Outward coupling suppression in waveguide display
A pupil replication waveguide for a projector display includes a slab of transparent material for propagating display light in the slab via total internal reflection. A diffraction grating is supported by the slab. The diffraction grating includes a plurality of tapered slanted fringes in a substrate for out-coupling the display light from the slab by diffraction into a blazed diffraction order. A greater portion of the display light is out-coupled into the blazed diffraction order, and a smaller portion of the display light is out-coupled into a non-blazed diffraction order. The tapered fringes result in the duty cycle of the diffraction grating varying along the thickness direction of the diffraction grating, to facilitate suppressing the portion of the display light out-coupled into the non-blazed diffraction order.
1. A pupil replication waveguide comprising:
a slab of transparent material for propagating display light therein via total internal reflection; and
a diffraction grating supported by the slab and comprising a plurality of fringes formed by a twisted nematic (TN) liquid crystal (LC) material in a polymer substrate, wherein the fringes are slanted for out-coupling the display light from the slab by diffraction into a blazed diffraction order, wherein a greater portion of the display light is out-coupled into the blazed diffraction order, and a smaller portion of the display light is out-coupled into a non-blazed diffraction order;
wherein the TN LC material has an ordinary refractive index n O for light polarized perpendicular to molecules of the TN LC material, and an extraordinary refractive index n E for light polarized parallel to the molecules of the TN LC material, wherein a refractive index contrast for polarized display light impinging onto the diffraction grating has a refractive index contrast profile along a thickness direction of the diffraction grating;
wherein a refractive index contrast is larger at a middle than at both sides of the refractive index contrast profile, whereby a portion of the impinging polarized display light out-coupled into the non-blazed diffraction order is lessened.
2. The pupil replication waveguide of claim 1 , wherein a refractive index material of the polymer substrate is closer to n O than to n E , whereby the refractive index contrast is larger at a middle than at both sides of the refractive index contrast profile.
3. The pupil replication waveguide of claim 1 , wherein the refractive index contrast profile is symmetrical along the thickness direction of the diffraction grating.
4. The pupil replication waveguide of claim 1 , wherein the refractive index contrast profile is substantially zero at both sides.
5. The pupil replication waveguide of claim 1 , wherein the TN LC material is polymer-stabilized.
6. The pupil replication waveguide of claim 1 , wherein the refractive index contrast profile is a smoothly varying function.
7. The pupil replication waveguide of claim 6 , wherein the smoothly varying function is a Gaussian function.
8. The pupil replication waveguide of claim 1 , further comprising an input grating for in-coupling the display light into the slab.
9. The pupil replication waveguide of claim 1 , further comprising an output grating for out-coupling the display light from the slab.
10. A near-eye display (NED) comprising:
a light source for providing a plurality of light beams;
a collimator optically coupled to the light source for collimating the plurality of light beams;
a scanner optically coupled to the collimator for scanning the plurality of light beams collimated by the collimator; and
a pupil replication waveguide optically coupled to the scanner for receiving and providing, at an eyebox of the NED, multiple laterally offset copies of the light beams scanned by the scanner, the pupil replication waveguide comprising:
a slab of transparent material for propagating display light therein via total internal reflection; and
a diffraction grating supported by the slab and comprising a plurality of fringes formed by a twisted nematic (TN) liquid crystal (LC) material in a polymer substrate, wherein the fringes are slanted for out-coupling the display light from the slab by diffraction into a blazed diffraction order, wherein a greater portion of the display light is out-coupled into the blazed diffraction order, and a smaller portion of the display light is out-coupled into a non-blazed diffraction order;
wherein the TN LC material has an ordinary refractive index n O for light polarized perpendicular to molecules of the TN LC material, and an extraordinary refractive index n E for light polarized parallel to the molecules of the TN LC material, wherein a refractive index contrast for polarized display light impinging onto the diffraction grating has a refractive index contrast profile along a thickness direction of the diffraction grating; and
wherein a refractive index contrast is larger at a middle than at both sides of the refractive index contrast profile.
11. The NED of claim 10 , wherein a refractive index material of the polymer substrate is closer to n O than to n E , whereby the refractive index contrast is larger at a middle than at both sides of the refractive index contrast profile.
12. The NED of claim 10 , wherein the refractive index contrast profile is symmetrical along the thickness direction of the diffraction grating.
13. The NED of claim 10 , wherein the refractive index contrast profile is substantially zero at both sides.
14. The NED of claim 10 , wherein the TN LC material is polymer-stabilized.
15. The NED of claim 10 , wherein the refractive index contrast profile is a smoothly varying function.
16. The NED of claim 15 , wherein the smoothly varying function is a Gaussian function.
17. The NED of claim 10 , further comprising an input grating for in-coupling the display light into the slab.
18. The NED of claim 10 , further comprising an output grating for out-coupling the display light from the slab.
19. The NED of claim 10 , wherein the scanner comprises a 2D microelectromechanical system (MEMS) scanner.