BLACES Grating for HMD Light Coupling Efficiency
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Solution Overview
Problem
Conventional blaze gratings used in head-mounted displays (HMDs) suffer from low diffraction efficiency due to shadowing effects at small grating widths, making them unsuitable for plane-plate HMDs, while binary gratings offer only 30% efficiency, and oblique gratings are expensive to produce.
Innovation Solution
The use of BLACES gratings with a diffraction efficiency of 65-75% over a large incidence range, which can be produced using electron beam lithography or NanoImprint technology, allowing for simple and cost-effective replication, and direct photolithographic transfer onto the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If conventional blaze gratings are used with small grating widths to achieve strong light deviation, then the angle of light with respect to the interfaces is increased, but shadowing effects occur that reduce diffraction efficiency
Solution Approach 1:
The patent transitions from surface gratings (2D) to volume gratings (3D) by etching periodic structures into the bulk of the transparent plate. This dimensional change allows light to interact with the grating structures throughout the material volume rather than just at the surface, eliminating shadowing effects and achieving high diffraction efficiency (65-75%) even with small grating widths that provide the necessary strong light deviation.
2Ease of manufacture
If binary gratings are used instead of blaze gratings, then manufacturing is simplified, but diffraction efficiency drops to at best 30 percent
Solution Approach 1:
The patent changes the fundamental parameter of grating structure from surface-level binary patterns to volumetric periodic refractive index modulations. By creating etched channels that extend through the material thickness with periodic variations in refractive index, the system achieves blaze grating-level diffraction efficiency (65-75%) while maintaining manufacturing simplicity through direct photolithographic patterning and etching processes.
3Loss of energy
If oblique gratings are used to achieve high diffraction efficiency, then luminous efficiency is improved, but production costs increase significantly
Solution Approach 1:
The patent employs a master template approach where a single high-precision master grating pattern is created using electron beam lithography, then replicated multiple times using NanoImprint technology. This copying method allows the complex volumetric grating structures to be reproduced with high fidelity across many devices at low cost, achieving oblique grating performance without the associated high manufacturing expenses.
4Ease of manufacture
If surface gratings are used for manufacturing reasons, then production is simplified, but diffraction efficiency is limited by shadowing effects
Solution Approach 1:
The patent transitions from surface gratings (2D) to volume gratings (3D) by etching periodic structures into the bulk of the transparent plate. This dimensional change allows light to interact with the grating structures throughout the material volume rather than just at the surface, eliminating shadowing effects and achieving high diffraction efficiency (65-75%) even with small grating widths that provide the necessary strong light deviation.
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
BLACES gratings achieve high diffraction efficiency for both light coupling into and out of the plane plate, reducing production costs and improving luminous efficiency in HMDs, making them suitable for entertainment electronics applications.
Implementation Method 1
The light is coupled into the plane-parallel plate by means of diffraction at a grating
Implementation Method 2
The angle of the light with respect to the interfaces of the planar light guide is greater than the angle of total reflection
Implementation Method 3
The light is coupled out again by means of diffraction at a grating
Data Source
AI summary
An optical display device, in particular for use in a head-up display or a head-mounted display, comprises an essentially planar light guide, an image-generating system, a first diffraction grating by which light that comes from the image-generating system can be coupled into the light guide, and a second diffraction grating, by which the light can be coupled out again from the light guide. At least one of the two diffraction gratings is a binary-blazed grating having a multiplicity of diffraction structures, which are composed of a multiplicity of individual substructures that ensure a blaze effect and in plan view have the shape of a closed geometrical surface.