3D Holographic Display Achieves Wide Viewing Angle

A Beihang University research team created a holographic 3D display system that widens its viewing angle and enlarges image size through the simultaneous implementation of two different hologram generation methods. The system features a tunable liquid crystal grating with an adjustable period to widen the viewing angle. It provides a secondary diffraction of the reconstructed image to increase the image size.

The holographic 3D display system is composed of a laser, a beam expander, a beamsplitter, a spatial light modulator (SLM), a 4f system with two lenses, a filter, a polarized light valve, and a signal controller, in addition to the tunable liquid crystal grating. The response time of the grating is 29.2 ms, which meets the requirements for synchronous control.

To achieve a wide viewing angle, the researchers apply voltage to the liquid crystal grating, causing the liquid crystal molecules to assume a periodic order and the diffraction image to undergo a secondary diffraction.

Conceptual drawing of the proposed holographic 3D display system with a tunable liquid crystal grating. Courtesy of Yi-Long Li et al.

The researchers generate M secondary diffraction images by tuning the period of the liquid crystal grating. To display the secondary diffraction image with uniform intensity, they adjust the state of the polarized light valve.

To enlarge the size of the display, the researchers generate a hologram of the 3D object and divide it into two subholograms that are equal in size. The first subhologram is loaded on the SLM before voltage is applied to the grating. The second subhologram is then loaded on the SLM while voltage is being applied, to generate the zero-order primary maximum and ±1 order secondary maximum on the spectral plane.

The researchers developed a signal controller for the system to control the switching speed of the hologram and the tuning of the liquid crystal grating. Adjustments to the polarized light valve ensure that only positive, first-order diffracted light can pass through.

When the switching time becomes fast enough, the reconstructed images of subhologram 1 and subhologram 2 can be spliced seamlessly in space to create a large-size holographic 3D display that is aligned with the visual persistence effect of the human eye.

(a) Viewing angle of the holographic display in the initial state. (b) Viewing angle when the voltage is applied to the tunable liquid crystal grating. Courtesy of Yi-Long Li et al.

In experiments, the holographic 3D display system demonstrated a viewing angle of 57.4 in., which is 7× that of a conventional system using a single SLM. When the team tested the system’s ability to reproduce large-size holographic images, the system demonstrated that it could magnify the size of an image by 4.2×.

The images produced by holographic 3D displays circumvent the uncomfortable side effects of traditional 3D viewing systems and present images that are nearly the same as what humans see in their actual surroundings. However, in traditional 3D holographic displays, the pixel pitch and size of the SLM limit the viewing angle and the size of the holographic image. Currently, the viewing angle of holographic reproduction based on a single SLM is usually less than 9°, and the size is less than 2 cm.

According to the researchers, the new holographic 3D display system has a simple structure and is easy to operate. The system reconstructs the details of the recorded object completely and ensures that the intensity distribution is uniform.

In addition to 3D holographic displays, the system can be used for augmented reality (AR) displays. The team expects the display system to have broad applicability, with applications including medical diagnostics, advertising, entertainment, and education.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-022-00880-y).