Gradient-Index Waveguide for Uniform AR Display Brightness
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Solution Overview
Problem
Augmented reality devices suffer from non-uniform brightness of virtual images due to varying replication distances of light components caused by different incidence and propagation angles, leading to inconsistent brightness across the viewing field.
Innovation Solution
A waveguide with a non-uniform refractive index profile, either continuous or stepwise, is used to minimize the difference in replication distances of light components, improving brightness uniformity by reducing the ratio of maximum to minimum replication distances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional waveguide with uniform refractive index is used, then the device structure is simple and easy to manufacture, but the brightness uniformity of the virtual image deteriorates due to varying replication distances
Solution Approach 1:
The waveguide employs a non-uniform refractive index distribution where the refractive index varies along the propagation direction. Specifically, the refractive index is higher near the entrance light-coupling element and lower near the exit light-coupling element. This local variation in optical properties compensates for the varying replication distances of different light components, ensuring uniform brightness across the viewing field while maintaining a relatively simple waveguide structure.
Solution Approach 2:
The invention changes the refractive index parameter of the waveguide material as a function of position. By designing the refractive index to decrease along the light propagation direction, the optical path lengths of different light components are equalized, which uniformizes the replication distances and consequently the brightness distribution. This parameter change approach resolves the brightness uniformity issue without requiring complex multi-element structures.
2Adaptability or versatility
If light components with different propagation angles are transmitted through the waveguide, then the field of view is covered, but the replication distances vary causing non-uniform brightness
Solution Approach 1:
The refractive index is designed to vary continuously or in steps along the propagation direction to compensate for the angle-dependent replication distances. Light components with different propagation angles experience different refractive index profiles, which equalizes their effective optical path lengths and replication distances, resulting in uniform brightness across the entire field of view.
Solution Approach 2:
The invention introduces a gradient in the refractive index along the propagation direction (adding a dimensional variation to the optical property). This dimensional change in the refractive index profile enables the waveguide to handle multiple propagation angles uniformly, as each angle experiences a tailored refractive index environment that compensates for its specific replication distance variation.
3Illumination intensity
If the replication distance ratio (γmax/γmin) is reduced to improve brightness uniformity, then the optical design becomes more complex
Solution Approach 1:
Instead of using multiple discrete optical elements or complex structures, the invention achieves the desired replication distance uniformization by simply changing the refractive index parameter as a continuous function of position. This single-parameter approach (refractive index gradient) is mathematically elegant and can be implemented through material composition gradients or layered structures, avoiding the need for complex multi-component optical designs.
Solution Approach 2:
The waveguide can be constructed using composite material structures with varying refractive indices, such as layers of different glass compositions or polymer blends. These composite materials provide the required non-uniform refractive index profile while maintaining manufacturability through established fabrication techniques for gradient-index materials.
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 non-uniform refractive index waveguide enhances brightness uniformity by ensuring consistent brightness levels across the viewing field, addressing the non-uniformity issues in conventional waveguides.
Implementation Method 1
The mechanism of propagation within the waveguide is typically total internal reflection
Implementation Method 2
The entrance light-coupling element diffracts the received light over a field of view defined by a first angular range
Implementation Method 3
the first exit light-coupling element diffracting the transmitted light out of the waveguide
Implementation Method 4
The waveguide features a non-uniform refractive index that improves the brightness uniformity of light diffracted from the outcoupling grating
Data Source
AI summary
A waveguide for augmented reality devices is described. The waveguide may be incorporated into an optical element that further includes an incoupling grating and an outcoupling grating. Imaging light is directed into the incoupling grating and diffracted into the waveguide. The diffracted light propagates within the waveguide to the outcoupling grating and is diffracted to the viewing field of a user of the device. The waveguide features a non-uniform refractive index profile that improves the brightness uniformity of light diffracted from the outcoupling grating.