Chia Rong Sheu

Using photopolymerization to achieve tunable liquid crystal lenses with coaxial bifocals

Liquid crystal (LC) lenses with circular hole-patterned electrodes possess the excellent capabilities of tunable focal lengths. In this paper, we demonstrate the performance of a specific LC lens with tunable coaxial bifocals (CB) synthesized via photopolymerization of LC cells. The characteristics of tunable CB are clearly exhibited when the voltage applied is continuously increased, eventually disappearing until only one focus is left when significantly higher voltages are applied. We simultaneously demonstrate two types of tunable CB LC lenses fabricated via different photocurable processes and determine their optical functions.

Preventing occurrence of disclination lines in liquid crystal lenses with a large aperture by means of polymer stabilization

Liquid crystal (LC) lenses with a circularly hole-patterned electrode possess excellent characteristics in optical performance, especially for the capability of tunable focal lengths. But, non-uniformly symmetrical electric fields in LC lenses usually induce disclination lines when operating. In general, the occurrence of disclination lines not only degrades their optical capability such as imaging performance, but also spends more time for tuning focal lengths. In this paper, we use a way of polymer stabilization to successfully prevent the disclination lines in LC lenses. Even arbitrarily adjusting the applied voltages in LC lenses, it seems no occurrence of disclination lines again. In addition, we compare the basic optical performance for LC lenses with or without polymer stabilization. From experimental results, it shows that they almost have the same optical performance.

Multimode optical demultiplexer for DWDM with liquid crystal enabled functionalities

We have developed a liquid-crystal-based multimode optical demultiplexer (DEMUX) with additional functionalities such as switching and power equalization. Demultiplexing 16-channel 100-GHz-spaced signals into a 62.5-/spl mu/m multimode-fiber array is demonstrated. The central wavelength of each channel is designed according to the International Telecommunication Union grid. Adjacent channel crosstalk is less than -30 dB. The average 1and 3-dB passbands of the DEMUX are 12.5 and 22.5 GHz, respectively. A maximum extinction ratio of 16.2 dB is achieved. Different channels can be switched with rise and fall times of /spl sim/10 and /spl sim/70 ms, respectively. The outputs of the channels are equalized to -65 dBm. The variation between different channels reduced from /spl sim/10 dB to less than 0.5 dB.

Holographic polymer networks formed in liquid crystal phase modulators via a He-Ne laser to achieve ultra-fast optical response

The holographic polymer network formed in liquid crystal (LC) phase modulators via a He-Ne laser in this study demonstrates ultra-fast optically response and low light scattering. These advantages are mainly caused by the small LC domains and uniform polymer network when processing LC cells via holographic exposure to a He-Ne laser. The use of this method to fabricate LC cells as phase modulators results in a decay time of 49 μs under 2π phase modulation at room temperature. The predicted fast optical response can be achieved when operating devices at high temperatures.

Ion-beam-processed SiO2 film for homogeneous liquid crystal alignment

Liquid crystals in our experiments could be aligned on a SiO2 surface bombarded by a neutral argon ion beam of an appropriate energy. Also, the pretilt angle of liquid crystals could be varied between 0 and 3.5°, by controlling the argon ion beam with an energy of 500 eV at an incident angle varying from 0 to 85° that was measured from the normal of the substrate. Finally, the twist nematic test cell of the 6 µm cell gap incorporating SiO2 alignment films exhibited a high optical transmission and a fast response time when the angle of the incident ion beam was more than 45°.

Small dosage of holographic exposure via a He-Ne laser to fabricate tunable liquid crystal phase gratings operated with low electric voltages

ABSTRACT The fabrication processes of tunable liquid crystal (LC) phase gratings via small dosage of holographic exposure of a He-Ne laser beam were investigated. The initial LC cells were filled with various ratios of ingredients mainly including LCs and photocurable monomers. The fabricated LC phase gratings shown in the Raman–Nath regime possessed a maximum value of first-order diffraction efficiency close to 33.3% at 5.8 Vrms. Furthermore, optimised grating was demonstrated and used as an interference recorder in holography. Graphical Abstract

Comparative Evaluation of the Imaging Performance of Multi-Primary Color LCDs With RGBCW and RGBCY Pixel Units by Simulation

Through a simulation study, the imaging performance of two types of multi-primary color liquid crystal displays (LCDs) with striped RGBCW (red, green, blue, cyan, and white) and RGBCY (red, green, blue, cyan, and yellow) pixel units is evaluated comparatively. Results show that color consistency and enhanced luminance can be achieved in image rendering with RGBCW rather than RGBCY pixels when the proposed color conversion algorithm is employed. Color measurement by spectrometry is conducted simultaneously to prove that with RGBCW pixels, the color imaging performance is almost as good as that with conventional RGB pixels; the luminance achieved with RGBCW pixel units is higher than that achieved with RGBCY pixel units.

Fast response time in liquid crystal cells doped with low concentrations of reactive mesogen via photopolymerization at low temperature

Liquid crystal (LC) cells with photopolymers usually exhibit a fast response time but inevitably present decreased optical transmittance and lower contrast ratio due to incomplete dark states. In this study, we show that this issue can be improved when photopolymerization at low temperature is considered. Comparing performance with the 4 wt% RM257-doped fringe-field switching (FFS) LC cell photopolymerized at room temperature, the 1.4 wt% RM257-doped FFS LC cell photopolymerized at low temperature (273 K) shows better contrast ratio and lower operating voltage. In addition, the electrostriction effect can be also reduced in LC cells with lower RM257-doped concentration. As a result, the 1.4 wt% RM257-doped FFS cell shows a response time as fast as that in the 4 wt% RM257-doped FFS cell. Meanwhile, the average response time for gray-to-gray switching of the optimal FFS cell is 4.9 ms.

Experimental Analysis to Avoid Migrating Zigzag Lines Occurring in Homogeneously Aligned Liquid Crystal Lenses with a Hole-Patterned Electrode

For our beginning study in large apertures of liquid crystal (LC) lenses, both disclination line and zigzag line are usually occurred in cells. Disclination lines possibly appear and then disappear if we slowly increase applied voltages and/or fabricate LC lens with thicker upper glass substrates. But, zigzag lines always appear near the edges of LC lens. Unfortunately, zigzag lines and disclination lines will possibly link each other, and permanently stay in cells to degrade performance of LC lens. In this paper, we experimentally study and conclude that the suitable rubbing conditions in cells will effectively prevent this problem of linked lines.

Wavelength tunable semiconductor laser with a liquid crystal pixel mirror

Tunable semiconductor lasers are compact, versatile sources used extensively in dense-wavelength-division-multiplexing (DWDM) optical communication systems, precision metrology, environmental monitoring, and laser spectroscopy. We have developed a twisted nematic liquid crystal device, the liquid crystal pixel mirror (LCPM), successfully as electronically tunable spectral filters for wavelength selection in external cavity semiconductor lasers. In this talk, we report recent advances in this class of electronically tunable single- and multiple-wavelength semiconductor lasers at 650 and 830 nm. Preliminary results of operating the laser at 1.5 microns will also be shown. The laser output can be locked to the ITU grid at 100 GHz intervals. Output power of the laser is as high as several hundred milliwatts, with a tuning range of several tens of nanometers. The laser can be operated either in the continuous-wave (CW) or mode-locked configuration. The linewidth of the laser in the free-running CW mode is about 30 MHz. Fine-tuning of the cw output wavelength can be achieved by changing the driving voltage to the desired pixels of the LCPM. In the mode-locked configuration, the laser design allows intra-cavity dispersion compensation and pulse compression.