Research on off-axis reflector-based Augmented Reality Near-Eye Displays (AR NED) is advancing. These displays use a single light source, resulting in a narrow Eyebox and Field of View (FOV), but they are lightweight and suitable for long-term use.
Holographic Optical Elements (HOEs) are used to enhance image quality with narrow-bandwidth laser light sources. Adding compensating HOEs can increase the Eyebox when using micro OLEDs with a wide spectral bandwidth. However, achieving a 15mm Eyebox remains challenging without user calibration for interpupillary distance.
This presentation explores increasing the Eyebox in the off-axis reflector AR NED using Holographic multi-configuration, which involves angular multiplexing HOEs at two different angles. The research team designed prototypes and compared models with and without Holographic Multi-configuration.
In conclusion, the study presents a prototype structure using a wide spectral bandwidth uOLED and two angularly multiplexed HOEs to achieve a larger Eyebox.
Many architectures of near eye display (NED) using a holographic optical element (HOE) come on the market. HOE has already been successfully industrialized due to its easy manufacturing process and small form factor. However, many studies are being conducted to solve degradation effect by the eye glow that the visibility of the user decreases occurred to external light. HOE is generally used as an element of NED for its good angular and wavelength selectivity characteristics. The parameters controlling those characteristics are the refractive index change and the thickness of HOE. Although the selectivity characteristics are optimized by regulating the two parameters, the eye glow occurs because the HOE reacts in parts other than the desired characteristics for sunlight and white light sources. For a fundamental reason, eye glow is further caused by a sudden refractive index change in boundary condition when incident into the HOE from the air. In this study, we figure out that the boundary condition changes continuously by apodization of the refractive index of HOE for eye glow reduction. Also, we calculate the angular and wavelength selectivity efficiency using scalar Fourier modal method (sFMM) based on rigorous coupled-wave analysis (RCWA) according to the thickness and refractive index change, and investigate the relationship between those parameters.
In our previous work, a meta-surface was designed using adjoint-based topology optimization for high-efficiency. However, since the design pattern was complicated and the size of element was small, it was hard to maintain in the actual process and was not appropriate for commercialization. To solve this problem, we use the adjoint inverse design method by applying the local curvature filter algorithm and curvature flow technique in this study. The meta-surface design was conducted using Fourier modal method, based on rigorous coupled-wave analysis. It is assumed that the meta-surface is composed of pixels having a relative dielectric constant of the dielectric material. The dielectric constant distribution in the meta-lattice region is a design variable for optimization, and the dielectric constant of the air layer is converted over the duration. Traditionally, Gaussian filter was used to change to a processable pattern. However, since it does not maintain high efficiency characteristics, we used the local curvature filter (LCF) method. As a result, the results of efficiently finding and filtering small and complex patterns while maintaining characteristics were acquired. The LCF detects it as a local area according to the degree of curvature. The detected area is filtered using a Bernstein filter, and then combined with the global pattern again. In addition, we re-progress filtering for smooth patterns using the curve flow to adjust the curve threshold value to design the meta-surface. Consequently, we propose the method and theory of novel algorithm as a way of maintaining the high efficiency properties of the meta-surface in practice.
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