Multistage spot size converter in silicon photonics

1. A device for optical mode spot size conversion to optically couple a semiconductor waveguide with an optical fiber, the device comprising:

a waveguide comprising a waveguide taper region, wherein:

the waveguide taper region comprises 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; and

the waveguide taper region comprises a first material; and

a mode converter coupled to the waveguide, 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 waveguide taper region; and

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

2. The device of claim 1 , wherein:

the mode converter comprises a first stage and a second stage of the plurality of stages, the second stage overlying the first stage;

the first stage comprises 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.

3. The device of claim 2 , wherein the first portion of the first stage of the mode converter and the shoulder portion of the waveguide have a common width.

4. The device of 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 of the first material;

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.

5. The device of claim 4 , wherein the waveguide extension region has a common thickness as the waveguide taper region and a first stage of the mode converter.

6. The device of claim 4 , 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.

7. The device of claim 1 , wherein the number of stages in the mode converter is greater than three and less than eleven.

8. The device of claim 1 , wherein the number of stages in the mode converter is seven.

9. The device of claim 1 , wherein the ridge portion 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.

10. The device of claim 1 , wherein, at an output end of the mode converter, the mode converter has a rectangular cross-section, and the plurality of stages has a common maximum width.

11. The device of claim 1 , wherein:

the mode converter comprises a first stage and a second stage of the number of stages,

the first stage has a first maximum width,

the second stage has a second maximum width, and

the first maximum width of the first stage is greater than the second maximum width of the second stage.

12. The device of claim 1 , wherein:

at a cross section of the mode converter at an output end of the device, 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 portion, has a wider width than a second stage of the plurality of stages, which is farther from the shoulder portion than the first stage.

13. The device of claim 12 , wherein:

the first stage of the mode converter has a first length,

the second stage of the mode converter has a second length, and

the first length is equal to the second length.

14. The device of claim 1 , wherein:

the waveguide taper region comprises crystalline silicon in a 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.

15. A method, for converting optical mode spot size between a semiconductor waveguide and an optical fiber, the method comprising:

receiving a light beam at a waveguide comprising a waveguide taper region, the waveguide taper region comprising 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, the waveguide taper region comprising a first material;

expanding an optical mode of the light beam in the waveguide taper region;

coupling the light beam from the waveguide taper region to a mode converter, 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 waveguide taper region; and

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

expanding the light beam through each of the plurality of stages in the mode converter; and

coupling the light beam to the optical fiber.

16. The method of claim 15 , further comprising transmitting the light beam through a first stage of the mode converter to a second stage, wherein:

the first stage comprises 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.

17. The method of claim 15 , further comprising transmitting the light beam through a waveguide extension region coupled between the waveguide taper region and the mode converter, wherein:

the waveguide extension region is made of the first material;

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 device for optical mode spot size conversion to optically couple a semiconductor waveguide with an optical fiber, the device comprising:

a waveguide comprising a waveguide taper region and a waveguide extension region coupled to the waveguide taper region, wherein:

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

the waveguide taper region is made of a first material;

the waveguide extension region is made of the first material;

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

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 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 device of claim 18 , wherein the ridge portion 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 device 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.