Waveguide Image Combiners for AR Displays
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Solution Overview
Problem
Current waveguide image combiners for augmented reality displays face limitations in field of view (FOV), eye box size, and light efficiency, particularly with broadband LED sources, leading to reduced user experience due to chromatic aberrations, astigmatism, and small light engine pupils.
Innovation Solution
The use of reflection volume holographic optical elements (VHOEs) with gradient diffraction efficiency and a Y-expander to expand the horizontal and vertical FOV, combined with multiple stacked substrates for multiplexing angular and spectroscopic ranges, enhances light transmission and reduces artifacts like 'ghost' images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If conventional waveguide image combiners are used, then the device structure is simple, but the field of view is limited (diagonal FOV < 40°) and eye box size is small (< 10 mm × 10 mm)
Solution Approach 1:
The waveguide substrate is divided into multiple discrete HOE elements (input HOE, output HOE, and intermediate HOEs) that are spatially separated and functionally distinct. Each HOE handles specific angular ranges, allowing the system to achieve large FOV and eye box while maintaining manageable structural complexity through modular design
Solution Approach 2:
The patent introduces intermediate HOEs positioned at angular deviations from the direct input-output path, utilizing the angular dimension to expand the eye box area. By routing light through multiple angular stages rather than direct transmission, the system achieves 20 mm × 10 mm eye box while keeping the physical waveguide thickness manageable
2Illumination intensity
If broadband LED sources are used, then the illumination intensity is high, but chromatic aberrations and color balance issues occur
Solution Approach 1:
The broadband spectrum is segmented into multiple wavelength bands, with dedicated HOE elements optimized for specific color ranges (blue, green, red). Each HOE is engineered to handle its assigned wavelength range with appropriate diffraction efficiency, preventing chromatic aberrations while maintaining high overall illumination intensity from the LED source
Solution Approach 2:
Different regions of the waveguide and individual HOE elements have locally optimized properties tailored to specific wavelength ranges. The diffraction efficiencies, grating periods, and orientations are locally adjusted to match the spectral characteristics of broadband LED sources, achieving accurate color balance across the entire visible spectrum
3Area of stationary object
If the horizontal FOV is expanded to ≥40°, then the viewer's field of view is improved, but the eye box size becomes difficult to maintain
Solution Approach 1:
The patent utilizes the angular dimension by introducing intermediate HOEs that redirect light at specific angles (e.g., 30° horizontal deviation) to create multiple lateral image paths. This angular staging allows the system to achieve 40° horizontal FOV while maintaining a 20 mm × 10 mm eye box, as each angular stage contributes to both FOV expansion and eye box positioning
4Area of stationary object
If multiple HOE layers are stacked to expand FOV, then the field of view increases, but light transmission efficiency decreases due to multiple diffraction events
Solution Approach 1:
Each HOE element in the stacked configuration is locally optimized with peak diffraction efficiency at its specific operating wavelength and angle. The intermediate HOEs are positioned and oriented to capture light at specific angular ranges with high efficiency, minimizing losses despite multiple diffraction events. This local optimization ensures that each stage contributes maximally to FOV expansion while preserving overall light transmission
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
This approach achieves a diagonal FOV of ≥50°, horizontal FOV of ≥40°, and vertical FOV of ≥25° with a large eye box of 20 mm × 10 mm, maintaining high light efficiency and resolving issues of astigmatism and color balance, thereby improving the overall augmented reality display experience.
Implementation Method 1
The incoupling VHOEs are configured to diffract light from the light engine into the substrate
Implementation Method 2
where the light travels through total internal reflection (TIR) to the outcoupling VHOEs
Implementation Method 3
the outcoupling VHOEs which diffract the light to the viewer
Implementation Method 4
A Y-expander is configured to duplicate a pupil of the light engine vertically N time
Implementation Method 5
The outcoupling VHOEs are configured with a gradient in their diffraction efficiency in the horizontal to provide uniform light intensity in the horizontal FOV
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 substrates and multiple pairs of incoupling and outcoupling VHOEs 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. The approach resolves issues with current embodiments including astigmatism, image overlap, color balance, and small light engine pupils leading to reduced eye boxes.