Deep learning-based computer-generated holography (CGH) has recently demonstrated tremendous potential in three-dimensional (3D) displays and yielded impressive display quality. However, current CGH techniques are mostly limited on generating and transmitting holograms with a resolution of 1080p, which is far from the ultra-high resolution (16K+) required for practical virtual reality (VR) and augmented reality (AR) applications to support a wide field of view and large eye box. One of the major obstacles in current CGH frameworks lies in the limited memory available on consumer-grade GPUs which could not facilitate the generation of highdefinition holograms. Moreover, the existing hologram compression rate can hardly permit the transmission of high-resolution holograms over a 5G communication network, which is crucial for mobile application. To overcome the aforementioned challenges, we proposed an efficient joint framework for hologram generation and transmission to drive the development of consumer-grade high-definition holographic displays. Specifically, for hologram generation, we proposed a plug-and-play module that includes a pixel shuffle layer and a lightweight holographic super-resolution network, enabling the current CGH networks to generate high-definition holograms. For hologram transmission, we presented an efficient holographic transmission framework based on foveated rendering. In simulations, we have successfully achieved the generation and transmission of holograms with a 4K resolution for the first time on an NVIDIA GeForce RTX 3090 GPU. We believe the proposed framework could be a viable approach for the evergrowing data issue in holographic displays.
Guest editors Yi-Hsin Lin, Victor Reshetnyak, Kai-Han Chang, and Yan Li introduce the Special Section on Micro-optical Systems Based on Liquid Crystals.
Current learning-based Computer-Generated Holography (CGH) algorithms often utilize Convolutional Neural Networks (CNN)-based architectures. However, the CNN-based non-iterative methods mostly underperform the State-Of-The-Art (SOTA) iterative algorithms such as Stochastic Gradient Descent (SGD) in terms of display quality. Inspired by the global attention mechanism of Vision Transformer (ViT), we propose a novel unsupervised autoencoder-based ViT for generating phase-only holograms. Specifically, for the encoding part, we use Uformer to generate the holograms. For the decoding part, we use the Angular Spectrum Method (ASM) instead of a learnable network to reconstruct the target images. To validate the effectiveness of the proposed method, numerical simulations and optical reconstructions are performed to compare our proposal against both iterative algorithms and CNN-based techniques. In the numerical simulations, the PSNR and SSIM of the proposed method are 26.78 dB and 0.832, which are 4.02 dB and 0.09 higher than that of the CNN-based method, respectively. Moreover, the proposed method contains less speckles and features a higher display quality than other CGH methods in experiments. We suggest the improvement might be ascribed to the ViT’s global attention mechanism, which is more suitable for learning the cross-domain mapping from image (spatial) domain to hologram (Fourier) domain. We believe the proposed ViT-based CGH algorithm could be a promising candidate for future real-time high-fidelity holographic displays.
Augmented reality (AR) has attracted great attention from academia and industry for its potential applications in diversified fields. In order to eliminate the accommodation-vergence conflict in AR display systems, which causes 3D visual fatigue after prolonged use, several approaches have been proposed. This paper presented three types of true three-dimensional (3D) AR display techniques. The first is multi-plane volumetric display based on liquid crystal (LC) devices. We utilized fast switching polymer stabilized LC (PSLC) scattering films and polarization selective cholesteric LC (CLC) reflective films, respectively, realizing magnified 3D images augmented on the real world. The second is holographic display based on two holographic optical elements (HOEs), functioning as an optical combiner, an ocular lens and a beam expander simultaneously. For the third technique, we proposed two super-multi-view display approaches based on polarizing glasses and geometry phase optical elements (GPOEs), respectively.
Augmented reality (AR) technology, which integrates virtual computer-generated information into the real world scene, is believed to be the next-generation human-machine interface. However, most AR products adopt stereoscopic 3D display technique, which causes the accommodation-vergence conflict. To solve this problem, we have proposed two approaches. The first is a multi-planar volumetric display using fast switching polymer-stabilized liquid crystal (PSLC) films. By rapidly switching the films between scattering and transparent states while synchronizing with a high-speed projector, the 2D slices of a 3D volume could be displayed in time sequence. We delved into the research on developing high-performance PSLC films in both normal mode and reverse mode; moreover, we also realized the demonstration of four-depth AR images with correct accommodation cues. For the second approach, we realized a holographic AR display using digital blazed gratings and a 4f system to eliminate zero-order and higher-order noise. With a 4k liquid crystal on silicon device, we achieved a field of view (FOV) of 32 deg. Moreover, we designed a compact waveguidebased holographic 3D display. In the design, there are two holographic optical elements (HOEs), each of which functions as a diffractive grating and a Fresnel lens. Because of the grating effect, holographic 3D image light is coupled into and decoupled out of the waveguide by modifying incident angles. Because of the lens effect, the collimated zero order light is focused at a point, and got filtered out. The optical power of the second HOE also helps enlarge FOV.
Blue phase liquid crystal (BPLC) has been attractive for display and photonic applications for its sub-millisecond response time, no need for surface alignment, and an optically isotropic dark state. Because of these advantages, diffractive devices based on blue phase liquid crystals have great potential for wide applications. In this work, we present several BPLC diffractive devices. The operation principles, fabrication and experimental measurements will be discussed in details for two BPLC gratings realized by holographic method and a BPLC Fresnel lens using a spatial light modulator projector. All of these devices exhibit several attractive features such as sub-millisecond response, relatively high spatial resolution and polarization-independence.
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