗An optical coupler is formed of a low index material and exhibits a mode field diameter suitable to provide efficient coupling between a free space optical signal (of large mode field diameter) and a single mode high index waveguide formed on an optical substrate. One embodiment comprises an antiresonant reflecting optical waveguide (ARROW) structure in conjunction with an embedded (high index) nanotaper coupling waveguide. Another embodiment utilizes a low index waveguide structure disposed in an overlapped arrangement with a high index nanotaper coupling waveguide. The low index waveguide itself includes a tapered region that overlies the nanotaper coupling waveguide to facilitate the transfer of the optical energy from the low index waveguide into an associated single mode high index waveguide. Methods of forming these devices using CMOS processes are also disclosed.
1. A silicon-based optical coupler for providing optical coupling between a free space optical signal exhibiting a large mode field diameter and a single mode optical waveguide formed on an optical substrate and exhibiting a mode field diameter smaller than the optical signal, the silicon-based optical coupler comprising:
an antiresonant reflecting optical waveguide (ARROW) structure disposed at an endface of the optical substrate and comprising a low index core region and a high index reflective boundary layer having a refractive index higher than the low index core region, the arrow structure exhibiting a large mode field diameter to provide coupling for the free space optical signal, the high index reflective boundary layer disposed under the low index core region forming in combination a resonant cavity; and
a nanotaper coupling waveguide including a core region having a greater refractive index than the low index core region of the ARROW structure, the nanotaper coupling waveguide disposed between the ARROW structure and the single mode optical waveguide for providing coupling therebetween, the nanotaper coupling waveguide includes a narrow tip termination, tapering outwardly therefrom to an endface, the single mode waveguide coupled to the endface of the core region of the nanotaper coupling waveguide, wherein the nanotaper coupling waveguide performs mode field diameter conversion between the large mode field diameter ARROW structure and the small mode field diameter single mode optical waveguide.
2. A silicon-based optical coupler as defined in claim 1 wherein the ARROW structure low index waveguide core region comprises a dielectric material.
3. A silicon-based optical coupler as defined in claim 2 wherein the dielectric material is further utilized as at least one interlevel dielectric layer.
4. A silicon-based optical coupler as defined in claim 2 wherein the reflective boundary layer comprises at least one layer of material selected from the group consisting of: amorphous silicon, polysilicon, single crystal silicon and silicon nitride.
5. A silicon-based optical coupler as defined in claim 1 wherein the nanotaper coupling waveguide comprises silicon.
6. A silicon-based optical coupler as defined in claim 2 wherein the nanotaper coupling waveguide comprises silicon nitride.
7. A silicon-based optical coupler as defined in claim 1 wherein the ARROW structure comprises a tapered end disposed in an overlapping arrangement with the narrow tip termination of the nanotaper coupling waveguide.
8. A silicon-based optical coupler as defined in claim 7 wherein the ARROW structure further comprises an angled endface along the surface opposite of the tapered end section.
9. A silicon-based optical coupler as defined in claim 7 wherein the ARROW structure comprises silicon oxynitride.
10. A silicon-based optical coupler as defined in claim 7 wherein the coupler further comprises a dielectric confinement region surrounding the ARROW structure, the dielectric confinement region having a refractive index less than that of silicon oxynitride.
11. A silicon-based optical coupler as defined in claim 7 wherein the ARROW structure exhibits a graded refractive index profile sufficient to create the tapered end section.
12. A silicon-based optical coupler as defined in claim 1 wherein the ARROW structure comprises a rib waveguide.
13. A silicon-based optical coupler as defined in claim 1 wherein the ARROW structure comprises an inverted rib waveguide.
14. A silicon-based optical coupler as defined in claim 1 wherein the ARROW structure comprises a strip waveguide.
15. A silicon-based optical coupler as defined in claim 1 wherein the ARROW structure comprises a buried strip waveguide.