Slanted Grating Waveguide for Rainbow Reduction
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Solution Overview
Problem
Waveguide displays in artificial reality systems suffer from glare and ghost images due to the wavelength-dependent characteristics of diffraction gratings, which negatively impact user experience.
Innovation Solution
A slanted grating with a specific grating period and slant angle is used in the waveguide display, along with an anti-reflective coating and over-coat layer, to cause destructive interference of ambient light, reducing glare and ghost images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a diffraction grating is used to couple light in and out of the waveguide, then light coupling efficiency is improved, but wavelength-dependent ghost images and rainbow artifacts are generated
Solution Approach 1:
The grating structure is segmented into multiple ridges with specific geometric parameters (slant angle, height, period) that create multiple diffracted beams. These segmented structures enable selective diffraction where the zeroth-order beam couples display light efficiently while higher-order beams are directed away from the user's eye, reducing ghost images and rainbow artifacts.
Solution Approach 2:
The grating ridges are designed with specific local geometric properties (slant angle between 0-85 degrees, height between 10-1000 nm, period between 100-1000 nm) that optimize diffraction behavior. The local quality of each ridge structure is tailored to achieve wavelength-selective diffraction, allowing efficient coupling of display light while minimizing diffraction of ambient light at harmful angles.
2Illumination intensity
If the grating diffracts ambient light, then some light is redirected, but ghost images appear at different locations for different wavelengths
Solution Approach 1:
The grating parameters (slant angle, height, period) are precisely controlled within specific ranges to change the diffraction behavior. By adjusting these parameters, the diffraction angles for different wavelengths are controlled such that ghost images are directed away from the user's eye while maintaining efficient display light coupling. The slant angle specifically controls the angular separation of diffracted orders.
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
The solution significantly reduces rainbow artifacts and improves the clarity of virtual and real-world content presented to the user, enhancing the overall user experience by minimizing unwanted light diffraction.
Implementation Method 1
The height of the plurality of ridges, the grating period, and the slant angle are configured to cause destructive interference between ambient light diffracted by the slanted grating
Implementation Method 2
The substrate is configured to reflect display light at the two opposite surfaces by total internal reflection
Implementation Method 3
The grating may diffract both the light of the projected image and light from the surrounding environment
Data Source
AI summary
A waveguide display includes a substrate having two opposite surfaces, and a slanted grating at a first surface of the two opposite surfaces of the substrate. The slanted grating includes a plurality of ridges and is characterized by a grating period in one direction. The plurality of ridges is tilted at a slant angle with respect to a surface normal of the first surface and is characterized by a height. The height of the plurality of ridges, the grating period, and the slant angle are configured to cause destructive interference between ambient light diffracted by the slanted grating. In some embodiments, a difference between the height of the plurality of ridges and an integer multiple of the grating period divided by the tangent of the slant angle is less than a threshold value.