| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 93 === In recent years, the display industries keep showing great interests in liquid-crystal-on-silicon (LCOS) devices, especially in the application of projection display. The products of LCOS devices include data projectors, rear-projection TV and the head-mounted virtual display. Due to the advantage of intrinsic high electron mobility of crystalline silicon, LCOS devices can be fabricated with very high resolution. In addition, its peripheral driving circuits can be integrated on a single chip, which greatly reduces the cost of manufacturing. Technically, LCOS devices are based on two major domestic industries: the semiconductor and liquid crystal display industries. Since the fabrication of the silicon backplane is based on the standard manufacturing process of semiconductor, LCOS devices have great potential of low price. Therefore, many manufacturers have already invested in this industry.
However, there are still many challenges in the LCOS industry. The two major issues of LCOS panels are the fringing-field and diffraction effects. As the resolution increases, the pixel size and the inter-pixel gap will become very small. When the applied voltages between adjacent pixels are different, the electric fields near the pixel edges will be distorted. Hence, the liquid crystal molecules near this region are aligned abnormally, which, in turns, degrades the optical performance of the device significantly. This is the so-called fringe field effect.
In addition, as the pixel pitch becomes comparable to the wavelength of the visible light, the LCOS panel acts as a reflective grating. Therefore, obvious diffraction effect can be observed. The oblique diffracted light may not be able to enter the optical system, and consequently results in serious light loss.
In this dissertation, we investigate the fringing-field effects of eight commonly used liquid crystal modes. We also investigate the influence of the LC cell structure on the optical performance of LCOS devices. It is found that the mixed-mode twist nematic (MTN) has weaker fringing-field effect while the twist nematic mode (TN) and vertically aligned mode (VA) suffer from the effect significantly. The fringing-field effect is particularly severe in VA mode. It not only degrades the static image qualities but also deteriorates the dynamic response of the LCOS panel. The pixel pitch, cell gap, pretilt angle and electrode slope are all found critical to the fringing-field effect.
In order to design a high-contrast-ratio LCOS panel without fringing-field effect, we focus on the analyses of VA mode which possesses an excellent dark state. Based on the simulated results of the LC director profile, we find that, by utilizing the properties of circularly polarized light, the light loss caused by fringing-field effect can be preserved and the sharpness of the image can be enhanced dramatically. Moreover, the dynamic response is also improved and the imaging blurring effect is successfully eliminated. The results can be qualitatively illustrated by the de Vries theory.
With regard to the effect of diffraction, a rigorous simulator is needed to investigate the optical performance of a high-definition LCOS panel. The conventional Jones matrix method is no longer suitable in this condition. We extend the beam propagation method (BPM), which is commonly employed in waveguide calculations, to the optical simulation of LCOS devices. Two promising LC operation modes are analyzed by BPM, i.e. VA and finger-on-plane (FOP) modes. The calculated light efficiencies by Jones matrix method and BPM with respect to the pixel pitch are compared. It is shown that the diffraction effect is critical to the light efficiency. Using Jones matrix method may give rise to significant miscalculation. By using BPM, it is found possible to reduce the diffraction effect for certain waveband by slightly modifying the FOP cell structure, suggesting that the light efficiency can be boosted effectively in a two- or three-panel LCOS projection system.
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