Holographic Near-Eye Display System with Expanded Eyebox Based on Waveguide

Objective Holographic near-eye displays (NEDs) have attracted ever-increasing attention in recent years. Through wavefront modulation of the incident beam by the spatial light modulator (SLM), a holographic NED can achieve multiple functions that are not within the reach of conventional two-dimensional (2D) displays, such as controlling the depth of the displayed image and dynamically correcting the aberration. However, due to the limited space-bandwidth product of the SLM, the etendue of the entire system is small, leading to a long-standing trade-off between the field of view (FOV) and the eyebox. For example, Microsoft reported at the SIGGRAPH conference in 2017 that they had achieved a FOV of 80 degrees, but the eyebox was small. Two main types of methods have been proposed to expand the eyebox, i. e., active methods and passive methods. The active methods utilize a pupil-tracking system and move the eyebox subject to the position of the user's eye. Their energy efficiency can be high, contributing to lower power consumption of the system, longer battery life, and simpler thermal design. However, the main challenge for achieving high immersion in augmented reality (AR) use cases is low motion-to- photon latency, which is more difficult to obtain when the process of eye- tracking is incorporated. The passive solutions generally provide multiple discrete eyebox points simultaneously to expand the entire eyebox. However, they are exposed to the risk that no or two eyebox points may enter the pupil at a certain position as the user's eye moves. In such a case, missing of fields, low brightness uniformity, or ghost artifact occurs. In this paper, a holographic NED system with an expanded eyebox based on a surface relief grating (SRG) waveguide is investigated, showcasing a continuously and two-dimensionally expanded eyebox. Methods In the calculation of holograms, the angular spectrum method (ASM) and the stochastic gradient descent (SGD) algorithm are adopted because they can provide much better image quality than that offered by the traditional Gerchberg-Saxton (GS) algorithm. This advantage is confirmed by the comparison result of the peak signal- to-noise ratio (PSNR). To increase the etendue of the holographic NED, this paper utilizes a waveguide incorporating an in-coupling grating and a 2D surface relief out-coupling grating. The beam width corresponding to each image point in the holographic image is expanded two-dimensionally and continuously, and a two-dimensionally expanded eyebox is thereby obtained. Furthermore, the influence of pupil expansion on holographic display is assessed. By calculating the optical path difference between adjacent out-coupling beams, the paper finds that the angular period of the interference pattern is too small to be observed, and its impact on image quality is thus negligible. Results and discussions An experimental prototype (Fig. 6) is built to verify the effectiveness of the investigated system. Firstly, the display performance of the system on a monochromatic image verifies that the fine details of the resolution pattern can be reconstructed (Fig. 7). Then, the system's capability of displaying color images is demonstrated. Since the principle of the system's display of color images is time-division multiplexing, the monochromatic images of three colors are acquired independently and then synthesized ( Fig. 8). The color image looks reasonably well despite a certain amount of stray light. Next, the paper verifies the aim of the 2D expansion of the eyebox. The baseline case without the waveguide is assessed first. The results (Fig. 9) reveal that when the eye relief is 20 mm, only a small part of the target image can be captured at each position within the range of 4 mm x 4 mm. In contrast, when the waveguide is added to the system, the entire image can be observed across an eyebox range of 8 mm x 6 mm ( Fig. 10). In this range, 15 points are sampled continuously and uniformly, with the horizontal sampling points located at 0, +/- 2 mm, +/- 4 mm and the vertical sampling points at 0, +/- 3 mm, respectively. Furthermore, an AR display test (Fig. 11) is conducted, and the results demonstrate that the user can observe virtual and real scenes simultaneously. After that, the stray light and uniformity of the system are discussed. The stray light in the upper part of the displayed image is mainly due to the scattering at the defects on the waveguide and the higher energy of the beam transmitting in the waveguide in the upper part. A uniformity of 39. 47% is obtained by evaluating the average grayscale of nine points uniformly sampled in the display area. Finally, the possibility of displaying 3D scenes is discussed. Conclusions To mitigate the challenge of obtaining a sufficiently large eyebox under a proper FOV for holographic NEDs, this paper investigates a holographic NED system with an expanded eyebox. A waveguide incorporating a 2D surface relief out-coupling grating is utilized to expand the beam width of the holographic image two- dimensionally and continuously. Experiments confirm that when the eye relief is 20 mm and the FOV is 38. 6 degrees, an eyebox of 8 mm x 6 mm can be obtained. The problem of the incompetence of the FOV is thereby effectively mitigated. In the follow-up work, research will be conducted on deep learning- based computer-generated holography (CGH) algorithms, which can provide suitable pre-compensation of phase for the image coupled into the waveguide, to achieve the high- quality reconstruction of 2D and three- dimensional (3D) scenes.