레이저 출력 세기에 따른 난류 모사 위상판 측정

오한결,강필성,이재현,이혁교,김영식 한국광학회 2023 한국광학회지 Vol.34 No.3

The performance of astronomical telescopes can be negatively affected by atmospheric turbulence. To address this issue, techniques for atmospheric turbulence correction have been developed, requiring the simulation of atmospheric turbulence in the laboratory. The most practical way to simulateatmospheric turbulence is through the use of a phase plate. When measuring a phase plate that simulates strong turbulence, a Shack-Hartmann wavefront sensor is commonly used. However, laser power decreases as it passes through the phase plate, potentially leading to a weak laser signal at the sensor. This paper investigates the need to control laser-power intensity when measuring a phase plate that simulates strong atmospheric turbulence, and examines the effects of laser-power intensity on the measured wavefront. For phase plates with relatively high Fried parameter r 0 , the laserpower intensity causes a variation of over 10% in r 0 . For phase plates with relatively low r 0 , the laser-power intensity causes a variation of less than5%, which means that the influence of laser-power intensity is negligible for phase plates that simulate strong atmospheric turbulence. Based on thesystem described in this paper, a phase plate simulating strong atmospheric turbulence can be measured with a laser-power intensity of 5 mW or higher. Therefore, controlling the laser’s output power is necessary when measuring a phase plate for simulating atmospheric turbulence, especially for phase plates with low r 0 values.

Atmospheric Turbulence Simulator for Adaptive Optics Evaluation on an Optical Test Bench

이준호,신선미,박규남,이형규,양호순 한국광학회 2017 Current Optics and Photonics Vol.1 No.2

An adaptive optics system can be simulated or analyzed to predict its closed-loop performance. However,this type of prediction based on various assumptions can occasionally produce outcomes which are farfrom actual experience. Thus, every adaptive optics system is desired to be tested in a closed loop onan optical test bench before its application to a telescope. In the close-loop test bench, we need anatmospheric simulator that simulates atmospheric disturbances, mostly in phase, in terms of spatial andtemporal behavior. We report the development of an atmospheric turbulence simulator consisting of twopoint sources, a commercially available deformable mirror with a 12×12 actuator array, and two randomphase plates. The simulator generates an atmospherically distorted single or binary star with varying stellarmagnitudes and angular separations. We conduct a simulation of a binary star by optically combining twopoint sources mounted on independent precision stages. The light intensity of each source (an LED witha pin hole) is adjustable to the corresponding stellar magnitude, while its angular separation is preciselyadjusted by moving the corresponding stage. First, the atmospheric phase disturbance at a single instance,i.e., a phase screen, is generated via a computer simulation based on the thin-layer Kolmogorov atmosphericmodel and its temporal evolution is predicted based on the frozen flow hypothesis. The deformable mirroris then continuously best-fitted to the time-sequenced phase screens based on the least square method. Similarly, we also implement another simulation by rotating two random phase plates which were manufacturedto have atmospheric-disturbance-like residual aberrations. This later method is limited in its abilityto simulate atmospheric disturbances, but it is easy and inexpensive to implement. With these two methods,individually or in unison, we can simulate typical atmospheric disturbances observed at the Bohyun Observatoryin South Korea, which corresponds to an area from 7 to 15 cm with regard to the Fried parameterat a telescope pupil plane of 500 nm.

대기외란시 비전센서를 활용한 구조물 동적 변위 측정 성능에 관한 연구

공준호 한국구조물진단유지관리공학회 2024 한국구조물진단유지관리공학회 논문집 Vol.28 No.3

본 연구는 대기외란 조건에서 비전센서를 활용하여 구조물의 동적 변위 측정을 위하여 멀티스케일 템플릿 매칭 기법 (TMI: Template Matching with Image pyramids)을 제안하고 제안기법의 변위 측정 성능을 조사하기 위해 진행되었다. 촬영거리에 따른 변위 측정 성능을 평가하기 위해 3층 전단 구조물을 설계하였으며, FHD(1920×1080)급 카메라를 준비하여 변위 계측에 사용하였다. 최초 촬영거리를 10m 로 설정하였고, 10m씩 멀어지면서 최대 40m까지 변위 측정 실험을 진행하였다. 실내 조도 조건(450lux)에서 발열 기구를 활용하여 대기외란을 발생시켰으며, 대기외란으로 이미지를 왜곡시켰다. 사전실험을 통해 대기외란시 특징점 기반 변위 측정 방법과 제안기법의 변위 측정 타당성을 비교 검증하였으며, 검증 결과 제안기법의 낮은 측정 에러율을 나타냈다. 대기외란 환경에서 변위 측정 성능평가 결과, 인공 타겟을 활용한 TMI는 대기외란 유무에 따라 변위 측정 성능에 큰 차이가 없었다. 하지만 자연 타겟을 활용하였을 때, 20m 이상의 촬영거리에서 RMSE가크게 상승하여 제안기법의 운용 한계를 보여줬다. 이는 촬영거리 증가에 따라 자연 타겟의 해상도가 저하되며, 대기외란으로 인한 이미지 왜곡이 템플릿 이미지 추정에 오류가 발생 되어 변위 측정 오차가 높게 발생하는 경향을 나타냈다. This study proposes a multi-scale template matching technique with image pyramids (TMI) to measure structural dynamic displacement using a vision sensor under atmospheric turbulence conditions and evaluates its displacement measurement performance. To evaluate displacement measurement performance according to distance, the three-story shear structure was designed, and an FHD camera was prepared to measure structural response. The initial measurement distance was set at 10m, and increased with an increment of 10m up to 40m. The atmospheric disturbance was generated using a heating plate under indoor illuminance condition, and the image was distorted by the optical turbulence. Through preliminary experiments, the feasibility of displacement measurement of the feature point-based displacement measurement method and the proposed method during atmospheric disturbances were compared and verified, and the verification results showed a low measurement error rate of the proposed method. As a result of evaluating displacement measurement performance in an atmospheric disturbance environment, there was no significant difference in displacement measurement performance for TMI using an artificial target depending on the presence or absence of atmospheric disturbance. However, when natural targets were used, RMSE increased significantly at shooting distances of 20 m or more, showing the operating limitations of the proposed technique. This indicates that the resolution of the natural target decreases as the shooting distance increases, and image distortion due to atmospheric disturbance causes errors in template image estimation, resulting in a high displacement measurement error.

Characteristics and Potential Sources of Atmospheric Turbulence in the Free Atmosphere Retrieved using High-Resolution Radiosonde Data in USA

Han-Chang Ko,Hye-Yeong Chun 한국기상학회 2021 한국기상학회 학술대회 논문집 Vol.2021 No.4

The eddy dissipation rate (ε) and turbulence-layer thickness (TLT) are retrieved in the free atmosphere (z=3-30 km) using a Thorpe method and high-resolution radiosonde data for 6 tears (Jan. 2012-Dec. 2017) at 68 USA operational stations. The turbulence occurs more frequently in the troposphere than in the stratosphere, with log_10ε ranging from -4 to 0 (from -4 to -0.5) m² s^-3 in the troposphere (stratosphere). The maximum TLT in the troposphere is about 4,000 m, which is larger than in the stratosphere of 2,000 m. However, the layer-mean log_10ε over 3- km altitude bin is found to be larger in the stratosphere, due to less turbulence cases than in the troposphere. For better representation of the layer-mean turbulence, we suggest a new quantity, named layer-mean effective ε (EE), by combining ε and TLT. We also investigate potential sources of the observed turbulence by analyzing four turbulence indices: squared Brunt-Vaisala frequency (N²), vertical wind shear (VWS), orographic gravity-wave drag (OGWD), and precipitation, calculated using ERA5 reanalysis. N² shows clear seasonal variations although differently with each altitude range: the largest in DJF and the smallest in JJA in z=3-15 but is opposite in z=15-21km. Strong VWS appears in eastern USA in DGF (JJA) in z=9-12 (12-18) km. Non-zero OGWD appears mostly in Rocky and Appalachian mountain regions, and its magnitude increases with altitude, while precipitation is concentrated in western coast and mid-eastern USA. The EE and N² (precipitation) are negatively (positively) correlated through all altitude and most regions. Contrary, VWS and OGWD are correlated with EE under specific conditions and locations: VWS is positively correlated under the strong static-stability condition, and OGWD is positively correlated in western mountainous regions in z=15-21 km.

Development and Characterization of an Atmospheric Turbulence Simulator Using Two Rotating Phase Plates

주지용,한석기,이준호,이혁교,허준,이기훈,박상영 한국광학회 2022 Current Optics and Photonics Vol.6 No.5

We developed an adaptive optics test bench using an optical simulator and two rotating phase plates that mimicked the atmospheric turbulence at Bohyunsan Observatory. The observatory was reported to have a Fried parameter with a mean value of 85 mm and standard deviation of 13 mm, often expressed as 85 ± 13 mm. First, we fabricated several phase plates to generate realistic atmospheric-like turbulence. Then, we selected a pair from among the fabricated phase plates to emulate the atmospheric turbulence at the site. The result was 83 ± 11 mm. To address dynamic behavior, we emulated the atmospheric disturbance produced by a wind flow of 8.3 m/s by controlling the rotational speed of the phase plates. Finally, we investigated how closely the atmospheric disturbance simulation emulated reality with an investigation of the measurements on the optical table. The verification confirmed that the simulator showed a Fried parameter of 87 ± 15 mm as designed, but a little slower wind velocity (7.5 ± 2.5 m/s) than expected. This was because of the nonlinear motion of the phase plates. In conclusion, we successfully mimicked the atmospheric disturbance of Bohyunsan Observatory with an error of less than 10% in terms of Fried parameter and wind velocity.

Numerical simulation of high-energy laser propagation through the atmosphere and phase correction based on adaptive optics

Ahn Kyohoon,Lee Sung-Hun,Park In-Kyu,Yang Hwan-Seok 한국물리학회 2021 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.79 No.10

The high-energy laser (HEL) has recently been in the spotlight for the most challenging military applications because it allows the warfighter to be capable of directly damaging or destroying faraway targets immediately. However, because of the atmospheric effect, the ability of the HEL to focus on the target can be significantly degraded. For improved focus on the target, this atmospheric effect should be accurately analyzed and mitigated in the HEL. Atmospheric turbulence and thermal blooming have been studied for decades to maximize the efficiency of the HEL. The aim of this study is to maximize the efficiency of the HEL through an accurate analysis of the atmospheric effects and mitigation using adaptive optics (AO). An accurate analysis of the HEL’s beam propagation through the atmosphere is implemented based on the split-step beam propagation method, and the phase correction of atmospheric distortions, such as turbulence and thermal blooming, is simulated using an AO simulation tool that is independently developed by LIG Nex1. The result of the simulation of the split-step beam propagation showed a high accuracy compared with the analytical result of beam diffraction without atmospheric turbulence. Furthermore, that result confirmed that atmospheric distortions could be effectively mitigated using the AO system in the phase correction simulation.

광 무선통신시스템에서 대기 교란으로 인한 광 가우시안 펄스의 펄스 퍼짐과 부호 간 간섭에 관한 연구

정진호,Jung, Jin-Ho 한국광학회 2005 한국광학회지 Vol.16 No.5

광 펄스가 대기 채널을 통해 전송될 때, 광 펄스는 대기 교란에 의해 감쇄되고 퍼지게 된다. 이러한 펄스 퍼짐이 인접 펄스간의 부호 간 간섭을 일으키고, 그 결과 인접한 펄스들은 중첩이 되어 비트전송률 및 무중계 전송거리를 제한하게 된다. 이에, 본 논문에서는 시간적 모멘트 함수를 이용하여 대기 교란상태에서 부호 간 간섭을 교란 상태를 나타내는 굴절률 구조상수로 구하고, SONET 광 전송방식에서 교란상태에 따른 부호 간 간섭을 수치해석하였다. 그 결과, 교란 정도가 심할수록 부호 간 간섭은 OC-192(9.953 Gb/s) 시스템 이하의 전송률에서는 점차적으로 증가하나, OC-768(39.813 Gb/s) 시스템 이상의 전송률에서는 급격히 증가 후 서서히 수렴함을 알 수 있었다. 또한, OC-48(2.488 Gb/s) 시스템에서는 어떠한 교란상태 하에서도 10 [km] 정도까지 정확한 정보 전송이 가능하나, 100 Gb/s 시스템에서는 $10^{-14}[m^{-2/3}]$ 이상, OC-768 시스템에서는 $10^{-13}[m^{-2/3}]$이상, OC-192 시스템에서는 $10^{-12}[m^{-2/3}]$ 이상의 교란상태에서 심한 부호 간 간섭이 발생하여 정확한 정보 전송이 불가능함을 알 수 있었다. When an optical pulse propagates through the atmospheric channel, it is attenuated and spreaded by the atmospheric turbulence. This pulse broadening produces the intersymbol interference(ISI) between adjacent pulses. Therefore, adjacent pulses are overlapped, and the bit rates and the repeaterless transmission length are limited by the ISI. In this paper, the ISI as a function of the refractive index structure constant that presents the strength of atmospheric turbulence is found using the temporal momentum function, and is numerically analyzed fer the basic SONET transmission rates. The numerical results show that ISI is gradually increasing at the lower transmission rate than the OC-192(9.953 Gb/s) system and is slowly converging after rapid increasing at the higher transmission rate than the OC-768(39.813 Gb/s) system as the turbulence is stronger. Also, we know that accurate information transmission is possible to 10[km] at the OC-48(2.488 Gb/s) system under any atmospheric turbulence, but is impossible under the stronger turbulence than $10^{-14}[m^{-2/3}]$ at the 100 Gb/s system, $10^{-13}[m^{-2/3}]$ at the OC-768 system, and $10^{-12}[m^{-2/3}]$ at the OC-192 system, because the ISI is seriously induced.

Restoration of Ghost Imaging in Atmospheric Turbulence Based on Deep Learning

Chenzhe Jiang,Banglian Xu,Leihong Zhang,Dawei Zhang 한국광학회 2023 Current Optics and Photonics Vol.7 No.6

Ghost imaging (GI) technology is developing rapidly, but there are inevitably some limitations such as the influence of atmospheric turbulence. In this paper, we study a ghost imaging system in atmospheric turbulence and use a gamma-gamma (GG) model to simulate the medium to strong range of turbulence distribution. With a compressed sensing (CS) algorithm and generative adversarial network (GAN), the image can be restored well. We analyze the performance of correlation imaging, the influence of atmospheric turbulence and the restoration algorithm’s effects. The restored image’s peak signal-to-noise ratio (PSNR) and structural similarity index map (SSIM) increased to 21.9 dB and 0.67 dB, respectively. This proves that deep learning (DL) methods can restore a distorted image well, and it has specific significance for computational imaging in noisy and fuzzy environments.

Application of Artificial Neural Networks to Predict Dynamic Responses of Wing Structures due to Atmospheric Turbulence

Anh Tuan Nguyen,Jae-Hung Han,Anh Tu Nguyen 한국항공우주학회 2017 International Journal of Aeronautical and Space Sc Vol.18 No.3

This paper studies the applicability of an efficient numerical model based on artificial neural networks (ANNs) to predict the dynamic responses of the wing structure of an airplane due to atmospheric turbulence in the time domain. The turbulence velocity is given in the form of a stationary Gaussian random process with the von Karman power spectral density. The wing structure is modeled by a classical beam considering bending and torsional deformations. An unsteady vortex-lattice method is applied to estimate the aerodynamic pressure distribution on the wing surface. Initially, the trim condition is obtained, then structural dynamic responses are computed. The numerical solution of the wing structure’s responses to a random turbulence profile is used as a training data for the ANN. The current ANN is a three-layer network with the output fed back to the input layer through delays. The results from this study have validated the proposed low-cost ANN model for the predictions of dynamic responses of wing structures due to atmospheric turbulence. The accuracy of the predicted results by the ANN was discussed. The paper indicated that predictions for the bending moments are more accurate than those for the torsional moments of the wing structure.

Application of Artificial Neural Networks to Predict Dynamic Responses of Wing Structures due to Atmospheric Turbulence

Nguyen, Anh Tuan,Han, Jae-Hung,Nguyen, Anh Tu The Korean Society for Aeronautical and Space Scie 2017 International Journal of Aeronautical and Space Sc Vol.18 No.3

This paper studies the applicability of an efficient numerical model based on artificial neural networks (ANNs) to predict the dynamic responses of the wing structure of an airplane due to atmospheric turbulence in the time domain. The turbulence velocity is given in the form of a stationary Gaussian random process with the von Karman power spectral density. The wing structure is modeled by a classical beam considering bending and torsional deformations. An unsteady vortex-lattice method is applied to estimate the aerodynamic pressure distribution on the wing surface. Initially, the trim condition is obtained, then structural dynamic responses are computed. The numerical solution of the wing structure's responses to a random turbulence profile is used as a training data for the ANN. The current ANN is a three-layer network with the output fed back to the input layer through delays. The results from this study have validated the proposed low-cost ANN model for the predictions of dynamic responses of wing structures due to atmospheric turbulence. The accuracy of the predicted results by the ANN was discussed. The paper indicated that predictions for the bending moments are more accurate than those for the torsional moments of the wing structure.