• Journal of the Korean Solar Energy Society
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• v.34 no.1
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• pp.48-57
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• 2014

The transient and synchronization behaviors of a TA (thermo acoustic) laser pair were investigated experimentally for various crossing angles and different separation distances between the laser openings. Sound waves generated by the lasers were measured and analyzed at or near the focusing point by means of microphones, SPL meters, and a commercial software called Signal-Express. The two TA lasers were acoustically coupled through the air mass between their openings, and the only mode-locking operation that could be achieved was the one that was nearly $180^{\circ}C$ out of phase. The time to achieve synchronization was found to be dependent upon the initial mistuning of the frequencies and the crossing angle between the laser axes. The synchronization process could also be accelerated by turning on the laser with the lower power input first.

• International Journal of Precision Engineering and Manufacturing
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• v.8 no.2
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• pp.14-19
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• 2007

Angle sensors based on the principle of autocollimation, which are usually called autocollimators, can accurately measure small tilt angles of a light-reflecting flat surface. This paper describes a prototype micro-angle sensor that is based on the laser autocollimation technique. The new angle sensor is compact and consists of a laser diode as the light source and a quadrant photodiode as a position-sensing device. Because of its concise design, the microangle sensor facilitates dynamic measurements of the angular error motions of a precision stage without influencing the original dynamic properties of the stage. This is because the sensor only requires a small extra target mirror to be mounted on the stage. The sensitivity of the angle detection is independent of the focal length of the objective lens; therefore, an objective lens with a relatively short focal length is employed to reduce the size of the device. The micro-angle sensor uses a single lens for the both the laser collimation and focusing, which distinguishes it from the conventional laser autocollimation method that has separate collimate and objective lenses. The new micro-angle sensor has dimensions of $15.1\times22.0\times14.0mm$ and its resolution is better than 0.1 arc-second The optical design and performance of this micro-angle sensor were verified by experimental results.

• Current Optics and Photonics
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• v.2 no.2
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• pp.147-152
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• 2018

We propose a novel bi-focal metallic Fresnel zone plate (MFZP) with shallow depth-of-field (DOF) characteristics. We design the specific annular slit patterns, exploiting the phase-selection-rule method along with the particle swarm optimization algorithm, which we have recently proposed. We numerically investigate the novel characteristics of the bi-focal MFZP in comparison with those of another bi-focal MFZP having equivalent functionality but designed by the conventional multi-zone method. We verify that whilst both bi-focal MFZPs can produce dual focal spots at $15{\mu}m$ and $25{\mu}m$ away from the MFZP plane, the former exhibits characteristics superior to those of the latter from the viewpoint of axial resolution, including the axial side lobe suppression and axial DOF shallowness. We expect the proposed bi-focal MFZP can readily be fabricated with electron-beam evaporation and focused-ion-beam processes and further be exploited for various applications, such as laser micro-machining, optical trapping, biochemical sensing, confocal sensing, etc.

• Transactions on Electrical and Electronic Materials
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• v.1 no.1
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• pp.32-39
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• 2000

Langmuir probe measurements of electron density, electron temperature and potential are mad in the early stage (<5${\mu}$s) of a laser ablated plasma plume, in which a positive current form positive ions and a electron current are overlapped. The plasma wes produced by focusing the second harmonic, 532 nm, of Q-switched Nd:YAG laser on a high purity carbon target. Then the laser intensity on the target of ~1.6${\times}$10$\^$15/ W/$\textrm{cm}^2$. The measured electron desities and temperatures are ~2${\times}$10/sip 11/ cm$\^$-3/ and -3 eV. In particluar , the phenomenon that the electron temperature decreased and then increased was observed,. It could be well explained that this phenomenon occurred in the process of inverse Bremsstrahlung of free electrons in plasma. Additionally, the plasma potential(>11V) was higher than the published values.

• Design & Manufacturing
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• v.17 no.2
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• pp.15-21
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• 2023

In this study, a diamond turning machine and a laser-assisted machining module were utilized for the complex combined cutting of aspheric shapes and fine patterns on the surface of high-hardness brittle material, silicon. The analysis of material's form accuracy and corrective machining was conducted based on key factors such as laser output, rotational speed, feed rate, and cutting depth to achieve form accuracy below 1 ㎛ and surface roughness below 0.1 ㎛. The cutting condition and corrective machining methods were investigated to achieve the desired form accuracy and surface roughness. The rotational speed of the spindle and the linear feed rate of the diamond turning machine were varied in five stages for the cutting condition test. Surface roughness and form accuracy were measured using both a contact surface profilometer and a non-contact surface profilometer. The experimental results revealed a tendency of improved surface roughness with increased rotational speed of the workpiece, and the best surface roughness and form accuracy were observed at a feed rate of 5 mm/min. Furthermore, based on the cutting condition experiments, corrective machining was performed. The experimental results demonstrated an improvement in form accuracy from 0.94 ㎛ to 0.31 ㎛ and a significant reduction in the average value of the surface roughness curve from 0.234 ㎛ to 0.061 ㎛. This research serves as a foundation for future studies focusing on the machinability in relation to laser output parameters.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.7
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• pp.106-115
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• 2002

Growth characteristics of micro carbon structures fabricated by laser-assisted chemical vapor deposition are studied. Argon ion laser and ethylene were used as the energy source and reaction gas, respectively, to grow micro carbon rod through pyrolytic decomposition of the reaction gas. Experiments were performed at various conditions to investigate the influence of process parameters on growth characteristics such as the diameter or growth rate of the micro carbon rod with respect to reaction gas pressure and incident laser power. Reaction gas pressure in experiments ranges from 200 to 600Torr and the incident laser power from 0.3 to 3.8W. For these conditions, the diameter of the rod increases linearly with respect to the laser power but is almost independent of the reaction gas pressure. Growth rate of the rod changes little with gas pressure when the laser power remains below IW. For a constant reaction gas pressure, the growth rate increase with Increasing laser power, but the rate of increase decreases gradually, implying that the chemical vapor deposition condition changes from a kinetically-limited regime to a mass-transport-limited regime. When the carbon rod was grown at near threshold laser power, a very smooth surface is obtained on the rod. By continuously moving the focusing lens in the direction of growth, a micro carbon rod with a diameter of 287${\mu}{\textrm}{m}$ and aspect ratio of 100 was fabricated..

• Design & Manufacturing
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• v.14 no.3
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• pp.44-49
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• 2020

One of display trends today is development of high pixel density. To get high PPI, a small size of pixel must be developed. RGB pixel is arranged by evaporation process which determines pixel size. Normally, a fine metal mask (FMM; Invar alloy) has been used for evaporation process and it has advantages such as good strength, and low thermal expansion coefficient at low temperature. A FMM has been manufactured by chemical etching which has limitation to controlling the pattern shape and size. One of alternative method for patterning FMM is laser micromachining. Femtosecond laser is normally considered to improve those disadvantages for laser micromachining process due to such short pulse duration. In this paper, a femtosecond laser drilling for thickness of 16 ㎛ FMM is examined. Additionally, we introduce experimental results for controlling taper angle of hole by vibration module adapted in laser system. We used Ti:Sapphire based femtosecond laser with attenuating optics, co-axial illumination, vision system, 3-axis linear stage and vibration module. By controlling vibration amplitude, entrance and exit diameters are controllable. Using vibrating objective lens, we can control taper angle when femtosecond laser hole drilling by moving focusing point. The larger amplitude of vibration we control, the smaller taper angle will be carried out.

• Medical Lasers
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• v.10 no.1
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• pp.31-36
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• 2021

Background and Objectives Several predicted optimal parameters for laser treatment have been suggested, making the settings subject to variation. Thus, picosecond lasers may require more data and studies to optimize the laser parameters, increase the efficacy of each session, and minimize the total number of sessions. This study evaluated the results of picosecond laser tattoo removal in variable locations, focusing on the aesthetic outcome, number of procedures, and adverse effects. Materials and Methods Nine patients who underwent picosecond laser treatment from February 2014 to July 2020 were enrolled. Before the procedures, the required number of sessions was estimated using the Kirby-Desai scale. At the end of the treatment, the patients assessed their satisfaction. The patient and two plastic surgeons assessed the clearance of the tattoo. Results The mean of the clearance was 86.6% in both the patient and investigators assessment. The patients reported satisfaction for an average score of 5.1 ± 0.78 for the aesthetic outcome, 4.5 ± 0.78 for pain, 4.1 ± 0.92 for the number of procedures, and 4.7 ± 0.97 for adverse effects. The average number of actual procedures was 6.7 ± 1.20. The average Kirby-Desai score was 7.1 ± 1.45. Their correlation coefficient was 0.803, which is considered a strong positive correlation. Conclusion Picosecond lasers have a remarkable ability to degrade smaller tattoo pigments through a photoacoustic effect. Moreover, a picosecond laser treatment for tattoo removal can be a safe and effective method. Picosecond lasers are a promising technology with the potential to optimize the treatment of tattoos.

• Proceedings of the KSME Conference
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• 2001.06c
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• pp.513-518
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• 2001

[ $Al_[2}O_{3}$ ] ceramics are generally used as components in processing equipment, devices or machinery. But it's difficult to machining as being machanical because $Al_[2}O_{3}$ ceramics are brittle materials. This study described a basic study of the input parameters effect on the dimension of the microhole at the $Al_[2}O_{3}$ ceramics using Nd:YAG laser. Major input parameters are peak power, pulse frequency and pulse duration in the laser microhole machining of $Al_[2}O_{3}$ ceramics. We will get a smaller microhole and diameter rate by an appropriate peak power, pulse duration.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.05a
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• pp.104-107
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• 2002

Direct Metal Manufacturing (DMM) is a new additive process that aims to take die making and metalworking in an entirely new direction. It is the blending of five common technologies : lasers, computer-aided design (CAD), computer-aided manufacturing (CAM), sensors and powder metallurgy. The resulting process creates parts by focusing an industrial laser beam onto a tool-steel work piece or platform to create a molten pool of metal. A small stream of powdered tool-steel metal is then injected into the melt pool to increase the size of the molten pool. By moving the laser beam back and forth, under CNC control, and tracing out a pattern determined by a computerized CAD design, the solid metal part is built line-by-line, one layer at a time. DMM produces improved material properties in less time and at a lower cast than is possible with traditional fabrication.