Interleaved Rectangular Grating for AR Waveguide Eyebox Expansion
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Solution Overview
Problem
Existing diffractive waveguide combiners for augmented reality displays face challenges such as limited eyebox expansion, manufacturing tolerances, rainbow artefacts, and increased device size due to constrained optical structure dimensions, which affect image quality and user experience.
Innovation Solution
The use of an interleaved rectangular grating (IRG) with spatially offset and differently configured optical structures in a diffractive waveguide combiner to achieve two-dimensional pupil replication and efficient light coupling, minimizing rainbow artefacts and reducing device size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If conventional diffraction gratings are used in diffractive waveguide combiners, then light coupling is achieved, but eyebox expansion is limited and device size increases
Solution Approach 1:
The diffraction grating is segmented into multiple zones with different grating vectors, allowing different regions to direct light to different spatial locations. This segmentation enables two-dimensional pupil replication and eyebox expansion without proportionally increasing the overall device volume, as each zone independently contributes to the expanded eyebox.
Solution Approach 2:
The patent transitions from conventional one-dimensional grating structures to two-dimensional grating patterns with varying grating vectors. By introducing spatial variation in both x and y directions, the system achieves two-dimensional eyebox expansion, effectively utilizing the waveguide's full aperture area rather than expanding in a single dimension.
2Ease of manufacture
If diffraction gratings with fixed dimensions are used, then manufacturing is simplified, but rainbow artefacts increase
Solution Approach 1:
Different regions of the diffraction grating are assigned different local properties, specifically different grating vectors and periodicities. This local variation allows optimization of diffraction efficiency for different wavelength components in different zones, reducing chromatic dispersion and rainbow artefacts while maintaining manufacturability through zone-based fabrication approaches.
Solution Approach 2:
The grating parameters (period, orientation, depth) are varied spatially across the waveguide surface. By changing these parameters as a function of position, the system compensates for wavelength-dependent diffraction effects, minimizing rainbow artefacts. The parameters follow specific functional forms that can be implemented using standard lithographic and etching processes.
3Volume of stationary object
If optical structures are constrained in size, then device compactness is maintained, but image fidelity decreases
Solution Approach 1:
The input pupil image is replicated multiple times across the waveguide output face through the diffractive structures. Each replica carries the full image information, allowing the system to maintain high image fidelity at multiple output locations simultaneously. This copying approach effectively increases the functional imaging area without requiring proportionally larger optical structures.
Solution Approach 2:
The diffraction grating structures serve multiple functions simultaneously: they couple light into the waveguide, guide it through total internal reflection, replicate the pupil image, and couple it out to the viewer. This multi-functionality allows compact design while maintaining image fidelity, as a single grating structure performs what would otherwise require multiple separate optical components.
4Use of energy by moving object
If conventional grating configurations are used, then light coupling efficiency is achieved, but luminance uniformity across gaze angles deteriorates
Solution Approach 1:
The grating's local properties (period, orientation, depth) are optimized for different viewing angles and spatial locations. Each zone is designed to provide uniform luminance for its specific output direction, compensating for angular-dependent diffraction effects. This local optimization ensures that luminance remains uniform across the entire field of view despite variations in gaze angle.
Solution Approach 2:
The grating design creates equipotential conditions for light propagation by ensuring that all rays, regardless of their input angle or wavelength, experience equivalent optical path lengths and diffraction efficiencies. This is achieved through carefully designed grating parameter variations that compensate for angular and spectral deviations, resulting in uniform luminance output across all gaze directions.
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 IRG enhances eyebox expansion, improves image fidelity, reduces rainbow artefacts, and maintains a compact form factor by optimizing light scattering properties and efficiency, ensuring high luminance uniformity and contrast across various gaze angles.
Implementation Method 1
light coupled into the waveguide is expanded in two dimensions by an diffractive optical element
Implementation Method 2
waveguide in which light coupled into the waveguide is expanded in two dimensions by an diffractive optical element as well as coupled out of a waveguide towards a viewer
Data Source
AI summary
A diffraction grating is disclosed for use as an output element of a diffractive waveguide combiner for an augmented reality or virtual reality display. The grating includes an interleaved rectangular grating (1213) comprising a first rectangular periodic array of optical structures (1211) and a second rectangular periodic array of optical structures (1212) arranged on the plane. The first array of optical structures (1211) and the second array of optical structures (1212) differ from one another in at least one characteristic or the first array of optical structures (1211) are offset from the second array of optical structures (1212) by a factor which is different to half the period of the first or second rectangular array, such that the first array of optical structures (1211) and the second array of optical structures (1212) are configured to receive light from an input direction and to couple orders of the light in directions that are at angles to the input direction and to couple out orders of the light towards a viewer.