近日，美国中佛罗里达大学的Shin-Tson Wu及其研究小组与东南大学的张云升等人合作并取得一项新进展。经过不懈努力，他们成功突破偏振体积光栅波导增强现实显示器的耦合效率限制。相关研究成果已于2024年8月12日在国际知名学术期刊《光：科学与应用》上发表。

本文首先分析了基于波导的增强现实（AR）显示器光学效率低和均匀性差的根本原因。随后，研究人员发现并阐明了当入射光方向不满足布拉格条件时，偏振体积光栅（PVG）所固有的异常偏振转换现象。这一新特性被有效利用，以解决耦合效率和眼盒均匀性之间的权衡问题。通过可行性演示实验，研究人员使用激光源和硅基液晶光引擎测量了不同厚度PVG的光泄露。

实验结果证实了偏振转换现象，并与仿真结果高度一致。为进一步探索该偏振转换现象的潜力，研究人员设计并模拟了一款具有50°视场的波导显示器。通过在PVG中实现一阶偏振转换，相较于传统耦合器，耦合效率和均匀性分别提高了2倍和2.3倍。这一突破性发现有望为下一代基于波导的AR显示器带来革命性变革，有望实现更高的效率和更优的图像均匀性。

据悉，增强现实(AR)显示器被誉为空间计算、虚拟世界和数字孪生的下一代平台，使用户能够感知叠加在现实世界环境中的数字图像，从而促进更深层次的人与数字交互。随着耦合器的快速发展，基于波导的AR显示器简化了整个系统，具有纤薄的外形和高光学性能。然而，波导合成器面临的挑战仍然存在，包括光学效率低和图像均匀性差，严重阻碍了其长期使用和用户体验。

附：英文原文

Title: Breaking the in-coupling efficiency limit in waveguide-based AR displays with polarization volume gratings

Author: Ding, Yuqiang, Gu, Yuchen, Yang, Qian, Yang, Zhiyong, Huang, Yuge, Weng, Yishi, Zhang, Yuning, Wu, Shin-Tson

Issue&Volume: 2024-08-12

Abstract: Augmented reality (AR) displays, heralded as the next-generation platform for spatial computing, metaverse, and digital twins, empower users to perceive digital images overlaid with real-world environment, fostering a deeper level of human-digital interactions. With the rapid evolution of couplers, waveguide-based AR displays have streamlined the entire system, boasting a slim form factor and high optical performance. However, challenges persist in the waveguide combiner, including low optical efficiency and poor image uniformity, significantly hindering the long-term usage and user experience. In this paper, we first analyze the root causes of the low optical efficiency and poor uniformity in waveguide-based AR displays. We then discover and elucidate an anomalous polarization conversion phenomenon inherent to polarization volume gratings (PVGs) when the incident light direction does not satisfy the Bragg condition. This new property is effectively leveraged to circumvent the tradeoff between in-coupling efficiency and eyebox uniformity. Through feasibility demonstration experiments, we measure the light leakage in multiple PVGs with varying thicknesses using a laser source and a liquid-crystal-on-silicon light engine. The experiment corroborates the polarization conversion phenomenon, and the results align with simulation well. To explore the potential of such a polarization conversion phenomenon further, we design and simulate a waveguide display with a 50° field of view. Through achieving first-order polarization conversion in a PVG, the in-coupling efficiency and uniformity are improved by 2 times and 2.3 times, respectively, compared to conventional couplers. This groundbreaking discovery holds immense potential for revolutionizing next-generation waveguide-based AR displays, promising a higher efficiency and superior image uniformity.

DOI: 10.1038/s41377-024-01537-8

Source: https://www.nature.com/articles/s41377-024-01537-8