Broadband reflector for waveguide assembly in a head-mounted display
A partially reflective thin film coating is utilized on an optical substrate that is affixed to a waveguide-based optical combiner in a see-through display of a mixed-reality head-mounted display (HMD) device to partially reflect a forward propagating holographic image light back to the user's eye. The thin film coating may be implemented as a broadband reflector over the angular range associated with the holographic images that are rendered over the field of view (FOV) of the virtual portion of the see-through display to simplify manufacturing and reduce bulk and weight of the HMD device.
1 . A near-eye mixed-reality optical system in which holographic images are mixed in with views of a real world, comprising:
a see-through planar optical waveguide through which real-world images are viewable by a user of the mixed-reality optical system, the optical waveguide including a first planar side and a second planar side opposite the first planar side;
a first diffractive optical element (DOE) disposed on a surface of one of the planar sides of the optical waveguide, the first DOE having an input region and configured as an in-coupling grating to in-couple, at the input region, one or more optical beams associated with holographic images from a holographic image source;
a second DOE disposed on a surface of one of the planar sides of the optical waveguide and configured for pupil expansion of the one or more optical beams along a first direction;
a third DOE disposed on a surface of one of the planar sides of the optical waveguide, the third DOE having an input and an output region and configured for pupil expansion of the one or more optical beams along a second direction, and further configured as an out-coupling grating to out-couple, as an output display from the output region to an eye of the user, the one or more optical beams having expanded pupil relative to the input; and
a broadband reflector disposed on a planar side of the optical waveguide that is opposite the third DOE, the broadband reflector having a bandwidth including a range of visible wavelengths from 400 to 650 nm, wherein the broadband reflector reflects both wavelengths associated with holographic image light and wavelengths associated with the views of the real world,
wherein the broadband reflector comprises a single-layer thin-film reflective coating disposed on a planar substrate, and
wherein the substrate and optical waveguide are tightly coupled with a uniform gap to maintain parallelism between the plane of the substrate and the plane of the optical waveguide within a predetermined threshold.
2 . The near-eye mixed-reality optical system of claim 1 in which the reflective coating comprises multiple layers of two or more dielectric materials, each of the two or more dielectric materials having a different refractive index.
3 . The near-eye mixed-reality optical system of claim 2 in which the dielectric materials comprise one of silicon dioxide (SiO 2 ), titanium dioxide (TiO 2 ), or aluminum oxide (Al 2 O 3 ).
4 . The near-eye mixed-reality optical system of claim 1 in which the one or more optical beams comprise one of a red wavelength range, blue wavelength range, or green wavelength range.
5 . The near-eye mixed-reality optical system of claim 1 further comprising a second see-through planar optical waveguide and a third see-through planar optical waveguide, the see-through planar optical waveguides being configured in a stack to form an optical combiner wherein each see-through planar optical waveguide propagates one or more optical beams for the holographic images for a different color in an RGB (red, green, blue) color model, the optical combiner having an eye side and a real-world side, wherein the broadband reflector is tightly coupled to the real-world side.
6 . The near-eye mixed-reality optical system of claim 1 in which the uniform gap is maintained using one of spacer or structural fitting that is disposed along one or more peripheral edge of the planar substrate or planar waveguide.
7 . The near-eye mixed-reality optical system of claim 1 in which the planar substrate of the broadband reflector and the see-through planar optical waveguide each comprises a glass material.
8 . The near-eye mixed-reality optical system of claim 6 in which the broadband reflector is configured to meet a threshold range of reflectance across a range of angles that comprise a field of view (FOV) of an HMD device in which the near-eye mixed-reality optical system is incorporated.