• Progress in Superconductivity and Cryogenics
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• v.15 no.1
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• pp.11-13
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• 2013

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for transmission and reflection measurements of metallic thin films. Using our THz-TDS system, we studied the conventional superconductor niobium (Nb) in the normal state in the spectral range from 5 to $50cm^{-1}$. Both the real and imaginary parts of the conductivity are acquired without Kramers-Kronig analysis. Nb exhibits a nearly frequency independent real conductivity spectrum in the terahertz range, with a very small imaginary part.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.21 no.6
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• pp.1587-1595
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• 1996

We obtained the predicted and measured results for the reflection coefficient and radiation pattern of Ka-band (20- GHz) corrugated horn, which is usually used for feeder of reflector antenna for satellite communication, using suface impedance condition. In order to predict the reture losses of corrugated horn, we analyzed propagation constant of hybrid mode in the corrugated waveguide and then obtained the total reflection coefficient using the circuti theory of multi-step transformer. We also got the radiation pattern of corrugated horn with small flare angle, considering the phase deviation and integrate transverse field on aperture. A test model of corrugated horn antenna for Ka-band designed using theory and program displayed performance and the results agree with the theoretical prediction.

• Journal of the Korean Solar Energy Society
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• v.26 no.1
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• pp.57-64
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• 2006

Cr-CrO cermet solar selective coatings with a double cermets layer film structure were prepared using a special direct current (dc) magnetron sputtering technology. The typical films structures from surface to bottom substrate were measured to be an $Al_2O_3$ anti-reflection layer on a double Cr-CrO cermet layer on an Al metal infrared reflection layer. Optical properties of optimized Cr-CrO cermet solar selective coating were absorptance (${\alpha}$) = 0.95 and emittance (${\varepsilon}$) = 0.10 ($100^{\circ}C$). Atomic force microscopy (AFM) image showed that Cr-CrO cermet film was very smooth and their grain size was also very small The results of thermal stability test showed that the Cr-CrO cermet solar selective coatings were stable for use at temperature of $400^{\circ}C$.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.11a
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• pp.720-725
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• 2000

A new ultrasonic pulse-echo technique was developed and described for vibration measurement of the suspension assembly in hard disk drives. The method is based upon the difference in magnitude of two consecutive reflection waves from a moving object(suspension), while the traditional ultrasonic vibrometer uses the phase shift of the reference wave and the reflection wave. A cone-shape delay line is designed to access the small structure. A simple mathematical model is presented and analyzed with experimental results to show the feasibility of the method. The advantages of this ultrasonic vibration measurement method are relatively high resolution, low cost, and ease of implementation comparing with the Laser Doppler vibrometer.

• Korean Journal of Optics and Photonics
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• v.9 no.1
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• pp.1-4
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• 1998

A compact scanning confocal microscope has been constructed using a quad-detector in a CD-player. The variation of error signal on depth which obtained by a quad-detector is used in the scanning confocal microscope. Bipolar error signals, which is sensitive near a focus, give a surface following effect whiout following the surface. In the case of small depth difference, the difference of materials through the reflection signal has been identified. The image which uses reflection signal only, has been obtained with the same setup. And, the results obtained in two different way were compared and analyzed.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.3
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• pp.221-230
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• 1996

The Suh and Park's analytical model. originally developed to calculate wave reflection from a conventional fully perforated caisson breakwater, is applied to a partially perforated caisson breakwater by approximating the vertical wall of the lower part of the front face of the caisson as a very steep sloping wall. Also, in the model, the inertial resistance term at the perforated wall is modified by using the blockage coefficient proposed by Kakuno and Liu. The model is compared against the hydraulic experimental data reported by Park et al. in 1993. Both the experimental data and the analytical model results show that the influence of inertial resistance is important so that wave reflection becomes minimum when B/L. is approximately 0.2 (in which R : wave chamber width, and 1, : wave length inside the wave chamber), which is somewhat smaller than the theoretical value B/L, : 0.25 obtained by assuming that the influence of inertial resistance is negligible. It is also shown that the analytical model based on a linear wave theory tends to overpredict the reflection coefficient as the wave nonlinearity increases, thus the model is preferably to be used for ordinary waves of small steepness.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.10a
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• pp.405-416
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• 2005

The rubblization technique is breaking the aged concrete pavement slab into rubblized concrete aggregate, and use it as an base material at its original position, then builds overlay above the rubblized base. This method has been successively used in USA due to the advantage of good contructibility, cost-effectiveness as well as the capability of preventing of reflection cracks. However, constructibility and economic performance of rubblization on typical Korean concrete pavements needed to be investigate since to typical Korean concrete pavements have thick slab, as well include lean concrete subbase course. This stud explored optimum breaking depth and suggested minimum 10cm based on reflection crack simulation test. Also proper head shape and impact energy were investigated based on small breaking field tests. It was found that $127kg/cm^2$ of stress with 52.3% of head contact area are breaking requirement. Also, Multi-head type breaker suitable for Korean condition was designed and developed. This multi-head type breaker was designed to rubblize old concrete to the suggested optimum rubblized-depth and rubblized-concrete-aggregate size to prevent reflection crack and maintain high bearing capacity. This machine was used for the test of rubblization of old concrete pavement on a non-use old concrete and a in-serviced road. In these two tests, engineering properties of rubblized base and constructability and cost were investigated. In both tests, the old concrete rubblized to targeted size and depth, and high-level bearing capacity was achieved. Also, superior constructability and lower cost compared with traditional reconstruction was examined.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.44 no.2
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• pp.46-54
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• 2007

We have fabricated and characterized MMI (Multimode Interference) coupled ring resonator with the total internal reflection mirrors and the semiconductor optical amplifier for the integration of the WDM (Wavelength Division Multiplexing) system. The TIR (Total Internal Reflection) mirrors were fabricated by self-aligned process and had losses of about 0.71 dB per mirror. Coupling in and out of a resonator was achieved using the extremely small MMI couplers. The MMI length and width used in the experiment were $119{\mu}m$ and $9{\mu}m$, respectively. The resulting FSR (Free Spectral Range) and on-off ratio were approximately 1.333 nm (162 GHz) and 13 dB, respectively.

• The Journal of the Korea institute of electronic communication sciences
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• v.14 no.3
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• pp.489-496
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• 2019

In this paper, we design and fabricate an ultra small dual crossed dipole antenna operating at 2.4 GHz frequency. In order to miniaturize the size of the antenna so that it can be applied to a mobile device, a cross dipole is disposed on the upper two layers and a reflection plane, a horizontal matching circuit and a ground plane are arranged on each layer. The circuit was connected by a vertical through-hole. The size of the fabricated antenna is $21.61mm{\times}16.88mm{\times}1.27mm$, the measured reflection coefficient is -31.5 dB, and the bandwidth below -10 dB is 112 MHz. In addition, since the gain of the antenna is -4 dBi, it has the omnidirectional radiation characteristic, so it can be applied to various fields as an antenna for mobile devices.

• Journal of the Optical Society of Korea
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• v.20 no.6
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• pp.739-744
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• 2016

We have designed a new TIR(Total Internal Reflection) structure for generating an LED lens which can produce a rectangular beam with small divergence angle in two perpendicular directions for an optical guidance system. The lens can control the divergence angle in the horizontal direction to be a small value of about $8^{\circ}$ with a $1mm{\times}1mm$ LED source, also in the vertical direction it can be about $7^{\circ}$, with optical collection efficiency higher than 0.83. After the lens is manufactured, the work demonstrates that the lens is suitable for an optical guidance system.