As a virtual 3D space parallel to reality, metaverse can greatly enrich human life and work and has received extensive attention. Augmented reality (AR) and virtual reality (VR) are considered the gateway to the metaverse. True 3D display without visual fatigue caused to human eyes is the key to AR and VR displays. Among the 3D displays such as light field 3D display, the holographic 3D display is the only way to completely reconstruct the phase and amplitude information of 3D scenes.
The phase-only hologram is more attractive for its higher diffraction efficiency. The spatial light modulator (SLM), especially the phase-only liquid crystal on silicon (LCoS), is an ideal display panel for dynamic holographic 3D displays with phase-only holograms. However, due to the pixel structure of the LCoS, there are zero-order and high-order lights, which are inevitable and very annoying for holographic near-eye 3D displays and are always filtered with 4f optical systems. The enlarged virtual 3D image is viewed with an eyepiece and the presence of 4f optical systems increases the size of the display system. Since the complex amplitude distribution of diffracted 3D scenes is difficult to be fully described by phase-only data, the computation of phase-only holograms is a big challenge. Different optimization algorithms including Gerchberg-Saxton (GS) algorithm, patterned phase-only hologram, double phase method, gradient descent algorithm and deep learning algorithm have been proposed for the phase-only hologram calculation. The above algorithms have their advantages and disadvantages. Most of the algorithms are not related to the display systems and cannot be employed to reduce the volume of the display system.
This paper demonstrates a compact holographic near-eye 3D display only with one projection lens and one eyepiece after the SLM, thereby avoiding the utilization of 4f optical systems and reducing the display system. In the hologram calculation, an interactive method by considering the parameters of holographic near-eye display systems is designed and implemented. The quadratic phases related to the focal length of the projection lens and the depth of each layer of the 3D model are adopted as compensation phase factors, and only the Fourier transform and inverse Fourier transform are leveraged in the iterations to obtain the 3D phase-only hologram. Finally, the 3D image with multiple layers can be projected near the focus plane of the projection lens and the enlarged virtual image can be watched with the eyepiece for VR near-eye 3D display and an extra beam splitter for AR near-eye 3D display.
Four layers with a resolution of 1080 pixel×1080 pixel non-overlapped 3D model are designed, and the phase-only holograms with random phase and quadratic phase as initial phase are calculated and reconstructed. The peak signal-to-noise ratio (PSNR) and correlation coefficient (CC) are employed to evaluate the proposed calculation method and point out that when the initial phase is random, the speckle noise of the reproduced image is worse but the 3D effect of out-of-focus focusing is more obvious. However, when the quadratic phase is used as the initial phase, the speckle noise can be suppressed under certain circumstances, but the out-of-focus change is not obvious for the 3D display. In addition, a simulation study is conducted on the multi-depth images with front and back occlusion, which proves that the proposed algorithm is also effective in this case. The optical reconstructions of the phase-only holograms are implemented and the holographic near-eye VR and AR 3D display results are verified with experiments in the proposed compact display system.
In this paper, a compact holographic near-eye display is proposed for holographic near-eye VR and AR 3D displays. The proposed iterative algorithm is a display system-related algorithm, which is conducive to reducing the volume of the holographic near-eye display system. The effectiveness of the proposed method is proved by simulations and optical experiments. The combination of the proposed method and the optical waveguide has the potential to be applied to the waveguide-type holographic AR 3D display to promote the early arrival of the metaverse.
