Display device having diffraction gratings with reduced polarization sensitivity
Blazed diffraction gratings provide optical elements in head-mounted display systems to, e.g., incouple light into or out-couple light out of a waveguide. These blazed diffraction gratings may be configured to have reduced polarization sensitivity. Such gratings may, for example, incouple or outcouple light of different polarizations with similar level of efficiency. The blazed diffraction gratings and waveguides may be formed in a high refractive index substrate such as lithium niobate. In some implementations, the blazed diffraction gratings may include diffractive features having a feature height of 40 nm to 120 nm, for example, 80 nm. The diffractive features may be etched into the high index substrate, e.g., lithium niobate.
1. An optical waveguide comprising:
a substrate configured to guide light coupled into the waveguide within the waveguide via total internal reflection; and
a diffraction grating formed in the substrate or in a layer disposed over the substrate, the diffraction grating comprising a plurality of protrusions that each includes a first sidewall and a second sidewall opposite the first sidewall, wherein the first sidewall forms a first tilt angle relative to a surface of the substrate, wherein the second sidewall forms a second tilt angle relative to the surface of the substrate, and wherein the first tilt angle is greater than the second tilt angle,
wherein the diffraction grating has a first diffraction efficiency for light having a first state over a range of angles for light incident thereon and the diffraction grating has a second diffraction efficiency for light having a second state over the range of angles for light incident thereon,
wherein at least one of the first state or the second state is a polarization state, and
wherein the first diffraction efficiency is between 1 and 2 times the second diffraction efficiency.
2. The optical waveguide of claim 1 , wherein the substrate has an index of refraction of approximately at least 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, or up to 2.7 or a value in any range between any of these values.
3. The optical waveguide of claim 1 , wherein the first state is a transverse-magnetic polarization state and the second state is a transverse-electric polarization state, or the second state is a transverse-magnetic polarization state and the first state is a transverse-electric polarization state.
4. The optical waveguide of claim 1 , wherein the first state or the second state is an unpolarized state.
5. The optical waveguide of claim 1 , wherein the diffraction grating comprises diffractive features having a peak height or groove depth of approximately 35 nm, and wherein a ratio of the first diffraction efficiency to the second diffraction efficiency is between approximately 1.5 and approximately 2.0 over the range of angles.
6. The optical waveguide of claim 1 , wherein the diffraction grating comprises diffractive features having a peak height or groove depth of approximately 80 nm, and wherein a ratio of the first diffraction efficiency to the second diffraction efficiency is between approximately 1.0 and approximately 1.3 over the range of angles.
7. The optical waveguide of claim 1 , wherein the substrate comprises one or more of a lithium-based oxide, silicon carbide, zirconium dioxide or titanium dioxide.
8. The optical waveguide of claim 1 , wherein the diffraction grating is formed in the layer disposed over the substrate, and wherein the layer comprises one or more of silicon nitride, zirconium dioxide, titanium dioxide or silicon carbide.
9. The optical waveguide of claim 1 , wherein the diffraction grating is formed in the layer disposed over the substrate, and wherein the layer has a refractive index that is lower than the refractive index of the substrate.
10. The optical waveguide of claim 1 , wherein the diffraction grating comprises diffractive features having a peak height or groove depth of 10 to 150 nm.
11. The optical waveguide of claim 1 , wherein the diffraction grating comprises the plurality of protrusions arranged in a two-dimensional (2D) array.
12. The optical waveguide of claim 11 , wherein the diffraction grating comprises the plurality of protrusions arranged in a square array.
13. The optical waveguide of claim 11 , wherein the diffraction grating comprises the plurality of protrusions arranged in a checkered board pattern.
14. The optical waveguide of claim 1 , wherein the diffraction grating is configured to direct light in two or more directions.
15. The optical waveguide of claim 1 , wherein the diffraction grating comprises one or more of:
an in-coupling optical element disposed so as to receive light from an image source and couple the light into the substrate to be guided therein;
an out-coupling optical element disposed so as to receive light from an image source that is guided in the substrate and couple the light out of the substrate; and
a light distributing optical element disposed so as to receive light from an image source that is guided in the substrate and one or more of:
direct the light to an out-coupling optical element to be coupled out of the substrate; and
spread the light within the waveguide to increase beam size or eye box size.
16. The optical waveguide of claim 1 , wherein the plurality of protrusions are tapered in thickness with height.
17. The optical waveguide of claim 1 , wherein the plurality of protrusions are arranged to direct light preferentially based on the first tilt angle and the second tilt angle.