Exit pupil expanding diffractive optical waveguiding device
An optical device is disclosed for expanding input light in two dimensions in an augmented reality display. The device comprises a waveguide ( 12 ) and three linear diffraction grat-ings H 0 , H 1 , H 2 . An incident beam from a projector illuminates an input grating H 0 with polychromatic light, and the light is coupled into the waveguide ( 12 ). The other two gratings H 1 , H 2 are overlaid on top of one another. Light can be diffracted by one grating H 1 into a first diffracted order and towards the other grating H 2 which can couple the light out of the waveguide ( 12 ) towards a viewer. In another arrangement the crossed gratings H 1 , H 2 may be replaced by a photonic crystal ( 19 ) having a regular array of pillars ( 20 ) which create a number effective diffraction gratings.
1. An optical device for expanding input light in two dimensions in an augmented reality display, comprising:
a waveguide;
an input diffractive optical element configured to couple input light into the waveguide; and
two diffractive optical elements that are crossed relative to one another so that their grating vectors extend in different directions and that are entirely overlaid on one another in or on the waveguide, wherein each of the two diffractive optical elements is configured to receive light from the input diffractive optical element and couple it towards the other diffractive optical element which can then act as an output diffractive optical element providing outcoupled orders towards a viewer, and wherein the two diffractive optical elements are configured to diffract light received from the input diffractive optical element in different respective directions, since their grating vectors extend in the different directions.
2. The optical device of claim 1 wherein each diffractive optical element comprises grooves and a grating vector in the plane of the grooves, having a direction that is normal to the grooves and a magnitude which is inversely related to the pitch of the grooves, wherein the input and output diffractive optical elements respectively have grating vectors with a substantially equal magnitude.
3. The optical device of claim 2 wherein a combination of the respective grating vectors of the input diffractive optical element and the two diffractive optical elements is a resultant vector with substantially zero magnitude.
4. The optical device of claim 3 wherein the grating vectors of the two diffractive optical elements subtend equal and opposite angles to the grating vector of the input diffractive optical element.
5. The optical device of claim 2 wherein the grating vectors of the input diffractive optical element and the two diffractive optical elements have substantially the same magnitude.
6. The optical device of claim 5 wherein the grating vectors of the input diffractive optical element and the two diffractive optical elements can be combined in a triangle that is substantially equilateral.
7. The optical device of claim 1 wherein the input grating is spaced apart from the two diffractive optical elements.
8. The optical device of claim 1 comprising a projector configured to project light towards the input diffractive optical element.
9. The optical device of claim 8 wherein the projected light is provided at a first angle relative to the input diffractive optical element and wherein light coupled out of the waveguide by the output diffractive optical element is also provided at the first angle.
10. The optical device of claim 1 wherein the two overlaid diffractive optical elements are provided in or on the waveguide in different planes.
11. The optical device of claim 10 wherein the diffractive optical elements are provided on opposing surfaces of the waveguide.
12. The optical device of claim 1 wherein at least two diffractive optical elements are provided in substantially the same plane in the waveguide.
13. The optical device of claim 12 wherein at least two diffractive optical elements are provided in a photonic crystal.