Waveguide with polarization volume hologram grating

1. An optical waveguide comprising:

a substrate of optically transparent material comprising two opposing outer surfaces for guiding a beam of light in the substrate by reflections therefrom; and

a polarization volume hologram (PVH) grating disposed in or upon the substrate, the PVH grating comprising a stack of PVH layers, at least one PVH layer of the stack comprising a periodic sequence of helically-birefringent fringes, wherein adjacent PVH layers of the stack have different birefringence;

wherein the PVH grating is configured to:

transmit therethrough light of a first polarization while diffracting light of a second, orthogonal polarization; and

diffract out of the substrate a portion of the beam having the second polarization.

2. The optical waveguide of claim 1 wherein the first and second polarizations are circular polarizations of opposite handedness.

3. The optical waveguide of claim 2 wherein the substrate further comprises an input region for coupling the beam into the substrate, and an output region at a distance from the input region for coupling the beam out of the substrate to form an output beam; wherein the PVH grating is disposed in at least one of the input or output regions.

4. The optical waveguide of claim 3 wherein the PVH grating is disposed in the output region.

5. The optical waveguide of claim 1 wherein the stack of PVH layers comprises a first PVH layer between second and third PVH layers, the first PVH layer having a higher birefringence than the second and third PVH layers.

6. The optical waveguide of claim 5 comprising an anti-reflection (AR) coating at an interface between the PVH grating and the substrate.

7. The optical waveguide of claim 5 wherein the PVH grating extends across the output region along the two opposing outer surfaces.

8. The optical waveguide of claim 7 wherein the PVH grating is configured to have an increasing diffraction efficiency in a direction of propagation of the beam across the output region.

9. The optical waveguide of claim 5 comprising a folding grating disposed in the substrate in an optical path of the beam from the input region to the output region.

10. The optical waveguide of claim 9 comprising a beam splitter disposed in the substrate in the optical path of the beam.

11. The optical waveguide of claim 5 comprising an a-plate retarder disposed in the substrate in an optical path of the beam.

12. A display apparatus comprising:

a display panel; and

an illuminator for the display panel, the illuminator comprising:

a light source for emitting a beam of light; and

an optical waveguide for relaying the beam to illuminate the display panel, the optical waveguide comprising:

a substrate of optically transparent material, the substrate comprising:

an input region for coupling the beam into the substrate, and an output region at a distance from the input region for coupling the beam out of the substrate to form an output beam;

two opposing outer surfaces for guiding the beam in the substrate by reflections therefrom; and

a first polarization volume hologram (PVH) grating comprising a periodic sequence of helically birefringent fringes and configured to diffract light having a first circular polarization while transmitting therethrough light of a second circular polarization of opposite handedness;

wherein the first PVH grating extends along the outer surfaces in the output region to diffract out of the substrate, as the output beam, a portion of the beam incident thereon having the first circular polarization, the first PVH grating comprising a stack of PVH layers, each PVH layer of the stack comprising a periodic sequence of helically birefringent fringes, the stack comprising a middle PVH layer between two outer PVH layers, the middle PVH layer having a greater birefringence than the outer PVH layers.

13. The display apparatus of claim 12 further comprising a projection lens disposed opposite the display panel with the output region of the optical waveguide therebetween, wherein the display panel is configured to reflect at least a portion of the output light toward the projection lens as image light, and wherein the first PVH grating is configured to transmit therethrough the image light to propagate to the projection lens.

14. The display apparatus of claim 13 comprising a steering reflector configured to direct the beam from the light source toward the input region at a variable angle of incidence.

15. The display apparatus of claim 14 comprising a second PVH grating configured to transmit light of one of the first and second circular polarizations and to diffract light of the other of the first and second circular polarizations, the second PVH grating disposed in the input region between the light source and the steering reflector, the light source configured to emit circularly polarized light of the one of the first and second circular polarizations.

16. A method for illuminating a display panel, the method comprising:

using an optical waveguide to propagate a beam of light toward the display panel;

using a polarization volume hologram (PVH) grating comprising a periodic array of helically birefringent fringes and configured to diffract incident light of a first polarization while transmitting incident light of a second, orthogonal polarization, to perform at least one of: coupling the beam into the optical waveguide or out of the optical waveguide; and

using a birefringence-graded stack of PVH layers in the PVH grating, the stack comprising an inner PVH layer between two outer PVH layers of lower birefringence, to lessen leakage of light of the first polarization through the PVH grating.

17. The method of claim 16 , comprising using a first PVH grating in an input region of the optical waveguide to transmit circularly polarized light therethrough from an optical source to a steering reflector, and using a second, graded-birefringence PVH grating disposed in an output region of the optical waveguide to diffract light out of the waveguide toward a reflective display panel, and to transmit light reflected from the reflective display panel toward a projection lens disposed opposite the reflective display panel.