Displays with dispersion-compensating interleaved gratings
View Patent ↗A display may include a waveguide, a diffractive input coupler, and an output coupler. The input coupler may diffract image light into the waveguide. The output coupler may diffract the image light out of the waveguide and towards an eye box. The output coupler may include a first volume hologram with a first grating vector and a second volume hologram with a second grating vector. The first and second grating vectors may be oriented at the same angle from opposing sides of an axis. The input coupler may have a first pitch. The first and second volume holograms may have a second pitch. The second pitch may be constant across the output coupler. In order to mitigate dispersion by the input coupler, the second pitch may be equal to twice the first pitch times a cosine of the angle.
1 . A display system comprising:
a waveguide having a lateral surface;
an input coupler having a first diffractive grating configured to couple image light into the waveguide, the first diffractive grating having a first grating vector with a first grating vector projection onto the lateral surface of the waveguide and having a first pitch; and
an output coupler configured to couple the image light out of the waveguide, the output coupler comprising:
a second diffractive grating having a second pitch and a second grating vector, wherein the second grating vector has a second grating vector projection onto the lateral surface of the waveguide and the second grating vector projection is oriented at an angle with respect to the first grating vector projection, and
a third diffractive grating that at least partially overlaps the second diffractive grating and that has the second pitch and a third grating vector, wherein the third grating vector has a third grating vector projection onto the lateral surface of the waveguide and the third grating vector projection is oriented at a negative of the angle with respect to the first grating vector, wherein
the second pitch is constant,
the second pitch is equal to twice the first pitch times a cosine of the angle, and
the second and third diffractive gratings each diffract the image light.
2 . The display system of claim 1 , wherein the first, second, and third diffractive gratings comprise surface relief gratings.
3 . The display system of claim 1 , wherein the first, second, and third diffractive gratings comprise volume holograms.
4 . The display system of claim 1 , wherein the second diffractive grating is configured to diffract the image light to produce diffracted light and wherein the third diffractive grating is configured to diffract the diffracted light out of the waveguide.
5 . The display system of claim 1 , wherein the image light coupled into the waveguide propagates towards the output coupler along an axis, the second grating vector projection is oriented at the angle from a first side of the axis, and the third grating vector projection is oriented at the angle from a second side of the axis opposite the first side of the axis.
6 . The display system of claim 5 , wherein the second and third diffractive gratings have fringes with planes that are non-parallel and non-perpendicular with respect to a normal axis of the lateral surface of the waveguide, the output coupler being configured to couple the image light out of the waveguide through the lateral surface.
7 . The display system of claim 5 , wherein the second diffractive grating is configured to diffract the image light as first diffracted light, the third diffractive grating is configured to diffract the image light as second diffracted light, the third diffractive grating is configured to diffract the first diffracted light out of the waveguide, and the second diffractive grating is configured to diffract the second diffracted light out of the waveguide.
8 . The display system of claim 7 , wherein the first diffractive grating comprises a surface relief grating, the second diffractive grating comprises a first volume hologram, and the third diffractive grating comprises a second volume hologram.
9 . The display system of claim 1 , wherein the second and third diffractive gratings are configured to cancel out a dispersion in the image light produced by the first diffractive grating.
10 . The display system of claim 1 , wherein the waveguide comprises a grating medium having a thickness orthogonal to the lateral surface, the second and third diffractive gratings are formed in the grating medium, and the second and third diffractive gratings extend across an entirety of the thickness of the grating medium.
11 . A head mounted display device comprising:
a waveguide having a planar surface;
an input coupler that includes a first diffractive grating configured to couple light into the waveguide, the first diffractive grating having a first pitch and being characterized by a first grating vector with a first grating vector projection onto the planar surface of the waveguide; and
an output coupler configured to couple the light out of the waveguide, the output coupler comprising:
a second diffractive grating having a second pitch and characterized by a second grating vector, wherein the second grating vector has a second grating vector projection onto the planar surface of the waveguide and the second grating vector projection is oriented at an angle with respect to the first grating vector projection, and
a third diffractive grating that at least partially overlaps the second diffractive grating, that has the second pitch, and that is characterized by a third grating vector, wherein the third grating vector has a third grating vector projection onto the planar surface of the waveguide and the third grating vector projection is oriented at a negative of the angle with respect to the first grating vector, wherein
the second pitch is constant,
the second pitch is equal to twice the first pitch times a cosine of the angle, and
the second and third diffractive gratings each diffract the image light.
12 . A display comprising:
a waveguide configured to propagate light via total internal reflection;
a first diffractive grating configured to couple the light into the waveguide, wherein the first diffractive grating has a first pitch and is characterized by a first grating vector that has a first projection onto a lateral surface of the waveguide;
a second diffractive grating configured to couple at least some of the light out of the waveguide, wherein the second diffractive grating has a second pitch, the second diffractive grating is characterized by a second grating vector that has a second projection onto the lateral surface of the waveguide, and the second projection is oriented at an angle with respect to the first projection; and
a third diffractive grating configured to couple at least some of the light out of the waveguide, wherein the third diffractive grating has the second pitch, the third diffractive grating is characterized by a third grating vector that has a third projection onto the lateral surface of the waveguide, and the third projection is oriented at a negative of the angle with respect to the first projection, wherein
the second pitch is constant,
the second pitch is equal to twice the first pitch times a cosine of the angle, and
the second and third diffractive gratings each diffract the image light.