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Credit: by Jianghao Xiong, En-Lin Hsiang, Ziqian He, Tao Zhan, and Shin-Tson Wu
AR and VR are widely regarded as next-generation displays that can provide deeper interactions with digital world than current flat panel displays. However, it is challenging to satisfy the demanding requirements of human vision, while keeping a glasses-like formfactor. The main reason is that the underlying etendue conservation poses a tradeoff between field of view and exit pupil. To increase the system etendue generally requires a bulkier optics, which would increase the size and weight of the near-eye displays. Encouragingly, recent advances in holographic optical elements (HOEs), surface relief grating, metasurface, and micro light-emitting diodes (micro-LEDs) offer new optical architectures to break the etendue limitation.
In a recent paper published in Light: Science & Applications, a team of scientists, led by Prof. Shin-Tson Wu from the College of Optics and Photonics, University of Central Florida, USA, reviewed the recent progress of HOEs and lithography-enabled devices and their applications in AR/VR displays. They gave detailed descriptions on the device structures and working principles, and implementations in AR/VR headsets.
Another promising architecture in AR is Maxwellian-type system. Depending on different operation mode, it can adopt computational holography, Maxwellian-view, and multi-view, all of which are potential candidates to solve the VAC issue in AR. The major challenge for Maxwellian-type system is the small exit pupil. In the paper, the Central Florida team reviewed several methods for implementing pupil steering and pupil duplication with emerging HOE technologies.
Meanwhile, light engine in AR is another critical topic that is often overlooked in most review papers. In this paper, Wu’s team compared the performance of various types of light engines quantitively, including laser beam scanner, liquid-crystal-on-silicon, and micro-LEDs. Afterwards, they also summarized the performance of various combiner optics. These hard-to-find data offer a comprehensive understanding of AR system performance with different types of light engine and combiner. Finally, the future perspectives of AR/VR display systems are outlined.
To reduce the size of optics without sacrificing system performance, novel folding optical structures like pancake optics and waveguide display can be adopted in VR and AR separately. Adopting thin-film HOEs, pancake optics can be further shrunk to the eyeglasses-like formfactor. Moreover, combined with dynamic liquid crystal HOE lenses, the knotty vergence-accommodation conflict (VAC) issue can be solved elegantly within a reasonable overall system size. As for waveguide architectures, various types of coupler gratings can be adopted, such as polarization volume grating, volume holographic grating, and surface relief grating. Wu’s team gave in-depth description of the waveguide design principles with k-diagram and associated challenges like diffraction artifacts.
Journal
Light Science & Applications