Sang Kyeong Yun

Speckle suppression in laser display using several partially coherent beams.

An optical scheme for speckle suppression using two or three partially coherent beams in a projection system is proposed. Diffractive optical elements (DOE) placed in the intermediate image plane create several beams carrying the image to a screen. Transparent plates of different thicknesses are placed in the Fourier plane of the projective lens and used for beam decorrelation. The coherence matrix algorithm for speckle suppression is used to calculate the speckle contrast ratio. It is shown that for a small decorrelation length and using the same maximum thickness of the transparent plates, two partially coherent beams would provide better suppression than three beams with different diffraction orders. However, for a large decorrelation length, the three beam setup provides better speckle suppression for all three colors examined with a suppression coefficient close to theoretical limits. Verification of speckle suppression using three-beam decorrelation is reported.

A study of image contrast restriction in displays using diffractive spatial light modulators

The peculiarities of 1D projection displays using spatial light modulators (SLM1) are considered. The conditions for high contrast images in SLM are investigated. It is shown that an amplitude imbalance between the light reflected from the passive and active parts of SLM gratings imposes a limitation on the achievable contrast ratio. This imbalance may come from errors in SLM structure sizes and from diffraction effects inside SLM layers. The key point is the relationship between ridge and hole sizes in SLM diffraction structures. It is shown that odd diffraction orders (especially 1st order) have a significantly decreased sensitivity to ridge/hole tolerances. This phenomenon affects the SLM’s contrast ratio. The optimal ridge/hole ratio for different SLM diffraction orders has been found. To test the theory, experimental samples were manufactured and contrast ratio measurements were carried out. The experimental data are in good agreement with the theoretical results.

Fine-pitch high-efficiency spatial optical modulator for mobile display applications

Diffractive spatial optical modulators (SOM) employing a fine-pitch pixel array were introduced. The micromechanical designs of the lead zirconate titanate (PZT) actuator and mirror ribbon structure were optimized for a small volume while maintaining the same level of performance. The same design rule and fabrication equipment were used for a new 10-µm-pitch and conventional 16-µm-pitch SOM. The optical efficiency of the new SOM was 78% (zeroth-order diffraction), which is an improvement over that of the 16-µm-pitch SOM (73%). The full on/off contrast ratio showed no differences, and a high displacement of 500 nm was achieved. The stress of the Pt/PZT/Pt actuating layer was the main parameter affecting the initial gap height and displacement of the ribbon. The required ribbon flatness could be achieved by applying a stress gradient on the SiN layer. The temperature-sensitive characteristics, which degrade image quality, could be minimized by a mechanical compensation method that takes advantage of the thermal expansion effect of Si substrates. The estimated lifetime of the device is >4000 h. The developed fine-pitch SOM device has sufficient response time and ribbon displacement to be suitable for high-quality embedded laser-projection displays. The VGA optical module was successfully demonstrated in a mobile laser projection display.

Numerical Simulation and Thermal Failure Analysis of SOM Package

This paper presents a numerical analysis and experimental study on failure of the spatial optical modulators (SOM) package and its preventing from thermal damage. The SOM package has been developed for projection displays. Most heat sources of the SOM package were mounted on a glass plane that was coated with SiO2 and TiO2 for optical performance. The heat sources were distributed by entering the laser beam to the operating mirror, heating of IC chip and ohmic heating in the circuit of glass plane. In designing the SOM package, it is important to consider the thermal influence to the SOM packages performance and reliability. In this study, the electrical and thermal simulation by CFD and experiments using infrared camera are carried out for the enhancement of thermal reliability. The results are in good agreement with the experiments results and the maximum temperature of glass plane was decreased by the change of glass plane structure change.

Wafer Level Hermetic Packaging for RF-MEMS Devices Using Electroplated Gold Layers

Thermocompression bonding of electroplated gold is a promising technique for achieving low temperature, wafer level hermetic bonding without the application of an electric field or high temperature. Silicon wafers were completely bonded at 320 at a pressure of 2.5. The interconnection between the packaged devices and external terminal did not need metal filling and was made by gold films deposited on the sidewall of the via-hole. In the hermeticity test, packaged wafers had the leak rate of 2.74 ± 0.61 × 10-11 Pa m3/s. In the result of application in packaging of FBAR filter, the insertion loss is increased from -0.75dB to -1.09dB at 1.9.