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1. (WO2019005823) MILLIMETER SCALE LONG GRATING COUPLER
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CLAIMS

What is Claimed:

1. A millimeter scale weak grating coupler comprising a waveguide having a plurality of bars of overlay material of length (a) disposed periodically at a period (Λ) adjacent the silicon waveguide.

2. The grating coupler of claim 1 , wherein a duty cycle of (aM) is uniform along the top of the waveguide.

3. The grating coupler of claim 1 , wherein a duty cycle of (aM) is varied along the top of the waveguide.

4. The grating coupler of claim 3, wherein the duty cycle increases along the silicon

waveguide as a grating strength decreases.

5. The grating coupler of any one of claims 1-4, further comprising a stop layer disposed between the overlay material and the waveguide.

6. The grating coupler of any one of claims 1-5, wherein a dimension of one or more bars of overlay material along at least one axis is varied across the plurality of bars.

7. The grating coupler of any one of claims 1 -6, wherein the overlay material has an index of refraction that is between an index of refraction of the waveguide and an index of refraction of a cladding material disposed adjacent the waveguide.

8. The grating coupler of any one of claims 1 -7, wherein the overlay material comprises Si3N4.

9. A method of forming a grating coupler comprising:

a. depositing on a wafer a stop layer;

b. depositing a grating layer on the stop layer;

c. patterning desired gratings; and

d. etching, based on the patterning, the grating layer to create the desired gratings, whereby bars of the remaining grating layer of width "w" and length "a" are disposed periodically at a period "Λ" on the wafer.

10. The method of claim 9, wherein a duty cycle of (aM) is uniform along the top of the wafer.

11. The method of claim 9, wherein a duty cycle of (aM) is varied along the top of the wafer.

12. The method of any one of claims 9-11, further comprising patterning and etching a

waveguide from the wafer whereby the duty cycle of aM increases along the waveguide moving away from a light source.

13. The method of any one of claims 9-12, wherein the stop layer comprises AI2O3 or S1O2, or both.

14. The method of any one of claims 9-13, wherein the wafer comprises Silicon On Insulator (SOI) and wherein the grating layer comprises S13N4.

15. The method of any one of claims 9-14, wherein a material forming the stop layer is

selected such that it will not etch during the etching step, effectively stopping the etch from penetrating the waveguide layer.

16. The method of any one of claims 9-14, wherein etch chemistry and process parameters of the etching step are selected such that an etch rate of the stop layer is lower than an etch rate of the grating layer.

17. The method of any one of claims 9-16, further comprising depositing a cladding material on the grating coupler.

18. The method of claim 17, wherein the grating layer has an index of refraction that is between an index of refraction of the wafer and an index of refraction of the cladding material.

19. The method of any one of claims 9-18, further comprising analytically mapping duty cycles of the gratings to a required strength set forth by a predetermined function so as to produce a profile of duty cycles per period for an entire length of the gratings.

20. The method of claim 19, wherein the predetermined function is dependent on an emission intensity profile or phase profile as a function of the direction of propagation.