• Proceedings of the KSRS Conference
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• 2005.10a
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• pp.628-632
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• 2005

Remote sensing through atmospheric turbulence had been hard works for a long time, because wavefront distortion due to the Earth's atmospheric turbulence deteriorates image quality. But due to the appearance of adaptive optics, it is no longer difficult things. Adaptive optics is the technology to correct random optical wavefront distortions in real time. For past three decades, research on adaptive optics has been performed actively. Currently, most of newly built telescopes have adaptive optical systems. Adaptive optical system is typically composed of three parts, wavefront sensing, wavefront correction and control. In this work, the wavefront sensing technology for adaptive optical system is treated. More specifically, shearing interferometers and Shack-Hartmann wavefront sensors are considered. Both of them are zonal wavefront sensors and measure the slope of a wavefront. . In this study, the shearing interferometer is made up of four right-angle prisms, whose relative sliding motions provide the lateral shearing and phase shifts necessary for wavefront measurement. Further, a special phase-measuring least-squares algorithm is adopted to compensate for the phase-shifting error caused by the variation in the thickness of the index-matching oil between the prisms. Shack-Hartmann wavefront sensors are widely used in adaptive optics for wavefront sensing. It uses an array of identical positive lenslets. And each lenslet acts as a subaperture and produces spot image. Distortion of an input wavefront changes the location of spot image. And the slope of a wavefront is obtained by measuring this relative deviation of spot image. Structures and measuring algorithms of each sensor will be presented. Also, the results of wavefront measurement will be given. Using these wavefront sensing technology, an adaptive optical system will be built in the future.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.16 no.6
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• pp.133-139
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• 2007

In this paper, optics polishing technology using adaptive tool and eccentric motion mechanism was suggested. Optics polishing can make high reflective and accurate surface. The optics polishing process based on the eccentric motion mechanism has been used to manufacture the ophthalmic lens mold. Also ophthalmic lens mold factory hold conventionally a lot of the curved polishing tools for the versatile mold curves of eye diopters and want to reduce tool numbers. Therefore, a polishing machine with adaptive airbag tool was developed and experimentally verified in view of surface roughness and form accuracy.

• Current Optics and Photonics
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• v.5 no.2
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• pp.101-113
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• 2021

Adaptive optics (AO) systems are becoming more complex to improve their optical performance and enlarge their field of view, so it is a hard and time consuming process to set up and optimize the components of AO systems with actual implementation. However, simulations allow AO scientists and engineers to experiment with different optical layouts and components without needing to obtain and prepare them physically. In this paper, we introduce a new AO simulation named the Korea Adaptive Optics Simulation (KAOS), independently developed by LIG Nex1. We verified the performance of KAOS by comparing with other AO simulation tools. In the comparison simulation, we confirmed the results from KAOS and other AO simulation tools were very similar. Also, we proposed a laser tomography AO system with five Rayleigh laser guide stars (LGSs) optimized by using KAOS to overcome the disadvantages of the AO system with a single sodium LGS for the satellite imaging system. We verified the performance of the proposed AO system using KAOS, and the simulation result showed averaged Strehl ratio of 0.37.

• ETRI Journal
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• v.41 no.3
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• pp.316-325
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• 2019

Accurate suppressive jamming is a prominent problem faced by radar equipment. It is difficult to solve signal detection problems for extremely low signal to noise ratios using traditional signal processing methods. In this study, a joint sensing dictionary based compressed sensing and adaptive iterative optimization algorithm is proposed to counter suppressive jamming in information domain. Prior information of the linear frequency modulation (LFM) and suppressive jamming signals are fully used by constructing a joint sensing dictionary. The jamming sensing dictionary is further adaptively optimized to perfectly match actual jamming signals. Finally, through the precise reconstruction of the jamming signal, high detection precision of the original LFM signal is realized. The construction of sensing dictionary adopts the Pei type fast fractional Fourier decomposition method, which serves as an efficient basis for the LFM signal. The proposed adaptive iterative optimization algorithm can solve grid mismatch problems brought on by undetermined signals and quickly achieve higher detection precision. The simulation results clearly show the effectiveness of the method.

• Korean Journal of Optics and Photonics
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• v.27 no.3
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• pp.106-113
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• 2016

In this paper, we deal with the wavefront compensation capability of a silicon carbide (SiC) deformable mirror (DM) with 37 actuators for adaptive optics. The wavefront compensation capability of the SiC DM is predicted by computer simulation and examined by actual experiments with a closed-loop adaptive optics system consistsing of a light source, a phase plate, a SiC DM, a high speed Shack-Hartmann sensor, and a control computer. Distortion of wavefront is caused by the phase plate in the closed-loop adaptive optics system. The distorted wavefront has a peak-to-valley (PV) wavefront error of $0.3{\mu}m{\sim}0.9{\mu}m$ and root-mean-square (RMS) error of $0.06{\mu}m{\sim}0.25{\mu}m$. The high-speed Shack-Hartmann sensor measures the wavefront error of the distortion caused by the phase plate, and the SiC DM compensates for the distorted wavefront. The compensated wavefront has residual errors lower than $0.1{\mu}m$ PV and $0.03{\mu}m$ RMS. Consequently, we conclude that we can compensate for the distorted wavefront using the SiC DM in the closed-loop adaptive optics system with an operating frequency speed of 500 Hz.

• Journal of the Optical Society of Korea
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• v.20 no.3
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• pp.368-380
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• 2016

There is no doubt that adaptive optics (AO) is the most promising method to compensate wavefront disturbance in free space optics communication (FSO). In order to improve the performance of the AO system described by discrete-time linear system model with time-delay and implicit phase turbulent model, new controllers based on a Kalman filter and its extensions are proposed. Based on the standard Kalman filter, we propose a fading memory filter to deal with the ruleless strong interference; sequential and U-D filters are applied to reduce implementation complexity for the embedded controllers. Theoretical analysis and the numerical simulations show that the proposed fading memory filter can upgrade the performance for AO systems in consideration of the unforeseen strong pulse interference, and the sequential and U-D filters perform well compared with a Kalman filter.

• Smart Structures and Systems
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• v.21 no.6
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• pp.777-791
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• 2018

This paper discusses the shape control of deformable mirrors for Adaptive Optics in the dynamic range. The phenomenon of control-structure interaction appears when the mirror becomes large, lowering the natural frequencies $f_i$, and the control bandwidth $f_c$ increases to improve the performance, so that the condition $f_c{\ll}f_i$ is no longer satisfied. In this case, the control system tends to amplify the response of the flexible modes and the system may become unstable. The main parameters controlling the phenomenon are the frequency ratio $f_c/f_i$ and the structural damping ${\zeta}$. Robustness tests are developed which allow to evaluate a lower bound of the stability margin. Various passive and active strategies for damping augmentation are proposed and tested in simulation.

• Journal of the Korea Society of Computer and Information
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• v.23 no.2
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• pp.1-7
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• 2018

The adaptive optics for compensating for optical wavefront distortion due to atmospheric turbulence has recently been used in systems that improve beam quality by eliminating the aberrations of high power laser beam wavefront. However, unseen-mode, which can not be measured in the wavefront sensor, increases the instability of the laser beam wavefront compensator on the adaptive optics system. As a method for improving such instability, a mathematical method for limiting the number of singular values is used when generating the command matrix involved in generation of the drive command of the wavefront compensator. In the past, however, we have relied solely on experimental methods to determine the limiting range of the singular values. In this paper, we propose a criterion for determining the limiting range of the singular values using the driving characteristics and the correlation technique of the wavefront compensator's actuators and have proved its performance experimentally.

• Current Optics and Photonics
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• v.5 no.6
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• pp.641-651
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• 2021

To handle the servo delay in a real-time adaptive optics system, a linear subspace system identification algorithm was employed to model the system, and the accuracy of the system identification was verified by numerical calculation. Experimental verification was conducted in a real test bed system. Through analysis and comparison of the experimental results, the convergence can be achieved only 200 times with prediction and 300 times without prediction. After the wavefront peak-to-valley value converges, its mean values are 0.27, 4.27, and 10.14 ㎛ when the communication distances are 1.2, 4.5, and 10.2 km, respectively. The prediction algorithm can effectively improve the convergence speed of the peak-to-valley value and improve the free-space optical communication performance.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.53.3-53.3
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• 2021

Adaptive Optics (AO) was first studied in the field of astronomy, and its applications have been extended to the field of laser, microscopy, bio, medical, and free space laser communication. AO modelling and simulation are required throughout the system development process. It is necessary not only for proper design but also for performance verification after the final system is built. In KASI, we are trying to develop the AO Python Package for AO modelling and simulation. It includes modelling classes of atmosphere, telescope, Shack-Hartmann wavefront sensor, deformable mirror, which are the components for an AO system. It also includes the ability to simulate the entire AO system over time. It is being developed in the Super Eye Bridge project to develop a segmented mirror, an adaptive optics, and an emersion grating spectrograph, which are future telescope technologies. And it is planned to be used as a performance analysis system for several telescope projects in Korea.