Input/Output Coupling Grating with Asymmetric Blaze for Low Back Reflection
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Solution Overview
Problem
Existing augmented and virtual reality display systems face challenges in providing a comfortable and natural presentation of virtual image elements amidst real-world imagery due to issues like high back reflection, polarization dependence, and limited field of view, leading to discomfort and reduced image quality.
Innovation Solution
The development of grating structures for input and output coupling in waveguides that are insensitive to polarization, have low back reflection, and allow operation over a wide range of input angles, including asymmetric blazed gratings with high index materials, which enhance light coupling and directionality, and are used in head-mounted displays with microLED projection systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If high index material is used in grating structures to reduce optical losses, then light coupling efficiency is improved, but back reflection increases causing unwanted coupling and ghosting
Solution Approach 1:
The patent employs asymmetric blazed grating profiles where the blaze angle is specifically optimized to be less than the critical angle for total internal reflection. This asymmetric geometry allows the grating to efficiently couple light into the waveguide while minimizing back reflection, as the asymmetric profile directs reflected light away from the coupling interface. The blaze angle optimization creates an asymmetric response that favors forward coupling over backward reflection.
Solution Approach 2:
The patent changes the critical parameter of the blaze angle to be less than the critical angle for total internal reflection. This parameter change fundamentally alters the optical behavior by ensuring that light coupled into the waveguide remains trapped through total internal reflection, while simultaneously reducing back reflection. The specific parameter range (blaze angle < critical angle) creates optimal conditions for both low optical loss and low back reflection.
2Loss of energy
If conventional gratings are used to couple light into waveguides, then light coupling is achieved, but polarization dependence reduces efficiency for unpolarized light
Solution Approach 1:
The patent designs a grating structure that performs effectively for multiple polarization states simultaneously. By using a blaze angle less than the critical angle and optimizing the grating profile, the system achieves universal light coupling efficiency regardless of whether the incident light is horizontally polarized, vertically polarized, or unpolarized. This multi-functional design allows the same grating structure to efficiently couple different polarization states without requiring separate optimization for each polarization type.
3Loss of energy
If standard grating structures are used, then light coupling is achieved, but field of view is limited reducing immersion experience
Solution Approach 1:
The patent employs a blazed grating profile with optimized blaze angle that dynamically adapts to different incident angles. The asymmetric blaze profile creates a broader angular acceptance window, allowing the grating to maintain efficient light coupling across a wider range of input angles. This dynamic response enables a larger field of view while preserving coupling efficiency, as the grating profile continuously optimizes the coupling condition for varying incident angles rather than being optimized for a single angle only.
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
These gratings improve light efficiency, reduce unwanted reflections, and enhance the presentation of virtual images, providing a more comfortable and realistic augmented reality experience with improved image quality and wider field of view.
Implementation Method 1
a grating structure optically coupled to the waveguide, the grating structure being configured to couple light from the light projection system into the waveguide
Implementation Method 2
Such high index layers can provide grating structures with relatively low optical losses. Due to the high index film, the reflected light can be significant (e.g., >10% for some incident angles)
Implementation Method 3
which can cause undesirable results, e.g., unwanted back reflection coupling and ghosting, reduced contrast, etc. in the virtual images
Data Source
AI summary
A head-mounted display system includes a waveguide configured to guide light from a light projection system coupled into the waveguide; a grating structure optically coupled to the waveguide, the grating structure being configured to couple light from the light projection system into the waveguide. The grating structure includes a grating layer having a grating with multiple ridges having ablaze profile in at least one cross-section, the blaze profile having an anti-blaze angle of 85° or less; and one or more additional layers on the grating layer, the additional layers including a first layer of a material having a refractive index of 1.5 or less at an operative wavelength of the head-mounted display, the first layer being an outermost layer of the grating structure.