• Current Optics and Photonics
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• v.7 no.6
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• pp.708-713
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• 2023

A robust all-fiber nonlinear amplifying loop-mirror-based mode-locked femtosecond laser is demonstrated. Power-dependent nonlinear phase shift in a Sagnac loop enables stable and power-efficient mode-locking working as an artificial saturable absorber. The pump power is adjusted to achieve the lowest intensity noise for stable long-term operation. The minimum pump power for mode-locking is 180 mW, and the optimal pump power is 300 mW. The lowest integrated root-mean-square relative intensity noise of a free-running mode-locked laser is 0.009% [integration bandwidth: 1 Hz-10 MHz]. The long-term repetition-rate instability of a free-running mode-locked laser is 10^-7 over 1,000 s averaging time. The repetition-rate phase noise scaled at 10-GHz carrier is -122 dBc/Hz at 10 kHz Fourier frequency. The demonstrated method can be applied as a seed source in high-precision real-time mid-infrared molecular spectroscopy.

• Journal of the Korean Society for Precision Engineering
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• v.25 no.9
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• pp.27-31
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• 2008

• Proceedings of the KWS Conference
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• 1994.05a
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• pp.152-154
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• 1994

• Journal of Powder Materials
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• v.25 no.3
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• pp.220-225
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• 2018

The microstructure and mechanical characteristics of SUS630 specimens fabricated using the direct energy deposition (DED) process are investigated. In DED, several process parameters such as laser scan speed, chamber gas flow, powder carrier gas flow, and powder feed rate are kept fixed; the laser power is changed as 150 W, 180 W, and 210 W. As the laser power increases, the surface becomes smooth, the thickness uniformity improves, and the size and number of pores decreases. With the increase in laser power, the hardness deviation decreases and the average hardness increases. The microstructure of the material is columnar; pores are formed preferentially along the columnar interface. The lath-martensite phase governs the overall microstructure. The volumetric fraction of the retained austenite phase is measured to increase with the increase of laser input power.

• Journal of ILASS-Korea
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• v.2 no.4
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• pp.7-14
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• 1997

Laser Raman scattering method has been applied to measure equivalence ratio of methane/air mixture in injected spray. We used high power KrF excimer laser$(\lambda=248nm)$ and a high gain ICCD camera to capture low intensity signal. Raman shifts and Raman scattering cross -sections of $H_2,\;O_2,\;N_2,\;CO_2,\;CH_4\;and\;C_3H_8$ are measured precisely. Our results show an excellent agreement with those of other groups. Mole fraction measurement of $O_2\;and\;N_2$ from air shows that $O_2:N_2=0.206:0.794$. We used gas injector which was operated at 1 bar. Methane is used as a fuel. Spray region is $10mm\times37mm$ and this region is divided into 80 points. In Raman signals are obtained and ensemble averaged for each point. 3-d and contour plot of distribution of equuivalence ratio is presented. Our measured results show that the equivalence ratio of methane/air mixture in methane-rich region is reasonable. However, more study is necessary for methane-lean region because background noise level is almost same as Raman intensity of methane.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.25 no.5
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• pp.329-337
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• 2016

The demand for the eco-friendly vehicles is skyrocketing because of the increasing $CO_2$ emissions and global warming. In the industrial field, the battery process, a core part of an eco-friendly vehicle, is drawing increased attention; its weight lightening as well as high energy density are becoming increasingly important. In this study, pure Ni plates that were used as the battery pole plate were welded using the laser. The lab joint welding was conducted on ten pure Ni plates at a laser power of 1900 W and a feed speed of 2.8-3.4 m/min. As observed in the experiment, a faster feed speed reduced the bead width, but the laser did not penetrate all ten specimen plates. In addition, pores were trapped when protective gas was used, but they were not trapped when the welding was conducted in atmospheric condition.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.16 no.6
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• pp.60-65
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• 2002

The purpose of this study is to measure and compare the output power characteristics and optical gain for selenium vapour laser depending on the He gas purity. The purity of the He gas was improved with a special He-filter. During the measurement the individual wavelengths were selected with a birefringent filter. The result shows that compared with those of laser without He-filter, the output-coupling power and small signal gain of laser with He-filter increase in the most of the lines. Especially, the output-coupling power and small signal gain for the strong lines (497.6 nm, 499.3 nm, 506.9 nm, 517.6 nm, 522.8 nm, 530.5 nm), blue(460.4 nm, 464.8 nm) and red(644.4 nm 649.1 nm) lines lies notably higher.

• Journal of Welding and Joining
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• v.16 no.1
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• pp.77-87
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• 1998

This paper deals with the effect of laser welding parameters on the weld formation. Thin steels for automotive application were prepared so as to be welded with high power carbon dioxide laser system. Major process parameters were position of focus and travel speed. The effect of shielding gas was also discussed by employing the high speed photometry. Test results showed that the optimal position of focus varied in accordance with the joint configuration; bead-on-plate, butt or lap welding. It was recommended that the position of focus for the lap welding be located at slightly inner part of the material to be welded. In this case, however, it was noticeable that the weld penetration ratio, d/t$_{0}$ dropped drastically at the critical region. Results also demonstrated that both the bead width and penetration reduced as the travel speed increased. The penetration ratio showed two distinct regions; stabilized zone at the lower range of the travel spped and sudden drop zone at the higher range of travel speed. Lower limit of the penetration for acceptable weld was proved to be about 90% of the parent metal thickness based on the physical properties of the weld. Mixed gas application for both the shielding of molten metal and laser induced plasma control was recommended as far as the penetration was concerned.d.

• Analytical Science and Technology
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• v.15 no.1
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• pp.7-14
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• 2002

Laser ablation ion trap mass spectrometry (LAITMS) was developed for the analysis of metal and ceramic samples. For this study, XeCl excimer laser (308 nm) was used for ablating the samples and ITMS was used as a detector. Samples were introduced from outside of a ring electrode and this way of sample introduction was very effective for solid samples when laser ablation was employed. Helium gas was used as a buffer gas, and its effect on sensitivity and some parameters (buffer gas pressure, ion storage time, and cut-off RF voltage) were studied. The optimized conditions were $1{\times}10^{-4}$ Torr of buffer gas pressure, 100 ms of ion storage time and $1150V_{p-p}$ of cut-off RF voltage. From that results, copper (Cu) and molybdenum (Mo) metals were tested with LAITMS and the mass spectra of these pure metals were compared with the natural abundance of isotope ratio. We also examined ceramic samples ($Al_2O_3$, $ZrO_2$) and represented the result of elemental analysis.

• International Journal of Automotive Technology
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• v.7 no.4
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• pp.391-397
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• 2006

The soot yield has been studied by a premixed propane-oxygen-inert gas combustion in a specially designed disk-type constant-volume combustion chamber to investigate the effect of pressure, temperature and turbulence on soot formation. Premixtures are simultaneously ignited by eight spark plugs located on the circumference of chamber at 45 degrees intervals in order to observe the soot formation under high temperature and high pressure. The eight converged flames compress the end gases to a high pressure. The laser schlieren and direct flame photographs with observation area of 10 mm in diameter are taken to examine the behaviors of flame front and gas flow in laminar and turbulent combustion. The soot volume fraction in the chamber center during the final stage of combustion at the highest pressure is measured by the in-situ laser extinction technique and simultaneously the corresponding burnt gas temperature by the two-color pyrometry method. The changes of pressure and temperature during soot formation are controlled by varying the initial charging pressure and the volume fraction of inert gas compositions, respectively. It is found that the soot yield increases with dropping the temperature and raising the pressure at a constant equivalence ratio, and the soot yield in turbulent combustion decreases as compared with that in laminar combustion because the burnt gas temperature increases with the drop of heat loss for laminar combustion.