Anisotropically formed diffraction grating device
A diffraction grating includes a substrate and an array of triangular ridges extending from the substrate. The ridges run parallel to one another and have triangular cross-sections such that first sides of the ridges face in a first direction and adjacent second sides of the ridges face in a second, different direction. An array of grating lines is disposed over the first sides of the array of ridges, each grating line of the array of grating lines comprising a slab of transparent material supported by the first side of a corresponding ridge of the array of ridges. A refractive index of the array of grating lines is different from a refractive index of the array of ridges.
1. A diffraction grating comprising:
a substrate;
an array of ridges extending from the substrate, the ridges running parallel to one another in a plane parallel to a plane of the substrate and having triangular cross-sections, such that first sides of the ridges face in a first direction and adjacent second sides of the ridges face in a second, different direction; and
an array of grating lines having a different refractive index than the array of ridges, each grating line of the array of grating lines comprising a slab of transparent material supported by the first side of a corresponding ridge of the array of ridges, wherein the array of grating lines has a duty cycle of between 0.01 and 0.3 or between 0.7 and 0.99.
2. The diffraction grating of claim 1 , wherein no grating lines are present on the second sides of the ridges.
3. The diffraction grating of claim 1 , wherein at least one of the substrate or the array of ridges comprises a different material than the array of grating lines.
4. The diffraction grating of claim 1 , wherein the array of grating lines has a spatially variant duty cycle.
5. The diffraction grating of claim 1 , further comprising an overcoat layer over the array of grating lines, the overcoat layer having a substantially flat top surface.
6. The diffraction grating of claim 5 , wherein a difference between a refractive index of the overcoat layer and a refractive index of the array of ridges is less than 0.05.
7. The diffraction grating of claim 5 , wherein the overcoat layer and the array of ridges are made of a same material, thereby forming a transparent matrix supporting the array of grating lines over the substrate, such that each grating line is disposed at an angle to the substrate.
8. The diffraction grating of claim 1 , wherein vertical projections of neighboring grating lines of the array of grating lines onto the substrate overlap each other.
9. The diffraction grating of claim 1 , wherein each grating line slab of the array of grating lines covers only a portion of a corresponding first side of the array of ridges.
10. A pupil-replicating waveguide comprising:
a substrate;
an array of ridges extending from the substrate, the ridges running parallel to one another in a plane parallel to a plane of the substrate and having triangular cross-sections, such that first sides of the ridges face in a first direction and adjacent second sides of the ridges face in a second, different direction;
an array of grating lines of a different material than the array of ridges, each grating line of the array of grating lines comprising a slab of transparent material supported by the first side of a corresponding ridge of the array of ridges; and
an overcoat layer over the array of grating lines, the overcoat layer having a substantially flat top surface;
wherein the substrate comprises opposed first and second surfaces, wherein the array of ridges extends from the first surface, and wherein the substrate is configured to guide display light by total internal refection from the second surface and the top surface of the overcoat layer;
wherein at least one of a pitch, height, slant angle, or a refractive index of the array of grating lines is selected to output portions of the display light at a plurality of offset locations along an optical path of the display light, wherein the array of grating lines has a duty cycle of between 0.01 and 0.3 or between 0.7 and 0.99.
11. The pupil-replicating waveguide of claim 10 , wherein at least one of the pitch, the duty cycle, the height, the slant angle, or the refractive index of the array of grating lines is selected to lessen non-zero-order diffraction of non-display external light to an eyebox of the pupil-replicating waveguide.