Ho-chi Huang

Simulation and Analysis of Silicon Electro-optic Modulators Utilizing the Carrier-dispersion Effect and Impact-ionization Mechanism

A new type of Si guided‐wave electro‐optic modulator is proposed and analyzed. The modulator makes use of the impact‐ionization mechanism for carrier generation, and the carrier‐dispersion effect for electro‐optic conversion. Both electrical and wave propagation properties of the modulator were examined by a two‐dimensional device simulator and a three‐dimensional waveguide simulator, respectively. Numerical estimates of phase modulation due to refractive‐index change and intensity modulation due to optical absorption and radiation loss were obtained. One of important features of the prospected modulator is speed. The simulated turn‐on and turn‐off time of the modulator was less than 1 ns. GHz modulation is, therefore, possible for this class of modulators with device structure and doping profiles optimized for fiber coupling.

Reflective twisted nematic liquid crystal displays. I. Retardation compensation

Reflective twisted nematic displays where the output polarizer is eliminated are analyzed and demonstrated. The absence of the rear output polarizer has many advantages, including much higher brightness and higher resolution. In this article, we examine the solution space for the design of such reflective displays. Both the cases of a simple reflective display and the use of a retardation film within the display to compensate for color dispersion are explored. It is shown that excellent black and white displays can be obtained using film compensation. Experimentally, over 90% reflectance from a compensated reflective display has been obtained with low color dispersion. The optical response of this reflective display was also found to be quite fast and suitable for video rate applications.

LED-Based Projection Systems

A novel design methodology for LED-array-based projection displays has been developed. By combining etendue limitation, system intensity, and efficiency requirements, a novel parameter space is proposed. Using this parameter space, LED lens-array and compound parabolic concentrator (CPC)-array illumination systems have been designed. A 1000-lm LED light source is built. Based on these lens-array and CPC-array illuminators, several LED-based liquid crystal on silicon (LCOS) projection systems are suggested. Among them, a one-panel LCOS projection system is proposed and tested. The method discussed here should be useful in the design of LED-array illumination systems for projectors in general.

Three-dimensional optical modeling and optimizations of color filter liquid-crystal-on-silicon microdisplays

We have developed a three-dimensional (3D) optical modeling of small color pixels in color filter liquid-crystal-on-silicon (CF-LCOS) microdisplays. The 3D optical modeling includes a LC electromechanical analysis of color LC cells, a calculation of optical reflectance using the extended Jones matrix, and a standard RGB (sRGB) representation of the optical reflectance in the pixel array. The simulated optical reflectance agreed well with the experiments. With this 3D optical analysis as a tool, the relation of the lateral color fringing field with pixel size and thickness of color filter were studied. Minimizations of the fringing field in the CF-LCOS microdisplays were obtained by pixel arrangement, rubbing direction, and LC mode. The color purity of the CF-LCOS microdisplays could attain 63% National Television System Committee (NTSC) level for a typical pixel size of 15μm. With an optimized LC mode, the color purity could still be maintained at 54% NTSC level even when the pixel size was reduced to 9μm...

Application of Photoalignment Technology to Liquid-Crystal-on-Silicon Microdisplays

We applied photoalignment technology to liquid-crystal-on-silicon microdisplays and characterized their performance. A three-step photoexposure method was developed to solve problems with alignment disturbances on reflective silicon panels. The azimuthal and polar anchoring energies of photoalignment layers produced by this method were high and comparable with that of rubbed polyimide layers. We assembled both the rubbed and photoaligned microdisplays on a MTN 90° configuration for characterization and comparison. These two kinds of silicon microdisplays had good and comparable characteristics in terms of contrast, response time, voltage holding ratio and residual DC charge. The rubbed microdisplays were more uniform, which we attributed to a more mature fabrication technology, while the photoaligned microdisplays had far fewer defects due to the noncontact nature of the photoalignment technology.

Efficient polarization converter for projection displays

In the waveguiding limit, a twisted nematic liquid crystal cell behaves as an achromatic polarization rotator. We propose and demonstrate the application of such a polarization rotator to convert unpolarized light into linearly polarized light with almost 100% efficiency. This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions. It can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve. Both transmittive and reflective light valves can be used. The temperature dependence of the achromatic polarization rotator is also studied.

35.4: Three-Dimensional Optical Analyses of Fringing Effect in Small Color Pixels

a three-dimensional optical analysis of fringing effect in small color pixels. In addition to the simulation of optical reflectance in visible spectrum, we further expressed the reflectance in spatial color. Color leakage was easily located and it agreed well with the experiments. 1. Introduction developed a novel but practical color liquid-crystal-on- silicon (LCOS) microdisplay that integrated color filters on silicon as illustrated in Figure 1. The projection optics is greatly simplified since color is already available on the display. What it needs is just a polarizing beam splitter (PBS) to direct a polarized light into the display and back to a projection lens. In order to have high resolution for this spatial color microdisplay, pixels have to be very small. This is not a problem for silicon panels, which were fabricated by sub-micron very large-scale integration (VLSI) processes. But the lateral field effect among small color pixels could be severe (1-3), and degrade contrast and color of this microdisplay.

Two-Dimensional Optical Analysis of Small Pixels in Reflective Silicon Microdisplay

We present a two-dimensional optical analysis of small pixels in reflective liquid-crystal-on-silicon microdisplay. The optical analysis is divided into two parts. In the first part, the two-dimensional (2D) director configuration of the liquid crystal display (LCD) in rectangular mesh was calculated by simultaneously solving the 2D Poisson and continuum theory equations. In the second part, generalized Jones matrix was used to simulate for optical reflectance through the LCD with the 2D director configuration obtained from the first part. This optical modeling was applied to analyze optical reflectance of small pixels in reflective mixed-mode twisted nematic and birefrigence (MTB) cells on silicon substrate. Both the simulated and experimental data of the MTB cells under frame and column inversions were presented. It was found that the frame inversion could give rise to a higher contrast ratio than the column inversion at the same driving voltage. It was also found that the optical reflectance decreased as the pixel size decreased due to the lateral electric field between the small pixels.

Trichroic prism assembly for separating and recombining colors in a compact projection display

A trichroic prism assembly design, believed to be new, is proposed and demonstrated. This new design has the advantages of low s- and p-polarization dependence in the reflectance spectra of the optical coatings. Hence it can be used for both color separation and color recombination with polarization change. This new trichroic prism assembly is especially useful in a compact color projector employing reflective liquid-crystal light valves.

Generalized mixed-mode reflective LCDs with large cell gaps and high contrast ratios

Abstract— A generalized mixed twisted-nematic/birefringent-effect mode reflective LCD is presented. This new mode encompasses all previously published reflective nematic LCD modes and is clearly shown on the parameter space diagrams. We also discuss how to obtain operating conditions of this generalized mixed-mode display with large cell gaps, low color dispersion, and high contrast at low voltages. This operating mode is particularly suitable for reflective crystalline-silicon-backplane active-matrix LCDs.