Stacked Waveguide Image Combiner for AR Displays
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Solution Overview
Problem
Current waveguide image combiners for augmented reality displays have limited field-of-view (FOV) and eye box sizes, leading to a less-than-ideal user experience due to mechanical weight and thickness issues, as well as inefficiencies in light transmission and image quality.
Innovation Solution
A full-color, large-FOV waveguide image combiner is achieved by stacking multiple individual waveguides with optically transparent substrates and volume holographic optical elements (VHOEs) that use total internal reflection (TIR) for mechanical protection, allowing for increased FOV and eye box sizes while maintaining high light efficiency and minimizing system weight and thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If traditional waveguide image combiners are used, then the system structure is simple, but the field-of-view and eye box sizes are limited
Solution Approach 1:
The waveguide is divided into multiple discrete transparent substrates (first substrate, second substrate, third substrate, etc.) that are optically coupled together. Each substrate can be independently manufactured and optimized, allowing the overall system to achieve larger eye box and FOV without requiring a single complex monolithic structure.
Solution Approach 2:
The system uses multiple different transparent substrates with potentially different optical properties (refractive indices, thicknesses, materials) that are optically coupled to form a composite waveguide system. This allows optimization of each substrate's contribution to the overall eye box and FOV performance.
2Reliability
If protective layers are added to protect VHOEs, then reliability improves, but weight and thickness increase
Solution Approach 1:
The transparent substrates serve multiple functions simultaneously: they act as mechanical protective layers for the VHOEs, serve as optical waveguide media for TIR, and provide structural support for the overall device. This multi-functionality eliminates the need for separate protective layers that would add weight and thickness.
Solution Approach 2:
The protective function and the waveguide function are merged into a single integrated structure. The transparent substrates that form the waveguide also provide mechanical protection to the VHOEs, combining what would traditionally be separate components into one unified system.
3Area of stationary object
If multiple stacked waveguides are used, then FOV and eye box increase, but light transmission efficiency decreases
Solution Approach 1:
The system optimizes key parameters including the refractive indices of the substrates, the thickness of each substrate, the spacing between substrates, and the positioning of VHOEs to maximize light transmission efficiency while achieving the desired FOV and eye box dimensions.
Solution Approach 2:
Different regions of the optical path are optimized for different functions: some substrates are optimized for maximum TIR efficiency, others for minimal absorption, and specific regions are positioned to optimize the eye box and FOV. The VHOEs are strategically positioned at specific locations to maximize outcoupling efficiency.
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 provides a diagonal FOV of 50° or greater, horizontal eye box of 20 mm or greater, and high light efficiency (>80%) for augmented reality displays, addressing limitations in current technologies regarding astigmatism, image overlap, color balance, and reduced eye boxes.
Implementation Method 1
uses an input Holographic Optical Element (HOE) to trap through total internal reflection (TIR) auxiliary content generated by an image engine inside a transparent substrate
Data Source
AI summary
A waveguide image combiner is used to transmit a monochrome or full-color image in an augmented reality display. The combiner uses multiple stacked waveguides, each having top and bottom substrates. The combiner also uses multiple pairs of incoupling and outcoupling VHOEs, which are sandwiched between the top and bottom substrates, to expand a first FOV and an image expander to expand the second or perpendicular FOV. This suitably provides an expanded FOV that offers a diagonal FOV≥50°, a horizontal FOV≥40 and a vertical FOV≥25°. The combiner also delivers a large horizontal eye box up to 20 mm and a vertical eye box of 10 mm while maintaining high light efficiency of the real scene (e.g. >80%). The system is able to use a light engine based on broadband (10 nm≤Δλ≤40 nm) LEDs and maintain a large horizontal field of view and high transmission of the real imagery.