• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.8
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• pp.967-973
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• 2013

An accelerated ultrasonic fatigue test(UFT) has been used for analyzing very high cycle fatigue( VHCF, $N_f$ > $10^7$). This study reviews how the test specimen is to be determined. We focus on UFT using a resonance of 20 kHz. The specimen geometry is determined by selecting test materials by using a dynamic Young's modulus and wavelength of 20 kHz. The dynamic Young's modulus is calculated at the resonant frequency. Through a resonant vibration test at 20 kHz, the length of the specimen is calculated. By determining the shape of the specimen, the stress during the UFT is calculated. The UFT results should be comparable at the test frequency and the specimen geometry obtained by the conventional fatigue tests.

• Proceedings of the Optical Society of Korea Conference
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• 2000.02a
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• pp.274-275
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• 2000

광결정(photonic crystal)으로 광원의 자발 방출을 조절하면 문턱전류 없는 레이저, 고효율 다이오드, 파장 크기에서 손실 없이 급격히 꺾을 수 있는 광도파로 등 기존의 광소자에서 얻을 수 없는 좋은 성능을 얻을 수 있을 것으로 예상된다. 이러한 광결정은 유전체를 파장정도 크기에서 주기적으로 배치시킨 인공적인 결정인데 고체에서 원자의 주기적인 배치로 전자가 전파할 수 없는 진동수 영역, 즉 밴드갭이 생기는 것과 유사하게 빛에 대해서 빛이 전파할 수 없는 진동수 영역인 광밴드갭(photonic bandgap)을 가진다. 그런데 관심있는 광영역에서 3차원 모든 방향으로 광밴드갭이 있는 구조물은 마이크로미터보다 작은 내부 구조를 가지는 복잡한 3차원 구조물로 제작이 어렵다. 이러한 어려움을 극복하기 위해 제작이 비교적 용이한 3차원 광밴드갭 구조물이 찾아지고 있다. 다른 접근 방법으로 평면(x-y)에서는 2차원 광밴드갭을 이용하고 제 3의 방향(z축)으로는 전반사를 이용하는 구조는 제작이 용이할 뿐만 아니라 처음부터 광원의 편광을 TE 또는 TM 모드로만 방출 되도록 준비해 줄 수 있으면 거의 3차원 광결정에서 얻을 수 있는 효과를 낼 수 있는 것으로 발표되었다.$^{(1)}$ 이 방법을 이용하여 최근에 미국의 캘리포니아 공과대학(Caltech)을 중심으로 레이저 동작을 보여 주었다.$^{(2.3)}$ 공기로 둘러싸인 얇은 유전체 평판에서 생기는 전반사와 평판 위에 2차원 삼각형살창(triangular lattice)에 구멍을 뚫어 얻는 2차원 광밴드갭을 이용해 3차원 공진모드를 형성하였다. 이러한 구조에서 1개만 구멍을 매워서 만든 공진기는 저온(143 K)에서 레이저 발진을 보였고 여러 개의 구멍을 매운 경우는 상온에서 펌프 펄스의 유지시간이 0.5% 인 경우 레이저가 동작하는 것을 보여주었다. 이는 구조내에서 열전도가 문제가 된다는 것을 의미하는데 위아래가 공기로 둘러 싸여 있어 발생한 열이 가는 유전체 네트웍을 통해서만 전달 될 수 있기 때문이다. (중략)

• Korean Journal of Optics and Photonics
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• v.6 no.2
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• pp.142-147
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• 1995

Soliton pulse outputs are generated with figure '8' type erbium-doped fiber laser mode-locked by using a fiber loop mirror. The fiber loop mirror consists of an erbium-doped fiber amplifier at the one end of the loop, and 504 m-long dispersion-shifted fiber as a nonlinear medium. By pumping with a $1.48{\mu}m$ wavelength laser diode and adjusting the polarization controllers inside the loop, soliton pulses are generated with 1574 nm center wavelength and 1.2 nm linewidth. The soliton pulses are found randomly spaced within the fundamental period corresponding to cavity round trip time. The autocorrelation trace shows that the pulse width is 2.4 ps, which is in good agreement with the theoretical prediction. The pulsewidth- bandwidth product is found to be 0.348 which means that the pulses are nearly transform-limited.imited.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.28 no.10
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• pp.761-768
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• 2017

In this paper, to implement the electromagnetic wave environment and solve electromagnetic compatibility(EMC) problem, miniaturization of ultra-wide band log periodic dipole antenna for measurement was investigated. In addition, in oder to improve the signal-to-noise ratio in high frequency band, balun was connected to the antenna to stabilize the operation of the differential mode antenna and the single mode coaxial cable. To minimize the total size and to increase bandwidth of the antenna, a fat dipole structure was used for the resonance frequency band below 4 GHz and a general dipole shape was used for that above 4 GHz. The bandwidth of the proposed antenna was represented from 0.6 GHz to 8.0 GHz with a ratio bandwidth of 12.3 : 1. Measured peak gain varies from 5.7 dBi to 9.1 dBi, and a half power beamwidth was presented from $29.4^{\circ}$ to $100.2^{\circ}$ in operating range.

• The Proceedings of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.7 no.3
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• pp.57-66
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• 1993

This research proposes a high frequency series resonant inverter consisting of equivalent half - bridge model in combination with two L-C linked full-bridge inverter circuits using MOSFET. As a output power control strategy, the sequential gate control method is applied. Also, analysis of operating MODE and state equation is described. From the computer simulation results, the inverters and devices can be shared properly voltage and current rating of the system in accordance with series and parallel operations. And it is confirmed that the proposed system has very stable performance.

• 한국가시화정보학회:학술대회논문집
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• 2004.11a
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• pp.18-21
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• 2004

Lock-on characteristics of the flow around a circular cylinder performing a rotationally oscillation with a relatively high forcing frequency have been investigated experimentally using flow visualization and hot-wire measurements. Dominant parameters are Reynolds number (Re), amplitude of oscillation $(\theta_A)$, and frequency ratio $F_R=f_f\;/\;f_n$, where $f_f$ is the forcing frequency and if is the natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14\times10^3,\;\pi/15\leq\theta_A\leq\pi/3$, and $F_R=1.0$. The effects of this active control technique on the lock-on flow regime of the cylinder wake were evaluated through wake velocity measurements and spectral analysis of hot-wire signals. The rotary oscillation modified the flow structure of near wake significantly. The lock-on phenomenon was found to occur in the range of frequency encompassing the natural vortex shedding frequency. In addition, when the amplitude of oscillation is less than a certain value, the lock-on phenomenon was occurred only at $F_R=1.0$. The lock-on range expanded and vortex formation length decreased as the amplitude of oscillation increases. The rotary oscillation generated small-scale vortex structure just near the cylinder surface.

• Journal of the Society of Naval Architects of Korea
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• v.38 no.4
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• pp.48-55
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• 2001

In this paper, sensitivity analysis for the coupled axial and torsional undamped free vibration of ship propulsion shafting is proposed. The purpose of this study is to effectively and optimally design the resonance frequencies of propulsion shafting affecting barred speed range of main engine by modifying the diameters of intermediate and propeller shafts. The presented method is validated by the sensitivity analysis for the natural frequencies of propulsion shafting of two real large merchant ships. In addition, the changes of natural frequency and resonance main engine speed are discussed in case that the diameter is varied within the range regulated by the rule of shipping register.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.12 no.1
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• pp.51-56
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• 2012

The resonance properties of circular vertical-cavity surface-emitting lasers (VCSELs) are studied by using a newly developed equivalent network approach. Optical parameters, such as the stop-band or the reflectivity of periodic Bragg mirrors and the resonance wavelength, are explored for the design of these structures. To evaluate the differential quantum efficiency and the threshold current density, a transverse resonance condition of circular modal transmission-line theory is also utilized. This approach dramatically reduces the computational time as well as gives an explicit insight to explore the optical characteristics of circular VCSELs.

• Journal of the Korean Magnetics Society
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• v.22 no.5
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• pp.183-187
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• 2012

In this paper, fluid movement of magnetic fluid which has free surface is investigated in a container subjected to a horizontal oscillation. Here, the vertical magnetic field is applied from the bottom of this container. The experiment is performed on the magnetic fluid in a rectangular and a cylindrical container and the effects of magnetic force exerted on the magnetic fluid are investigated on the resonance frequency and liquid surface displacement. The increase of magnetic field affects on the maximum resonance point and the liquid surface displacement. In result, it changes the amplitude of the surface wave and the period of sloshing fluid movement.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.1
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• pp.82-89
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• 2014

This paper discusses the methods for reducing the sampling time quantization errors of the body dither type ring laser gyroscope. A ring laser gyroscope has the angle quantization error which is generated by the frequency counting method of the laser beat signal and sampling time quantization error which is generated by the demodulation method for eliminating the body dithering in which the sampling periods are fitted to the dither periods. Generally, because the dither periods are longer than the calculation periods of the inertial navigation system, vehicle navigation errors are produced by long time attitude update missing during the vehicle move with a high dynamical motion. In this paper, the double demodulation method is proposed for reducing the sampling time quantization error and its effects under the dynamic situation are confirmed by simulation.