Multistage spot size converter in silicon photonics

1. A method for manufacturing a device for optical mode spot size conversion, the method comprising:

providing a substrate having a device layer disposed on the substrate, wherein the device layer comprises a first material;

etching the device layer to form a waveguide;

etching the device layer to form a waveguide taper region, wherein:

the waveguide taper region comprises a shoulder portion and a ridge portion, the ridge portion oriented along a direction of beam propagation, the ridge portion having a width that tapers to meet a width of the shoulder portion; and

forming a mode converter coupled with the waveguide taper region, wherein:

the mode converter comprises a plurality of stages;

each of the plurality of stages tapers in a direction similar to a direction of taper of the ridge portion of the waveguide taper region; and

the mode converter is made of a second material different from the first material.

2. The method claim 1 , wherein the ridge portion of the waveguide taper region is disposed directly above the shoulder portion of the waveguide.

3. The method claim 1 , further comprising:

applying a 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 a second material different from the first material;

etching the device layer to define the waveguide, and

etching the second material to form a first stage of the mode converter, such that the first stage of the mode converter is aligned with the waveguide;

covering the substrate with a first cladding;

applying photoresist on the first cladding;

etching the first cladding to form a second recess, the second recess having a shape of a second pattern;

removing photoresist from the first cladding; and

filling the second recess with the second material to forms a second stage of the mode converter.

4. The method claim 3 , wherein:

the waveguide taper region comprises crystalline silicon in the device layer disposed on a buried-oxide layer of an SOI (silicon-on-insulator) substrate; and

the mode converter comprises non-crystalline silicon formed on the buried-oxide layer of the SOI substrate.

5. The method claim 3 , wherein etching the first cladding uses a highly selective etch such that the first cladding is more easily etched than the first material and the second material.

6. The method claim 3 , wherein:

the first stage of the mode converter comprises a first portion that does not taper and a second portion that tapers; and

the second stage of the mode converter extends over the first portion and the second portion of the first stage.

7. The method claim 6 , wherein the first portion of the first stage of the mode converter and the shoulder portion of the waveguide have a common width.

8. The method claim 3 , further comprising:

applying a second cladding on the second stage of the mode converter;

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 second material to form a third stage of the mode converter.

9. The method claim 8 , wherein the number of stages in the mode converter is seven.

10. The method claim 1 , wherein the waveguide further comprises a waveguide extension region coupled between the waveguide taper region and the mode converter, wherein:

the waveguide extension region is made from the device layer;

the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region; and

the waveguide extension region and the shoulder portion of the waveguide taper region have a common width.

11. The method claim 10 , wherein the waveguide extension region has a common thickness as the waveguide taper region and a first stage of the mode converter.

12. The method claim 10 , wherein the waveguide extension region has a common width as an output end of the waveguide taper region and an input end of a first stage of the mode converter.

13. A method for manufacturing a device for optical mode spot size conversion, the method comprising:

providing a substrate having a device layer disposed on the substrate, the device layer comprising a first material;

etching the device layer to form a waveguide and a first stage of a mode converter coupled to the waveguide, wherein the waveguide comprises a waveguide taper region having a shoulder portion and a ridge portion, the ridge portion disposed directly above the shoulder portion and oriented along a direction of beam propagation, the ridge portion having a width that tapers to meet a width of the shoulder portion; depositing a cladding layer over the substrate;

forming a first cladding on the device layer;

applying photoresist on the first cladding;

etching the first cladding to form a first recess, the first recess having a shape of a first pattern;

removing photoresist from the first cladding;

filling the first recess with a second material to form a second stage of the mode converter.

14. The method of claim 13 , wherein the first material is crystalline silicon and the second material is non-crystalline silicon.

15. The method of claim 13 , wherein the device layer comprises crystalline silicon disposed on a buried-oxide layer of an SOI (silicon-on-insulator) substrate.

16. The method of claim 13 , wherein the waveguide further comprises a waveguide extension region coupled between the waveguide taper region and the mode converter.

17. The method of claim 16 , wherein:

the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region; and

the waveguide extension region and the shoulder portion of the waveguide taper region have a common width.

18. A method for manufacturing a device for optical mode spot size conversion, the method comprising:

providing a substrate having a device layer disposed on the substrate, wherein the device layer comprises a first material;

etching the device layer to form a waveguide and a waveguide extension region;

etching the device layer to form a waveguide taper region, the waveguide taper region coupled to the waveguide extension region, wherein:

the waveguide taper region comprises a shoulder portion and a ridge portion, the ridge portion having a width that tapers to meet a width of the shoulder portion; and

the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region; and

the waveguide extension region and the shoulder portion of the waveguide taper region have a common width; and

forming a mode converter coupled to the waveguide extension region, wherein:

the mode converter comprises a plurality of stages;

each of the plurality of stages tapers in a direction similar to a direction of taper of the ridge portion of the waveguide taper region;

the mode converter comprises a first stage and a second stage of the plurality of stages, the first stage comprising a first portion that does not taper and a second portion that tapers; and

the second stage extends over the first portion and the second portion of the first stage; and

the mode converter is made of a second material different from the first material.

19. The method of claim 18 , wherein the ridge portion of the waveguide taper region tapers from a narrower width of 0.75-2.75 μm near an input end to a wider width of 2.0-4.5 μm near an output end.

20. The method of claim 18 , wherein the waveguide extension region has a common thickness as the waveguide taper region and the first stage of the mode converter.