Waveguide including volume Bragg gratings

1. A waveguide for conveying image light, the waveguide comprising:

a first input port for receiving a first beam of image light carrying a first color channel of an image in a first wavelength band;

opposed first and second optical surfaces for propagating the first beam therebetween; and

first and second diffraction gratings disposed in the waveguide between the first and second optical surfaces and offset laterally from each other;

wherein the first diffraction grating comprises a plurality of volume Bragg gratings (VBGs) configured to expand the first beam along a first axis and to redirect the first beam towards the second diffraction grating; and

wherein the second diffraction grating comprises a plurality of VBGs configured to receive the first beam from the first diffraction grating and to out-couple portions of the first beam at a plurality of locations along a second axis, thereby expanding the first beam along the second axis for observation of the image by a user.

2. The waveguide of claim 1 , wherein projections of the first and second diffraction gratings onto the first optical surface are non-overlapping.

3. The waveguide of claim 1 , wherein the first diffraction grating comprises between 300 and 1000 VBGs, and wherein the second diffraction grating comprises between 10 and 200 VBGs.

4. The waveguide of claim 1 , wherein in operation, different portions of the first wavelength band of the first beam are out-coupled at different locations of the plurality of locations along the second axis.

5. The waveguide of claim 1 , further comprising:

a second input port for receiving a second beam of image light carrying a second color channel of the image in a second wavelength band;

a third diffraction grating disposed in the waveguide between the first and second optical surfaces and offset laterally from the first and second diffraction gratings;

wherein the third diffraction grating comprises a plurality of VBGs configured to expand the second beam along the first axis and to redirect the second beam towards the second diffraction grating; and

wherein the VBGs of the second diffraction grating are configured to receive the second beam from the third diffraction grating and to out-couple portions of the second beam at a plurality of locations along the second axis, thereby expanding the second beam along the second axis for observation of the image by the user.

6. The waveguide of claim 5 , further comprising:

a third input port for receiving a third beam of image light carrying a third color channel of the image in a third wavelength band;

a fourth diffraction grating disposed in the waveguide between the first and second optical surfaces and offset laterally from the first to third diffraction gratings;

wherein the fourth diffraction grating comprises a plurality of VBGs configured to expand the third beam along the first axis and to redirect the third beam towards the second diffraction grating; and

wherein the VBGs of the second diffraction grating are configured to receive the third beam from the fourth diffraction grating and to out-couple portions of the third beam at a plurality of locations along the second axis, thereby expanding the third beam along the second axis for observation of the image by the user.

7. The waveguide of claim 6 , wherein the first, second, and third input ports are offset from each other along the second axis.

8. The waveguide of claim 6 , wherein in operation, different portions of the first, second, and third wavelength bands of the first, second, and third beams respectively are out-coupled at different locations along the second axis.

9. The waveguide of claim 1 , wherein the VBGs of the first and second diffraction gratings are disposed in a same layer spaced apart from the first and second optical surfaces.

10. The waveguide of claim 1 , wherein the VBGs of the first diffraction grating have grating periods spatially varying along the first axis.

11. The waveguide of claim 10 , wherein the grating periods of the VBGs of the first diffraction grating are varying within a range of 100 nm to 500 nm.

12. The waveguide of claim 1 , wherein the VBGs of the second diffraction grating have grating periods spatially varying along the second axis.

13. The waveguide of claim 12 , wherein the grating periods of the VBGs of the second diffraction grating are varying within a range of 100 nm to 300 nm.

14. The waveguide of claim 1 , wherein the VBGs of the first diffraction grating are configured to redirect the first and second beams of image light by reflective diffraction.

15. The waveguide of claim 14 , wherein the VBGs of the first diffraction grating comprise a plurality of fringes forming an angle with the first optical surface of between 34 degrees and 54 degrees.

16. The waveguide of claim 15 , wherein the VBGs of the second diffraction grating comprise a plurality of fringes forming an angle with the first optical surface of between 20 degrees and 38 degrees.

17. The waveguide of claim 1 , wherein the VBGs of the first diffraction grating are configured to redirect the first and second beams of image light by transmissive diffraction.

18. The waveguide of claim 17 , wherein the VBGs of the first diffraction grating comprise a plurality of fringes forming an angle with the first optical surface of greater than 80 degrees.

19. The waveguide of claim 18 , wherein the VBGs of the second diffraction grating comprise a plurality of fringes forming an angle with the second optical surface of between 50 degrees and 70 degrees.

20. The waveguide of claim 1 , wherein the image has a field of view (FOV), wherein the first beam of image light carries a first portion of the FOV of the image, the waveguide further comprising a second input port for receiving a second beam of image light carrying a second portion of the FOV of the image in the first wavelength band;

wherein the VBGs of the first diffraction grating are configured to expand the second beam along the first axis and to redirect the second beam towards the second diffraction grating; and

wherein the VBGs of the second diffraction grating are configured to receive the second beam from the first diffraction grating and to out-couple portions of the second beam at different locations along the second axis, thereby expanding the second beam along the second axis for observation of the image by the user.