• 비파괴검사학회지
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• 제32권4호
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• pp.347-354
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• 2012

Using a single-line pulsed laser beam is well known as a useful noncontact method to generate a directional surface acoustic wave. In this method, different laser beam energy profiles produce different waveforms and frequency characteristics. In this paper, we considered two typical kinds of laser beam energy profiles, Gaussian and square-like, to find out a difference in the frequency characteristics. To achieve this, mathematical models were proposed first for Gaussian laser beam profile and square-like respectively, both of which depended on the laser beam width. To verify the theoretical models, experimental setups with a cylindrical lens and a line-slit mask were respectively designed to produce a line laser beam with Gaussian spatial energy profile and square-like. The frequency responses of the theoretical models showed good agreement with experimental results in terms of the existence of harmonic frequency components and the shift of the first peak frequencies to low.

• Journal of the Optical Society of Korea
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• 제13권1호
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• pp.60-64
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• 2009

We attempted the first measurement of the spectral width of the nickel-like molybdenum x-ray laser (${\lambda}\;=\;18.895\;nm$) by use of a high-resolution spectrometer in order to determine the strength of the magnetic field required for the generation of a circularly polarized x-ray laser. The spectral width was measured to be ${\Delta}{\lambda}\;=\;18\;m{\AA}$ under the substantial lasing condition. The magnetic field required for the generation of a circularly polarized x-ray laser was 40 T. The splitting of the x-ray laser line was clearly obtained under 15 T external magnetic field. The strength of the magnetic field estimated from the splitting of the x-ray laser line was large compared with the external magnetic field. It implies that there might be an alternative mechanism for enhancement of the magnetic field in the gain medium plasma.

• 반도체디스플레이기술학회지
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• 제18권3호
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• pp.72-76
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• 2019

A Q-switched diode-pumped neodymium-doped yttrium vanadate (YVO₄, λ =1064nm) laser was used for the direct patterning of indium tin oxide (ITO) films on glass substrate. During the laser direct patterning, the laser beam was incident on the two different directions of glass substrate and the laser ablated patterns were compared and analyzed. At a low scanning speed of laser beam, the larger laser etched lines were obtained by laser beam incident in reverse side of glass substrate. On the contrary, at a higher scanning speed, the larger etched pattern sizes were found in case of the beam incidence from front side of glass substrate. Furthermore, it was impossible to find no ablated patterns in some laser beam conditions for the laser beam from reverse side at a much higher scanning speed and repetition rate of laser beam. The laser beam is expected to be transferred and scattered through the glass substrate and the laser beam energy is thought to be also dispersed and much more influenced by the overlapping of each laser beam spot.

• 한국전기전자재료학회논문지
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• 제29권11호
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• pp.707-711
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• 2016

ITO/Ag/ITO conductive films on PET (polyethylene terephthalate) was etched by a Q-switched diode-pumped neodymiun-doped yttrium vanadate (Nd:YVO4, ${\lambda}=1064nm$) laser. During the laser direct etching, the laser beam was incident on the two different directions of PET and the etching patterns were investigated and analyzed. At a lower repetition rate of laser pulse, the larger laser etched patterns were obtained by laser beam incident on reverse side of PET substrate. On the contrary, at a higher repetition rate, it was possible to find the larger etched patterns in case of the laser beam incidence on forward side of PET substrate. For the laser beam incidence on reverse side, the laser beam is expected to be transferred and scattered through the PET substrate and the laser beam energy is thought to be dependent on the etch laser pulse beam energy.

• 한국레이저가공학회지
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• 제11권3호
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• pp.5-9
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• 2008

The effect of a laser peening on the surface residual stress of SUS 304 was investigated using a second harmonic Nd:YAG laser beam. The energy density and the diameter of the laser beam were $400mJ/mm^2$ and about 1mm, respectively. According to the test results, the effect of a laser peening for improving the surface residual stress was not big enough to induce a high compressive stress on the SUS 304 surface. This is thought to be attributed to the small radius of the laser beam used in this study, even though its energy density is big enough. From this study, it can be concluded that to induce a recognizably high compressive stress on a metal surface, the energy density as well as the size (diameter) of the laser beam should be large enough to generate surface plasma with a high energy to have a big impact to a metal surface.

• 한국레이저가공학회지
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• 제13권2호
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• pp.10-16
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• 2010

Laser-assisted machining is dependent on absorbed energy density into workpiece. Generally, the absorptivity of laser beam is dependent on wave length of laser, materials, surface roughness, etc. Various shapes and energy densities for beam irradiation can be used to laser-assisted machining. In this thesis, efficient method of heat source modeling was developed and designed by using one fundamental experimental trials. And then, laser-assisted machining of rod-shaped cast iron was simulated by using commercial FEM code MARC. Simulations and experiments with various conditions were carried out to determine suitable condition of pre-heating for laser-assisted turning process. Temperature distribution of cutting zone could be predicted by simulation.

• Journal of the Optical Society of Korea
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• 제20권3호
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• pp.407-412
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• 2016

We analyzed through numerical simulations the properties of a square beam homogenizer near the target for laser shock peening. The efficiency was calculated near the target by considering the plasma threshold of the metals. We defined the depth of focus of the square beam homogenizer with a given efficiency near the target. Then, we found the relationship between the depth of focus for the laser shock peening and four main parameters of the square beam homogenizer: the plasma threshold of the metal, the number of lenslets in the array-lens, the focal length of the condenser lens and the input beam size.

• Journal of the Optical Society of Korea
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• 제18권4호
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• pp.382-387
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• 2014

An all-reflective, simple noncollinear second harmonic (SH) autocorrelator is described for monitoring the shot-to-shot behavior of ultrashort high-power laser pulses. Two mirrors are used for the dispersion-free splitting of a pulse into two halves. One of the mirrors is able to adjust the delay time and angle between two halves of the laser pulse in a nonlinear crystal. We present the possibility of real-time measurement of the pulse duration, peak intensity (or energy), and the pointing jitters of a laser pulse, by analyzing the spatial profile of the SH autocorrelation signal measured by a CCD camera. The measurement of the shot-to-shot variation of those parameters will be important for the detailed characterization of laser accelerated electrons or protons.

• 방사성폐기물학회지
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• 제19권3호
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• pp.289-295
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• 2021

A laser scabbling experiment was performed using a high-power fiber laser to investigate the removal rate of the concrete block and the scabbled depth. Concrete specimens with a 28-day compressive strength of 30 MPa were used in this study. Initially, we conducted the scabbling experiment under a stationary laser beam condition to determine the optimum scan speed. The laser interaction time with the concrete surface varied between 3 s and 40 s. The degree of spalling and vitrification on the surface was primarily dependent on the laser interaction time and beam power. Furthermore, thermal images were captured to investigate the spatial and temporal distribution of temperature during the scabbling process. Based on the experimental results, the scan speed at which the optical head moved over the concrete was set to be 300 mm·min^-1 or 600 mm·min^-1 for the 4.8-kW or 6.8-kW laser beam, respectively. The spalling rates and average depth on the concrete blocks were measured to be 87 cm³·min^-1 or 227 cm³·min^-1 and 6.9 mm or 9.8 mm with the 4.8-kW or 6.8-kW laser beams, respectively.

• 한국기계가공학회지
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• 제18권5호
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• pp.74-81
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• 2019

This study reports the CW $CO_2$ laser scribing of soda lime glass. In this study, scribing experiments are carried out at different laser powers, scan speeds, and focal positions to investigate the effect of the process parameters on the interaction characteristics between a laser beam and glass. In particular, the interaction characteristics are analyzed and described with the input laser energy per unit length. According to the experimental results, the damage threshold for the glass surface was found to exist between 0.072 and 0.08 J/mm. The input laser energy in this region induced partial melting of the surface and grain-shaped cracks. These cracks tended to increase as the input laser energy increased. At the laser input energy larger than 1 J/mm, a huge crack propagating along the scan direction was produced, and the volume below the scribed area was fully melted. The growth of this crack finally resulted in the complete cutting of the glass at the input laser energy above 8 J/mm. It was found that both the width and depth of the scribed line increased with increasing input laser energy. For the beam focusing at the rear surface, the width of the scribed line varied irregularly. This could be ascribed to the increased asymmetry of the beam intensity distribution when the laser beam was focused at the rear surface. Under this condition, a large burr was only produced on one side of the scribed line.