Achromatic Prism Waveguide Coupling for AR Displays
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Solution Overview
Problem
Designing electronic devices with displays that present images close to the user's eyes can result in unsightly, bulky, or uncomfortable components that do not achieve desired optical performance if not properly optimized.
Innovation Solution
Incorporating a waveguide with surface relief gratings and an achromatic prism that redirects image light from the display module to the input coupler, allowing the display module to be positioned ergonomically within the device housing without compromising optical performance, and using offset collimating optics to mitigate ghost artifacts and conserve space.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the display module is positioned close to the user's eyes to present images with high immersion, then the optical performance is improved, but the device becomes bulky and uncomfortable for wear
Solution Approach 1:
An achromatic prism is introduced as an intermediary optical element between the display module and the waveguide. The prism redirects light from the display module at an angle that is not parallel to the waveguide normal, enabling the display module to be positioned ergonomically within the housing while still achieving proper optical coupling into the waveguide. This intermediary component resolves the contradiction by allowing optimal display positioning without compromising optical performance.
2Ease of operation
If the display module is positioned within the housing to improve ergonomics, then wear comfort is improved, but the optical performance may be compromised
Solution Approach 1:
The achromatic prism serves as a mediator that enables the display module to be positioned within the housing for ergonomic reasons while maintaining proper optical coupling. The prism redirects light at the appropriate angle to match the input vector of the surface relief grating, ensuring that both ergonomic positioning and optical performance are achieved simultaneously.
Solution Approach 2:
The system changes the angular parameter of light propagation by introducing the achromatic prism, which redirects light from a direction non-parallel to the waveguide normal into a direction parallel to the input vector of the surface relief grating. This parameter change enables the display module to be positioned within the housing while maintaining optimal optical coupling.
3Device complexity
If conventional optics are used to direct light into the waveguide, then the structure is simple, but ghost artifacts are produced and optical performance is reduced
Solution Approach 1:
The achromatic prism acts as an intermediary that eliminates ghost artifacts by properly redirecting light into the waveguide. The prism ensures that light enters the waveguide at the correct angle parallel to the input vector of the surface relief grating, preventing higher order diffractive modes from reflecting light off pixels and creating ghost images. This adds minimal complexity while eliminating harmful artifacts.
Solution Approach 2:
The achromatic prism is formed from composite materials with different refractive indices (first and second optical wedges made from different materials) to mitigate dispersion. This composite structure enables the prism to redirect light accurately across different wavelengths, eliminating ghost artifacts while maintaining optical performance across the visible spectrum.
4Device complexity
If the optical axis is aligned with the center of the field of view, then the optics are simple, but ghost artifacts are produced due to higher order diffractive modes
Solution Approach 1:
The system introduces asymmetric optical alignment by offsetting the optical axis of the collimating optics from the center of the field of view. This asymmetric positioning, combined with the achromatic prism, prevents higher order diffractive modes from reflecting light off pixels and creating ghost artifacts. The asymmetric alignment is necessary to eliminate the harmful ghost images while maintaining proper optical coupling.
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 enables the placement of display modules within device housings without discomfort to the user while maintaining optimal optical performance and reducing the production of ghost artifacts, thus enhancing user experience and device aesthetics.
Implementation Method 1
An achromatic prism may be optically interposed between the display module and the first SRG. The achromatic prism may redirect the image light from the display module to the first SRG in a direction parallel to the input vector.
Implementation Method 2
The achromatic prism may include first and second optical wedges formed from different materials to mitigate dispersion.
Implementation Method 3
The input coupler may include a first surface relief grating (SRG). The display module may produce image light that is coupled into the waveguide by the first SRG.
Implementation Method 4
The output coupler may include a second SRG. Image light that is coupled out of the waveguide by the second SRG.
Implementation Method 5
a waveguide, an input coupler, and an output coupler. The waveguide may have a lateral surface with a normal axis.
Data Source
AI summary
A display system may include a waveguide, an input coupler with a first surface relief grating (SRG), and an output coupler with a second SRG. A display module may produce image light that is coupled into the waveguide by the first SRG. The first SRG may have an input vector non-parallel with respect to a normal axis of the waveguide. The display module may have an optical axis tilted with respect to the input vector by a non-zero angle. A prism may redirect the image light from the module to the first SRG in a direction parallel to the input vector. The module may include lens elements with an optical axis offset with respect to the center of the field of the image light. This may cause the lens elements to output the image light in a direction parallel to the input vector of the first SRG.