• Korean Journal of Optics and Photonics
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• v.17 no.3
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• pp.223-230
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• 2006

We demonstrate a distributed fiber sensor system based on spatially-selective Brillouin scattering, using a single laser diode as a light source whose optical frequency is directly modulated by the injection current. The pump and the counter-propagating probe lights, which are sinusoidally frequency-modulated, are superposed in the fiber so that stimulated Brillouin scattering takes places only at a specific location along the fiber. Brillouin gain peak position is controlled by varying the modulation frequency. Distributions of Brillouin shift frequency are measured for the case of concatenated optical fibers of two different kinds and also for the case of temperature distribution. The temperature coefficient of the Brillouin shift frequency is measured to be $1.33MHz/^{\circ}C$.

• Korean Journal of Optics and Photonics
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• v.4 no.4
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• pp.420-427
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• 1993

Dielectric thin film mirrors are embedded in multimode and single-mode fibers by a fusion splicing technique. The fibers with $45{\circ}$ angled embedded mirrors serve as ultra-compact directional couplers with low excess optical loss of 0.2 dB for multimode and 0.5 dB for single mode at 1.3 ${\mu}m$ and excellent mechanical properties. The reflectance is wavelength dependent and strongly polarization depencient. Far-field scans of the reflected output power measured with a white-light source show a pattern which is almost circularly symmetric with aspect ratio of 1.09 at 5% of the peak power. The splitting ratio in a multimode coupler measured with a diode laser source is much less dependent on input coupling conditions than in conventional fused biconical-taper couplers, indicating that these couplers are less susceptible to modal noise occuring in optical fiber communication systems. Spectral properties of multilayer internal mirrors normal to the fiber axis have been investigated experimentally, and a matrix analysis has been used to explain the results.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.187-187
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• 2013

양자점(Quuantum dot, QD)은 0차원 특성을 가지는 구조로 양자 구속 효과로 인하여 bulk와 는 다른 구조적, 광학적, 전기적 특성을 가지고 있다. InAs QD는 size와 barrier의 bandgap 조절을 이용하여 쉽게 bandgap을 바꿀 수 있는 장점이 있어 solar cell, semiconductor laser diode, infrared photodetector 등으로 많은 연구가 이루어지고 있다. 일반적으로 Stranski-Krastanov (SK) mode로 성장한 InAs QD는 보통 GaAs epilayer와의 lattice mismatch (7%)를 이용하여 성장을 하고 이로 인하여 strain을 가지고 있고 QD의 density와 stack이 높을수록 strain이 커진다. 하지만 sub-monolayer (SML) QD 같은 경우 wetting layer가 생기는 지점인 1.7 ML이하에서 성장되는 성장 방식으로 SK-QD보다는 작은 strain을 가지게 된다. 또 QD의 size가 작아 SK-QD보다 큰 bandgap을 가지고 있다. 본 연구에서는 분자선 에피택시(molecular beam epitaxy, MBE)를 이용하여 semi-insulating GaAs substrate 위에 InAs QD를 0.5/1/1.5/1.7/2/2.5 monolayer로 성장을 하였다. GaAs과 InAs의 성장온도와 성장속도는 각각 $590^{\circ}C$, 0.8 ML/s와 $480^{\circ}C$, 0.2 ML/s로 성장을 하였으며 적층사이의 interruption 시간은 10초로 고정하였고 10주기를 성장하였다. Photoluminescence (PL)측정 결과 SML-QD는 size에 따라서 energy가 1.328에서 1.314 eV로 약간 red shift를 하였고 SK-QD의 경우 1.2 eV의 energy정도로 0.1 eV이상 red shift 하였다. 이는 QD size에 의하여 energy shift가 있다고 사료된다. 또 wetting layer의 경우 1.41 eV의 energy를 가지는 것으로 확인 하였다. SML-QD는 SK-QD 보다 반치폭(full width at half maximum, FWHM)이 작은 것은 확인을 하였고 strain field의 감소로 해석된다. 하지만 SML-QD의 경우 SK-QD보다 상대적으로 작은 PL intensity를 가지고 있었다. 이를 개선하기 위해서는 보다 높은 QD density를 요구하게 되는데 growth temperature, V/III ratio, growth rate 등을 변화주어서 연구할 계획이다.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.39 no.8
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• pp.1-11
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• 2002

A multi-section DFB-LD shows optical bistability subject to externally injected light signal, then it has potential applications such as wavelength conversion and optical logic gates. In this paper, we have studied the optical bistability in multi-section DFB-LD using split-step time-domain model. It is confirmed that the multi-section DFB-LD, which is excited inhomogeneously, shows bistability. The optical bistable characteristics are investigated when input light is injected into a absorptive region. Simulation results show that multi-section DFB-LD works as a flip-flop depending on the set-reset optical pulse which has a few ns in switching time and a few pj in switching energy, so that it can act as a optical logic device. Besides, if we change the carrier lifetime and the differential gain coefficient, it is expected that the response time of optical output signal can be reduced.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.21-22
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• 2008

In the paper, we report several experimental data capable of evaluating the phase transformation characteristics of $Ag_x(Ge_2Sb_2Te_5)_{1-x}$ (x =0, 0.05, 0.1) thin films. The $Ag_x(Ge_2Sb_2Te_5)_{1-x}$ phase change thin films have been prepared by thermal evaporation. The crystallization characteristics of amorphous$Ag_x(Ge_2Sb_2Te_5)_{1-x}$ thin films were investigated by using nano-pulse scanner with 658 nm laser diode (power; 1~17 mW, pulse duration; 10~460 ns) and XRD measurement. It was found that the more Ag is doped, the more crystallization speed was 50 improved. In comparision with $Ge_2Sb_2Te_5$ thin film, the sheet resistance$(R_{amor})$ of the amorphous $Ag_x(Ge_2Sb_2Te_5)_{1-x}$ thin films were found to be lager than that of $Ge_2Sb_2Te_5$ film($R_{amor}$ $\sim10^7\Omega/\square$ and $R_{cryst}$ 10 $\Omega/\square$). That is, the ratio of $R_{amor}/R_{cryst}$ was evaluates to be $\sim10^6$ This is very helpful to writing current reduction of phase-change random acess memory.

• Korean Journal of Optics and Photonics
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• v.18 no.3
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• pp.216-220
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• 2007

A widely tunable add/drop filter composed of double ring resonators is implemented with high-index-contrast polymer waveguide. To enhance the productivity, directional couplers are designed to have good fabrication tolerance. The refractive indices of the core and cladding in the 1550 nm wavelength are 1.51 and 1.378, respectively. Drop response in comparison with neighborhood peak gets enhanced by more than 2.9 dB at the wavelength where both rings resonate. This filter can be used to build widely tunable laser diode through hybrid-integration with reflective SOA.

• Transactions of the Korean hydrogen and new energy society
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• v.27 no.1
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• pp.127-134
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• 2016

Most of reconstruction algorithms for the calculation of temperature distributions in CT (computed tomography)-TDLAS (tunable diode laser absorption spectroscopy) are based upon two-line thermometry method. This method gives unstable calculation convergence due to signal noise, bias error, and signal mis-matches. In this study, a new reconstruction algorithm based on cross-correlation for temperature calculation is proposed. The patterns of the optical signals at all wave lengths were used to reconstruct the temperature distribution. Numerical test has been made using phantom temperature distributions. Using these phantom temperature data, absorption spectra for all wave lengths were constructed, and these spectra were regarded as the signals that would be obtained in an actual experiments. Using these virtually generated experimental signals, temperature distribution was once again reconstructed, and was compared with those of the original phantom data. Calculation errors obtained by the newly proposed algorithm were slightly large at high temperatures with small errors at low temperature.

• Transactions of the Korean hydrogen and new energy society
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• v.27 no.3
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• pp.318-327
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• 2016

In order to improve the performance of temperature field measurements by CT-TDLAS (Computer Tomography Tunable Diode Laser Absorption Spectroscopy), a new reconstruction algorithm, named two-ratios-of-three-peaks method is proposed in this paper. Further, two methods for selecting appropriate initial values of the iterative calculation of CT-TDLAS are proposed. One is MLOS (multiplicative line of sight) method and the other one is ALOS (additive line of sight) method. Two-ratios-of-three-peaks (2R3P) algorithm combined with MART (multiplicative algebraic reconstruction technique) is finally developed for the enhancements of reconstructive calculations. The results have been compared with those obtained by the conventional one-ratio-of-two-peaks (1R2P) algorithm. In order to evaluate the performance of this algorithm, numerical test has been performed using phantom Gaussian temperature distributions with $11{\times}11$ square mesh. The performance of the constructed algorithm has been demonstrated by comparing the results obtained in actual burner experiments with those obtained by thermocouples. It has been verified that 2R3P algorithm with MART and MLOS showed best performance than that of 1R2P algorithm.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.9B
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• pp.1542-1550
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• 2000

Fiber optic link systems based on SCM scheme are widely used as efficient and economic RF signal links between base station and a central station in mobile communication networks. However, its performance can be seriously limited depending on the operational conditions of not only the optical transmission system but also the wireless link in various environments. In this paper, we propose an analytic model for performance analyses of the SCM fiber optic link for CDMA RF signal transmission in various link environments. We present optimal operational conditions taking account of the nonlinear effects of the optical transmission system, and the multiple access interference produced at the wireless link. It has been shown through the BER analyses in this paper that the selection of optical modulation index of the SCM fiber optic links can be found optimally to minimize the fiber optic link noises and intermodulation distortion due to LD.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2009.06a
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• pp.467-470
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• 2009

A novel 8 V DC power source with an external series-parallel connection of 50 Dye-Sensitized Solar Cells(DSSCs) has been proposed. One DSC has the optimized length to width ratio of $5.2{\times}2.6$ cm and an active area 8 $cm^2$($4.62{\times}1.73$ cm) which attained a conversion efficiency of 4.2%. From the electrochemical impedance spectroscopic analysis, it was found that the resistance elements related to the Pt electrode and electrolyte interface behave like that of diode and the series resistance corresponds to the sum of the other resistance elements. In addition, the TEMoo mode pulsed Nd:YAG laser beam is used to improve the incident photon to current efficiency(IPCE) of DSSC. From this result, this novel 8V-0.38A DC power source shows stable performance with an energy conversion efficiency of about 4.5% under 1 sun illumination(AM 1.5, Pin of 100 $mW/cm^2$).