Gao Wei Chang

Electro-optic periodically poled lithium niobate Bragg modulator as a laser Q-switch

We report an electro-optic Bragg modulator using a periodically poled lithium niobate (PPLN) crystal. We measured a half-wave voltage of 160 V when transmitting a 1064 nm laser through a 14.2 mm long, 780 microm thick, 20.13 microm period PPLN crystal at the Bragg angle. We also demonstrated a Q-switched Nd:YVO(4) laser using such a PPLN Bragg modulator as its Q-switch, producing 7.8 ns, 201 microJ pulses at a 10 kHz repetition rate when pumped by a 19.35 W diode laser at 808 nm.

A Progressive Design Approach to Enhance Project-Based Learning in Applied Electronics Through an Optoelectronic Sensing Project

In this paper, a progressive design learning approach is proposed in the course of ldquoapplied electronicsrdquo to help students to develop system design skills through an optoelectronic sensing project. This project offers a progressive guideline to lead student teams to design, build, and troubleshoot their optoelectronic systems. Such a system contains a light-source current stabilizer, a photodiode amplifier, a microcontroller system (including input and output), and optomechanical devices. In this approach, students are motivated to learn the required knowledge and skills in a future professional capacity. Those skills include designing specification, realizing teamwork and communication, and developing a variety of optoelectronic sensing techniques. Through this project, an optoelectronic sensing system is established called a spatial radiance distribution measurement system for various light sources. In particular, this kind of system is useful and important for LED industries. The course evaluations have been obtained from classmates and instructors and these results indicate that the objectives of this course are achieved.

Monolithically integrated laser Bragg Q-switch and wavelength converter in a PPLN crystal

We report a periodically poled lithium niobate (PPLN) crystal for both temperature-insensitive laser Q-switching and temperature-tuned wavelength conversion. The PPLN crystal consists of two sections, a 20.3-µm period section functioning as an electro-optic Bragg grating for Q-switching a diode-pumped Nd:YVO4 laser at 1064 nm and a 31-µm-period section functioning as an optical parametric generator for down converting the generated 1064-nm laser. When driving the PPLN Bragg grating with 170-V voltage pulses, we measured 181 µJ pulse energy at 1064 nm from the Nd:YVO4 laser pumped by 20.4 W diode power. The 181-µJ pulsed laser was further converted into mid-infrared radiation in the monolithic PPLN crystal with 35% parametric efficiency. The wavelengths were broadly tunable in the range of 1.75–1.88 µm (signal) and 2.7–2.44 µm (idler) via temperature without affecting the performance of the PPLN Bragg Qswitch.

Neural plant inverse control approach to color error reduction for scanner and printer

The process of eliminating the color errors from the gamut mismatch, resolution conversion, quantization and nonlinearity between scanner and printer is usually recognized as an essential issue of color reproduction. A new formulation based on the inverse plant control for the color error reduction process is presented. In this formulation, the printer input and scanner output correspond to the input and output of a system plant respectively. If the printer input equals the scanner output, there are no color errors involved in the entire system, i.e., the plant becomes an identity system. To achieve this goal, a plant inverse should be identified and added to the original system. Since the system of a combination of both scanner and printer is highly nonlinear, multilayer backpropagation neural networks, which have the capability to learn arbitrary nonlinearity, are applied to identify the plant inverse. A number of test samples are conducted to verify the effectiveness of the proposed method.<<ETX>>

Spectral estimation of color CCD cameras

The spectral sensitivity of a CCD camera is a prominent electro-optical characteristic for color image formation. In this paper, spectral estimation of color CCD cameras generally involves the assessment of overall spectral sensitivities, which are composed of those of optics, filters, and CCD sensors. To this end, we develop an automatic testing/measurement system with an optical platform. And, the spectral characterization of a CCD camera imaging process is presented in the approach of vector representation for the estimation problem. This technique examines the modulated incident colored lights, which are regarded as a set of color stimuli candidates, to be the effective color stimuli, such that they can be successfully applied to generic estimation algorithms. To show the feasibility of this work, a realization of a basic set of color stimuli candidates is generated in the numerical and graphical forms. In our approach the use of a spectrophotometer is needed only for the system calibration. It would be pleasing that this technique will practically simplify the complexity of the spectral estimation problem of CCD cameras.

Colorimetric modeling for vision systems

A colorimetric modeling technique is proposed to give a computational model associated with colorimetry so that the repre- sentation of color acquired from camera imaging is accurate and meaningful. First of all, the camera spectral responses are esti- mated and the colorimetric quality is evaluated to reveal the feasi- bility of this work. In the modeling process, we present a spectral matching method and an approach of determining a reference-white luminance. As a result, the acquired color and the true (or mea- sured) color can be well coordinated, with the strength of a global illumination or display white, in a perceptually uniform color space, e.g., in CIE 1976 L*a*b* space (abbreviated as CIELAB). Then, lower-degree polynomial regression is employed to eliminate color errors due to the mismatch between spectral response functions. Experimental results indicate that the root-mean-square DEab value (i.e., color error) from the degree-3 polynomial regression is less than a just-noticeable difference (about 2.3) in CIELAB. It appears that the proposed technique can establish an accurate colorimetric model for vision systems.

Accurate and cost-effective MTF measurement system for lens modules of digital cameras

For many years, the widening use of digital imaging products, e.g., digital cameras, has given rise to much attention in the market of consumer electronics. However, it is important to measure and enhance the imaging performance of the digital ones, compared to that of conventional cameras (with photographic films). For example, the effect of diffraction arising from the miniaturization of the optical modules tends to decrease the image resolution. As a figure of merit, modulation transfer function (MTF) has been broadly employed to estimate the image quality. Therefore, the objective of this paper is to design and implement an accurate and cost-effective MTF measurement system for the digital camera. Once the MTF of the sensor array is provided, that of the optical module can be then obtained. In this approach, a spatial light modulator (SLM) is employed to modulate the spatial frequency of light emitted from the light-source. The modulated light going through the camera under test is consecutively detected by the sensors. The corresponding images formed from the camera are acquired by a computer and then, they are processed by an algorithm for computing the MTF. Finally, through the investigation on the measurement accuracy from various methods, such as from bar-target and spread-function methods, it appears that our approach gives quite satisfactory results.

White light interferometric profile measurement system using spectral coherence

It is well known that white light interferometry (WLI) is important to nano-scale 3-D profile measurement technology. To archive cost-effective and accurate measurements, the researches for WLI are widely spreading. Our objective is to build up a 3-D micro-structure profile measurement system based on WLI, for micro-mechatronic, micro-optical, and semi-conductor devices. This paper briefly reviews related WLI theory and then the principle of spectral coherence is employed to improve the system design. Specifically, proper spectral filters can be used to extend the coherence length of the light source to the order of several ten micrometers. That is, the coherence length of the filtered light source is longer than that of the original source. In this paper, Michelson interference experiments are conducted with filtered and unfiltered white light sources, to show the feasibility of the concept of spectral coherence. The Michelson interferometer is adopted due to its convenience of optical installation and its acceptable tolerance to noise. The experiment results indicate that our approach is feasible and thus it can improve the WLI measurement performance.

Multiple-functional and cost-effective liquid-crystal cell parameter measurement system

For years, the technology of TFT-LCDs (thin-film-transistor liquid crystal displays) has grown very rapidly, especially in the market share and technical development of FPD industries. To effectively promote the industrys capacity for the mass production and quality control, it is urgent to design and develop LC cell optical parameter measurement systems. The goal of this paper is to develop a multiple-functional and cost-effective measurement system to lower the manufacturing cost for the industry. The optical parameters includes the pretilt angle, liquid crystal (LC) cell gap (or phase retardation), and twist angle, which highly influence the display quality. In this paper, we first study the past approaches and analyze their measurement performance. Then, a simple and cost-effective method is proposed to achieve the multiple functions. That is, in addition to the precise measurement of the three important optical parameters, the proposed system can measure the voltage-transmittance (V-T) curve. In our approach, the theoretical study, simulation, and experiment are performed to show the feasibility of the system implementation. Finally, the proposed system is developed to automatically measure the LC cell parameters. Experimental results indicate that the proposed measurement system gives a satisfactory result.

DSP-based MTF measurement system for optical imaging modules

With the increasing demand on miniaturization of optical modules, it is important to measure their image quality for consumer electronic devices, such as video cameras, mobile phones, etc. This is because the effect of diffraction arises from the miniaturized optical modules and as a result, it degrades the resolution of the imaging processes. Modulation transfer function (MTF) has been widely recognized as a useful and important tool for assessing the image quality. Therefore, the objective of this paper is to develop a MTF measurement system for the optical modules, on the basis of digital signal processing. In our approach, a spatial light modulator (SLM) is employed to spatially modulate the light, which is emitted from a white-light LED, with the bar patterns of variable frequencies, the modulated light then goes through the module under test, and it is eventually detected by a charge-couple-device (CCD). Obviously, the mapping from the collection of the modulated light to the output images is treated as a computation kernel of the MTF measurement. The corresponding images formed by the optical module are acquired by a digital signal processor (DSP) based system and they are processed by an algorithm for computing the MTF, which is implemented in the system. Finally, it is found that our experiments give quite satisfactory results.