Waveguide Image Combiner for AR Displays
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Solution Overview
Problem
Current waveguide image combiners for augmented reality displays have limited horizontal and vertical fields of view (FOV) and eye box sizes, leading to a suboptimal user experience due to chromatic aberrations, astigmatism, image overlap, and small light engine pupils, which restrict the diagonal FOV to less than 50 degrees and eye box to less than 10 mm×10 mm.
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, respectively, while maintaining high light transmission efficiency, by replicating the light engine's pupil vertically and using multiple VHOEs with different angular ranges to eliminate cross-talk and chromatic aberrations.
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 to less than 40 degrees diagonal and eye box is less than 10 mm×10 mm
Solution Approach 1:
The waveguide substrate is divided into multiple functional zones with different HOE configurations. The input HOE is segmented into multiple regions to accept light from different angles, and the output HOE is segmented to direct light to different output angles, enabling expanded FOV through spatial segmentation of optical functions
Solution Approach 2:
The patent transitions from conventional 2D surface HOEs to 3D volume HOEs embedded within the waveguide substrate. This dimensional transition enables light to be trapped and propagated through total internal reflection in three dimensions, significantly expanding the angular acceptance and output ranges beyond what surface HOEs can achieve
2Ease of operation
If the eye box is enlarged to accommodate eye movement and tolerances, then user comfort improves, but the FOV decreases due to the geometric relationship between eye box size and FOV angle
Solution Approach 1:
The patent changes the optical parameters by using volume HOEs with specific diffraction efficiency profiles and angular selectivity characteristics. These parameter changes enable the system to maintain high diffraction efficiency over wider angular ranges, decoupling the traditional trade-off between eye box size and FOV angle
3Illumination intensity
If transmission HOEs are used to maintain high light transmission, then real imagery visibility is improved, but chromatic aberrations and astigmatism occur
Solution Approach 1:
The patent replaces conventional transmission HOE mechanics with reflection-based volume HOE mechanics. The volume HOEs use total internal reflection to trap and guide light, substituting the transmission mechanism that causes chromatic aberration and astigmatism with a reflection-based TIR mechanism that eliminates these optical defects while maintaining high light transmission
4Area of stationary object
If the light engine pupil is enlarged to increase FOV, then the angular range expands, but the device size and weight increase
Solution Approach 1:
The patent uses the third dimension (depth within the waveguide substrate) to expand the angular acceptance range. Volume HOEs embedded in the substrate utilize the z-dimension to trap and guide light at multiple angles, enabling large FOV without proportionally increasing the x-y plane dimensions of the device
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 achieves a diagonal FOV of at least 50 degrees, horizontal FOV of at least 40 degrees, and a large eye box of 20 mm×10 mm, while maintaining high light efficiency and resolving issues of astigmatism and color balance, thereby enhancing the user experience in augmented reality displays.
Implementation Method 1
The incoupling VHOEs are configured to diffract light from the light engine in first and second non-overlapping but contiguous angular ranges horizontally 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 with a horizontal FOV equal to the extent of the first and second angular ranges
Implementation Method 4
The reflective VHOEs transmit light from the real imagery across the entire visible spectrum with an approximately 20% reduction at the wavelengths of light from the light engine
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 pairs of overlapping incoupling and outcoupling VHOEs to expand the horizontal FOV and a Y expander to expand the vertical FOV. This suitably provides an expanded horizontal and vertical 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≤30 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.