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 method for manufacturing a waveguide mode expander, the method comprising:
providing a substrate having a device layer disposed on the substrate;
applying photoresist on the device layer;
etching the device layer to form a first recess, the first recess having a shape of a first pattern;
removing photoresist from the device layer;
filling the first recess with non-crystalline silicon to form a shoulder;
etching the device layer to define a waveguide;
etching the shoulder to align with the waveguide;
covering the shoulder with cladding;
applying photoresist on the cladding;
etching the cladding to form a second recess, the second recess having a shape of a second pattern;
removing photoresist from the cladding; and
filling the second recess with non-crystalline silicon, wherein:
the non-crystalline silicon forms a ridge of the waveguide mode expander;
the shoulder is between the substrate and the ridge; and
the ridge has a narrower width than the shoulder.
2. The method for manufacturing a waveguide mode expander of claim 1 , wherein etching the cladding uses a highly selective etch such that the cladding is more easily etched than the shoulder.
3. The method for manufacturing a waveguide mode expander of claim 1 , wherein the second pattern comprises a triangle taper.
4. The method for manufacturing a waveguide mode expander of claim 1 , wherein the second pattern comprises a parabolic taper.
5. The method for manufacturing a waveguide mode expander of claim 1 , further comprising:
applying, wherein the cladding is a first cladding, a second cladding on both the first cladding and the non-crystalline silicon;
etching the second cladding to form a third recess, the third recess having a shape of a third pattern; and
filling the third recess with additional non-crystalline silicon to form a second stage of the ridge, wherein filling the second recess formed a first stage of the ridge.
6. The method for manufacturing a waveguide mode expander of claim 5 , wherein:
the first stage is wider than the second stage; and
the second stage is thicker than the first stage.
7. The method for manufacturing a waveguide mode expander of claim 5 , further comprising:
applying, a third cladding the second cladding;
etching the third cladding to form a fourth recess, the fourth recess having a shape of a fourth pattern; and
filling the fourth recess with additional non-crystalline silicon to form a third stage of the ridge.
8. The method for manufacturing a waveguide mode expander of claim 1 , wherein filling the first recess with non-crystalline silicon comprises filling the first recess with amorphous silicon.
9. The method for manufacturing a waveguide mode expander of claim 8 , the method further comprising converting the amorphous silicon to polycrystalline silicon.