• Korean Journal of Optics and Photonics
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• v.14 no.6
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• pp.663-668
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• 2003

We studied a localized hologram recording method that can be used in a disk-shaped medium. In this method, the reference beam is focused by use of a cylindrical lens to get a thin spot in the medium, and then a hologram is recorded in that spot by illuminating the signal beam. Many localized holograms are multiplexed by shifting the medium by a distance more than the thin spot size of the reference beam. This method does not need recording-time scheduling for uniform diffraction efficiencies. We show that a minimal required thickness of the recording medium exists for a given spot size of the signal beam. We performed experiments for data storage and retrieval, and obtained a storage density of 20 bits/${\mu}{\textrm}{m}$$^2$ and a raw-bit error rate (RBER) of 2.5${\times}$10$^{-3}$ , when a 2 mm-thick Fe-doped LiNbO$_3$ crystal was used.

• Journal of Magnetics
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• v.8 no.4
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• pp.146-148
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• 2003

Multilayered films of Co/Pd have been studied as a candidate material for a high density perpendicular recording medium due to higher anisotropy energy. However, high exchange coupling among grains results in large transition noise. To reduce the exchange coupling and grain size, addition of 3rd elements and physical separation of grains have been attempted. In the present paper, effects of sputtering pressure, Co sublayer thickness and Pd underlayer thickness on magnetic properties and microstructures were studied. It was found that by increasing sputtering pressure from 5 mTorr to 25 mTorr, Ms decreased to one half and coercivity increased more than 5000 Oe. The increase of the coercivity is associated with physical separation of grains by high pressure sputtering. Ms per volume of Co for Co/Pd multilayered film deposited at 25 mTorr shows continuous decrease with increasing Co sublayer thickness. This was related to void formation and intermixing of Co/Pd interface. Also, effect of Pd underlayer thickness on magnetic properties will be discussed.

• Korean Journal of Optics and Photonics
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• v.13 no.3
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• pp.215-222
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• 2002

Using an in-situ ellipsometer, we monitored the growth curve of optical recording media in real time. For confirmation of the thickness control using in-situ ellipsometry, we analyzed the deposited multi-layer sample made of Ge-Sb-Te alloy film and ZnS-Si0$_2$ dielectric films using an exsitu spectroscopic ellipsometer. The target material in the first sputtering gun is ZnS-SiO$_2$ as the protecting dielectric layer and that in the second gun is Ge$_2$sb$_2$Te$_{5}$ as the receding layer. While depositing ZnS-SiO$_2$, Ge$_2$Sb$_2$Te$_{5}$ and ZnS-SiO$_2$ films on c-Si substrate in sequence, we measured Ψ $\Delta$ in real time. Utilizing the complex refractive indices of Ge$_2$Sb$_2$Te$_{5}$ and ZnS-SiO$_2$ obtained from the analysis of spectroscopic ellipsometry data, the evolution of ellipsometric constants Ψ, $\Delta$ with thickness is calculated. By comparing the calculated evolution curve of ellipsometric constants with the measured one, and by analyzing the effect of density variation of the Ge$_2$Sb$_2$Te$_{5}$ recording layer on ellipsometric constants with thickness, we precisely monitored the growth rate of the Ge-Sb-Te multilayer and controlled the growth process. The deviation of the real thicknesses of Ge-Sb-Te multilayer obtained under the strict monitoring is post confirmed to be less than 1.5% from the target structure of ZnS-SiO$_2$(1400 $\AA$)IGST(200 $\AA$)$\mid$ZnS-SiO$_2$(200$\AA$).(200$\AA$).

• Korean Journal of Optics and Photonics
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• v.12 no.1
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• pp.61-66
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• 2001

Defocused Fourier plane holograms are stored in disk-type holographic memories where thin recording media are used, the areal storage density per hologram and the intensity uniformity of the signal beam at the recording plane are studied. As the pixel pitch of the spatial light modulator that represents binary data increases, the storage density per hologram increases if exact Fourier holograms are stored. When defocused Fourier plane holograms are stored, however, we show that there exists an optimal pixel pitch that maximizes the area storage density per hologram in general, to increase the areal storage density per hologram, f/# of the Fourier transform lens that focuses the data image should be as small as possible. In this case, not only the intensity distribution at the recording plane but also the recording area becomes very sensitive to the degree of defocusing. Therefore, even if the exact Fourier plane holograms are stored, the defocusing effect owing to the medium thickness should be taken into account to achieve the maximal areal storage density per hologram.logram.

• Transactions of the Society of Information Storage Systems
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• v.1 no.1
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• pp.78-83
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• 2005

The Blu-ray disc as a next generation high density optical medium uses an objective lens of high numerical aperture(NA 0.85) and blue laser in order to increase the recording density. The double layer is also useful for doubling the density. The spherical aberration of Blu-ray disc is very sensitive to the thickness of the disk because of high numerical aperture. This paper suggests a method to compensate the spherical aberration caused by the change of disk thickness, by using the Liquid Crystal lens instead of the Liquid Crystal panel. It was possible to develop both the LC panel generating the optical phase difference and LC lens changing the optical power. In this paper, we analyzed the aberration performance and a position sensitivity of the two type LC elements when the samples deviate from the optical axis of the objective lens. The results of applying this analysis show that the LC lenses rather than the LC panels have a significant assembling tolerance. The theoretical characteristics of the two elements are calculated and compared with the measurement data.

• Korean Journal of Optics and Photonics
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• v.8 no.6
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• pp.445-449
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• 1997

The complex refractive indices of $Ge_2Se_2Te_5$ which show reversible phase change between the crystalline phase and an amorphous one depending upon the annealing process have been determined in the spectral range of 0.7-4.5 eV. The $Ge_2Se_2Te_5$ films were DC sputter deposited on the crystalline silicon substrate. The spectro-ellipsometry data of a thick film were analyzed following the modelling procedure where the quantum mechanical dispersion relation were used for the complex refractive indices of both the cryastalline phase $Ge_2Se_2Te_5$ and and amorphous phase $Ge_2Se_2Te_5$, respectively. On the other hand, with the surface micro-roughness layer whose effective thickness was determined from AFM analysis, the spectro-ellipsometry data were numerically inverted to yield the complex refractive index of $Ge_2Se_2Te_5$ at each wavelength. With these set of complex refractive indices, the reflectance spectra were calculated and those spectra obtained from the numerical inversion showed better agreement with the experimental reflection spectra for both the cryastalline phase and an amorphous phase. Finally, the thin $Ge_2Se_2Te_5$ film which has the optimum thickness of 26 nm as the medium for optical recording was also analyzed and the quantitative result of the film thickness and the surface microroughness has been reported.

• KCI Concrete Journal
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• v.13 no.2
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• pp.10-18
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• 2001

The spectral analysis of surface waves (SASW) method, which is an in-situ seismic technique, has mainly been developed and used for many years to determine the stiffness profile of layered media (such as asphalt concrete and layered soils) in an infinite half-space. This paper presents a modified experimental technique for nondestructive evaluation of in-place cement mortar compressive strength in single-layer concrete slabs of rather a finite thickness through a correlation to surface wave velocity. This correlation can be used in the quality control of early age cement mortar structures and in evaluating the integrity of structural members where the infinite half space condition is not met. In the proposed SASW field test, the surface of the structural members is subjected to an impact, using a 12 mm steel ball, to generate surface wave energy at various frequencies. Two accelerometer receivers detect the energy transmitted through the medium. By digitizing the analog receiver outputs, and recording the signals for spectral analysis, surface wave velocities can be identified. Modifications to the SASW method includes the reduction of boundary reflections as adopted on the surface waves before the point where the reflected compression waves reach the receivers. In this study, the correlation between the surface wave velocity and the compressive strength of cement mortar is developed using one 36"x36"x4"(91.44$\times$91.44$\times$91.44 cm) cement mortar slab of 2,000 psi (140 kgf/$\textrm{cm}^2$) and two 36"x36"x4"(91.44$\times$91.44$\times$91.44 cm) cement mortar slabs of 3,000 psi (210 kgf/$\textrm{cm}^2$).