BEIJING, July 15, 2022 — A research team from Beihang University has created a 3D holographic display system that widens its viewing angle and enlarges the image size through the simultaneous implementation of two generation methods. different holograms. The system features a tunable liquid crystal array with an adjustable period to widen the viewing angle. It provides secondary diffraction of the reconstructed image to increase image size.
The 3D holographic display system is composed of a laser, a beam expander, a beam splitter, a spatial light modulator (SLM), a 4f system with two lenses, a a filter, a polarized light valve and a signal controller, in addition to the tunable liquid crystal network. The network response time is 29.2ms, which meets the requirements of synchronous control.
To achieve a wide viewing angle, researchers apply a voltage to the liquid crystal lattice, which causes the liquid crystal molecules to adopt periodic order and the diffraction image to undergo secondary diffraction.
Concept drawing of the proposed 3D holographic display system with a tunable liquid crystal array. Courtesy of Yi-Long Li et al.
Researchers generate M secondary diffraction images by adjusting 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 split it into two equal-sized sub-holograms. The first sub-hologram is loaded onto the SLM before voltage is applied to the array. The second sub-hologram is then loaded onto the SLM during voltage application, 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 array. The polarized light valve settings ensure that only first-order positive diffracted light can pass.
When the switching time becomes fast enough, the reconstructed images of sub-hologram 1 and sub-hologram 2 can be spatially seamlessly merged to create a large holographic 3D display aligned with the visual persistence effect of the human eye.
(a) Angle of view of the holographic display in the initial state. (b) Viewing angle when voltage is applied to the tunable liquid crystal array. Courtesy of Yi-Long Li et al.
In experiments, the 3D holographic display system demonstrated a viewing angle of 57.4 inches, which is 7 times that of a conventional system using a single SLM. When the team tested the system’s ability to reproduce large holographic images, the system demonstrated that it could enlarge the size of an image by 4.2×.
The images produced by holographic 3D displays bypass the uncomfortable side effects of traditional 3D viewing systems and present images that are nearly identical to what humans see in their real environment. However, in traditional 3D holographic displays, the pixel pitch and size of the SLM limit the viewing angle and size of the holographic image. Currently, the angle of view of the holographic reproduction based on a single SLM is generally less than 9° and the size is less than 2 cm.
According to the researchers, the new 3D holographic display system has a simple structure and is easy to operate. The system completely reconstructs the details of the recorded object 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 diagnosis, advertising, entertainment and education.
The research has been published in Light: science and applications (www.doi.org/10.1038/s41377-022-00880-y).