Spatially multiplexed volume Bragg gratings with varied refractive index modulations for waveguide display

1. A waveguide display comprising:

a substrate;

a first reflection volume Bragg grating (VBG) on the substrate and characterized by a first refractive index modulation; and

a second reflection VBG on the substrate and including a first region and a second region,

wherein the first reflection VBG is configured to diffract display light in a first wavelength range such that the display light in the first wavelength range propagates in the substrate through total internal reflection to the first region and the second region of the second reflection VBG;

wherein the first region of the second reflection VBG is characterized by a second refractive index modulation lower than the first refractive index modulation and is configured to diffract display light in a second wavelength range that is within the first wavelength range; and

wherein the second region of the second reflection VBG is characterized by a third refractive index modulation greater than the second refractive index modulation and is configured to diffract display light in a third wavelength range that includes and is larger than the second wavelength range.

2. The waveguide display of claim 1 , wherein the first region and the second region of the second reflection VBG are arranged such that the display light in the first wavelength range reaches the first region before reaching the second region.

3. The waveguide display of claim 1 , wherein:

the third refractive index modulation is equal to or less than the first refractive index modulation; and

the first wavelength range is the same as or includes the third wavelength range.

4. The waveguide display of claim 1 , wherein the first reflection VBG and the second reflection VBG have a same grating vector in a plane perpendicular to a surface normal direction of the substrate.

5. The waveguide display of claim 1 , wherein the second reflection VBG further includes a third region between the first region and the second region, the third region characterized by a fourth refractive index modulation greater than the second refractive index modulation but lower than the third refractive index modulation and configured to diffract display light in a fourth wavelength range that includes the second wavelength range and is within the third wavelength range.

6. The waveguide display of claim 1 , further comprising:

a third reflection VBG multiplexed with the first reflection VBG and characterized by a fourth refractive index modulation; and

a fourth reflection VBG multiplexed with the second reflection VBG in the first region and second region,

wherein the third reflection VBG is configured to diffract display light in a fourth wavelength range such that the display light in the fourth wavelength range propagates in the substrate through total internal reflection to the first region and the second region of the fourth reflection VBG;

wherein the first region of the fourth reflection VBG is characterized by a fifth refractive index modulation lower than the fourth refractive index modulation and is configured to diffract display light in a fifth wavelength range that is within the fourth wavelength range; and

wherein the second region of the fourth reflection VBG is characterized by a sixth refractive index modulation greater than the fifth refractive index modulation and is configured to diffract display light in a sixth wavelength range that includes and is larger than the fifth wavelength range.

7. The waveguide display of claim 1 , wherein the first reflection VBG is configured to:

diffract display light in the first wavelength range and a first field of view (FOV) range; and

diffract display light in a fourth wavelength range and a second FOV range different from the first FOV range.

8. The waveguide display of claim 1 , wherein:

the substrate is transparent to visible light; and

the second reflection VBG is transparent to visible light from an ambient environment.

9. The waveguide display of claim 1 , wherein:

the first reflection VBG is configured to couple the display light in the first wavelength range into the substrate; and

the second reflection VBG is configured to couple the display light in the first wavelength range out of the substrate.

10. The waveguide display of claim 9 , further comprising a third grating and a fourth grating, wherein:

the third grating is configured to diffract the display light in the first wavelength range from the first reflection VBG to the fourth grating; and

the fourth grating is configured to diffract the display light in the first wavelength range at two or more regions of the fourth grating to the second reflection VBG.

11. The waveguide display of claim 10 , wherein the third grating and the fourth grating have a same grating vector in a plane perpendicular to a surface normal direction of the substrate.

12. The waveguide display of claim 1 , further comprising:

an input coupler configured to couple the display light in the first wavelength range into the substrate; and

an output coupler configured to couple the display light diffracted by the second reflection VBG out of the substrate.

13. The waveguide display of claim 12 , wherein the input coupler and the output coupler include multiplexed VBGs.

14. The waveguide display of claim 1 , further comprising:

a light source configured to generate the display light; and

projector optics configured to collimate the display light and direct the display light to the first reflection VBG.