Optical layered composite having a reduced content of highly refractive layers and its application in augmented reality
An optical layered composite includes: a substrate having a front face, a back face, a thickness ds between the front face and the back face and a refractive index n s ; and a coating applied to the front face. The coating comprises a group A of coating layers having a refractive index of 1.7 or more and a group B of coating layers having a refractive index of less than 1.7. The sum of the thicknesses of the layers of the group A is at most 55% of the total thickness d c of the coating.
1. A device, comprising:
at least one layered composite, the at least one layered composite comprising:
a substrate having a front face, a back face, a thickness d s between the front face and the back face and a refractive index n s ; and
a coating applied to the front face, the coating comprising a group A of coating layers having a refractive index of 1.7 or more and a group B of coating layers having a refractive index of less than 1.7, a sum of thicknesses of the layers of the group A being at most 40% of the total thickness de of the coating, wherein a total thickness variation is less than 2 μm, wherein an in-plane optical loss measured perpendicular to the front face is at most 20%; and
a projector configured to generate an overlaid image that passes through the at least one layered composite, wherein the projector is arranged such that the overlaid image passes through the at least one layered composite transverse to the front face and exits through the back face.
2. The device of claim 1 , wherein the at least one layered composite comprises a grouping of x layered composites, x being an integer that is at least 2, wherein the x layered composites are arranged in a stack, their front faces being parallel and oriented in a same direction and wherein a spacer region made of a material having a refractive index below 1.3 is present between each pairing of front face with adjacent back face.
3. A process for making an augmented reality device, comprising the following steps:
providing a wafer, the wafer comprising a layered composite, the layered composite comprising:
a substrate having a front face, a back face, a thickness ds between the front face and the back face and a refractive index ns; and
a coating applied to the front face, the coating comprising a group A of coating layers having a refractive index of 1.7 or more and a group B of coating layers having a refractive index of less than 1.7, a sum of thicknesses of the layers of the group A being at most 40% of the total thickness de of the coating, wherein a total thickness variation of the layered composite is less than 2 μm, wherein an in-plane optical loss measured perpendicular to the front face is at most 20%;
reducing the surface area of the front face to obtain a portion; and
providing the portion as a viewing screen in the augmented reality device, the augmented reality device comprising a projector configured to generate an overlaid image that passes through the layered composite, wherein the projector is arranged such that the overlaid image passes through the layered composite transverse to the front face and exits through the back face.
4. The device of claim 1 , wherein the coating fulfils at least one of the following criteria:
the coating comprises nitrogen and the coating has a total thickness of less than 85 nm for coating layers comprising nitrogen;
the coating comprises nitride and the coating has a total thickness of less than 85 nm for coating layers comprising nitride;
the coating comprises Si 3 N 4 and the coating has a total thickness of less than 85 nm for coating layers comprising Si 3 N 4 ;
the coating comprises AlN and the coating has a total thickness of less than 85 nm for coating layers comprising AlN;
the coating comprises crystalline TiO 2 and the coating has a total thickness of less than 85 nm for coating layers comprising crystalline TiO 2 ; or
the coating comprises TiO 2 and the coating has a total thickness of less than 85 nm for coating layers comprising TiO 2 .
5. The device of claim 1 , wherein the substrate has a refractive index of 1.6 or more.
6. The device of claim 1 , wherein at least one of the following is satisfied:
the thickness d s is in the range from 10 to 1500 μm;
a root mean squared (RMS) surface roughness of the substrate is less than 5 nm;
a root mean squared (RMS) surface roughness of the coating is less than 5 nm;
a warp is less than 350 μm; or
a bow is less than 300 μm.
7. The device of claim 1 , wherein the coating comprises a coating layer having a refractive index in the range from 1.70 to 2.60.
8. The device of claim 1 , wherein the coating comprises a coating layer having a refractive index in the range from 1.37 to 1.60.
9. The device of claim 1 , wherein the substrate is selected from glass, polymer, optoceramics or crystals.
10. The device of claim 1 , further comprising a device for coupling light into or decoupling light out of the at least one layered composite.
11. The device of claim 1 , wherein the at least one layered composite is a wafer.
12. The device of claim 11 , wherein at least one of the following criteria is satisfied:
the front face has a surface area in the range from 0.010 to 0.500 m 2 ;
the thickness d s is in the range from 10 to 1500 μm;
a root mean squared (RMS) surface roughness of the substrate is less than 5 nm;
a root mean squared (RMS) surface roughness of the coating is less than 5 nm;
a warp is less than 350 μm;
a bow is less than 300 μm; or
a circular shape.
13. The device of claim 1 , wherein the coating has a total thickness of less than 85 nm for coating layers comprising TiO 2 .
14. The device of claim 1 , wherein the back face is parallel with the front face so the substrate is a flat substrate and the coating extends in a plane that is parallel to the substrate.