Aperture multiplier with depolarizer
An optical aperture multiplier includes a first optical waveguide ( 10 ) having a rectangular cross-section and including partially reflecting surfaces ( 40 ) at an oblique angle to a direction of elongation of the waveguide. A second optical waveguide ( 20 ), also including partially reflecting surfaces ( 45 ) at an oblique angle, is optically coupled with the first optical waveguide ( 10 ). An image coupled into the first optical waveguide with an initial direction of propagation at an oblique coupling angle advances by four-fold internal reflection along the first optical waveguide, with a proportion of intensity of the image reflected at the partially reflecting surfaces so as to be coupled into the second optical waveguide, and then propagates through two-fold reflection within the second optical waveguide, with a proportion of intensity of the image reflected at the partially reflecting surfaces so as to be directed outwards from one of the parallel faces as a visible image.
1. An optical arrangement comprising:
(a) an image projector configured to project image illumination corresponding to a collimated image via an output surface, said image projector comprising at least one polarizing element such that an entirety of said image illumination propagating from said at least one polarizing element towards said output surface is polarized with a single polarization;
(b) an optical aperture expansion arrangement comprising at least one waveguide having at least one pair of parallel external faces, said image illumination propagating within said at least one waveguide by internal reflection at said at least one pair of external faces, said optical aperture expansion arrangement including a plurality of mutually-parallel partially-reflective surfaces within said at least one waveguide, said plurality of partially-reflective surfaces being deployed within said waveguide and angled at an oblique angle to said at least one pair of external faces, said image illumination being progressively redirected outwards from a coupling-out region of the optical aperture expansion arrangement towards a viewer; and
(c) a depolarizer deployed in a path of said image illumination after said at least one polarizing element of said image projector and before said at least one waveguide.
2. The optical arrangement of claim 1 , wherein said depolarizer is a passive depolarizer.
3. The optical arrangement of claim 1 , wherein said depolarizer comprises at least one layer of birefringent material deployed in a path of said image illumination.
4. The optical arrangement of claim 1 , wherein said depolarizer is a depolarizer selected from a group consisting of: a Cornu depolarizer; a Lyot depolarizer; and a wedge depolarizer.
5. The optical arrangement of claim 1 , wherein said depolarizer is an active depolarizer.
6. The optical arrangement of claim 1 , wherein said depolarizer includes at least one liquid crystal element switchable between at least two polarization-rotating states.
7. The optical arrangement of claim 6 , wherein said image projector projects a video image with a frame rate, and wherein the depolarizer alters a state of said at least one liquid crystal element at a cycle rate that is greater than said frame rate.
8. The optical arrangement of claim 1 , wherein said plurality of mutually-parallel partially-reflective surfaces is located at said coupling-out region and provides a coupling-out arrangement that progressively deflects the image illumination towards the viewer.
9. The optical arrangement of claim 1 , further comprising at least one diffractive optical element associated with said coupling-out region of said at least one waveguide.
10. The optical arrangement of claim 1 , wherein said optical aperture expansion arrangement comprises two waveguide portions, wherein said coupling-out arrangement is associated with a second of said two waveguide portions, and wherein a first of said two waveguide portions includes an image redirecting arrangement configured to redirect said image illumination from a first direction of propagation within said first waveguide portion to a second direction of propagation within said second waveguide portion.
11. The optical arrangement of claim 10 , wherein said first waveguide portion has two pairs of parallel external faces.
12. The optical arrangement of claim 1 , wherein said at least one polarizing element of said image projector comprises at least one polarizing beam splitter.