Surface relief grating in an optical waveguide having a reflecting surface and dielectric layer conforming to the surface

1. A display device for displaying an image, the device comprising:

an optical waveguide arrangement for directing image-bearing light, received from an image source over a range of angles relative to an injection axis, through the optical waveguide arrangement for output to form a viewable image; and

an input grating for diffracting into the optical waveguide arrangement the received image-bearing light such that all of the diffracted light is totally internally reflected within the optical waveguide arrangement and so that image bearing light output from the optical waveguide arrangement has a field of view corresponding to said range of angles,

wherein the input grating is a surface relief grating for diffracting light over said range of angles into the optical waveguide arrangement, the input grating having a profiled surface and a layered structure applied thereto comprising a layer of reflective material providing a reflective surface and at least one layer of dielectric material, each of the reflective surface and the at least one layer of dielectric material being conformal in profile with said profiled surface and disposed such that the received image-bearing light interacts with both the at least one layer of dielectric material and the reflective surface, and

wherein the thickness of the at least one layer of dielectric material with which the light interacts is configured to control the angular bandwidth of the input grating such that the image bearing light that is incident upon the input grating over said range of angles is diffracted into the optical waveguide arrangement.

2. The display device as claimed in claim 1 , wherein the at least one layer of dielectric material comprises one or more of titanium dioxide, silicon dioxide or magnesium oxide.

3. The display device as claimed in claim 1 , wherein said profiled surface is formed on a surface of a waveguide of the optical waveguide arrangement, the layered structure comprising the at least one layer of dielectric material deposited on the profiled waveguide surface so as to reproduce the profile thereof and the layer of reflective material deposited onto the at least one layer of dielectric material.

4. The display device as claimed in claim 1 , wherein said profiled surface of the input grating is formed upon a surface of a substrate, the layered structure comprising the layer of reflective material deposited onto the profiled substrate surface so as to reproduce the profile thereof and the at least one layer of dielectric material deposited onto the layer of reflective material, the resulting surface of the input grating provided by the at least one layer of dielectric material being is optically contacted to a surface of a waveguide of the waveguide arrangement.

5. The display device as claimed in claim 3 , wherein the layer of reflective material comprises a metallic or other material having a complex refractive index.

6. The display device as claimed in claim 1 , wherein the optical waveguide arrangement further comprises:

an output grating having a profiled surface and at least one layer of dielectric material conforming to the profiled surface of the output grating for diffracting light, passing through the waveguide arrangement over said range of angles, out of the optical waveguide arrangement for displaying an image with a field of view corresponding to said range of angles.

7. The display device as claimed in claim 1 wherein the display device is one of a head-mounted display, a head-up display or a helmet-mounted display.

8. The display device as claimed in claim 6 , wherein the output grating has an angular bandwidth such that the angular bandwidth of the input grating is matched substantially with, or less than, the angular bandwidth of the output grating.

9. A method for controlling the angular bandwidth of a surface-relief diffraction grating, having a profiled surface, which when applied to a waveguide is arranged to diffract light received over a range of angles into the waveguide such that the received light may propagate through the waveguide by total internal reflection, the method comprising:

applying a layered structure to said profiled surface, the layered structure comprising a layer of reflective material providing a reflective surface and at least one layer of dielectric material, each of the reflective surface and the at least one layer of dielectric material being conformal in profile with said profiled surface and disposed such that the received light interacts with both the at least one layer of dielectric material and the reflective surface; and

selecting the thickness of the at least one layer of dielectric material with which the light interacts thereby to define the angular bandwidth of the diffraction grating such that the light that is incident upon the diffraction grating over said range of angles is diffracted into the waveguide.

10. The method as claimed in claim 9 , further including outputting the light diffracted into the waveguide to a display device, wherein the display device is one of a head-mounted display, a head-up display or a helmet-mounted display.