Apodized Diffraction Grating for Waveguide Light Leakage
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Solution Overview
Problem
Current pupil replication waveguides in near-eye displays suffer from light leakage into non-blazed diffraction orders, causing unwanted light to be directed outside the display, which can be distracting and compromise privacy by allowing others to see the displayed content.
Innovation Solution
A diffraction grating with a refractive index contrast profile that is apodized at the extremities, reducing diffraction efficiency into non-blazed orders by varying the refractive index contrast across the thickness of the grating, specifically at the top and bottom surfaces, to minimize light leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a diffraction grating is used to out-couple display light from the waveguide, then light extraction efficiency is improved, but light leakage into non-blazed diffraction orders occurs causing privacy issues
Solution Approach 1:
The patent applies local quality by varying the refractive index contrast at different positions within the diffraction grating structure. Specifically, the refractive index contrast is reduced at the top and bottom surfaces (extremities) of the grating while maintaining higher contrast in the middle region. This spatial variation in optical properties selectively suppresses diffraction into non-blazed orders at the grating extremities where such diffraction occurs, while preserving efficient light extraction in the central region. This resolves the contradiction by making different parts of the grating serve different functions: efficient extraction in the center and leakage suppression at the edges.
Solution Approach 2:
The patent employs parameter changes by modifying the refractive index contrast parameter throughout the diffraction grating structure. The refractive index contrast is not uniform but varies continuously or in steps from the center to the extremities of the grating. By changing this optical parameter spatially, the grating achieves different diffraction efficiencies at different locations, suppressing unwanted non-blazed diffraction orders while maintaining effective light out-coupling in the desired directions.
2Volume of moving object
If the waveguide is made compact for head-mounted display, then device size and weight are reduced, but directivity control becomes more challenging
Solution Approach 1:
The patent applies local quality by implementing position-dependent refractive index contrast within the compact waveguide structure. The diffraction grating features varying optical properties at different locations, with reduced refractive index contrast at the extremities and higher contrast in the middle. This spatial variation enables precise control of light diffraction directions within the compact form factor, maintaining strong directivity despite the reduced overall device size.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces the amount of display light out-coupled into unwanted directions, minimizing light leakage and enhancing user privacy by ensuring that the displayed content is primarily visible to the wearer.
Implementation Method 1
A diffraction grating is supported by the slab. The diffraction grating includes a plurality of fringes suspended in a substrate material. The fringes are slanted for out-coupling display light from the slab by diffraction into a blazed diffraction order.
Implementation Method 2
The refractive index of the fringes varies in a thickness direction of the diffraction grating such that a refractive index contrast profile of the diffraction grating along the thickness direction is symmetrical, and a refractive index contrast is larger at a middle point of the thickness than at both sides of the refractive index contrast profile.
Data Source
Figure 1~2
Figure 3A~3C
Figure 4A~4C
AI summary
A pupil replication waveguide for a projector display includes a slab of transparent material for propagating display light in the slab via total internal reflection. A diffraction grating is supported by the slab. The diffraction grating includes a plurality of slanted fringes in a substrate for out-coupling the display light from the slab by diffraction into a blazed diffraction order. A greater portion of the display light is out-coupled into the blazed diffraction order, and a smaller portion of the display light is out-coupled into a non-blazed diffraction order. A refractive index contrast profile of the diffraction grating along a thickness direction of the diffraction grating is symmetrical, and a refractive index contrast is larger at a middle than at both sides of the refractive index contrast profile, whereby the portion of the display light out-coupled into the non-blazed diffraction order is decreased.