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Large viewing angle holographic 3D display system based on maximum diffraction modulation

Guide

The release of Apple’s first headset, Vision Pro, has pushed the consumer market’s enthusiasm for virtual reality (VR), augmented reality (AR) and mixed reality (MR) to another peak. 3D display technology is one of the core technologies to realize VR/AR/MR. In 3D display technology, holographic 3D display technology can provide all the depth information required by the human eye to achieve an immersive and comfortable viewing experience, which is a potential true 3D display technology. However, existing holographic 3D displays generally have the problem of small field of view, which greatly limits their application.

In view of this, the research team of Professor Wang Qionghua of Beihang University proposed a large-view holographic 3D display system based on maximum diffraction modulation. In this system, based on the maximum diffraction modulation of the spatial light modulator, the researchers proposed a large-size hologram calculation method, and used the optimized liquid crystal grating for secondary diffraction modulation, which effectively expanded the viewing angle of the holographic 3D display. The system is expected to promote the application of holographic 3D displays in fields such as education, medical care, culture and art, and national defense and security.

The results are currently published in Light: Advanced Manufacturing under the title “Large viewing angle holographic 3D display system based on maximum diffraction modulation”. Associate Professor Wang Di and PhD student Nannan Li of Beihang University are co-first authors of the paper, and Postdoctoral Fellow Chu Fan and Professor Wang Qionghua are the corresponding authors.

The ideal holographic 3D display technology provides a viewing experience with large viewing angles, full color, and low speckle noise. However, due to the limitation of the pixel pitch and size of the spatial light modulator, the viewing angle of the holographic 3D display is difficult to meet the actual viewing needs. Some researchers have proposed temporal multiplexing or spatial multiplexing methods to expand the perspective of holographic display, but the temporal multiplexing method has high requirements for the refresh rate of the spatial light modulator, and the spatial multiplexing method requires a more complex system for 3D display, which has certain limitations. Some researchers have proposed the use of cylindrical holograms or spherical holograms to solve the problem of small viewing angle, but because the existing spatial light modulators are difficult to achieve non-planar structures, the application of this scheme is difficult. Nowadays, how to achieve a large-view holographic 3D display system with simple structure and low cost has become one of the key problems to be solved in holographic 3D display.

In view of the above problems, the researchers proposed a large-view holographic 3D display system based on maximum diffraction modulation. As shown in Figure 1, the core modulation components of the system mainly include spatial light modulators and liquid crystal gratings. In order to maximize the diffraction modulation ability of spatial light modulators, the researchers proposed a large-size hologram calculation method. In addition, the researchers also designed a structure-optimized liquid crystal grating to perform secondary diffraction modulation on the holographic reproduction image, thereby further expanding the viewing angle of the holographic 3D display system. Finally, the proposed system realizes a holographic 3D display with a viewing angle of 73.4°.

Figure 1 Schematic diagram of a large-view holographic 3D display system based on maximum diffraction modulation

The implementation principle of the large-view holographic 3D display is shown in Figure 2. In the traditional hologram recording process, when the object size, diffraction distance and viewing distance and other parameters are determined, the diffraction angle of the holographic reproduction image does not fully reach the maximum diffraction angle of the spatial light modulator. In the proposed system, the maximum diffraction angle of the spatial light modulator is recorded as φ, in order to expand the viewing angle, so that each image point of the holographic reproduction image reaches the maximum diffraction angle of the spatial light modulator when diffraction, the researchers calculate the maximum interference map of each image point, and then superimpose the interference map of all image points to generate a large-size hologram, while expanding the size of the spatial light modulator. Therefore, the viewing angle of holographic reproductions based on large-size holograms is larger than that of traditional holograms.

Figure 2 Implementation schematic of large-view holographic 3D display

In addition, the researchers designed and fabricated a structure-optimized liquid crystal grating to perform secondary diffraction modulation on the holographic reproduction image to further improve the viewing angle of the holographic reproduction image. As shown in Figure 3, when a voltage is applied to the electrodes on the liquid crystal grating, a special electric field distribution is formed between the pixel electrodes and the common electrodes. This electric field induces the deflection of the pointing vector of the liquid crystal molecule, forming a centrally symmetrical gradient refractive index distribution. At this time, the light incident on the liquid crystal layer is diffracted, and the holographic reproduction image is modulated by secondary diffraction to produce N secondary diffraction images. After determining the size and diffraction distance of the holographic reproduction image, the continuous expansion of the viewing angle can be realized by designing the parameters of the liquid crystal grating. The viewing angle of the proposed holographic 3D system is expanded to 2φ×N. At present, the liquid crystal grating can achieve 9 levels of uniform diffraction, and achieve a viewing angle of 73.4° without affecting the holographic reproduction image quality. (Source: Advanced Manufacturing WeChat public account)

Figure 3 Schematic diagram of the structure of the liquid crystal grating

Related paper information:https://doi.org/10.37188/lam.2023.018

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