Waveguide structure
An optical waveguide device for use in a head up display. The waveguide device provides pupil expansion in two dimensions. The waveguide device comprise a primary waveguide and a secondary waveguide, the secondary waveguide being positioned on a face of the primary waveguide. The secondary waveguide has a diffraction grating on a face opposite to the face which contacts the primary waveguide. The diffraction grating diffracts light into more than one diffraction order. Rays diffracted into a non-zero order are trapped in the secondary waveguide by total internal reflection.
1. An optical waveguide system for a head-up display, the system comprising:
first and second optical waveguides mounted such that the inner and outer faces of each of the first and second optical waveguides are all parallel, the first optical waveguide comprising a first diffraction grating, and the second optical waveguide comprising a second diffraction grating, wherein the first and second diffraction gratings are arranged to be at right angles to each other and to partially overlap each other;
wherein an input region of the first optical waveguide is arranged such that light projected into the first optical waveguide within an input angle range strikes the first diffraction grating without passing through the second diffraction grating;
wherein an area of overlap where the first and second diffraction gratings overlap is arranged such that the first diffraction grating diffracts a portion of light trapped in the first optical waveguide by total internal reflection to an angle at which the diffracted light is not trapped by total internal reflection within the first optical waveguide and at which the diffracted light propagates out of the first optical waveguide to strike the second diffraction grating, and such that the second diffraction grating diffracts a portion of light trapped in the second optical waveguide by total internal reflection to an angle at which the diffracted light is not trapped by total internal reflection within the second optical waveguide and at which the diffracted light propagates out of the second optical waveguide to re-enter the first optical waveguide and strike the first diffraction grating; and
wherein an output region of the first or second optical waveguide where the first and second diffraction gratings do not overlap is arranged such that light trapped in the first or second optical waveguide by total internal reflection and which strikes the first or second diffraction grating is diffracted into the input angle range to exit the optical waveguide system to an exit pupil without striking the other of the first or second diffraction grating.
2. The optical waveguide system according to claim 1 , wherein the output region of the first optical waveguide is oriented in a direction of propagation of light in the first optical waveguide from the input region.
3. The optical waveguide system according to claim 1 , wherein the output region of the second optical waveguide is oriented perpendicular to a direction of propagation of light in the first optical waveguide from the input region.
4. The optical waveguide system according to claim 1 , wherein the first and/or second diffraction gratings diffract reflectively.
5. The optical waveguide system according to claim 1 , wherein the first and/or second diffraction gratings diffract in transmission.
6. The optical waveguide system according to claim 1 , wherein the first and/or second diffraction gratings are coated with a reflective coating to preferentially reflect light that is not trapped by total internal reflection.
7. The optical waveguide system according to claim 1 , wherein the first and/or second diffraction gratings include a rectangular profiled material that has a low area of phase and an area of high optical phase, the said areas being rectangular in shape.
8. The optical waveguide system according to claim 7 , wherein the first and/or second diffraction gratings comprise overlapping layers of rectangular profiled material that combine to create a phase profile.
9. The optical waveguide system according to claim 8 , wherein the overlapping layers of rectangular profiled material are arranged to be approximately the shape of a blazed grating.
10. An optical waveguide system comprising:
first and second parallel optical waveguides, the first optical waveguide comprising a first diffraction grating, and the second optical waveguide comprising a second diffraction grating, wherein the first and second diffraction gratings are arranged to be at right angles to each other and to partially overlap each other,
wherein light projected into the first optical waveguide within an input angle range strikes the first diffraction grating without passing through the second diffraction grating, wherein an area of overlap where the first and second diffraction gratings overlap is arranged such that the first diffraction grating diffracts a portion of light trapped in the first optical waveguide to an angle at which the diffracted light propagates out of the first optical waveguide to strike the second diffraction grating, and such that the second diffraction grating diffracts a portion of light trapped in the second optical waveguide to an angle at which the diffracted light propagates out of the second optical waveguide to re-enter the first optical waveguide and strike the first diffraction grating, and
wherein, where the first and second diffraction gratings do not overlap, light trapped in the first or second optical waveguide that strikes the first or second diffraction grating is diffracted to exit the optical waveguide system via an exit pupil without striking the other of the first or second diffraction grating.
11. The optical waveguide system according to claim 10 , wherein an output region of the first optical waveguide is oriented in a direction of propagation of light in the first optical waveguide with respect to an input region of the first optical waveguide.
12. The optical waveguide system according to claim 11 , wherein an output region of the second optical waveguide is oriented perpendicular to a direction of propagation of light in the first optical waveguide from the input region of the first optical waveguide.
13. The optical waveguide system according to claim 10 , wherein the first and/or second diffraction gratings diffract reflectively.
14. The optical waveguide system according to claim 10 , wherein the first and/or second diffraction gratings diffract in transmission.
15. The optical waveguide system according to claim 10 , wherein the first and/or second diffraction gratings are coated with a reflective coating to preferentially reflect light that is not trapped by total internal reflection.
16. The optical waveguide system according to claim 10 , wherein the first and/or second diffraction gratings include a rectangular profiled material that has a low area of phase and an area of high optical phase, the said areas being rectangular in shape.
17. The optical waveguide system according to claim 16 , wherein the first and/or second diffraction gratings comprise overlapping layers of rectangular profiled material that combine to create a phase profile.
18. The optical waveguide system according to claim 17 , wherein the overlapping layers of rectangular profiled material are arranged to be approximately the shape of a blazed grating.
19. A head-up display, comprising:
a projector configured to project light, and
an optical waveguide system configured to receive the light from the projector, the optical waveguide system comprising:
first and second parallel optical waveguides, the first optical waveguide comprising a first diffraction grating, and the second optical waveguide comprising a second diffraction grating, wherein the first and second diffraction gratings are arranged to be at right angles to each other and to partially overlap each other,
wherein light projected into the first optical waveguide within an input angle range strikes the first diffraction grating without passing through the second diffraction grating, wherein an area of overlap where the first and second diffraction gratings overlap is arranged such that the first diffraction grating diffracts a portion of light trapped in the first optical waveguide to an angle at which the diffracted light propagates out of the first optical waveguide to strike the second diffraction grating, and such that the second diffraction grating diffracts a portion of light trapped in the second optical waveguide to an angle at which the diffracted light propagates out of the second optical waveguide to re-enter the first optical waveguide and strike the first diffraction grating, and
wherein, where the first and second diffraction gratings do not overlap, light trapped in the first or second optical waveguide that strikes the first or second diffraction grating is diffracted to exit the optical waveguide system via an exit pupil without striking the other of the first or second diffraction grating.
20. The head-up display of claim 19 , wherein the first optical waveguide and the second optical waveguide are separated by an airgap.