Sub-wavelength Grating Waveguide for AR Glasses Reflection Reduction
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Solution Overview
Problem
Conventional wearable head-mounted displays (HMDs) experience reflections from waveguides, particularly when embedded in augmented reality glasses with curved prescription lenses, leading to jarring flashes for both the user and external viewers due to the alignment of light sources and viewers at specific angles.
Innovation Solution
A system incorporating a waveguide with a sub-wavelength grating on its surface, featuring rows of three-dimensional primitive structures that are smaller than the wavelength of visible light, which imparts destructive interference to reflected light and constructive interference to transmitted light, minimizing reflections while maintaining display quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a flat waveguide surface is used in HMD with curved prescription lens, then the waveguide can maintain simple structure and good light guidance, but reflections appear as jarring flashes at specific angles
Solution Approach 1:
The patent applies local quality by adding sub-wavelength grating structures only at specific locations on the waveguide surface where reflections occur, rather than modifying the entire waveguide. The grating is positioned at the curved prescription lens interface to locally address the reflection problem while maintaining the simplicity of the rest of the waveguide structure.
Solution Approach 2:
The patent changes the physical parameters of the waveguide surface by introducing sub-wavelength grating structures with specific geometric parameters (grating period smaller than wavelength, controlled depth and shape). This parameter change transforms the flat surface into a structured surface that manipulates light through diffraction and interference effects, converting harmful reflections into useful transmitted light.
2Object-affected harmful factors
If sub-wavelength grating is added to reduce reflections, then reflection reduction is achieved, but manufacturing complexity increases
Solution Approach 1:
The patent segments the anti-reflection function into discrete sub-wavelength grating structures that can be independently designed and fabricated. These grating elements are arranged in patterns that can be applied to existing waveguide manufacturing processes, allowing the anti-reflection functionality to be added as a separate layer or surface treatment rather than requiring complete redesign of the waveguide fabrication process.
3Object-affected harmful factors
If sub-wavelength grating structures are used, then destructive interference reduces reflected light, but the structures must be precisely smaller than wavelength requiring high manufacturing precision
Solution Approach 1:
The patent applies partial action by using sub-wavelength grating structures where the grating period is smaller than the wavelength of light, but not necessarily extremely small. This partial application of the sub-wavelength concept achieves sufficient destructive interference for reflection reduction while maintaining manufacturable dimensions. The grating depth and other parameters are optimized to provide adequate anti-reflection performance without requiring ultra-precise nanoscale fabrication.
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 sub-wavelength grating effectively reduces visible reflections from the waveguide, enhancing the appearance of HMDs by minimizing aberrations, making them appear like conventional lenses, without impacting the display or 'see-thru' properties experienced by the user.
Implementation Method 1
The sub-wavelength grating is configured to impart a phase that destructively interferes with light that reflects off the sub-wavelength grating
Implementation Method 2
The sub-wavelength grating is configured to impart a phase that constructively interferes with light that is transmitted through the sub-wavelength grating
Implementation Method 3
Once the light beams have been coupled into the waveguide, the light beams are 'guided' through the substrate, typically by multiple instances of total internal reflection (TIR)
Data Source
AI summary
A head-mounted display system (100) includes a lens element (110) supported by a support structure (102). The lens element (110) includes a waveguide (212) to couple light from an image source. The waveguide (212) includes a waveguide surface (207) and a grating (250). The grating (250) is disposed onto the waveguide surface (207) and includes rows of three-dimensional, 3D, primitive structures (435), with a height of the 3D primitive structures being smaller than a wavelength of visible incident light at a surface of the sub-wavelength grating.