• Journal of Biomedical Engineering Research
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• v.20 no.3
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• pp.275-281
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• 1999

Conventional PW Doppler systems suffer from the ambiguity of measured blood velocities due to the spectrum aliasing when the corresponding Doppler frequencies are greater than the Nyquist frequency. Base-line shift is a customary method for dealiasing the Doppler spectrums. I lowever, Doppler audio signals still remain unchanged even when the base-line shift method is applied. This paper de scribes an method for dealiasing both the Doppler spectra and audio signals by using sampling rate expansion, frequency shifting, and filtering poerations. For undirectional flows, the method can increase the maximum detectable Doppler frequency from the Nyquist limit of one-half of the Pulse Repetition Frequency(PRF) to the PRF. Experiments with real data have been performed to verify the proposed method.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2000.07a
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• pp.340-343
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• 2000

ITO(Indium Tin Oxide) films for transparent electrodes of FPD(Flat Panel Display) were patterned in atmosphere using laser. A pulse type(repetition rate of 10 Hz) Q-switched Nd:YAG laser which can generate the fundamental wavelength at 1064 nm or its harmonics(532, 266 nm) was used for Patterning of the ITO film. In case of using the second harmonic(532 nm) of Nd:YAG laser, the ITO film(thickness of 20 nm) was removed clearly with a laser fluence of 5.2 J/$\textrm{cm}^2$ and a beam scan speed of 200${\mu}{\textrm}{m}$/s. But the glass substrate was damaged when the laser fluence was over 5.2 J/$\textrm{cm}^2$. We discussed the etching mechanism of the ITO film using Nd:YAG laser with observation of the etching characteristics including a depths and widths of ITO films as a function of laser fluence using SEM(Scanning Electron Microscopy) and surface profiler($\alpha$-step 500).

• Journal of Astronomy and Space Sciences
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• v.36 no.2
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• pp.97-103
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• 2019

An automated signal-acquisition method for the NASA's space geodesy satellite laser ranging (SGSLR) system is described as a selection of two system parameters with specified probabilities. These parameters are the correlation parameter: the minimum received pulse number for a signal-acquisition and the frame time: the minimum time for the correlation parameter. The probabilities specified are the signal-detection and false-acquisition probabilities to distinguish signals from background noise. The steps of parameter selection are finding the minimum set of values by fitting a curve and performing a graph-domain approximation. However, this selection method is inefficient, not only because of repetition of the entire process if any performance values change, such as the signal and noise count rate, but also because this method is dependent upon system specifications and environmental conditions. Moreover, computation is complicated and graph-domain approximation can introduce inaccuracy. In this study, a new method is proposed to select the parameters via a conditional equation derived from characteristics of the signal-detection and false-acquisition probabilities. The results show that this method yields better efficiency and robustness against changing performance values with simplicity and accuracy and can be easily applied to other satellite laser ranging (SLR) systems.

• Journal of the Korea Institute of Military Science and Technology
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• v.25 no.2
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• pp.169-176
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• 2022

Laser ablation propulsion(LAP) is the method to create impulse by laser ablation. It can be used to deorbit the space debris(SD), as its long-range property and versatility on any material. In this paper, we find out several requirements of the LAP system(LAPS) to deorbit the SD by simple numerical calculations of the SD orbit and laser beam flux. As a result, minimum operable altitude angle turned out to be a crucial variable to the LAPS. Moreover, if minimum operable altitude angle is 10°, and if the minimum distance between the LAPS and the SD is below 450 km, 1 m/s² is sufficient to deorbit the SD by once. With 18 kJ/3 ns pulsed laser and cube shaped 100 kg SD, 1 m/s² acceleration can be achieved by increasing the pulse repetition rate over 34~53 Hz, depending on the size of the SD. This capability could compare with the conceptual design of the Japan Establishment for a Power-laser Community Harvest(J-EPoCH) facility, which include 8 kJ, 5 PW@100 Hz laser.

• Nuclear Engineering and Technology
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• v.54 no.2
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• pp.437-441
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• 2022

In this study, we developed a neutron time-of-flight (nTOF) measurement system for a 1.7-MV tandem proton accelerator with a target covered with 300-nm-thick lithium (Li) layer. With implementation of beam chopping module after its ion source, the accelerator is configured to operate in pulsed-beam mode with a pulse width <50 ns at 20-kHz repetition rate. This enables the gamma flash-type nTOF measurement system to identify the neutron generated with 3-MeV proton beam energy. The nTOF system consists of a 30" cylindrical NaI(Tl) and four stilbene scintillation detectors. The NaI(Tl) scintillator is placed 50 cm from the Li target to measure the time of beam irradiation on the target, and the stilbene detectors are placed 2 and 2.4 m away to measure nTOF at each location. The nTOF system successfully measured the generated neutron energy at irradiated proton energies of 2.6 and 3.0 MeV with an average energy resolution of 15%.

• Current Optics and Photonics
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• v.5 no.6
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• pp.721-729
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• 2021

We demonstrate a Pr³⁺:YLF 639.7-nm laser with single-frequency output based on the Q-switched pre-lase technology, pumped by a fiber-coupled GaN blue laser diode. The pre-lase technology is realized by the step-type loss of the acousto-optical Q-switched device. The conclusions of the theoretical research are verified experimentally. The mode-suppression ratio was 44 dB at the single-frequency laser output. Detection by interferometer verified the realization of the stable single-frequency laser. In addition, the emission spectrum had a linewidth of 139.9 MHz, measured by Fabry-Perot interferometer. The single-frequency laser's single-peak power was over 19.7 W with 98.8-ns pulse duration, obtained under an absorption power of 1.74 W.

• Review of KIISC
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• v.33 no.4
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• pp.13-22
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• 2023

Controller area network (CAN) 네트워크로 대표되는 자동차 내부네트워크와 비교하여 자동차 스마트키 시스템은 상대적으로 소수의 연구가 진행되어오고 있다. 하지만, 현실 세계에서는 스마트키 시스템의 취약점으로 인해 많은 피해사례가 발생하고 있다. 대표적으로, 2010년 NDSS 학회에 소개된 신호 중계 공격 (signal relay attack)은 현재까지도 수많은 자동차 절도 사건들에 악용되고 있다. 이와 같은 문제를 근본적으로 해결하기 위해 초광대역 통신(ultra-wideband communication, UWB)을 사용한 디지털 키 (Digital Key) 기술이 일부 최신 자동차들에 탑재되고 있다. 하지만, 2022년USENIX Security 학회에서 애플, 삼성과 같은 글로벌 기업이 채택한 high rate pulse repetition frequency (HRP) UWB 측위 시스템에 대한 거리 단축 공격 (distance reduction attack)이 가능함이 소개되었다. 이는 디지털 키 시스템 또한 신호중계 공격과 같은 보안 위협에 노출될 수 있다는 점을 시사한다. 본 논문에서는 자동차 스마트키 시스템을 대상으로 수행된 공격 연구 사례들을 살펴본다. 먼저, remote keyless entry (RKE) 시스템 및 passive keyless entry and start (PKES) 시스템으로 대표되는 기존 스마트키 시스템을 대상으로 하는 보안 위협에 대해 살펴본다. 다음으로 차세대 스마트키 시스템으로 주목받고 있는 디지털키 시스템을 구성하는 초광대역 통신기술의 동작 원리 및 이에 대한 보안위협 연구 동향을 살펴본다.

• Journal of the Korean Society for Precision Engineering
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• v.26 no.3
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• pp.25-31
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• 2009

Femto-second laser ablation with the various feed velocities of the Invar alloy and the micro surface milling for the processing condition were studied. We used a regenerative amplified Ti:sapphire laser with a 1kHz repetition rate, 184fs pulse duration time and 785nm wavelength. Femto-second laser pulse was irradiated on the Invar alloy with the air blowing at the condition of various laser peak powers and feed velocities. An ablation characteristic according to feed velocity of the Invar alloy was appeared as the non-linear type at different zone of energy fluence. The micro surface milling of the Invar alloy using a mapping method was investigated. The optimal condition of micro surface milling was laser peak power of 22.8mW, feed velocity of 1 mm/s, beam gap of $1{\mu}m$. With the optimal processing condition, the fine rectangular shape without burr and thermal damage was achieved. Using the femto-second laser system, it demonstrates excellent tool for micro surface milling of the Invar alloy without heat effects and poor edge.

• Laser Solutions
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• v.14 no.2
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• pp.17-23
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• 2011

Active thin films are ubiquitous in the manufacture of all forms of flat panel display (FPD). One of the most widely employed thin films is indium tin oxide (ITO) and metal films used electrically conductive materials in display industries. ITO is widely used for fabrication of LCD, OLED device, and many kinds of optical applications because of transparency in visible range and its high conductivity and metal films are also widely employed as electrodes in various electric and display industries. It is important that removing specific area of layer, such as ITO or metal film on substrate, to fabricate and repair electrode in display industries. In this work, we demonstrate efficient selective ablation process to ITO and aluminum film on glass using a femtosecond laser (${\lambda}p=1025nm$) respectively. The femtosecond laser with wavelength of 1025nm, pulse duration of 400fs, and the repetition rate of 100kHz was used for selectively removing ITO and Al on glass in the air. We can successfully remove the ITO and Al films with various pulse energies using a femtosecond laser.

• Korean Journal of Optics and Photonics
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• v.4 no.1
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• pp.47-56
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• 1993

Practical efficiencies of pulsed gas discharge lasers utilizing vaporized Au, Ba, Cu, Fe, Mn, Pb, TI atoms are numerically optimized for a discharge pulse of 30 ns rise time at 5 kHz pulse repetition rate. Calculated optimum operating temperatures are close to the values at which the vapor pressure of each atom is about 1 Torr and are in good agreement with experimental values reported elasewhere. The optimum charging voltage was about 4 kV for all atomic vapor lasers except the TI laser. The calculated maximum efficiencies that have been discussed for possible limiting efficiencies for each laser were considerably higher than the reported experimental values. Especially, they were as high as 6.8% for Mn laser and 15% for Ba laser that were comparable to the 8.5% of copper vapor laser, from which considerably higher efficiencies can be expected for these two lasers experimentally. But, the present calculation shows a very low efficiency of 0.058% and requires an exceptional operating condition for the TI laser.