• 천문학회지
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• 제47권6호
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• pp.235-253
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• 2014

Intensity interferometry, based on the Hanbury Brown-Twiss effect, is a simple and inexpensive method for optical interferometry at microarcsecond angular resolutions; its use in astronomy was abandoned in the 1970s because of low sensitivity. Motivated by recent technical developments, we argue that the sensitivity of large modern intensity interferometers can be improved by factors up to approximately 25 000, corresponding to 11 photometric magnitudes, compared to the pioneering Narrabri Stellar Interferometer. This is made possible by (i) using avalanche photodiodes (APD) as light detectors, (ii) distributing the light received from the source over multiple independent spectral channels, and (iii) use of arrays composed of multiple large light collectors. Our approach permits the construction of large (with baselines ranging from few kilometers to intercontinental distances) optical interferometers at the cost of (very) long-baseline radio interferometers. Realistic intensity interferometer designs are able to achieve limiting R-band magnitudes as good as $m_R{\approx}14$, sufficient for spatially resolved observations of main-sequence O-type stars in the Magellanic Clouds. Multi-channel intensity interferometers can address a wide variety of science cases: (i) linear radii, effective temperatures, and luminosities of stars, via direct measurements of stellar angular sizes; (ii) mass-radius relationships of compact stellar remnants, via direct measurements of the angular sizes of white dwarfs; (iii) stellar rotation, via observations of rotation flattening and surface gravity darkening; (iv) stellar convection and the interaction of stellar photospheres and magnetic fields, via observations of dark and bright starspots; (v) the structure and evolution of multiple stars, via mapping of the companion stars and of accretion flows in interacting binaries; (vi) direct measurements of interstellar distances, derived from angular diameters of stars or via the interferometric Baade-Wesselink method; (vii) the physics of gas accretion onto supermassive black holes, via resolved observations of the central engines of luminous active galactic nuclei; and (viii) calibration of amplitude interferometers by providing a sample of calibrator stars.

• 한국광학회지
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• 제17권5호
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• pp.469-474
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• 2006

본 논문에서는 실리카 기반의 다중모드 간섭기를 이용하여 적은 초과손실을 갖는 $1{\times}16$ 마하젠더 스위치에 대한 설계 및 측정결과에 대하여 논하였다. 제작된 $1{\times}16$ 마하젠더 스위치는 마하젠더 간섭계(Mach-Zehnder Interferometer, MZI) 구조를 갖는 $2{\times}2$ 열광학스위치를 단위소자로 하였으며, 15개의 단위소자를 이용하여 4단(stage)으로 구성하였다. 먼저 광분배기와 $2{\times}2$ MZI 열광학 스위치등의 개별적인 특성을 파악하였고, 그 결과를 전체 소자의 설계에 적용함으로써 보다 좋은 성능을 얻을 수 있었다. 제작된 다중모드 간섭기를 이용한 MZI 구조의 단위스위치 당 초과손실은 최소 -0.5dB로 측정되었다.

• 비파괴검사학회지
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• 제26권6호
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• pp.390-402
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• 2006

The elastic properties and thickness of mullite environmental barrier coatings grown through chemical vapor deposition (CVD) on silicon carbide substrates were measured using frequency domain photoacoustic microscopy. In this technique, extremely narrow bandwidth surface acoustic waves are generated with an amplitude modulated laser source. A photorefractive crystal based interferometer is used to detect the resulting surface displacement. The complex displacement field is mapped as a function of source-to-receiver distance in order to extract the wavelength of the surface acoustic wave at a given excitation frequency, and the phase velocity is determined. The coatings tested exhibited spatial variations in thickness and mechanical properties. The measured surface wave dispersion curves were used to extract an effective value for the elastic modulus and the coating thickness. Nanoindentation was used to validate the measurements of the effective elastic modulus. The average elastic modulus measured through the coating thickness using nanoindentation is compared to the effective modulus found using the photoacoustic system. Optical microscopy is used to validate the thickness measurements. The results indicate that the photoacoustic microscopy technique can be used to estimate the effective elastic properties in coatings exhibiting spatial inhomogeneities, potentially providing valuable feedback for the optimization of the CVD growth process.

• 한국정밀공학회지
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• 제26권5호
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• pp.72-78
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• 2009

We propose the separation algorithm to simultaneously measure two-dimensional refractive index distribution and thickness profile of transparent samples using three wavelengths. The optical system was based on the Mach-zehnder interferometer with LD (Laser Diode)-based multi-wavelength sources. A LCR (Liquid Crystal Retarder) was used to obtain interference images at four phase states and then the optical phase of the object is calculated by four-bucket algorithm. Experimental results with a glass rod are provided at the different wavelengths of 635nm, 660nm and 675nm. The refractive indices of the sample are distributed with accuracy of less than 0.0005 and the thickness profile of sample was cylindrical type. This result demonstrates that it is possible to separate refractive index distribution and thickness profile of samples in two dimensions using the proposed algorithm.

• Journal of Astronomy and Space Sciences
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• 제31권4호
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• pp.341-346
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• 2014

A prototype of the GMT FSM has been developed to acquire and to enhance the key technology - mirror fabrication and tip-tilt actuation. The ellipsoidal off-axis mirror has been designed, analyzed, and fabricated from light-weighting to grinding, polishing, and figuring of the mirror surface. The mirror was tested by using an interferometer together with CGHs, which revealed the surface error of 13.7 nm rms in the diameter of 1030 mm. The SCOTS test was employed to independently validate the test results. It measured the surface error to be 17.4 nm rms in the diameter of 1010 mm. Both tests show the optical surface of the FSMP mirror within the required value of 20 nm rms surface error.

• 한국정밀공학회지
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• 제28권3호
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• pp.323-329
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• 2011

Certified reference materials (CRMs) have been used to calibrate surface profilers for reliable measurements. In this paper, we present a newly designed step height CRM which has a step height pattern with two different widths and various special patterns for checking radial magnification, distortion of optical viewing systems, etc. Especially, it could be useful for multi-probe inspection instruments in the manufacturing lines. The fabrication was done by conventional optical lithography and dry etching process with optimized conditions. To verify the step height values, a white-light scanning interferometer was used with objective lenses having magnification of $10{\times}$ and $100{\times}$. CRMs with nominal step heights of $0.5\;{\mu}m$, $1\;{\mu}m$, $3\;{\mu}m$, $5\;{\mu}m$, $7\;{\mu}m$, and $10\;{\mu}m$ were fabricated and the uniformity of these CRMs was evaluated to be less than 3 nm ($1{\sigma}$).

• 한국항행학회논문지
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• 제16권2호
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• pp.401-408
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• 2012

측정, 통신, 광정보전달학 등의 분야에서 광 장치를 이용한 광소자의 역할이 점점 증대됨에 따라 이들을 특징 지을 명확한 근거가 필요하게 되었다. 특히 영상시스템에서 이러한 필요를 만족시켜줄 아주 기본적이고도 극히 중요한 측정 파라미터로서 MTF가 있다. 지난 수십년에 걸쳐서 레이저 간섭계, CCD카메라, 컴퓨터를 포함한 새로운 기기들이 등장하여 MTF를 측정하고 계산하는 데에 획기적인 변화를 초래하였다. 그런데 이 MTF를 계산하려면 지루하고도 더딘 과정을 거쳐야 한다. 따라서 우리는 여기서 심프슨 근사법을 이용하여 MTF를 간단히 계산하고 그래프로 구현할 수 있는 방법을 제시하였다. 이 방법은 근사법이긴 하나 정밀값과의 오차가 극히 적어서 실현하기에 매우 유익한 기법이다.

• 한국정밀공학회지
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• 제15권10호
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• pp.156-164
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• 1998

Electronic Speckle Pattern Interferometry(ESPI) has been used to measure surface deformations of engineering components and materials in industrial areas. ESPI, a non-contact and non-destructive technique, is capable of providing full-field results with high spatial resolution and high speed. One of the important application using electronic speckle pattern interferometry is electronic speckle contouring of a diffused object for 3-D shape analysis and topography measurement. Generally the electronic speckle contouring is suitable for providing measurement range from millimeters to several centimeters. In this study, we introduce the contouring method by modified dual-beam speckle pattern interferometer and the shift of the two illumination beams through optical fiber in order to obtain the contour fringe patterns. We also describe formation process of depth contour fringes and grid contour fringes by shifting direction of the two illumination beams. Before the experiments, we performed the geometric analysis for dual-beam-shifted ESPI contouring, and then, the electronic speckle contouring experiment with various specimens. For quantitative analysis of the contour fringes, we used 4-frame phase shifting method with PZT Finally, good agreement between the geometric analysis and experimetal results is obtained.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2001년도 춘계학술대회 논문집 유기절연재료 전자세라믹 방전플라즈마 연구회
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• pp.189-192
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• 2001

In this paper, an novel optical measuring system based on the electro-optic effect has been proposed and realized using Pockels cell with a view to detecting partial discharge taking place at the needle plane electrode. This system has the following advantages ; nonmetallic probe sensor, immune to external EMI noise and broad band response of the Pockels cell from DC to GHz. This system is constructed by He-Ne laser, Mach-Zehnder interferometer with Pockels sensor, balanced photo receiver, data acquisition board and PC. The response characteristics of the developed proto type sensor are examined for AC and corona discharge.

• Journal of the Optical Society of Korea
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• 제18권2호
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• pp.146-150
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• 2014

We propose an experimental holographic imaging scheme combining compressive sensing (CS) theory with digital holography in phase-shifting conditions. We use the Mach-Zehnder interferometer for hologram formation, and apply the compressive sensing (CS) approach to the holography acquisition process. Through projecting the hologram pattern into a digital micro-mirror device (DMD), finally we will acquire the compressive sensing measurements using a photodiode. After receiving the data of two holograms via conventional communication channel, we reconstruct the original object using certain signal recovery algorithms of CS theory and hologram reconstruction techniques, which demonstrated the feasibility of the proposed method.