Waveguide mode expander having an amorphous-silicon shoulder
A waveguide mode expander couples a smaller optical mode in a semiconductor waveguide to a larger optical mode in an optical fiber. The waveguide mode expander comprises a shoulder and a ridge. In some embodiments, the ridge of the waveguide mode expander has a plurality of stages, the plurality of stages having different widths at a given cross section.
1. A waveguide mode expander comprising:
a substrate;
a waveguide disposed on the substrate, wherein the waveguide comprises crystalline silicon;
a shoulder optically coupled with the waveguide, wherein:
the shoulder is disposed on the substrate; and
the shoulder comprises non-crystalline silicon; and
a ridge, wherein:
the ridge comprises non-crystalline silicon;
the ridge is disposed on the substrate, such that the shoulder is between the ridge and the substrate;
the ridge has a narrower width than the shoulder, wherein the ridge and the shoulder are configured to guide and expand an optical beam propagating from the waveguide and through the shoulder and the ridge.
2. The waveguide mode expander of claim 1 , wherein the waveguide has a rectangular cross section.
3. The waveguide mode expander of claim 1 , wherein:
the ridge comprises a plurality of stages;
at a cross section of the waveguide mode expander, each stage of the plurality of stages has a different width; and
a first stage of the plurality of stages, which is closer to the shoulder, has a wider width than a second stage of the plurality of stages, which is farther from the shoulder than the first stage.
4. The waveguide mode expander of claim 3 , wherein the first stage is thinner than the second stage.
5. The waveguide mode expander of claim 1 , wherein the non-crystalline silicon is amorphous silicon.
6. The waveguide mode expander of claim 1 , wherein:
the plurality of stages has a number of stages; and
the number of stages is three.
7. The waveguide mode expander of claim 1 , further comprising a cladding covering the ridge and the shoulder.
8. The waveguide mode expander of claim 7 , wherein the cladding is SiO2.
9. The waveguide mode expander of claim 1 , wherein the ridge expands the optical beam by tapering from a narrower width near an input end to a wider width near an output end.
10. The waveguide mode expander of claim 1 , wherein:
the substrate comprises buried-oxide layer and a handle layer;
the buried-oxide layer is disposed between the shoulder and the handle layer;
the buried-oxide layer is disposed between the waveguide and the handle layer; and
the buried-oxide layer acts as a cladding layer to the shoulder and the waveguide.
11. The waveguide mode expander of claim 1 , further comprising an interface between the shoulder and the waveguide, wherein the interface forms a plane that is angled with respect to an optical path of the waveguide such that the plane is not orthogonal to an optical path.