Diffractive gratings for eye-tracking illumination through a light-guide
Techniques for eye-tracking in a near-eye display system are disclosed. One example of a near-eye display system includes a substrate transparent to visible light and configured to be placed in front of a user's eye, one or more light sources configured to emit illumination light invisible to the user's eye, one or more input couplers configured to couple the illumination light into the substrate, and one or more grating couplers each configured to couple a portion of the illumination light out of the substrate and towards the user's eye at a different direction. The illumination light coupled into the substrate propagates within the substrate through total internal reflection. The one or more grating couplers include at least one of a chirped surface-relief grating coupler or a volume Bragg grating coupler.
1. A near-eye display system comprising:
a substrate transparent to visible light and configured to be placed in front of a user's eye;
one or more light sources that emit illumination light invisible to the user's eye for eye tracking;
one or more input couplers that couple the illumination light into the substrate, wherein the illumination light coupled into the substrate propagates within the substrate through total internal reflection; and
two or more output grating couplers formed on the substrate, the two or more output grating couplers including at least one of a chirped surface-relief grating coupler or a volume Bragg grating coupler, each output grating coupler of the two or more output grating couplers characterized by a different respective grating parameter and coupling a portion of the illumination light propagating within the substrate out of the substrate and towards the user's eye at a different respective angular direction to form a different respective virtual light source in a field of view of the user's eye for eye illumination,
wherein a distance between the respective virtual light source and the user's eye is greater than a distance between the output grating coupler and the user's eye.
2. The near-eye display system of claim 1 , wherein the one or more light sources are positioned in a peripheral of the field of view of the user's eye.
3. The near-eye display system of claim 1 , wherein:
the one or more light sources include two or more light sources;
the one or more input couplers include two or more input couplers, each input coupler corresponding to a respective light source and configured to couple the illumination light from the respective light source into the substrate; and
the two or more output grating couplers include two or more groups of grating couplers, each group of grating couplers aligned with a respective input coupler and configured to direct the illumination light from a corresponding light source towards the user's eye.
4. The near-eye display system of claim 1 , wherein:
the one or more light sources include one light source;
the one or more input couplers include one input coupler;
the two or more output grating couplers include two or more groups of grating couplers; and
the near-eye display system further includes two or more diffractive gratings formed on the substrate, each diffractive grating aligned with a respective group of grating couplers and configured to divert a portion of the illumination light from the light source towards the respective group of grating couplers.
5. The near-eye display system of claim 1 , wherein the one or more input couplers include a diffractive grating, a prism, a slanted transmissive surface, or a slanted reflective surface.
6. The near-eye display system of claim 1 , wherein each output grating coupler of the two or more output grating couplers has a varying grating period or tilt angle in a region of the output grating coupler.
7. The near-eye display system of claim 1 , wherein each of the two or more output grating couplers include multiple regions, each region including a grating configured to couple a portion of the illumination light propagating within the substrate out of the substrate and towards a different region on the user's eye.
8. The near-eye display system of claim 1 , wherein the two or more output grating couplers include two grating couplers, the two grating couplers configured to couple illumination light from a same light source out of the substrate such that the illumination light coupled out of the substrate by the two grating couplers forms an interference fringe pattern on the user's eye.
9. The near-eye display system of claim 1 , wherein the one or more light sources include:
a light emitting device including a light emitting diode (LED), a laser diode, or vertical cavity surface emitting lasers (VCSELs); and
collimating optics for collimating light emitted by the light emitting device.
10. A display device comprising:
a substrate transparent to visible light and configured to be placed in front of a user's eye;
one or more input couplers that couple illumination light for eye tracking from one or more light sources into the substrate, wherein the illumination light coupled into the substrate propagates within the substrate through total internal reflection, and wherein the illumination light is invisible to the user's eye; and
two or more output grating couplers formed on the substrate and transparent to visible light, the two or more output grating couplers including at least one of a chirped surface-relief grating coupler or a volume Bragg grating coupler, each output grating coupler of the two or more output grating couplers characterized by a different respective grating parameter and coupling a portion of the illumination light propagating within the substrate out of the substrate and towards the user's eye at a different respective angular direction to form a different respective virtual light source in a field of view of the user's eye for eye illumination,
wherein a distance between the respective virtual light source and the user's eye is greater than a distance between the output grating coupler and the user's eye.
11. The display device of claim 10 , wherein:
the one or more input couplers include two or more input couplers, each input coupler corresponding to a respective light source and configured to couple the illumination light from the respective light source into the substrate; and
the two or more output grating couplers include two or more groups of grating couplers, each group of grating couplers aligned with a respective input coupler and configured to direct the illumination light from a corresponding light source towards the user's eye.
12. The display device of claim 10 , wherein:
the one or more input couplers include one input coupler configured to couple the illumination light from one light source into the substrate;
the two or more output grating couplers include two or more groups of grating couplers; and
the display device further includes two or more diffractive gratings formed on the substrate, each diffractive grating aligned with a respective group of grating couplers and configured to divert a portion of the illumination light from the light source towards the respective group of grating couplers.
13. The display device of claim 10 , wherein each output grating coupler of the two or more output grating couplers has a varying grating period or tilt angle in a region of the chirped surface-relief grating coupler.
14. The display device of claim 10 , wherein each output grating coupler of the two or more output grating couplers includes multiple regions, each region including a grating configured to couple a portion of the illumination light propagating within the substrate out of the substrate and towards a different region on the user's eye.
15. The display device of claim 10 , wherein the two or more output grating couplers include two grating couplers, the two grating couplers configured to couple illumination light from a same light source out of the substrate such that the illumination light coupled out of the substrate by the two grating couplers forms an interference fringe pattern on the user's eye.
16. A method of illuminating an eye of a user of a near-eye display system, the method comprising:
coupling, by an input coupler, illumination light for eye tracking from a light source into a substrate of the near-eye display system, wherein the illumination light is invisible to the eye of the user;
reflecting the coupled illumination light by surfaces of the substrate through total internal reflection, such that the illumination light propagates within the substrate; and
diffracting, by each output grating coupler of a set of output grating couplers formed at a plurality of locations on the substrate and characterized by respective grating parameters, a portion of the illumination light propagating within the substrate out of the substrate and towards the eye of the user at a different respective angular direction to form a different respective virtual light source in a field of view of the eye of the user for eye illumination, wherein the set of output grating couplers includes at least one of a chirped surface-relief grating coupler or a volume Bragg grating coupler,
wherein a distance between the respective virtual light source and the eye of the user is greater than a distance between the output grating coupler and the eye of the user.
17. The method of claim 16 , further comprising:
splitting the coupled illumination light into multiple illumination light beams; and
directing each of the multiple illumination light beams towards a respective group of grating couplers in the set of output grating couplers.
18. The near-eye display system of claim 1 , wherein the two or more output grating couplers form a two-dimensional array of grating couplers on the substrate.
19. The near-eye display system of claim 1 , wherein:
the one or more light sources include one light source; and
each of the portion of the illumination light coupled out of the substrate and towards the user's eye at the different respective angular direction forms a respective image of the light source on the user's eye.
20. The near-eye display system of claim 1 , wherein:
the two or more output grating couplers are formed on a surface of the substrate; and
the illumination light propagating within the substrate is reflected at the surface of the substrate.