Young June Cho

An Optical Mixer and RGB Control for Fine Images using Grey Scale Distribution

We considered the issue of color mixing from the viewpoint of gray scale images and investigated the effect of the mixed light. The goal of this study was to improve the quality of the grey scale images by using mixed RGB illumination. The mixing mechanism and image quality were formulized and applied to a machine vision system with an RGB mixer. The RGB mixer was made of RGB LEDs, a power AMP, a mixing chamber, and a bundle of optical fibers. The intensity of the RGB light was adjusted by varying the 12 bit DAC voltage. The optical characteristics of the RGB LEDs were measured by a spectrometer. The color variation of the mixed light was monitored by a chromatic meter. Grey scale images were captured and analyzed under various lighting conditions provided by the mixer. The optimal illumination was determined by the distribution of the grey scale in the captured image. The chromatic diagram showed that various colors can be generated, and the image histogram showed that the mixed light can increase the quality of the grey scale image.

A study on driving waveform of a piezoelectric inkjet print head

Inkjet printing technology with a drop-on-demand (DOD) inkjet head has been recognized as one of the versatile and low cost manufacturing tools in the electronics industry. However, general strategy to optimize jetting stability has not been understood well, because of the inherent complex multi-physics nature in inkjet phenomena. In this paper, an experimental approach has been adopted to attack this problem. Based on the driving voltage amplitude and duration as the main and controllable parameter, the jetting map of an OLED ink has been constructed and the effect of phase matching between pressure and surface waves at the nozzle tip has been discussed.

Electrical sintering of inkjet-printed silver electrode for c-Si solar cells

Electrical sintering of the front electrode for crystalline silicon solar cells was performed applying a constant DC current to the printed lines. Conducting lines were printed on glass substrate by a drop-on-demand (DOD) inkjet printer and silver nanoparticle ink. Specific resistance and microstructure of sintered silver lines and were measured with varying DC current. To find the relation between temperatue increase with changing applied cuurent and specific resistance, temperature elevation was also calculated. Sintering process finished within a few milliseconds. Increasing applied DC current, specific resistance decreased and grain size increased after sintering. Achieved minimum specific resistance is approximately 1.7 times higher than specific resistance of the bulk silver.

Effects of substrate treatments on silver ink-jet printing

In this study, characteristics of silver ink-jet printing were investigated under various substrate treatments such as substrate heating, hydrophobic coating, and ultraviolet (UV)/ozone soaking. Fluorocarbon (FC) film was spin-coated on the polyimide (PI) film substrate to obtain a hydrophobic surface. The coated FC film surface was estimated to have a fairly hydrophobic nature in that it exhibited a large water contact angle of 110‐113°. Although hydrophobicity of the FC film could reduce the diameter of the printed droplets, the singlet images printed on the FC film surface showed irregularities in the pattern size and the position of the printed droplet along with droplet merging phenomenon. The proposed UV/ozone soaking of the FC film improved the uniformity of the pattern size and the droplet position after printing and substrate heating was very effective way in preventing droplet merging. By heating of the substrate after UV/ozone soaking of the coated FC film, silver conductive lines of 78‐116 μm line were successfully printed at relatively low substrate temperatures of 40 °C and 70 °C.

Waveguide-MEMS cascaded variable optical attenuator with enhanced performance

We present a novel micro optical waveguide (MOW) on micro actuating platform (MAP) structure that is used for a variable optical attenuator. The device is consists of a fused biconical taper (FBT) coupler mounted on an electromechanical system where an axial stress over the waist of FBT coupler is precisely controlled. Its operation is based on change of coupling constant by compressive stress induced photoelastic effects on the waist zone. We use two FBT couplers to implement an enhanced performance of variable optical attenuator. The couplers are made from a standard single mode fiber and have a circular cross-section in their waist with an enough heating temperature. Each FBT coupler is optimized at 1450nm where total insertion loss is 0.75dB. π phase shifts in the coupling constant have been observed at an axial displacement of 9.5μm. The spectral response between two output ports of the coupler is reciprocal. This allows the proposed device to achieve a high attenuation of >72dB and for 20dB attenuation a flat bandwidth of <1dB over 100nm. Both a low polarization dependent loss (PDL) of <0.07dB and a low operating voltage of 15.3V have been demonstrated with a micro-order actuation.