Multilayer Grating Rainbow Reduction Waveguide Displays
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Solution Overview
Problem
Conventional near-eye displays using diffractive optics suffer from a 'rainbow effect' due to stray ambient light being scattered and diffracted into the user's field of view, detrimental to augmented and mixed reality experiences.
Innovation Solution
A multilayer grating with a 2-dimensional diffraction pattern, comprising layers with inverted index profiles, is integrated into the waveguide display to create destructive interference between diffracted rays, mitigating the rainbow effect by using a first and second patterned layer with different material arrangements that produce first and second diffracted rays which interfere with each other.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If diffractive optics are used in near-eye displays, then image light can be coupled into the waveguide, but stray ambient light is scattered and diffracted into the user's field of view causing the rainbow effect
Solution Approach 1:
The waveguide is divided into multiple functional layers: an input grating for incoupling image light, a multilayer grating with alternating high and low refractive index layers for rainbow reduction, and an output grating for decoupling light to the eyebox. Each layer performs a specific function to resolve the contradiction between effective image light coupling and stray light rejection.
Solution Approach 2:
The multilayer grating uses the same diffraction mechanism that causes the rainbow effect to instead create destructive interference. By designing alternating layers with opposite index profiles, the system causes stray light diffracted by one layer to be canceled by the complementary layer, converting the harmful diffraction effect into a beneficial cancellation mechanism.
2Device complexity
If a single-layer diffraction grating is used, then the structure is simple, but it cannot effectively reduce the rainbow effect
Solution Approach 1:
The solution transitions from a single-layer grating to a multilayer grating structure, adding the dimension of layer stacking. The alternating high and low refractive index layers create a three-dimensional structure where light interacts with multiple interfaces, enabling destructive interference of stray light while maintaining a compact form factor suitable for waveguide integration.
Solution Approach 2:
The multilayer grating employs composite material construction with alternating layers of high refractive index material and low refractive index material. This composite structure creates the necessary index profile variations to generate complementary diffraction patterns that cancel each other, effectively reducing the rainbow effect while maintaining structural integrity.
3Illumination intensity
If ambient light is allowed to pass through the waveguide, then the display is bright, but stray light enters the field of view and degrades image quality
Solution Approach 1:
The multilayer grating applies local quality differentiation by creating regions with opposite index profiles within the waveguide structure. The high index layers and low index layers are spatially arranged to selectively affect different light paths: allowing image light to pass while creating destructive interference specifically for stray ambient light entering at angles outside the intended field of view.
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 multilayer grating effectively reduces the rainbow effect by ensuring that non-zero diffraction orders produced by different patterned layers destructively interfere, enhancing the clarity and quality of the augmented and mixed reality experience.
Implementation Method 1
the first diffracted ray and the second diffracted ray destructively interfere with each other based in part on the inverted index profile, thereby reducing the rainbow effect
Implementation Method 2
Ambient light incident on the first patterned layer and the second patterned layer creates a first diffracted ray and a second diffracted ray
Data Source
AI summary
A multilayer grating is a diffraction grating that includes a plurality of layers. The plurality of layers arranged to form a 2-dimensional grating, the layers including at least a first patterned layer and a second patterned layer. The first patterned layer includes a plurality of different materials that are arranged in a first pattern such that the first patterned layer has a first index profile. The second patterned layer includes a plurality of different materials that are arranged in a second pattern such that the second patterned layer has a second index profile that is inverted relative to the first index profile. Ambient light incident on the first patterned layer and the second patterned layer creates a first diffracted ray and a second diffracted ray, respectively, and the first diffracted ray and the second diffracted ray destructively interfere with each other based in part on the inverted index profile.