• Korean Journal of Optics and Photonics
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• v.10 no.2
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• pp.89-94
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• 1999

The 1.54 ${\mu}{\textrm}{m}$ forward and backward stimulated Raman scattering (SRS) have been studied in CH$_4$pumped by 1.06 ${\mu}{\textrm}{m}$ Nd:YAG laser. The forward and backward SRS output energy in a single pass were measured at dufferent CH$_4$pressures. Under steady state conditions, the pump input threshold energies and Raman gains in forward and backward directions were for Raman conversion at various CH$_4$pressures for a tight focusing geometry. The forward and backward slope efficiency for Raman conversion were 18% and 34% respectively. The pump input threshold energy of the backward SRS was lower than that of the forward. In backward SRS, the experimental input laser threshold and Raman gain values were in good agreement with the calculated values at different pressures of CH$_4$. The retio of the backward to the forward SRS gain was appoximately 1.4 times above 1200 psi. We obtained that the backward Raman gain coefficient was 0.32 cm/GW, and the forward Raman gain coefficient 0.23cm/GW at 1400 psi. Asymmetry of the forward and backward Raman gain is caused by the interaction between different pump intensities of each direction duting the amplification of the Stokers. The backward Raman gain is proportional to the average pump intensity. However, the forward SRS output grows by depleting the local pump intensity.

• Korean Journal of Optics and Photonics
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• v.10 no.2
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• pp.95-101
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• 1999

We have studied the 1.54$\mu\textrm{m}$ forward and backward stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SRS) for various $CH_4$pressures by 1.06$\mu\textrm{m}$ Q-switched Nd:YAG laser pumping under a repetition rate of Hz in single pass. We obtained that the output of backward SRS was more efficient than that of the forward SRS. The output energy and conversion efficiency of forward and backward SRS were higher than those of SBS since SRS is a steady state, but SBS is a transient state. In a $CH_4$gas uncirculating system, the output energy of the backward SRS and SBS were reduced the about 47% due to a thermal heating of $CH_4$medium in a focusing region for a repetition rate of 5 Hz. But, the output energy of forward SRS was slightly enhanced by about 8.5% due to the increase of the undepleted pump beam in the backward SRS generation. Inthe Raman half resonator using a dichromatic focusing lens, the conversion efficiency of SRS was more than 37% for a input pump laser energy of 40 mJ.

• Journal of the Mineralogical Society of Korea
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• v.1 no.2
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• pp.94-103
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• 1988

Diamond anvil cell (DAC) interfaced with a YAG laser heating system has been used to study the phase transformations on perovskite structured titanates ($BaTiO_3$, and $PbTiO_3$) and to synthesize the silicate perovskite phase from the orthopyroxenes of $MgSiO_3$ and $(Mg_{0.87},\;Fe_{0.13})SiO_3$. $BaTiO_3$ and $PbTiO_3$ transform from tetragonal phase to cubic at the pressures of approximately 2.6 GPa and 4.0 GPa at room temperature, respectively. Cubic phases of the both show wide range of stability in the extended in-situ high pressures and high temperature regions. Starting orthoenstatite of $MgSiO_3$ has yielded the perovskite phase as the major structure with ilmenite, gamma-spinel, betta-spinel and stishovite phases at ~38 GPa and ${\sim}1,000^{\circ}C$. $(Mg_{0.87},\;Fe_{0.13})SiO_3$ has shown the perovskite as the major phase with betta-spinel, stishovite and enstatite phases at ~35 GPa and ${\sim}1,000^{\circ}C$. The ilmenite phase does not occur at this condition.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.55 no.10
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• pp.467-474
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• 2006

The effect of pressureless-sintered temperature on the densification behavior, mechanical and electrical properties of the $SiC-TiB_2$ electroconductive ceramic composites was investigated. The $SiC-TiB_2$ electroconductive ceramic composites were pressureless-sintered for 2 hours at temperatures in the range of $1,750{\sim}1,900[^{\circ}C]$, with an addition of 12[wt%] $Al_2O_3+Y_2O_3(6:4\;mixture\;of\;Al_2O_3\;and\;Y_2O_3)$ as a sintering aid. The relative density, flexural strength, vicker's hardness and fracture toughness showed the highest value of 84.92[%], 140[MPa], 4.07[GPa] and $3.13[MPa{\cdot}m^{1/2}]$ for $SiC-TiB_2$ composites of $1,900[^{\circ}C]$ sintering temperature at room temperature respectively. The electrical resistivity was measured by the Pauw method in the temperature ranges from $25[^{\circ}C]\;to\;700[^{\circ}C]$. The electrical resistivity showed the value of $5.51{\times}10^{-4},\;2.11{\times}10^{-3},\;7.91{\times}10^{-4}\;and\;6.91{\times}10^{-4}[\Omega{\cdot}cm]$ for ST1750, ST1800, ST1850 and ST1900 respectively at room temperature. The electrical resistivity of the composites was all PTCR(Positive Temperature Coefficient Resistivity). The resistance temperature coefficient showed the value of $3.116{\times}10^{-3},\;2.717{\times}10^{-3},\;2.939{\times}10^{-3},\;3.342{\times}10^{-3}/[^{\circ}C]$ for ST1750, ST1800, ST1850 and ST1900 respectively in the temperature ranges from $25[^{\circ}C]\;to\;700[^{\circ}C]$. It is assumed that because polycrystallines, such as recrystallized $SiC-TiB_2$ electroconductive ceramic composites, contain of porosity and In Situ $YAG(Al_5Y_3O_{12})$ crystal grain boundaries, their electrical conduction mechanism are complicated. In addition, because the condition of such grain boundaries due to $Al_2O_3+Y_2O_3$ additives widely varies with sintering temperature, electrical resistivity of the $SiC-TiB_2$ electroconductive ceramic composites with sintering temperature also varies with sintering condition. It is convinced that ${\beta}-SiC$ based electroconductive ceramic composites for heaters or ignitors can be manufactured by pressureless sintering.

• Journal of the Korean Ceramic Society
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• v.40 no.9
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• pp.874-880
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• 2003

The effect of h-BN content on microstructure, mechanical properties, and machinability of AlN-BN based machinable ceramics were investigated. The relative density of sintered compact decreased with increasing h-BN content. The four-point flexural strength also decreased from 238 MPa of monolith up to 182 MPa by the addition of 30 vol％ h-BN. Both low Young's modulus and residual tensile stress, formed by the thermal expansion coefficient difference between AIN and h-BN, might cause the strength drop in AlN-BN composite. The crack deflection, and pull-out phenomena increased by the plate-like h-BN. However, the fracture toughness decreased with h-BN content. The second phases, consisted of YAG and ${\gamma}$-Al$_2$O$_3$, were formed by the reaction between Al$_2$O$_3$ and Y$_2$O$_3$. During end-milling process, feed and thrust forces measured for AlN-(10～30) vol％ BN composites decreased with increasing h-BN particles, showing excellent machinability. Also, irrespective of h-BN content, relatively good surfaces with roughness less than 0.5 m (Ra) could be achieved within short lapping time.

• Journal of the Korean Society of Propulsion Engineers
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• v.4 no.4
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• pp.18-25
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• 2000

A two-color particle image velocimetry (PIV) has been developed for measuring two dimensional velocity flowfields and applied to a Mach 2.0 supersonic nozzle. This technique is similar to a single-color PIV technique except that two different color laser beams are used to solve the directional ambiguity problem. A green-color laser sheet (532 nm: 2nd harmonic beam of YAG laser) and a red-color laser sheet (619 nm: output beam from YAG pumped Dye laser using Rhodamine 640) are employed to illuminate the seeded particles. A high resolution (3060${\times}$2036) digital color CCD camera is used to record the particle positions. This system eliminates the photographic-film processing time and subsequent digitization time as well as the complexities associated with conventional image shifting techniques for solving directional ambiguity problem. The two-color PIV also has the advantage that velocity distributions in high speed flowfields can be measured simply and accurately by varying the time interval between two different laser beams due to its high signal-to-noise ratio and thereby less requirement of panicle pair numbers for a velocity vector in one interrogation spot. The velocity distribution in the Mach 2.0 supersonic nozzle has been measured and the over-expanded shock cell structure can be predicted by the strain rate field. These results are compared and analyzed with the schlieren photograph for the velocity distributions and shock location.

• Korean Journal of Optics and Photonics
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• v.21 no.3
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• pp.123-128
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• 2010

Sum frequency generation was utilized to obtain a yellow laser with the wavelength of 578.4 nm for a probe laser of an Yb lattice clock. The output of an Nd:YAG laser with wavelength of 1319 nm and that of an Yb-fiber laser with wavelength of 1030 nm were passed through a waveguided periodically-poled lithium niobate (WG-PPLN) for sum frequency generation. It is required that the probe laser has a linewidth of the order of 1 Hz to fully resolve the Yb lattice clock transition. Thus, the linewidth of the probe laser was reduced by stabilizing the frequency to a super-cavity. This was made of ULE with a low thermal expansion coefficient, and was mounted on an active vibration-isolation table at the optimal point for the reduced sensitivity to vibration. Also, this was installed in a vacuum chamber, and the temperature was stabilized to 1 mK level. This system was installed in an acoustic enclosure to block acoustic noise. The finesse of the super-cavity was measured to be 380 000 from the photon life time of the cavity.

• Journal of the Optical Society of Korea
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• v.7 no.4
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• pp.230-233
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• 2003

This study describes the conceptual design of a soft x-ray microscope system based on a laserbased source for biomedical application with high resolution (${\leq}$50nm). The laboratory scale soft x-ray microscope consists of high power laser plasma x-ray source and grazing incidence mirrors with high reflectivity. The laser plasma source used for developing this system employs Q-switched Nd-YAG pulsed laser. The laser beam is focused on a tantalum (Ta) target. The Wolter type I mirror was used as condenser optics for sample illumination and as objective mirror for focusing on a detector. The fabrication of the Wolter type I mirror was direct internal cutting using ultraprecision DTM. A hydrated biological specimen was put between the two silicon wafers, the center of which was $Si_3N_4$ windows of 100㎚ thickness. The main issues in the future development work are to make a stable, reliable and reproducible x-ray microscope system.

• Journal of ILASS-Korea
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• v.17 no.4
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• pp.192-196
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• 2012

This paper reports on breakup characteristics of fuel droplet which includes metal nanoparticles. In order to develop a new injection system for nanoparticle-coated layers overcoming the conventional flame spray system, fundamental experiments were conducted to examine the interaction between a fuel droplet with nanoparticles and the external energy induced by the laser. In the experiments, this study used nickel nanoparticles whose size was under 100 nm to mix with kerosene as the fuel, and utilized a syringe pump and a metal needle to inject a fuel droplet. In particular, the Nd-YAG laser was adopted to give additional energy to the nanoparticles for evaporation of a fuel droplet containing nanoparticles. When the laser energy as 96 mJ was irradiated during the injection, it was observed that such an explosive evaporation occurred to break up a fuel droplet including nanoparticles, making the rapid increase in the ratio surface area to liquid volume. From this work, we suggest the possibility that the laser energy can be used for rapid evaporation of a fuel droplet.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.54-59
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• 2001

OH radical concentration have been measured in a methane-air partially premixed flames using PLIF. Excitation lines were selected $Q_{1}(6)$ branch, (1,0) band. The system is consisted of Nd:YAG laser, dye laser and frequency doubler to make pump beam for OH radical. On the direct photographs, flame height increases as fuel flow rate and equivalence ratio increase. And on the PLIF images, OH radical is distributed from premixed flame front to nonpremixed flame front through the flame structure with all equivalence ratio. OH overall concentrations increase with equivalence ratio. At the stoichiometric equivalence ratio, the peak of OH radical concentration exists strongly near the inner cone. As equivalence ratio is changed to richer, OH radical distribution goes thinly and the peak is increased as longitudinal direction. As the flow goes to the downstream, OH radical concentration decreases and broadens, because OH radical reacts with another species after OH formation at the initial oxidization. This phenomenon resembles radial distribution. At the l00cc fuel flowrate, the radial peak of OH radical exists from x/R=l.0 to 1.5.