Optical display device
An optical display device ( 600, 700 ) is disclosed. A diffractive optical element ( 618, 718 ) is mounted on a substrate ( 611, 711 ). The diffractive optical element comprises a blazed grating ( 619, 719 ) and a remnant layer ( 617, 717 ) which is formed of the same polymer-based material as the blazed grating. The remnant layer ( 617, 717 ) is positioned adjacent the substrate ( 611, 711 ) and a refractive index mismatch is provided between the two. The depth of the remnant layer is selected so that light diffracted by the grating and light reflected by the interface re-combine and interfere constructively for selected angles of incidence in order to undergo thin-film interference.
1. An optical display device comprising:
a substrate; and
a diffractive optical element comprising a surface relief grating and a remnant layer, wherein the remnant layer is provided between the surface relief grating and the substrate, and the remnant layer has an absolute thickness that is less than 500 nm;
wherein a partially reflective interface is provided between the substrate and the remnant layer, and wherein a thickness of the remnant layer is controlled to enable constructive thin-film interference for light having a predetermined wavelength that is received at the partially reflective interface from the substrate at a predetermined angle of incidence.
2. The optical display device of claim 1 wherein the partially reflective interface is provided between the substrate and the diffractive optical element, and the substrate and the diffractive optical element have respective refractive indices that are mismatched.
3. The optical display device of any of claim 2 wherein the refractive index of the substrate is less than the refractive index of the diffractive optical element.
4. The optical display device of claim 1 wherein the thickness of the remnant layer is controlled to enable thin-film interference for light in a range of wavelengths from a first wavelength to a second wavelength, and wherein an optical thickness of the remnant layer is approximately half of the average of the first and second wavelengths.
5. The optical display device of claim 1 wherein the diffractive optical element is an input diffractive optical element, configured to couple light into the substrate for total internal reflection.
6. The optical display device of claim 5 wherein a reflective coating is provided to the diffractive optical element to improve the efficiency with which light is coupled into the substrate.
7. The optical display device of claim 5 further comprising an output diffractive optical element comprising an output surface relief grating and an output remnant layer, wherein the output remnant layer is provided between the output surface relief grating and the substrate, and wherein an output partially reflective interface is provided between the substrate and the output remnant layer, and wherein a thickness of the output remnant layer is controlled to enable thin-film interference for light having a predetermined wavelength that is received at the output partially reflective interface from the substrate at a predetermined angle of incidence.
8. The optical display device of claim 1 wherein the diffractive optical element is an output diffractive optical element, configured to couple light out of the substrate for viewing by a user.
9. A method of manufacture for an optical display device, the method comprising:
providing a substrate;
providing a diffractive optical element comprising a surface relief grating and a remnant layer, wherein the remnant layer is provided between the surface relief grating and the substrate, and the remnant layer has an absolute thickness that is less than 500 nm;
providing a partially reflective interface between the substrate and the remnant layer; and
controlling a thickness of the remnant layer to enable constructive thin-film interference for light having a predetermined wavelength that is received at the partially reflective interface from the substrate at a predetermined angle of incidence.
10. The method of claim 9 wherein a mould is provided to position the diffractive optical element on the substrate.
11. The method of claim 10 wherein the mould comprises a first portion for creating the surface relief grating and a second portion for creating the remnant layer.
12. The method of claim 10 wherein the mould comprises at least one spacer for positioning the surface relief grating relative to the substrate and for controlling a depth of the remnant layer.
13. The method of claim 9 wherein providing a partially reflective interface comprises providing the substrate and the diffractive optical element with respective refractive indices that are mismatched.