The science fiction movies such as Star Wars and Avatar, paint a blooming future for the holographic 3D display. Compared with conventional 3D display technologies, holographic 3D display technology offers all the depth information of 3D objects, and therefore avoids the viewer from experiencing discomfort, nausea and other negative effects especially after prolonged use. However, we still have a long way to go before achieving the ideal holographic 3D display effect. Currently, it is still difficult to display 3D images with wide viewing angle and large size by using a simple holographic system. One of the major obstacles is that the pixel pitch of the spatial light modulator (SLM) is not small enough. For example, when the green light with a wavelength of 532nm is used for illumination, an SLM with a pixel pitch of about 614nm is required to achieve a 60° viewing angle. The larger the viewing angle of the holographic display system is expected, the smaller the SLM pixel pitch should be. Nevertheless, manufacturing such SLMs with sufficiently small pixel pitch is extremely hard.
In this work, we introduce a tunable liquid crystal grating in the holographic 3D display system to overcome the limitations. The proposed system allows for wide viewing angle and large size, thus can enhance the viewing experience. As shown in Fig. 1, the tunable liquid crystal grating is placed behind an SLM to allow for the secondary diffraction of the reconstructed image. According to our design, the tunable liquid crystal grating can be controlled by the voltage to provide an adjustable period and flexible operations for the reconstructed light. The tunable liquid crystal grating comprises a top glass substrate, a top electrode, a liquid crystal layer, bottom electrodes, a bottom glass substrate and wires. A direct voltage VDC is applied to the top electrode and an alternating voltage VAC is applied to the bottom electrode. The voltage of the ground electrode is V0.
Figure 1 Concept of the proposed system.
When the voltage is only applied to the top electrode, the bottom electrode is at zero potential. In this case, the function of the bottom electrode is similar to that of the ground electrode and the electric field distribution around the bottom electrode is similar to that around the ground electrode. The periodic pitch is the same as the base pitch (20 μm) and the distribution of the tunable liquid crystal grating is in the small periodic order. However, when an alternating voltage is applied to the bottom electrode, the bottom electrode is at a high potential. In this case, the bottom and top electrodes work together and generate a new electric field distribution. The periodic pitch is 40 μm and the distribution of the tunable liquid crystal grating is in the large periodic order. We also ensure that the response time of the tunable liquid crystal grating is fast enough to meet the requirement of synchronous control. The tunable liquid crystal grating has various benefits of being easily controllable, low-cost fabrication, high portability, thinness and lightness.
To realize wide viewing angle and large size display respectively, we apply different voltages to the tunable liquid crystal grating in cooperation with the corresponding hologram generation methods. In the wide viewing angle reproduction (Fig. 2), we adjust the voltage applied to the tunable liquid crystal and guarantee the liquid crystal molecules are arranged in the small periodic order. In this instance, the diffraction image is subjected to a secondary diffraction by the tunable liquid crystal grating and 7 secondary diffraction images can be generated. The proposed system achieves the viewing angle of 57.4°, which is 7 times that of the traditional system using a single SLM. It is worth noting that flexible viewing angle can be achieved by altering the parameters of the tunable liquid crystal grating. In the large size holographic reproduction (Fig. 3), we ensure that the tunable liquid crystal grating has different operations for the reconstructed image at different times by changing the applied voltages. At moment T1, we apply no voltage to the tunable liquid crystal grating, and it operates as a transparent glass. At moment T2, we apply suitable voltage to the liquid crystal grating to ensure that it diffracts the reconstructed image to the +1 order secondary maximum on the spectral plane. By taking advantage of the visual persistence effect of human eyes, we enlarge the reconstructed image seamlessly. The proposed system enlarges the size of the reconstructed image by 4.2 times. In both cases, the reconstructed images have sharp details and uniform intensity distribution.
Figure 2 Principle of the wide viewing angle holographic 3D display. 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.
Figure 3 Principle of the large size holographic 3D display.
Our proposed system sheds light on the viewing experience enhancement of 3D holographic display by using the tunable liquid crystal grating. We believe the proposed system has promising applications in advertising, education, entertainment and other fields. In the future, we plan to fold the system to a more compact configuration and realize color holographic 3D display.
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