Outcoupling grating for augmented reality system
An eyepiece for use in front of an eye of a viewer includes a waveguide having a surface and a diffractive optical element (DOE) optically coupled to the waveguide. The DOE includes a plurality of first ridges protruding from the surface of the waveguide and arranged as a periodic array having a period, each respective first ridge has a first height and a respective first width. The DOE also includes a plurality of second ridges, each respective second ridge protruding from a respective first ridge and having a second height greater than the first height and a respective second width less than the respective first width. At least one of the respective first width, the respective second width, or a respective ratio between the respective first width and the respective second width varies as a function of a distance from a first edge of the DOE.
1. An eyepiece for use in front of an eye of a viewer, the eyepiece comprising:
a waveguide having a surface; and
a diffractive optical element optically coupled to the waveguide, the diffractive optical element including:
a plurality of first ridges protruding from the surface of the waveguide and arranged as a periodic array having a period, each respective first ridge of the plurality of first ridges having a first height in a direction perpendicular to the surface of the waveguide and a respective first width in a direction of the period; and
a plurality of second ridges, each respective second ridge of the plurality of second ridges protruding from a respective first ridge and having a second height greater than the first height and a respective second width less than the respective first width;
wherein the diffractive optical element is configured to diffract a first portion of a light beam propagating in the waveguide toward the eye as a first order reflection, and to diffract a second portion of the light beam propagating in the waveguide away from the eye as a first order transmission, and
wherein at least one of the respective first width, the respective second width, or a respective ratio between the respective first width and the respective second width varies as a function of a distance from a first edge of the diffractive optical element.
2. The eyepiece of claim 1 wherein the respective ratio at a middle section between the first edge of the diffractive optical element and a second edge of the diffractive optical element opposite to the first edge is lower than the respective ratio at either the first edge or the second edge of the diffractive optical element.
3. The eyepiece of claim 1 wherein the respective first width ranges from 0.4 times the period to 0.6 times the period.
4. The eyepiece of claim 1 wherein the respective second width is less than 0.25 times the period.
5. The eyepiece of claim 1 wherein a ratio of the second height and the first height ranges from 8:7 to 8:3.
6. The eyepiece of claim 1 wherein the respective first width and the respective second width are selected such that the diffractive optical element has a first order reflection efficiency greater than a first order transmission efficiency.
7. The eyepiece of claim 6 wherein a ratio of the first order reflection efficiency and the first order transmission efficiency is greater than 2.
8. The eyepiece of claim 1 wherein the diffractive optical element is configured to diffract a third portion of the light propagating in the waveguide in a direction substantially opposite to a direction of incidence as a second order reflection, and the respective first width and the respective second width are selected such that the diffractive optical element has a second order reflection efficiency less than one percent.
9. The eyepiece of claim 1 wherein the first height ranges from 45 nm to 65 nm, and the second height ranges from 70 nm to 80 nm.
10. The eyepiece of claim 1 wherein each of the respective first width of the respective first ridge and each of the respective second width of the respective second ridge are varied such that a first order reflection efficiency of the diffractive optical element varies as a function of a position across the surface of the waveguide in the direction of the period.
11. The eyepiece of claim 10 wherein the first order reflection efficiency of the diffractive optical element increases as a function of the distance from the first edge of the diffractive optical element.
12. The eyepiece of claim 11 wherein the first order reflection efficiency of the diffractive optical element increases substantially linearly as a function of the distance from the first edge of the diffractive optical element.
13. An eyepiece for projecting an image to an eye of a viewer, the eyepiece comprising:
a waveguide having a first surface;
an incoupling grating optically coupled to a first region of the waveguide, the incoupling grating configured to diffract image light incident on the waveguide in a direction substantially perpendicular to the first surface into the waveguide to be propagated therein in a first direction;
a first diffractive optical element (DOE) optically coupled to a second region of the waveguide, the first DOE configured to diffract the image light propagating in the waveguide toward a second direction different from the first direction; and
a second DOE optically coupled to the second region of the waveguide, the second DOE including:
a plurality of first ridges protruding from the first surface of the waveguide and arranged as a periodic array having a period, each of the plurality of first ridges having a first height in a direction perpendicular to the first surface of the waveguide and a first width in a direction of the period; and
a plurality of second ridges, each of the plurality of second ridges protruding from a respective first ridge and having a second height greater than the first height and a respective second width less than the respective first width;
wherein the second DOE is configured to diffract a first portion of the image light diffracted by the first DOE toward the eye as a first order reflection, and to diffract a second portion of the image light diffracted by the first DOE away from the eye as a first order transmission, and
wherein at least one of the respective first width, the respective second width, or a respective ratio between the respective first width and the respective second width varies as a function of a distance from a first edge of the second DOE.
14. The eyepiece of claim 13 wherein the respective ratio at a middle section between the first edge of the second DOE and a second edge of the second DOE opposite to the first edge is lower than the ratio at either the first edge or the second edge of the second DOE.
15. The eyepiece of claim 13 wherein the respective first width and the respective second width are selected such that the second DOE has a first order reflection efficiency greater than a first order transmission efficiency.
16. The eyepiece of claim 15 wherein a ratio of the first order reflection efficiency and the first order transmission efficiency is greater than 2.
17. The eyepiece of claim 13 wherein the second DOE is configured to diffract a third portion of the image light propagating in the waveguide in a direction substantially opposite to a direction of incidence as a second order reflection, and the respective first width and the respective second width are selected such that the second DOE has a second order reflection efficiency less than one percent.
18. The eyepiece of claim 13 wherein each respective first width of the respective first ridge and each respective second width of the respective second ridge are varied such that a first order reflection efficiency of the second DOE varies as a function of a position across the first surface of the waveguide in the direction of the period.
19. The eyepiece of claim 18 wherein the first order reflection efficiency of the second DOE increases as a function of the distance from the first edge of the second DOE.
20. The eyepiece of claim 19 wherein the first order reflection efficiency of the second DOE increases substantially linearly as a function of the distance from the first edge of the second DOE.