Optical waveguide and display device
In a slab waveguide which expands an image-bearing pupil into a visible image, reflective edge surfaces are used to redirect once-diffracted light back through the same grating structure. The number of separate grating structures thereby can be reduced to two or even one, compared to three in the prior art.
1. An optical waveguide in which in operation light propagates by total internal reflection, comprising:
two parallel surfaces between which light propagates within the waveguide parallel to those surfaces by total internal reflection;
a grating structure having three diffraction regions, a first diffraction region being for diffracting image-bearing light into the waveguide, a second diffraction region being for further diffracting the image-bearing light so as to expand it in a first dimension and a third diffraction region being for further diffracting the image-bearing light so as to expand it in a second dimension and for releasing it from the waveguide as a visible image, wherein at least two of said diffraction regions have aligned gratings; and
at least one reflecting edge surface which reflects light received from one of said at least two diffraction regions back to the other of said at least two diffraction regions for a further diffraction, the reflecting edge surface being perpendicular to the two parallel surfaces of the waveguide.
2. The waveguide of claim 1 wherein the reflecting surface is a specular reflecting surface.
3. The waveguide of claim 1 wherein the reflecting surface is an edge surface of the waveguide.
4. The waveguide of claim 1 wherein the reflecting surface is embedded within the waveguide.
5. The waveguide of claim 1 wherein the reflecting surface has an optical coating whereby to control the reflectance of the surface and/or to control the planarization of light reflected therefrom.
6. The waveguide of claim 1 wherein a said reflecting surface is positioned to receive image-bearing light which has been expanded in the first dimension by diffraction by the second diffraction region, and to reflect it back to the grating structure for further diffraction to expand it in the second dimension by the third diffraction region.
7. The waveguide of claim 6 wherein the grating structure comprises identically-oriented diffraction regions which respectively diffract the light to expand it in the first and second dimensions.
8. The waveguide of claim 7 wherein the identically oriented gratings are parts of a single grating.
9. The waveguide of claim 6 wherein the first and second diffraction regions comprise identically oriented gratings which respectively diffract the light into the waveguide and expand it in the first dimension.
10. The waveguide of claim 9 wherein the grating structure comprises a further grating for diffracting the light into the waveguide.
11. The waveguide of claim 1 wherein a said reflecting surface is positioned to receive light which has been diffracted into the waveguide by the first diffraction region and to reflect it back to the grating structure for diffraction to expand it in the first dimension by the second diffraction region.
12. The waveguide of claim 11 wherein the grating structure comprises a further grating for expanding the light in the second dimension and releasing it from the waveguide as a visible image.
13. A helmet-mounted, head-mounted, head-up or other display device comprising the optical waveguide of claim 1 .