Appl Opt. 2023 May 1;62(13):3467-3476. doi: 10.1364/AO.485971.
The coupling efficiency of light beams is a crucial factor for waveguide displays. Generally, the light beam is not coupled with maximum efficiency in the holographic waveguide without employing a prism in the recording geometry. Use of prisms in recording geometry leads to restricting the propagation angle of the waveguide to a specific value only. The issue of efficient coupling of a light beam without using prisms could be overcome via Bragg degenerate configuration. In this work, the simplified expressions of the Bragg degenerate case are obtained for the realization of normally illuminated waveguide-based displays. Using this model, by tuning the parameters of recording geometry, a range of propagation angles can be produced for a fixed normal incidence of a playback beam. Numerical simulations and experimental investigations of the Bragg degenerate waveguides of different geometries are performed to validate the model. A Bragg degenerate playback beam is successfully coupled in four waveguides recorded with different geometries and yields good diffraction efficiency at normal incidence. The quality of transmitted images is characterized using the structural similarity index measure. The augmentation of a transmitted image in the real world is experimentally demonstrated through the fabricated holographic waveguide for near-eye display applications. Bragg degenerate configuration can provide flexibility in the angle of propagation while maintaining the same coupling efficiency achievable with a prism for holographic waveguide displays.
光束的耦合效率是波导显示器的一个关键因素。通常情况下，如果在记录几何形状中不使用棱镜，那么在全息波导中光束不会以最大效率耦合。在记录几何形状中使用棱镜会导致将波导的传播角度限制在特定值。通过布拉格简并配置，可以克服不使用棱镜高效耦合光束的问题。在这项工作中，获得了用于实现正常照明基于波导的显示器的布拉格简并情况的简化表达式。使用该模型，通过调整记录几何形状的参数，可以为固定的播放光束的正常入射产生一系列传播角度。对不同几何形状的布拉格简并波导进行了数值模拟和实验研究，以验证该模型。成功地在以不同几何形状记录的四个波导中耦合了布拉格简并的播放光束，并在正常入射时产生了良好的衍射效率。使用结构相似性指数测量来表征传输图像的质量。通过用于近眼显示应用的制造的全息波导，实验证明了在真实世界中传输图像的增强。布拉格简并配置可以在保持与棱镜相同的耦合效率的同时提供传播角度的灵活性，适用于全息波导显示器。
