The coherent backlight unit (C-BLU) using a diffractive optical element (DOE) for full-color flat-panel holographic display is proposed. The coherent backlight unit is composed of two diffractive optical elements (DOEs) that are imprinted on the same glass substrate. The illumination area of the backlight is 250 mm x 130 mm and the thickness is 2.2 mm, which is slim compared to other conventional coherent backlight units for holographic display systems. In experiments, the total efficiency is measured as 0.8% at red (638 nm), 3.9% at green (520nm), and 3.4% of blue (473 nm). As a result, we could get the 10 inch full color holographic display with 4k resolution.
The novel design for the compact augmented reality (AR) glasses that utilize holographic optical element (HOE) as a combiner is presented. The wide field of view (FoV) that is larger than 90°, full color and high contrast ratio (CR) are achieved based on the single layer HOE, which has the thickness of 25 μm. In order to implement compactness of AR glasses using HOE combiner, the combination of optical lenses is proposed. In this design, a chromatic aberration and astigmatism, which are caused by highly off-axis projection of the image onto HOE, and the precise wavefront reproduction that maximize the efficiency of the HOE are taken into account simultaneously. The geometrical image distortion is corrected by implementation of image pre-distortion algorithm. The interpupillary distance (IPD) adjustment is applied to compensate small eye box. Based on the design, wearable prototype is introduced. Through the experiments both on benchtop and prototype, at the distance of 2 m, large image with diagonal of 150 inches is displayed.
Future commercialization of glasses-free holographic real 3D displays requires not only appropriate image quality but also slim design of backlight unit and whole display device to match market needs. While a lot of research aimed to solve computational issues of forming Computer Generated Holograms for 3D Holographic displays, less focus on development of backlight units suitable for 3D holographic display applications with form-factor of conventional 2D display systems. Thereby, we report coherent backlight unit for 3D holographic display with thickness comparable to commercially available 2D displays (cell phones, tablets, laptops, etc.). Coherent backlight unit forms uniform, high-collimated and effective illumination of spatial light modulator. Realization of such backlight unit is possible due to holographic optical elements, based on volume gratings, constructing coherent collimated beam to illuminate display plane. Design, recording and measurement of 5.5 inch coherent backlight unit based on two holographic optical elements are presented in this paper.
We propose the coherent backlight unit (BLU) using Holographic Optical Element (HOE) for full-color flat-panel holographic display. The HOE BLU consists of two reflective type HOEs that change the optical beam path and shape by diffraction. The diverging incident beam is transformed to the collimated beam which has a very small diffraction angle (7.5°) by HOE 1 (H1) in order to illuminate the whole display. This collimated beam is converged to a point at a distance from the glass substrate by HOE 2 (H2). As a result, the diverging incident beam is converted to a point light by H1 and H2. When the high resolution Spatial Light Modulator (SLM) displaying Computer Generated Hologram (CGH) is illuminated by HOE BLU, the hologram image is displayed at a view point near focal point. Practically, we fabricated the full color HOE BLU for 5.5" flat panel holographic display by using the proposed design. At least 5.5" size of HOE is required to illuminate the whole panel. For this reason, we recorded 150 mm x 90 mm size HOE on the 10 mm thickness glass substrate. This HOE BLU exhibits a total efficiency of 8.0% at Red (660 nm), 7.7% at Green (532 nm), 3.2% at Blue (460 nm) using optimized recording conditions for each wavelength. Finally, a bright full color hologram image was achieved.
We propose slim coherent backlight unit for a mobile holographic display. This backlight unit consists of glass substrate for waveguide and two surface gratings produced by two-beam interference. The area of backlight illumination is 150 by 85 mm, and the thickness is 0.7 mm, which is thin compared to other conventional coherent backlight units. This backlight unit exhibits a total efficiency of 0.1%, preserving the collimation and a uniformity of 80% over the whole area. The proposed slim coherent backlight can be applied to a mobile holographic display.
We propose the effective viewing window enhancement method for a holographic display with an amplitude-only SLM by using algorithmic approach. The basic concept is the superposition principle of holography. The multiple computer generated holograms (CGH) can be displayed on the SLM, and multiple 3D images are reconstructed at different positions within a viewing window simultaneously. In the experiments, we have implemented the holographic display using an amplitude-only SLM, a field lens, and laser light sources. We can observe the holographic 3D image in the frustum formed by the field lens through the viewing window located in the Fourier plane of the hologram. To enhance the effective viewing window, we generate multiple CGHs with an observer’s eye positions, and then overlap them to make the final CGH. Multiple 3D images can be reconstructed in different positions within the theoretical viewing window from the CGH displayed on SLM. This makes the enlargement of viewing zone that can observe the holographic images. The multiple holograms can be also made for enlargement of the viewing window along both horizontal and vertical direction (2D enlargement viewing zone). We confirmed that the experimental results and the simulation based on Rayleigh-Sommerfeld theory match well.
We report on electrically-driven diffraction grating, where refractive index of a liquid crystal (LC) was modulated periodically at an interval of 700 nm by applying an external DC bias to a metallic nanograting (NG). The LC-NG structure exhibited a maximum refractive index variation (Δn) of 0.088 and a diffraction efficiency (η) change of 0-16% with a large diffraction angle of 64° for incident light of 633 nm wavelength. This approach, with the help of faster electronics, provides an opportunity of developing active holograms for real 3D display
We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates. We found that GaN morphology affected the EL and achieved enhanced EL of GaN-LEDs on glass by identifying the optimal GaN morphology having both high crystallinity and compatibility for device fabrication. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of ~ 9100 cd/m<sup>2</sup> at the peak wavelength of 495 nm. The EL color could be adjusted mainly by varying the quantum well temperature. In addition, new growth methods for achieving high GaN crystallinity at a low growth temperature (e.g. ~700°C) were briefly reviewed and attempted by adopting selective heating. We expect that performance of the GaN LEDs on glass can be much enhanced by enhancing GaN crystallinity and p-GaN coating, and evolvement of low-temperature growth of high-quality GaN might even customize ordinary glass as a substrate, which enables high-performance, low-cost lighting or display.
We propose compact holographic printer using RGB waveguide hologram while reducing overall device size and quantity of elements with integrated functionality of each optical element. For glasses-free 3D experience anywhere anytime, it is critical to make holography device that can be as compact and integrated as possible. Compared to the conventional optics-based structure, our RGB WGH-based one reduces the overall size by 20 times, the number of components by 10 times, and improves the optical efficiency by 3 times, with comparable holographic quality to the conventional optics-based approaches. Proposed research can be useful for both general consumers and professionals like 3D photography and medical 3D image printing applications.