• Current Optics and Photonics
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• 제5권2호
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• pp.141-146
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• 2021

We have evaluated the suppression effect of atmospheric-turbulence-induced optical scintillation in terrestrial free-space optical (FSO) communication systems using a gain-saturated erbium-doped fiber amplifier (EDFA). The variation of EDFA output signal power has been measured with different amounts of gain saturation and modulation indices of the optical input signal. From the measured results, we have found that the peak-to-peak power variation was decreased drastically below 2 kHz of modulation frequency, in both 3-dB and 6-dB gain compression cases. Then, the power spectral density (PSD) of optical scintillation has been calculated with Butterworth-type transfer function. In the calculation, different levels of atmospheric-turbulence-induced optical scintillation have been taken into account with different values of the Butterworth cut-off frequency. Finally, the suppression effect of optical scintillation has been estimated with the measured frequency response of the EDFA and the calculated PSD of the optical scintillation. From our estimated results, the atmospheric-turbulence-induced optical scintillation could be suppressed efficiently, as long as the EDFA were operated in a deeply gain-saturated region.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2006년도 6th International Meeting on Information Display
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• pp.1239-1242
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• 2006

Scintillation that is grainy patterns appeared on a screen has been one of a biggest issues in a rear projection TVs. In this paper, with focusing on the average size of random particle, it was proved that the particle size of calculated speckle and the one of measured scintillation are almost the equal. This result shows speckle phenomenon is an important factor of scintillation.

• 한국방사선학회논문지
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• 제15권4호
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• pp.525-531
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• 2021

양전자방출단층촬영기기의 검출기는 다수의 섬광 픽셀과 다수의 광센서를 사용하여 구성된다. 다수의 광센서를 사용함으로써 비용이 증가하고 신호처리가 복잡해진다. 본 연구에서는 적은 수의 광센서를 사용하여 비용의 감소와 간결한 신호처리가 가능한 검출기를 설계하였다. 섬광 픽셀과 적은 수의 광센서를 사용하고 모든 광센서에 빛을 전달하기 위해 광가이드를 사용하였다. 섬광 픽셀과 광가이드에는 광센서로 최대한의 빛을 전달하기 위해 반사체를 적용하였다. 반사체는 난반사체 및 거울반사체를 사용하였으며, 광가이드의 두께를 다르게 적용하여 평면 영상을 획득하였다. 획득된 평면 영상의 비교 분석을 통해 최적의 조합을 선택하였다. 그 결과 섬광 픽셀과 광가이드 모두 거울반사체를 사용하였을 경우, 모든 두께의 광가이드에서 우수한 평면 영상을 획득하였다. 광가이드는 섬광 픽셀의 영상이 형성된 지점의 분석 및 영상의 크기를 고려하여 3 mm 두께를 사용할 경우 최적의 영상이 획득되는 결과를 도출하였다.

• Current Optics and Photonics
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• 제8권3호
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• pp.239-245
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• 2024

This paper proposes single logarithmic amplification (single-LA) and deep learning (DL)-based fixed-threshold on-off keying (OOK) detection for free-space optical (FSO) communication. Multilevel LAs (MLAs) can be used to mitigate intensity fluctuations in the received OOK signal by their nonlinear gain characteristics; however, it is ineffective in the case of high scintillation, owing to degradation of the OOK signal's extinction ratio. Therefore, a DL technique is applied to realize effective scintillation compensation in single-LA applications. Fully connected (FC) networks and fully connected neural networks (FCNN), which have nonlinear modeling characteristics, are deployed in this work. The performance of the proposed method is evaluated through simulations under various scintillation effects. Simulation results show that the proposed method outperforms the conventional adaptive-threshold-decision, single-LA-based, MLA-based, FC-based, and FCNN-based OOK detection techniques.

• Nuclear Engineering and Technology
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• 제54권9호
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• pp.3398-3402
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• 2022

We fabricated a 15 m long position-sensitive plastic scintillation optical fiber (PSOF) detector consisting of a PSOF, two photomultiplier tubes, four fast amplifiers, and a digitizer. A single PSOF was used as a sensing part to estimate the gamma-ray source position, and ¹³⁷Cs, an uncollimated solid-disk-type radioactive isotope, was used as a gamma-ray emitter. To improve the sensitivity, accuracy, and measurement time of a PSOF detector compared to those of previous studies, the performance of the amplifier was optimized, and the digital signal processing (DSP) was newly designed in this study. Moreover, we could measure very low dose rates of gamma-rays with high sensitivity and accuracy in a very short time using our proposed PSOF detector. The results of this study indicate that it is possible to accurately and quickly locate the position of a very low dose rate gamma-ray source in a wide range of contaminated areas using the proposed position-sensitive PSOF detector.

• 한국광학회:학술대회논문집
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• 한국광학회 2009년도 동계학술발표회 논문집
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• pp.203-204
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• 2009

• 한국광학회:학술대회논문집
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• 한국광학회 2009년도 동계학술발표회 논문집
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• pp.213-214
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• 2009

• Current Optics and Photonics
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• 제1권1호
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• pp.1-6
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• 2017

Degradation of bit-error-rate (BER), caused by atmospheric turbulence, seriously hinders the performance of coherent Free Space Optical (FSO) communication systems. An adaptive optics system proves to be effective in suppressing the atmospheric turbulence. The holographic modal wavefront sensor (HMWFS) proposed in our previous work, noted for its fast detecting rates and insensitivity to beam scintillation, is applied to the coherent FSO communication systems. In this paper, based on our previous work, we first introduce the principle of the HMWFS in brief and give the BER of the coherent FSO with homodyne detection in theory, and then analyze the improvement of BER for a coherent FSO system based on our previous simulation works. The results show that the wavefront sensor we propose is better for weak atmospheric turbulence. The most obvious advantages of HMWFS are fast detecting rates and insensitivity to beam scintillation.

• 한국의학물리학회:학술대회논문집
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• 한국의학물리학회 2002년도 Proceedings
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• pp.244-247
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• 2002

We examined a possibility to use inorganic plastic scintillator, which has the effective atomic number close to that of human soft tissue, for the measurement of dose distributions in a shorter time period. The method was to irradiate a block of plastic scintillator as a phantom, and to measure the distribution of the scintillation light by a wave length analyzer through a thread of plastic optical fiber. By irradiating the diagnostic x-ray, we observed the emission spectrum of the scintillation light from the scintillator. It showed a peak at around 420nm with a full width of 140 nm. The emission spectrum was integrated to determine the total number of photons. The dependences of the amount of photons on the irradiated dose were measured. The results of the experiment show that the amount of emission light is in proportional to the irradiated dose. From this fact, we conclude that the present method can be used for the measurement of the depth dose distribution of the diagnostic x-rays.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1998년도 추계학술대회
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• pp.303-304
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• 1998

We investigated the effects of scintillation crystal surface treatment on gamma camera imaging. The NaI(Tl) and CsI(Tl) (20 mm (dia.) $\times10mm$ (thick) plate) scintillators were chosen for this study. Two different surface treatments, white and black reflectors, were applied to NaI(Tl) and CsI(Tl). The optical properties of generated scintillation light were evaluated using Monte Carlo simulation and postion sensitive photo multiplier tube (PSPMT). We measured sensitivity, energy resolution and spatial resolution of a gamma camera system with the scintillators coupled to a PSPMT. Based on the results, we concluded that the careful consideration of surface treatments of the scintillator was necessary in order to develop the gamma camera having good sensitivity and spatial resolution.