• Proceedings of the Optical Society of Korea Conference
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• 2001.02a
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• pp.218-219
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• 2001

본 실험에서는 RF 소자들과 650 nm InGaAIP FP LD를 사용하고 이득스위칭에 의하여 선폭이 35psec 되는 극초단 광펄스를 생성하였다. 또한 이렇게 생성된 극초단 광펄스를 이용하여 언덕형 굴절률 분포를 가지는 다중모드 광섬유의 대역폭을 빠른 푸리에 변환(Fast Fourier Transform)으로 계산하였다 따라서 본 실험에서 이용된 언덕형 굴절률 분포를 갖는 다중모드 광섬유의 대역폭은 670 MHz.km로 측정되었다.

• The Journal of the Acoustical Society of Korea
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• v.27 no.3
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• pp.140-148
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• 2008

In this study, a numerical method for a time-domain acoustic wave backscattering analysis is established based on a physical optics and a Fourier transform. The frequency responses of underwater targets are calculated based on physical optics derived from the Kirchhoff-Helmholtz integral equation by applying Kirchhoff approximation and the time-domain signals are simulated taking inverse fast Fourier transform to the obtained frequency responses. Particularly, the adaptive triangular beam method is introduced to calculate the areas impinged directly by acoustic incident wave and the virtual surface concept is adopted to consider the multiple reflection effect. The numerical analysis result for an acoustic plane wave field incident normally upon a square flat plate is coincident with the result by the analytic time-domain physical optics derived theoretically from a conventional physical optics. The numerical simulation result for a hemi-spherical end-capped cylinder model is compared with the measurement result, so that it is recognized that the presented method is valid when the specular reflection effect is predominant, but, for small targets, gives errors due to higher order scattering components. The numerical analysis of an idealized submarine shows that the established method is effectively applicable to large and complex-shaped underwater targets.

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.268-273
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• 2020

We analyze the characteristics of the coherent beam combination of lasers by monitoring the speckle pattern of the beam reflected from a scattering medium. Three collimated laser sources with high coherence are focused on a scattering target using a lens, and we then examine the speckle pattern of the returned beam in the Fourier domain. We observe that the size of the speckle pattern changes, depending on the focused-beam size or degree of spatial overlap of the three beams. Furthermore, through Fourier-domain analysis of the speckle pattern we obtain the monitoring variable to qualify the efficiency of the coherent beam combination.

• Journal of the Korean Institute of Telematics and Electronics S
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• v.35S no.10
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• pp.126-134
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• 1998

In this paper, we consider the problem of reconstructing a two-dimensional signal from its Fourier intensity and the Fourier intensities of two windowed signals: one by a window w(m,n) and the other by its complementary window [1-w(m,n)]. We develop several conditions under which a signal can be uniquely specified to within several trivial ambiguities such as sign, translation,and time-reversal from the given conditions. We present two possible reconstruction algorithms derived from the Gerchberg-Saxton algorithm.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.39 no.3
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• pp.34-40
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• 2002

In this paper, a new encryption method for a binary image using Phase modulation and Fourier transform is proposed. For decryption we use the characteristics of square law detector. In encryption process, a key image is obtained by phase modulation of 256 level random pattern and its Fourier transformation, and input image is encrypted by Fourier transforming the multiplication of the phase modulated random pattern and phase modulated input image. The encrypted image and key image have only phase information, so they can not be copied or counterfeited and the original image can not be decrypted without the key image. To reconstruct the original image, each phase mask of the key image and the encrypted image must be placed on each path of the Mach-Zehnder interferometry with Fourier transform lens and the output image is obtained in the form of intensity in the CCD(Charge Coupled Device) camera. The real-time decryption is possible in the proposed system by use of a LCD as a phase modulator and a CCD camera as an intensity detector. The proposed method shows a good performance in the computer simulation and optical experiment as an encryption scheme.

• Proceedings of the Optical Society of Korea Conference
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• 2000.02a
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• pp.296-297
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• 2000

프라운호퍼 회절에 의하여 주어진 세기 무늬를 발생시키는 회절광학소자(Diffractive Optical Element, DOE)는 회절무늬소자, 키노폼(kinoform), 컴퓨터 푸리에 홀로그램 (computer-generated Fourier hologram) 등으로 불리우며, 광정보처리, 광연결, 레이저가공에서 중요한 역할을 한다. 이 소자를 설계하는 매우 다양한 방법들이 제안되었는데, iterative Fourier transform 알고리즘(IFTA)과 이를 변형한 알고리즘들이 가장 널리 사용된다. IFTA는 fast Fourier transform(FFT)를 활용하므로 계산시간이 절감되지만 국소 최소점에 고착되는 stagnation문제가 있어 이를 해결하기 위한 많은 변형된 알고리즘들이 제안되었다. 본 연구에서는 최근에 제안한 new Pnoise algorithm with hybrid input-output algorithm(NPA-HIOA)$^{(1)}$ 의 설계 성능을 IFTA, hybrid input-output 알고리즘(HIOA), new Pnoise 알고리즘(NPA)$^{(2)}$ , Nonlinear Least-Square (NLS)$^{(3)}$ 등의 기존의 알고리즘들과 비교하고자 한다. (중략)

• Korean Journal of Optics and Photonics
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• v.16 no.4
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• pp.326-333
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• 2005

Phase-only encryption scheme using exclusive-OR rules in Fourier plane and a single path decryption system are presented. A zero-padded original image, multiplied by a random phase image, is Fourier transformed and its real-valued data is encrypted with key data by using XOR rules. A decryption is simply performed based on 2-1 setup with spatial filter by Fourier transform for multiplying phase-only encrypted data by phase-only key data, which are obtained by phase-encoding process, and spatial filtering for zero-order elimination in inverse-Fourier plane. Since the encryption process is peformed in Fourier plane, proposed encryption scheme is more tolerant to loss of key information by scratching or cutting than previous XOR encryption method in space domain. Compare with previous phase-visualization systems, due to the simple architecture without a reference wave, our system is basically robust to mechanical vibrations and fluctuations. Numerical simulations have confirmed the proposed technique as high-level encryption and simple decryption architecture.

• Korean Journal of Optics and Photonics
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• v.14 no.4
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• pp.377-382
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• 2003

We propose a new optical watermarking method, which can protect the copyright of digital data, using a binary phase hologram and a Mach-Zehnder interferometer. Using a simulated annealing algorithm, the binary phase hologram of the mark image to be hidden is designed. We obtained a watermarked image by linearly superposing the hologram, which is the watermark, in the original image. The extraction processing of the mark image from the watermarked image is achieved by placing the phase-modulated watermarked image on a LCD in one path and the phase-modulated original image on another LCD in the other path in the Mach-Zehnder interferometer. The mark image was obtained by inverse Fourier transforming the phase modulated interference intensity. We confirmed that the proposed method is robust for the cropped images through computer simulation, and we implemented it optically using LCDs which are phase modulation devices.

• Proceedings of the Optical Society of Korea Conference
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• 2009.02a
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• pp.167-168
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• 2009

We propose a new computing method for Fourier hologram of 3D objects captured by lens array. Fourier hologram of the two objects which positioned at different distances can be calculated using multiple orthographic view images. The size of the Fourier hologram is in proportion to the number of the orthographic view images. By repeating the orthographic view images, the size of the Fourier hologram can be increased. The principle is verified by numerically reconstructing the hologram which is synthesized from the orthographic images captured optically.

• Korean Journal of Optics and Photonics
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• v.3 no.1
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• pp.20-26
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• 1992

The fourier transform method of retrieving the phase of the test wavefront from a Twyman-Green interferometer was reviewed by numerical simulations and experiments. Of the two methods reviewed, Takeda's approach proved more reliable in reconstructing the deformation of the test surface. The application of this approach to a plane mirror showed the existence of the surface curvature, whose maximum deviation was about $\lambda$/6. The accuracy in the measurement was evaluated to be around $\lambda$/40.