Gradient refractive index grating for display leakage reduction
A waveguide display includes a waveguide, an input coupler configured to couple display light into the waveguide, and a surface-relief grating on the waveguide and configured to couple the display light out of the waveguide towards an eyebox of the waveguide display on a first side of the waveguide. The surface-relief grating is formed in a plurality of grating layers having uniform or non-uniform thickness profiles. The plurality of grating layers is characterized by a refractive index modulation that increases and then decreases as the distance of the grating layer from the waveguide increases.
1. A waveguide display comprising:
a waveguide;
an input coupler configured to couple display light into the waveguide; and
one or more surface-relief gratings on the waveguide and configured to couple the display light out of the waveguide towards an eyebox of the waveguide display on a first side of the waveguide, the one or more surface-relief gratings formed in a plurality of grating layers, wherein the plurality of grating layers includes:
a first grating layer characterized by a first thickness profile and a first refractive index;
a second grating layer characterized by a second thickness profile and a second refractive index different from the first refractive index; and
a third grating layer characterized by a third thickness profile and a third refractive index different from the second refractive index, the second grating layer between the first grating layer and the third grating layer,
wherein the one or more surface-relief gratings include a grating groove etched into the first grating layer, the second grating layer, and the third grating layer;
wherein at least one of the first grating layer, the second grating layer, or the third grating layer has a non-uniform thickness profile; and
wherein the first thickness profile, the first refractive index, the second thickness profile, the second refractive index, the third thickness profile, and the third refractive index are configured to reduce coupling of the display light to a second side of the waveguide opposing the first side.
2. The waveguide display of claim 1 , wherein the plurality of grating layers is characterized by a refractive index modulation that increases and then decreases with an increase in a distance from the waveguide.
3. The waveguide display of claim 1 , wherein the second refractive index is greater than each of the first refractive index, the third refractive index, and a fourth refractive index of the waveguide.
4. The waveguide display of claim 1 , wherein:
the second refractive index is lower than each of the first refractive index and the third refractive index; and
the first refractive index is the same as or different from the third refractive index.
5. The waveguide display of claim 1 , wherein the plurality of grating layers is characterized by a refractive index that increases and then decreases with an increase in a distance from the waveguide.
6. The waveguide display of claim 1 , wherein the plurality of grating layers is characterized by a refractive index that decreases and then increases with an increase in a distance from the waveguide.
7. The waveguide display of claim 1 , wherein the second grating layer is characterized by a refractive index modulation greater than 0.1.
8. The waveguide display of claim 1 , wherein an efficiency of the coupling of the display light to the second side of the waveguide is less than 1.0%.
9. The waveguide display of claim 1 , wherein the first thickness profile is different from the second thickness profile.
10. The waveguide display of claim 1 , wherein the first thickness profile includes a non-uniform thickness profile in one or two dimensions.
11. The waveguide display of claim 1 , wherein the one or more surface-relief gratings comprise a slanted surface-relief grating including a plurality of grating grooves in the plurality of grating layers.
12. The waveguide display of claim 11 , wherein the plurality of grating grooves is characterized by non-uniform depths.
13. The waveguide display of claim 12 , wherein each of the plurality of grating grooves extends through all grating layers in the plurality of grating layers.
14. The waveguide display of claim 11 , wherein the one or more surface-relief gratings further comprise an overcoat layer on the plurality of grating layers, the overcoat layer filling the plurality of grating grooves and characterized by a fourth refractive index different from the first refractive index, the second refractive index, and the third refractive index.
15. The waveguide display of claim 1 , wherein at least one surface-relief grating of the one or more surface-relief gratings is characterized by at least one of a non-uniform grating period or a non-uniform duty cycle.
16. A surface-relief grating coupler comprising:
a substrate;
a plurality of grating layers on the substrate, the plurality of grating layers including:
a first grating layer characterized by a first thickness profile and a first refractive index;
a second grating layer characterized by a second thickness profile and a second refractive index different from the first refractive index;
a third grating layer characterized by a third thickness profile and a third refractive index different from the second refractive index, the second grating layer between the first grating layer and the third grating layer; and
a plurality of grating grooves etched into the plurality of grating layers, the plurality of grating grooves slanted with respect to the substrate and characterized by non-uniform depths, wherein a grating groove of the plurality of grating grooves is etched into the first grating layer, the second grating layer, and the third grating layer, and wherein opposing sidewalls of the grating groove are slanted in a same direction with respect to the substrate; and
an overcoat layer on the plurality of grating layers, the overcoat layer filling the plurality of grating grooves.
17. The surface-relief grating coupler of claim 16 , wherein the plurality of grating layers is characterized by a refractive index modulation that increases and then decreases with an increase in a distance from the substrate.
18. The surface-relief grating coupler of claim 16 , wherein:
the first thickness profile includes a first non-uniform thickness profile; and
the second thickness profile includes a second non-uniform thickness profile.
19. The surface-relief grating coupler of claim 16 , wherein each of the plurality of grating grooves extends through all grating layers in the plurality of grating layers.
20. The surface-relief grating coupler of claim 16 , wherein the plurality of grating grooves is characterized by at least one of non-uniform depths, non-uniform pitches, or non-uniform widths.