• Journal of Astronomy and Space Sciences
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• 제33권3호
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• pp.177-183
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• 2016

Active galactic nuclei (AGN) are too compact to be resolved by any existing optical telescope facility, making it difficult to understand their structure and the emission processes responsible for their huge energy output. However, variability, one of their characteristic properties, provides a tool to probe the inner regions of AGN. Blazars are the best candidates for such a study, and hence a considerable amount of effort is being made to investigate variability in these sources across the electromagnetic spectrum. Here, using the Mt. Abu infrared observatory (MIRO) blazar monitoring program, we present intra-night, inter-night, and long term aspects of the variability in S5 0716+71, 3C66A, and OJ 287. These stars show significant variability on short (a few tens of mins, to a few hours, to a few days) to long term (months to years) timescales. Based on the light travel time argument, the shortest variability timescales (micro-variability) provide upper limits to the size of the emission region. While S5 0716 shows a very high duty cycle of variability (> 80 %), 3C66A shows a much lower intra day variability (IDV) duty cycle (< 20 %). All three show rapid variations within 2.5 to 3.5 hr, which, perhaps, are generated near the vicinity of black holes. Assuming this, estimates of the masses of the black holes are made at ~10⁹, 8×10⁸, and 2.7×10⁹ M_⨀ for S5 0716+71, 3C66A, and OJ 287, respectively. Multi-wavelength light-curves for the blazar PKS 1510-089 are discussed to infer the emission processes responsible for the recent flaring episodes in this source.

• 한국통신학회논문지
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• 제34권2B호
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• pp.178-187
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• 2009

인지 라디오 (Cognitive Radio: CR)란 할당된 주파수 대역 중에 우선 사용자가 사용하지 않는 유휴 주파수를 사용하여 주파수의 효율을 극대화 시킬 수 있는 기술이다. 그러나 CR 환경에서는 우선 사용자에 대한 간섭이 없는 상태에서만 자원을 공유하는 매우 엄격한 제한을 가지고 있다. 더욱이 유휴 주파수 자원은 시간, 주파수 별로 변화하기 때문에 이를 효율적으로 사용하기 위한 스케줄링 알고리즘이 필수적이다. 그러나 기존의 우선 사용자들만 존재하는 무선 통신 환경에서 적용되던 스케줄링 알고리즘은 CR 환경에 대한 제약 조건이 반영되지 않아 유휴 주파수 자원 사용의 효율성 문제가 발생한다. 이에 본 논문에서는 무선 채널 상태와 우선 사용자 출현 확률을 고려하는 하향링크 스케줄링 알고리즘을 제안한다. 제안 스케줄링 알고리즘의 성능 평가를 위해 모의실험을 수행하였으며, 그 결과 제안 알고리즘이 우선 사용자 출현 확률을 고려하지 않는 하향링크 스케줄링 알고리즘에 비해 우수한 성능이 제공됨을 확인하였다.

• 디지털융복합연구
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• 제19권11호
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• pp.341-349
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• 2021

최근 조명 산업이 중요한 분야로 인식되면서 PQR (Photonic Quantum Ring) 소자는 LED(Light Emitting Diode)를 대체할 수 있는 차세대 광원이 될 전망이다. 본 연구에서는 기존 연구와 유사한 결과를 검증하고, 소자의 광특성을 분석하기 위해 광섬유가 연결된 스테이지에 x, y, z 좌표를 입력하면 자동으로 이동하며, 또한, 소자에 광섬유를 근접시키는 NSOM (Near field scanning optical microscopy) 장치를 추가한 측정 시스템을 이용하여 소자의 광특성 실험과 공진 및 어레이 소자의 광특성 시뮬레이션을 통해 조명용 소자로 가능성을 검증하고자 하였다. 이를 위해 메사와 홀 형태가 동시에 존재하는 메사 직경 40㎛, 홀 직경 3㎛의 소자를 제작하여 소자의 근접장으로 PQR 소자는 ㎂에서 동작하며, 메사와 홀 소자는 서로 독립적으로 구동됨을 관찰하였다. 위치에 따른 소자의 광파장 스펙트럼을 측정하여 메사와 홀 소자에 의한 커플링 현상을 처음으로 확인하였다.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1997년도 춘계학술대회 논문집
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• pp.842-846
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• 1997

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratios larger than 10) is gaining increased attention in a wide spectrum of precision production industries. Alternative methods such as EDM, laser drilling, etc. can sometimes replace mechanical micro-hole drilling but are not acceptable in PCB manufacture because they yield inferior hole quality and accuracy. The major difficulties in micro-hold drilling are related to wandering motions during the inlet stage, high aspect ratios, high temperature,etc. However, of all the difficulties, the most undesirable one is the increase of drilling force as the drill penetrates deeper into hold. This is caused mainly by chip related effects. Peck-drilling is thus widely used for deep hole drilling despite the fact that it leads to low productivity. Therefore, in this paper, a method of cutting force regulation is proposed to achieve continuous drilling. A proportional plus derivative (PD) and a sliding modecontrol algorithm will be implemented for controlling the spinle rotational frequeency. Experimental results will show that sliding mode control reduces the nominal cutting force and its variation better than the PD control, resulting in a number of advantages such as an increase in drill life, fast stabilization of the wandering motion, and precise positioning of the hole.

• 대한기계학회논문집B
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• 제29권2호
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• pp.224-231
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• 2005

The present study investigates flow characteristics in an optical disc drive (ODD). Detailed knowledge of the flow characteristics is essential to analyze flow-induced noise and vibration, forced convection and flow friction loss. The ODD used in a personal computer is used for the experiment and rotating velocity of disc is under the 4500 rpm. Time-resolved velocity component and velocity spectrum are obtained using the laser Doppler anemometry (LDA), and the flow patterns induced by rotating disc in the ODD are calculated by a commercial finite volume method at the same time. The results show that the front holes reduce flow-induced noise and the position of pickup body only affects flow near the window. Furthermore, it is possible for cooling of heat sources in the drive through measuring the flow fields under the tray. In addition, the numerical results are well matched up to the experimental results, therefore, the validation of the numerical results can be achieved. From the validation of numerical results, it is possible to predict the flow characteristics of the region where it is unable to conduct the experiment.

• 한국정밀공학회지
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• 제15권1호
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• pp.117-129
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• 1998

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratio larger than 10) is recently having more attention in a wide spectrum of precision production industries. Alternative methods such as EDM. laser drilling, etc. can sometimes replace the mechanical micro-hole drilling but are not acceptable in PCB manufacture because of the inferior hole quality and accuracy. The major difficulties in micro-hole drilling are related to small signal to noise ratios, wandering motions of the inlet stage, high aspect ratios, high temperatures and so forth. Of all the difficulties. the most undesirable one is the increase of drilling force as the drill proceeds deeper into the hole. This is caused mainly from the chip effects. Peck-drilling is thus widely used for deep hole drilling despite that it suffers from low productivity. In the paper, a method of cutting force regulation is proposed to achieve continuous drilling. A PD and a sliding mode control algorithms were implemented through controlling the spindle rotating frequency. Experimental results show that the sliding mode control reduces the nominal cutting force and the variation of the cutting force better than the PD control. The advantages of the regulation, such as increase of drill life, fast stabilization of a wandering motion, and the precise positioning of the hole are verified in experiment.

• 천문학회지
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• 제33권2호
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• pp.81-88
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• 2000

The international ultraviolet explorer (IUE) spectra of a low dispersion $\~6{\AA}$, have been investigated for two Seyfert 1 galaxies, Mrk 335 and NGC 4051, well known for the line variability. The electron densities of broad line region (BLR) of these variable Seyfert 1 galaxies have been derived, which showed a non-linear abrupt variation from $10^8$ to $10^{10} cm-3$ within a month. We also found the excitation (or temperature) changes in the Mrk 335 BLR from the IUE broad line profiles analysis, but no such evidence in the NGC 4051. The large amount of mass inflow activity through the bar or spiral structure of host galaxies, may trigger the density change in BLR and emission line variability for both objects. Mass of the giant black holes appear to be order of $10^7\;M_{\bigodot}$ for both variable Seyfert l's.

• 천문학회지
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• 제36권3호
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• pp.177-187
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• 2003

Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electromagnetic spectrum, from radio waves to gamma-rays. There is much evidence that they are driven by gravitational accretion of stars, dust, and gas, onto central massive black holes (MBHs) imprisoning anywhere from $\~$1 to $\~$10,000 million solar masses; such objects may naturally form in the centers of galaxies during their normal dynamical evolution. A small fraction of AGNs, of the radio-loud type (RLAGNs), are somehow able to generate powerful synchrotron-emitting structures (cores, jets, lobes) with sizes ranging from pc to Mpc. A brief summary of AGN observations and theories is given, with an emphasis on RLAGNs. Preliminary results from the imaging of 10000 extragalactic radio sources observed in the MITVLA snapshot survey, and from a new analytic theory of the time-variable power output from Kerr black hole magnetospheres, are presented. To better understand the complex physical processes within the central engines of AGNs, it is important to confront the observations with theories, from the viewpoint of analyzing the time-variable behaviours of AGNs - which have been recorded over both 'short' human ($10^0-10^9\;s$) and 'long' cosmic ($10^{13} - 10^{17}\;s$) timescales. Some key ingredients of a basic mathematical formalism are outlined, which may help in building detailed Monte-Carlo models of evolving AGN populations; such numerical calculations should be potentially important tools for useful interpretation of the large amounts of statistical data now publicly available for both AGNs and RLAGNs.

• 한국전기전자재료학회논문지
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• 제15권5호
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• pp.429-434
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• 2002

In this paper, multiheterostructure white organic light-emitting device was fabricated by vacuum evaporation. The structure of white organic light-emitting device is ITO/CuPc/TPD/DPBi:DPA/$Alq_3/Alq_3$:DCJTB/BCT/$Alq_3$/Ca/Al. Three primary colors are implemented with DPVBi, Alq$_3$and DCJTB. The maximum EL wavelength of the fabricated white organic light-emitting device is 647nm. And the CIE coordinate is (0.33, 0.33) at 13 V. In the fabrication of white organic light-emitting devices with DCJTB, $Alq_3$, DPVBi, the EL spectrum has two peaks at 492nm, 647nm. Two peaks appeared because the blue light is combined with green light. The maximum wavelength of red light is not changed with applied voltage. After voltage applied, for the first time, the electrons met the holes in the red emission layer and emitted red light. And then the electrons moved to the green emission layer, and blue emission layer continuously. Finally, when all of the emission layer activated, the white light is emitted.

• Journal of the Optical Society of Korea
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• 제9권4호
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• pp.162-168
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• 2005

Extraordinary phenomena related to the transmission of light via metallic films with subwavelength holes and grooves are known to be due to resonant excitation and interference of surface waves. These waves make various surface structures to have optically effective responses. Further, a related study subject involves the control of light transmitted from a single hole or slit by surrounding it with diffractive structures. This paper reports on the effects of controlling light with a periodic groove structure with Fresnel-type chirping. In Fresnel-type chirping, diffracted surface waves are coherently converged into a focus, and it is designed considering the conditions of constructive interference and angular spectrum optimization under the assumption that the surface waves are composite diffracted evanescent waves with a well-defined in-plane wavenumber. The focusing ability of the chirped periodic structures is confirmed experimentally by two-beam attenuated total reflection coupling. Critical factors for achieving subwavelength foci and bounds on size of focal spots are discussed in terms of the simulation, which uses the FDTD algorithm.