In this paper, we introduce an approach—multiplexing gratings plus drive signal management scheme implemented on a micro-display device within an optical engine—to precisely adjust the color uniformity of an Augmented Reality (AR) eyewear display. This display is based on Volume Holographic Optical Elements (VHOEs) and a waveguide. Our method simplifies the complexity of multiplexing, requiring only a single optical waveguide and three RGB gratings for primary colors to achieve a full-color eyewear display with an expansive horizontal field of view (FOV) of nearly 30° and less than 3% ΔELab color non-uniformity.
In this study, a near-eye display system is presented based on a dual-layer holographic light-guide structure. Each holographic light-guide is generated with wavelength multiplexing for color display and is designed to offer half FOV for the whole system. The full FOV of the system can reach 50°. The holographic light-guide utilized Volume Holographic Optical Elements (VHOEs) with linear grating function as the in-coupler and out-coupler. For a light-guide display, the full-color information light must propagate in the light-guide within Total Internal Reflection (TIR) condition. Therefore, it is imperative that the device must employ the gratings with distinct periods for directing red, green, and blue information light. Then the information light with different colors must propagate in different light-guides, in order to prevent crosstalk and ghost noise induced by Raman-Nath gratings, such as surface relief grating, with different periods. In comparison, the VHOEs can record the gratings with distinct periods on a single material with low crosstalk because of their strong wavelength selectivity. In the wavelength multiplexing process, each photopolymer material recorded three gratings for red, green, and blue images. Each grating primarily affects the information light of its corresponding color and guides the information light with different colors into the same light-guide within the TIR condition. In our case, a single waveguide provided the color images with about 25 degrees horizontal FOV. The diffraction efficiency of the primary information light is much larger than crosstalk noise.
We proposed a Maxwellian-view display based on Volume Holographic Optical Element (VHOE) and light-guide for see-through Head-Mounted Display (HMD) system. The diffractive light-guide is advantageous because of compact construction that leads to smaller and lighter devices. Furthermore, the advantage of VHOEs lies in their potential to achieve reducing energy loss and increasing battery endurance, owing to the higher diffraction efficiency. The Maxwellian-view display is unique because the image quality is almost not affected by the observer's focus distance or the diopter of their pupils. The proposed system utilizes a VHOE with linear grating as the in-coupling device and a VHOE with convex lens function as the out-coupling. In order to achieve a high Field of View (FOV), a prefabricated holographic lens with a high Numerical Aperture (NA) was utilized to record the out-coupling. The proposed device achieved the diagonal FOV as 50°. In this study, the detailed fabrication method of the holographic light-guide based on VOHEs was presented. Furthermore, the design method in order to improve image quality was also proposed. The optical simulation for determining image quality and optimizing was achieved based on the ray tracing method. In this case, astigmatism aberration caused by the diffractive light-guide degraded the image quality. Therefore, a cylindrical lens is necessary if the compensation of astigmatism is desired.
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