Non-telecentric holographic optical element in an eye tracking system

1 . A lens assembly comprising:

a waveguide;

an out-coupling diffraction grating at least partially disposed within the waveguide and configured to out-couple light from the waveguide; and

a holographic optical element (HOE) at least partially disposed within the waveguide, wherein:

an input surface of the HOE shares an input surface of the waveguide;

the HOE is configured to gradually change diffraction angles of a plurality of incident light rays based on a plurality of diffraction grating planes comprising slanted grating planes with differing grating periods across the HOE from a first end to a second end of the HOE; and

the HOE is configured to map the plurality of incident light rays to a corresponding plurality of total internal reflection (TIR) angles, wherein the plurality of incident light rays include oblique light rays that are non-normal to the input surface of the HOE and extends laterally outside of a peripheral border of the input surface of the HOE.

2 . The lens assembly of claim 1 , wherein the first end is distal to the out-coupling diffraction grating, wherein the second end is proximal to the out-coupling diffraction grating, wherein the HOE is configured to receive first ones of the plurality of incident light rays at the first end, wherein the HOE is configured to diffract the first ones of the plurality of incident light rays with a first one of the TIR angles, wherein the HOE is configured to receive second ones of the plurality of incident light rays at the second end, wherein the HOE is configured to diffract the second ones of the plurality of incident light rays with a second one of the TIR angles.

3 . The lens assembly of claim 2 , wherein the HOE is configured to receive third ones of the plurality of incident light rays between the first end and the second end, wherein the HOE is configured to diffract the third ones of the plurality of incident light rays with third ones of the TIR angles, wherein the third ones of the TIR angles gradually change from the first end to the second end of the HOE.

4 . The lens assembly of claim 1 , wherein the HOE is a volume Bragg grating.

5 . The lens assembly of claim 1 , wherein the HOE is configured to diffractively operate in reflection.

6 . The lens assembly of claim 1 , wherein the HOE includes the plurality of diffraction grating planes configured to diffract the plurality of incident light rays.

7 . The lens assembly of claim 6 , wherein each of the plurality of diffraction grating planes is configured to diffract the plurality of incident light rays to a corresponding one of the plurality of TIR angles.

8 . The lens assembly of claim 7 , wherein a first of the plurality of diffraction grating planes includes first characteristics that are at least partially defined based on one of the oblique light rays originating from outside a periphery of a first end of the input surface of the HOE.

9 . The lens assembly of claim 8 , wherein a second of the plurality of diffraction grating planes includes second characteristics that are at least partially defined based on one of the oblique light rays originating from outside a periphery of a second end of the input surface of the HOE.

10 . The lens assembly of claim 6 , wherein at least some of the diffraction grating planes have characteristics that are defined at least partially based on an angular bandwidth of an adjacent one of the diffraction grating planes.

11 . The lens assembly of claim 6 , wherein each of the slanted grating planes includes a corresponding one of a plurality of slant angles, wherein each of the plurality of slant angles is greater or less than an adjacent one of the plurality of slant angles.

12 . A head mounted device comprising:

a frame; and

a lens assembly coupled to the frame, wherein the lens assembly includes:

a waveguide;

an out-coupling diffraction grating at least partially disposed within the waveguide and configured to out-couple light from the waveguide; and

a holographic optical element (HOE) at least partially disposed within the waveguide, wherein:

an input surface of the HOE shares an input surface of the waveguide;

the HOE is configured to gradually change diffraction angles of a plurality of incident light rays based on a plurality of diffraction grating planes comprising slanted grating planes with differing grating periods across the HOE from a first end to a second end of the HOE; and

the HOE is configured to in-couple oblique light rays from a field-of-view (FOV) of an eyebox region that extends laterally beyond a peripheral border of the input surface of the HOE.

13 . The lens assembly of claim 1 , wherein the HOE is configured to diffract the plurality of light rays to cause the light rays to exit through the input surface of the HOE.

14 . The head mounted device of claim 12 , wherein the HOE is configured to map the plurality of incident light rays to a corresponding plurality of total internal reflection (TIR) angles, wherein the plurality of incident light rays include the oblique light rays, wherein at least some of the oblique light rays are non-normal to the input surface of the HOE.

15 . The head mounted device of claim 14 , wherein the HOE includes a plurality of diffraction grating planes configured to diffract the plurality of incident light rays into the waveguide.

16 . The head mounted device of claim 15 , wherein each of the plurality of diffraction grating planes is configured to diffract the plurality of incident light rays to a corresponding one of a plurality of TIR angles.

17 . The head mounted device of claim 15 , wherein at least some of the diffraction grating planes have characteristics that are defined at least partially based on an angular bandwidth of an adjacent one of the diffraction grating planes.

18 . A method comprising:

illuminating a holographic optical element (HOE) with a first light ray that is oblique to an input surface of the HOE, the HOE partially disposed within a waveguide and sharing an input surface of the HOE with an input surface of the waveguide and the HOE configured to gradually change diffraction angles of a plurality of incident light rays based on a plurality of diffraction grating planes comprising slanted grating planes with differing grating periods across the HOE from a first end to a second end of the HOE, wherein the first light ray extends laterally beyond a peripheral border of the input surface of the HOE;

adjusting characteristics of a first grating plane in the HOE until a first diffraction angle of the first light ray reaches a first predetermined diffraction angle value;

determining a first angular bandwidth of the first grating plane;

illuminating the HOE with one or more additional light rays, wherein at least some of the one or more additional light rays originate from outside the peripheral border of the input surface of the HOE, wherein incident angles of the one or more additional light rays are at least partially based on the first angular bandwidth; and

adjusting characteristics of one or more additional grating planes in the HOE until one or more additional diffraction angles of the one or more additional light rays become one or more additional predetermined diffraction angle values.

19 . The method of claim 18 , wherein the characteristics of the first grating plane include a slant angle and a grating line period.

20 . The method of claim 18 , wherein the one or more additional light rays include a corresponding one of one or more slant angles, wherein each of the one or more slant angles is greater or lesser than an adjacent one of the one or more slant angles.