• Korean Journal of Optics and Photonics
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• v.13 no.3
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• pp.251-257
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• 2002

The measuring resolution and speed for wavefronts are important to improve the performance of an adaptive optics system. In this paper, we propose a fast measuring algorithm with high resolution in the Shack-Hartmann wavefront sensor for an adaptive optics system. We designed ground isolated electrical devices whose differential data signals are used to control the deformable mirror and tip/tilt mirror for robust control. The conventional mass centroid algorithm in the Shack-Hartmann sensor to measure wavefront has been widely used and provided good measurement results. In this paper, the proposed fast measuring algorithm for measuring the wavefront combines the conventional mass centroid algorithm with a weighting factor. The weighting factor is a real value estimating the real center of mass in a wavefront spot image. This proposed wavefront measuring algorithm provided fast measurement results with high resolution from experimental tests.

• Proceedings of the Optical Society of Korea Conference
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• 2001.08a
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• pp.218-219
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• 2001

• Korean Journal of Optics and Photonics
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• v.18 no.6
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• pp.375-382
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• 2007

The Shack-Hartmann wavefront sensor is composed of a lenslet array generating the spot images from which local slope is calculated and overall wavefront is measured. Generally the principle of wavefront reconstruction is that the spot centroid of each lenslet array is calculated from pixel intensity values in its subaperture, and then overall wavefront is reconstructed by the local slope of the wavefront obtained by deviations from reference positions. Hence the spot image of each lenslet array has to remain in its subaperture for exact measurement of the wavefront. However the spot of each lenslet array deviates from its subaperture area when a wavefront with large local slopes enters the Shack-Hartmann sensor. In this research, we propose a spot image searching method that finds the area of each measured spot image flexibly and determines the centroid of each spot in its area Also the algorithms that match these centroids to their reference points unequivocally, even if some of them are situated off the allocated subaperture, are proposed. Finally we verify the proposed algorithm with the test of a defocus measurement through experimental setup for the Shack-Hartmann wavefront sensor. It has been shown that the proposed algorithm can expand the dynamic range without additional devices.

• Korean Journal of Optics and Photonics
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• v.19 no.2
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• pp.144-149
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• 2008

Thermal distortion of the laser mirrors which are the Coude mirrors of the laser beam director and the wavefront error caused by the thermal distortion are studied. Coude mirrors consist of three relay mirrors and one fast steering mirror. The mirrors have reflectivity of 99.5% with respect to the laser wavelength of $3.8\;{\mu}m$, and absorption of 500 W per second. Thermal distortion and its related wavefront errors are studied with laser beam irradiation for 5 seconds. For the relay mirror, the wavefront error is 334 nm_PV, 98 nm rms and for fast steering mirror, $11.5\;{\mu}m$_PV, $3{\mu}m$ rms.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.132.2-132.2
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• 2011

We are developing Adaptive Optics (AO) system for astronomical use. The He-Ne laser works as an artificial light source. The tip-tilt correction servo is added to our AO system. The tip-tilt term, among the Zernike terms, is the biggest contributor of wavefront deformation caused by atmospheric turbulence at small telescopes. The tip-tilt correction servo consists of a Piezo tip-tilt platform with a mirror, a quadrant photodiode as a tip-tilt sensor, and controllers. The Shack-Hartmann wavefront sensor measures the residual wavefront errors and they are corrected by the MEMS (Micro Electro Mechanical System) deformable mirror. The MEMS deformable mirror allows the compact size at low cost compare to adaptive secondary mirror and other deformable mirrors. As the frame rates of the MEMS deformable mirror is about tens of kHz, the frame rates of the detector in wavefront sensor is the bottleneck of the wavefront correction speed. For faster performance, we replaced a CCD which provides frame rates only 70 Hz with a CMOS with frame rates up to 450 Hz.

• Proceedings of the Optical Society of Korea Conference
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• 2000.02a
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• pp.200-201
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• 2000

보상광학(Adaptive Optics)은 막 흐르는 대기 또는 매개체에 의한 왜곡을 제거 또는 보상하는 시스템이다$^{(1)}$ . 그 시스템의 구성요소 중 파동앞엣선 측정장치(Wavefront Sensor)는 그러한 왜곡을 측정하는 센서로, Shearing interferometer, Shack-Hartman, Curvature wavefront sensor등이 널리 사용되어 왔으며, Pugh$^{(2)}$ 와 Ragazzoni$^{(3)}$ 가 프라미드형의 프리즘을 이용한 동공면 파면 측정을 새로이 제시했다. 본 연구에서는 동공면 파면 측정을 전산모사하여, 제니케 다항식에 대한 센서의 결과를 얻고, 분석하였다. (중략)

• Journal of the Optical Society of Korea
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• v.19 no.1
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• pp.45-54
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• 2015

In this study, a novel parallel wavefront correction system architecture is proposed, and a model-based tabu search (MBTS) algorithm is introduced for this new system to compensate wavefront aberration caused by atmospheric turbulence in a free-space optical (FSO) communication system. The algorithm flowchart is presented, and a simple hypothetical design for the parallel correction system with multiple adaptive optical (AO) subsystems is given. The simulated performance of MBTS for an AO-FSO system is analyzed. The results indicate that the proposed algorithm offers better performance in wavefront aberration compensation, coupling efficiency, and convergence speed than a stochastic parallel gradient descent (SPGD) algorithm.

• Korean Journal of Optics and Photonics
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• v.12 no.3
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• pp.172-176
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• 2001

Optical performance depending on the input wavefront distortion is analyzed in terms of StreW ratio. The amplitude of wavefront distortion at the optic axis that gives the system diffraction limited optical performance is described quantatively from the analysis of the Strehl ratio, which is obtained at the image plane using the input wavefront that is characterized by low and high spatial frequency. uency.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.25-26
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• 2006

The method measuring the absolute position of a point diffraction source emitting a spherical wavefront in three-dimension is proposed. Two-dimensional interference of spherical wavefronts is used to overcome ambiguity of phase order. The spherical wavefront is explicated by Taylor series expansion, from which a radius of curvature of a spherical wavefront and its center position in three-dimension are obtainable. The spherical wavefront is reconstructed by a modified lateral shearing interferometer, which uses single-mode fiber as a point diffraction source.

• 정보저장시스템학회:학술대회논문집
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• 2005.10a
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• pp.160-161
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• 2005

Using a Shack-Hartmann sensor, we construct an optical testing system measuring the wavefront error of small optical components. The systematic error of the sensor is compensated with a reference plane-wave system that produces almost perfect plane waves. Several types of lenses are tested using a point source that generates spherical waves emitted from a pinhole. The results of the optical testing obtained with the Shack- Hartman sensor are compared with those measured with Zygo interferometer.