Anisotropic diffraction grating and waveguide
A diffraction grating includes a substrate and a plurality of fringes supported by the substrate. The fringes run parallel to each other in a first direction. A refractive index of a material of the plurality of fringes is anisotropic, whereby a refractive index contrast of the diffraction grating depends on direction of electric field of an impinging light beam, and through that dependence is a function of an azimuthal angle of the impinging light beam. A dependence of the diffraction efficiency on the azimuthal angle is affected by the dependence of the refractive index contrast on the direction of electric field of an impinging light beam. A pupil-replicating waveguide may use such a diffraction grating as a coupler for in- our out-coupling image light.
1. A diffraction grating comprising a substrate and a plurality of fringes supported by the substrate and running parallel to each other in a first direction, wherein:
a refractive index of a material of the plurality of fringes is anisotropic, whereby a refractive index contrast of the diffraction grating is anisotropic; and
the material comprises a uniaxial birefringent material having an optic axis parallel to the substrate.
2. The diffraction grating of claim 1 , wherein fringes of the plurality of fringes extend at an acute angle to the substrate.
3. The diffraction grating of claim 1 , wherein the substrate is transparent for visible light.
4. A diffraction grating comprising a substrate and a plurality of fringes supported by the substrate and running parallel to each other in a first direction, wherein:
a refractive index of a material of the plurality of fringes is anisotropic, whereby a refractive index contrast of the diffraction grating is anisotropic; and
the material comprises a uniaxial birefringent material having an optic axis forming an acute angle with the first direction.
5. A pupil-replicating waveguide comprising:
a substrate for propagating image light therein; and
a diffraction grating coupler supported by the substrate for in-coupling or out-coupling the image light, the diffraction grating coupler comprising a plurality of fringes running parallel to each other in a first direction, wherein:
a refractive index of a material of the plurality of fringes is anisotropic, whereby a refractive index contrast of the diffraction grating coupler is anisotropic; and
the material comprises a birefringent material having an optic axis forming an acute angle with the first direction.
6. The pupil-replicating waveguide of claim 5 , wherein the material is at least one of a liquid crystal material or a birefringent crystal.
7. The pupil-replicating waveguide of claim 5 , wherein the material comprises nematic liquid crystals.
8. The pupil-replicating waveguide of claim 5 , wherein the material comprises a birefringent crystal selected from a group consisting of lithium niobate, silicon carbide, or rutile.
9. The pupil-replicating waveguide of claim 5 , wherein fringes of the plurality of fringes extend from the substrate at an acute angle thereto.
10. The pupil-replicating waveguide of claim 9 , further comprising a layer filling gaps between the fringes of the plurality of fringes.
11. A pupil-replicating waveguide comprising:
a substrate for propagating image light therein; and
a diffraction grating coupler supported by the substrate for in-coupling or out-coupling the image light, the diffraction grating coupler comprising a plurality of fringes running parallel to each other in a first direction, wherein:
a refractive index of a material of the plurality of fringes is anisotropic, whereby a refractive index contrast of the diffraction grating coupler is anisotropic; and
the material comprises a birefringent material having an optic axis parallel to the substrate.
12. A near-eye display (NED) comprising:
an image source for providing image light carrying an image in angular domain;
a pupil-replicating waveguide optically coupled to the image source and configured to provide the image light to an eyebox of the NED, the pupil-replicating waveguide comprising:
a substrate for propagating the image light therein; and
a diffraction grating coupler supported by the substrate for in-coupling or out-coupling the image light, the diffraction grating coupler comprising a plurality of fringes running parallel to each other in a first direction, wherein:
a refractive index of a material of the plurality of fringes is anisotropic, whereby a refractive index contrast of the diffraction grating coupler is anisotropic; and
the material comprises a uniaxial birefringent material having an optic axis forming an acute angle with the first direction.
13. The NED of claim 12 , wherein fringes of the plurality of fringes extend from the substrate at an acute angle thereto.
14. The NED of claim 12 , wherein the pupil-replicating waveguide further comprises a layer filling gaps between the fringes of the plurality of fringes.