Refractive coating for diffractive optical elements
A waveguide configured for use with a near-eye display (NED) device can include a light-transmissive substrate configured to propagate light rays through total internal reflection and a diffractive optical element (DOE) on a surface of the substrate that is configured to input and/or output light rays to and/or from the substrate. According to some embodiments the DOE can include a diffraction grating made of first material having a first refractive index and a coating of a second material over the diffraction grating, the second material having a second refractive index that is not equal to the first refractive index.
1. An optical waveguide, comprising:
a light-transmissive substrate configured for use in a near-eye display (NED) device, the substrate including, a plurality of internally reflective surfaces, the substrate made of a first material having a first refractive index; and
a diffractive optical element (DOE) formed on a first surface of the plurality of surfaces of the substrate, the DOE configured to input light rays to the substrate or output light rays from the substrate, the DOE including:
a diffraction grating made of a second material having a second refractive index; and
a coating over the diffraction grating made of a third material having a third refractive index;
wherein the first refractive index is greater than the second refractive index and less than the third refractive index, and the difference between the second refractive index and the third refractive index is at least 0.4.
2. The optical waveguide of claim 1 , wherein the third refractive index is greater than the second refractive index.
3. The optical waveguide of claim 1 , wherein the coating decreases the sensitivity of the DOE to the wavelength and the angle of incidence of diffracted light rays.
4. The optical waveguide of claim 1 , wherein the coating, increases uniformity in an image propagated via the DOE across a particular field of view (FOV).
5. The optical waveguide of claim 1 , wherein the DOE has a diffraction efficiency of at least 30% over a 30-degree range of angle of incidence for diffracted light rays.
6. The optical waveguide of claim 1 wherein the diffraction grating is optimized to maximize diffraction efficiency over a particular range of angles of incidence of diffracted light rays.
7. The optical waveguide of claim 1 , wherein the DOE is an in-coupling element configured to input light rays received from an image generator of the NED device into the substrate.
8. The optical waveguide of claim 1 , wherein the DOE is an out-coupling element configured to output light rays from the substrate and to direct the light rays towards an eye of a user of the NED device.
9. The optical waveguide of claim 1 , wherein the diffraction grating is embedded in the coating.
10. The optical waveguide of claim 1 wherein the diffraction grating has been applied to the substrate by a curing process.
11. The optical waveguide of claim 1 , wherein the diffraction grating includes grating structures oriented at an oblique slant angle relative to the first surface of the substrate.
12. The optical waveguide of claim 1 , wherein the first material comprises glass.
13. The optical waveguide of claim 1 , wherein the second material comprises a UV-curable resin.
14. The optical waveguide of claim 1 , wherein the third refractive index is at least 1.9.
15. A near-eye display device comprising:
a light-emitting microdisplay imager configured to emit light rays associated with a generated image; and
a waveguide including:
a light-transmissive substrate configured to propagate light through internal reflection, the substrate made of a first material having a first refractive index;
an in-coupling element configured to input light rays received from the microdisplay imager into the substrate; and
an out-coupling element configured to output propagated light rays from the substrate and direct the light rays to an eye of a user of the near-eye display device;
wherein both the in-coupling element and out-coupling element include a diffractive optical element (DOE) formed on at least a portion of at least one of a plurality of surfaces of the substrate, the DOE including:
a diffraction grating made of a second material having a second refractive index; and
coating over the diffraction grating made of a third material having a third refractive index;
wherein first refractive index is greater than the second refractive index and less than the third refractive index, and the difference between the second refractive index and the third refractive index is at least 0.4.
16. A method of manufacturing a waveguide display, comprising:
forming a layer of a second material on a surface of a light-transmissive substrate made of a first material;
curing the second material to form a diffraction grating on the substrate, the diffraction grating configured to input light rays to the substrate or output light rays from the substrate; and
coating the diffraction grating with a third material;
wherein the third material has a refractive index that is greater than a refractive index of the second material after the second material has been cured, the refractive index of the first material is greater than a refractive index of the second material, and the difference between the refractive index of the second material and the refractive index of the third material is at least 0.4.
17. The method of claim 16 , wherein coating includes applying the third material to a surface of the diffraction grating using one or more of atomic layer deposition (ALD), chemical vapor deposition (CVD), spin coating, or dip coating.
18. The method of claim 16 , wherein:
the first material comprises glass;
the second material comprises UV-curable polymer resin; and
the third material has a refractive index of at least 1.9.