2D pupil expander using holographic Bragg grating

1. A device comprising:

a waveguide configured to guide light from an input element to an output element;

the input element is configured to couple light from a source into the waveguide; and

the output element is configured to out couple light from the waveguide, wherein:

the output element comprises a first super grating;

the first super grating comprises a first plurality of gratings;

each grating of the first plurality of gratings is defined by a grating vector;

grating vectors of the first plurality of gratings are oriented in a first direction;

the output element comprises a second super grating;

the second super grating comprises a second plurality of gratings;

each grating of the second plurality of gratings is defined by a grating vector;

grating vectors of the second plurality of gratings are oriented in a second direction; and

the first direction is different from the second direction to out couple light from the waveguide.

2. The device of claim 1 , wherein the first super grating is in a grating layer and the second super grating is in the grating layer, such that the first super grating is superimposed with the second super grating.

3. The device of claim 1 , wherein the input element is a prism.

4. The device of claim 1 , wherein the first plurality of gratings and the second plurality of gratings are Bragg gratings.

5. The device of claim 1 , wherein:

the device further comprises a frame configured to support a display for an artificial-reality system, and

the waveguide is part of the display for the artificial-reality system.

6. The device of claim 1 , wherein at least one grating of the first plurality of gratings diffracts red light, at least one grating of the first plurality of gratings diffracts green light, and at least one grating of the first plurality of gratings diffracts blue light.

7. The device of claim 1 , wherein a grating vector of the first plurality of gratings plus a grating vector of the second plurality of gratings sum to zero when projected onto an axis.

8. The device of claim 1 , wherein the first plurality of gratings comprises:

gratings defined by a first set of pitches at a first location, and

gratings defined by a second set of pitches a second location.

9. The device of claim 1 , wherein the first plurality of gratings comprises more than 600 gratings and less than 5000 gratings.

10. A method comprising:

obtaining a waveguide;

exposing the waveguide to electromagnetic radiation to create a first plurality of gratings of an output element, wherein:

the first plurality of gratings are defined by grating vectors, and

the grating vectors of the first plurality of gratings have an orientation in a first direction; and

exposing the waveguide to electromagnetic radiation to create a second plurality of gratings of the output element, wherein:

the second plurality of gratings are defined by grating vectors, and

the grating vectors of the second plurality of gratings have an orientation in a second direction, different from the first direction.

11. The method of claim 10 , further comprising:

exposing a first location of the waveguide to form the first plurality of gratings to have a first set of pitches; and

exposing a second location of the waveguide to form the first plurality of gratings to have a second set of pitches at the second location.

12. The method of claim 10 , wherein the first plurality of gratings is formed by exposing the waveguide a number of times to electromagnetic radiation, and the number is 2000, +/−10%.

13. The method of claim 10 , further comprising forming a prism on the waveguide for an input element.

14. The method of claim 10 , further comprising:

exposing the waveguide to electromagnetic radiation to create a first set of gratings at a first location; and

exposing the waveguide to electromagnetic radiation to create a second set of gratings at a second location, wherein:

the first grating is part of the first set of gratings;

the first set of gratings and the second set of gratings have grating vectors in a direction of the first direction;

at the first location, the first set of gratings includes at least one grating for diffracting red light, at least one grating for diffracting green light, and at least one grating for diffracting blue light; and

at the second location the second set of gratings includes at least one grating for diffracting red light, at least one grating for diffracting green light, and at least one grating for diffracting blue light.

15. The method of claim 10 , wherein the electromagnetic radiation is ultra violet (UV) light.

16. A device comprising:

a waveguide configured to guide light from an input element to an output element;

the input element is configured to couple light from a source into the waveguide; and

the output element is configured to out couple light from the waveguide, the output element comprising a super grating, wherein:

the super grating comprises a plurality of gratings;

the plurality of gratings are oriented in the same direction; and

each grating of the plurality of gratings varies spatially so that the super grating has a first set of pitches at a first location and a second set of pitches at a second location.

17. The device as recited in claim 16 , wherein:

the super grating is a first super grating;

the device comprises a second super grating; and

gratings in the second super grating are oriented in a different direction than the plurality of gratings in the first super grating.

18. The device as recited in claim 17 , wherein the first super grating and the second super grating are configured to out couple light from the waveguide in two dimensions.

19. The device as recited in claim 17 , wherein:

the plurality of gratings of the first super grating are Bragg gratings; and

gratings of the second super grating are Bragg gratings.