Development of the active feedback scheme for real-time polarization control in fiber lasers

학위논문 (석사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2014. 2. 정윤찬.초 록 비편광유지 광섬유 기반의 레이저 구조에선 레이저의 편광상태가 일정한 상태를 유지하기 힘들다. 이는 광섬유의 빠른 축상(fast axis)과 느린 축상(slow axis)의 유효 굴절률(effective refractive index)이 완전히 일치하지 못하며, 또한 광섬유 길이에 따라 그 값이 균일하지 못하기 때문에 발생한다. 광섬유가 완전히 대칭적으로 사출되었다 하더라도 외부의 물리적 충격 혹은 온도 변화 등에 의한 추가적인 외부 요인에 의해서도 복굴절성(birefringence)이 발생한다. 헌데 광섬유 구조 내에서, 편광 상태를 유지하는 것은 파라미터들을 측정하는 데에 있어 매우 중요하다. 때문에 편광 상태를 유지해주기 위해 편광 유지 광섬유(Polarization Maintaining optical fiber, PM Fiber)를 이용하여 시스템을 구축하거나 시스템 내부에 세 개의 파장판으로 이뤄진 인라인 광섬유 편광 조절기(in-line fiber polarization controller)를 포함시킨다. 본 논문에서는 동력화된 회전 스테이지를 이용해 편광판을 실시간으로 제어하여 편광 상태의 안정성을 제공하는 체계를 소개한다. 편광 제어 체계에 적용된 알고리즘을 알아보고, 이 편광 제어 기술이 출력과 편광의 변화 양상에 어떠한 영향을 주는지 살펴본다. 개발된 체계는 우선적으로 레이저 다이오드의 출력을 HI1060 광섬유만을 통해 바로 적용하여 이상적인 구동을 확인하였고, 알고리즘을 최적화한 후 1060nm 대역의 연속광(continuous wave) 출력을 갖는 이터븀 첨가 광섬유 증폭기 (Ytterbium doped fiber amplifier) 구조에 적용하였다. 각각의 경우에서 개발된 체계가 편광과 출력의 변화 양상에 어떤 차이를 주었는지 분석하고 그 결과에 대해 논한다.목 차 초 록 ⅰ 목 차 ⅲ 그림 목차 ⅴ 제 1 장 서 론 1 1.1 연구의 배경 1 1.2 연구의 목적 3 1.3 논문의 구성 4 제 2 장 배경이론 5 2.1 레이저 5 2.2 에너지 레벨 구조에 따른 펌핑 9 2.3 이터븀 첨가 광섬유 11 2.4 편광 15 제 3 장 실험 결과 21 3.1 능동형 편광 제어 체계의 구현 21 3.2 이터븀 첨가 광섬유 증폭기 구조에의 활용 28 제 4 장 결론 43 참고문헌 45 Abstract 47Maste

새로운 비대칭 상이동 촉매의 개발과 이를 이용한 알파 아미노산의 입체선택적 합성

Thesis (doctoral)--서울대학교 대학원 :제약학과,2003.Docto

對러 科學技術協力 政策執行에 관한 分析

학위논문(석사)--서울대학교 행정대학원 :행정학과 정책학전공,1997.Maste

Joint Demosaicing and Denoising Based on a Variational Deep Image Prior Neural Network

A joint demosaicing and denoising task refers to the task of simultaneously reconstructing and denoising a color image from a patterned image obtained by a monochrome image sensor with a color filter array. Recently, inspired by the success of deep learning in many image processing tasks, there has been research to apply convolutional neural networks (CNNs) to the task of joint demosaicing and denoising. However, such CNNs need many training data to be trained, and work well only for patterned images which have the same amount of noise they have been trained on. In this paper, we propose a variational deep image prior network for joint demosaicing and denoising which can be trained on a single patterned image and works for patterned images with different levels of noise. We also propose a new RGB color filter array (CFA) which works better with the proposed network than the conventional Bayer CFA. Mathematical justifications of why the variational deep image prior network suits the task of joint demosaicing and denoising are also given, and experimental results verify the performance of the proposed method

A new family of non-stationary hermite subdivision schemes reproducing exponential polynomials

In this study, we present a new class of quasi-interpolatory non-stationary Hermite subdivision schemes reproducing exponential polynomials. This class extends and unifies the well-known Hermite schemes, including the interpolatory schemes. Each scheme in this family has tension parameters which provide design flexibility, while obtaining at least the same or better smoothness compared to an interpolatory scheme of the same order. We investigate the convergence and smoothness of the new schemes by exploiting the factorization tools of non-stationary subdivision operators. Moreover, a rigorous analysis for the approximation order of the non-stationary Hermite scheme is presented. Finally, some numerical results are presented to demonstrate the performance of the proposed schemes. We find that the quasi-interpolatory scheme can circumvent the undesirable artifacts appearing in interpolatory schemes with irregularly distributed control points. (C) 2019 Elsevier Inc. All rights reserved

A non-uniform corner-cutting subdivision scheme with an improved accuracy

The aim of this paper is to construct a new non-uniform corner-cutting (NUCC) subdivision scheme that improves the accuracy of the classical (stationary and non-stationary) methods. The refinement rules are formulated via the reproducing property of exponential polynomials. An exponential polynomial has a shape parameter so that it may be adapted to the characteristic of the given data. In this study, we propose a method of selecting the shape parameter, so that it enables the associated scheme to achieve an improved approximation order (that is, three), in case that either the initial data or its derivative is bounded away from zero. In contrast, the classical methods attain the second-order accuracy. An analysis of convergence and smoothness of the proposed scheme is conducted. The proposed scheme is shown to have the same smoothness as the classical Chaikin's corner-cutting algorithm, that is, C1. Finally, some numerical examples are presented to demonstrate the advantages of the new corner-cutting algorithm. (c) 2021 Elsevier B.V. All rights reserved

Construction of Hermite subdivision schemes reproducing polynomials

The aim of this study is to present a new class of quasi-interpolatory Hermite subdivision schemes of order two with tension parameters. This class extends and unifies some of well-known Hermite subdivision schemes, including the interpolatory Hermite schemes. Acting on a function and the associated first derivative values, each scheme in this class reproduces polynomials up to a certain degree depending on the size of stencil. This is desirable property since the reproduction of polynomials up to degree d leads to the approximation order d+1. The smoothness analysis has been performed by using the factorization framework of subdivision operators. Lastly, we present some numerical examples to demonstrate the performance of the proposed Hermite schemes. © 2017 Elsevier Inc

APPROXIMATION OF MULTIVARIATE FUNCTIONS ON SPARSE GRIDS BY KERNEL-BASED QUASI-INTERPOLATION

In this study, we present a new class of quasi-interpolation schemes for the approximation of multivariate functions on sparse grids. Each scheme in this class is based on shifts of kernels constructed from one-dimensional radial basis functions such as multiquadrics. The kernels are modified near the boundaries to prevent deterioration of the fidelity of the approximation. We implement our scheme using the standard single-level method as well as the multilevel technique designed to improve rates of approximation. Advantages of the proposed quasi-interpolation schemes are twofold. First, our sparse approximation attains almost the same level convergence order as the optimal approximation on the full grid related to the Strang-Fix condition, reducing the amount of data required significantly compared to full grid methods. Second, the single-level approximation performs nearly as well as the multilevel approximation, with much less computation time. We provide a rigorous proof for the approximation orders of our quasi-interpolations. In particular, compared to another quasi-interpolation scheme in the literature based on the Gaussian kernel using the multilevel technique, we show that our methods provide significantly better rates of approximation. Finally, some numerical results are presented to demonstrate the performance of the proposed schemes

지수 B-스플라인 일반화 비정적 세분법

학위논문(박사) - 한국과학기술원 : 수리과학과, 2012.2, [ vii, 61p. ]An important capability for a subdivision scheme is the reproducing property of circular shapes or parts of conics that are important analytical shapes in geometrical modeling. In this regards, this thesis first provides necessary and sufficient conditions for a non-stationary subdivision to have the reproducing property of exponential polynomials. Then, the approximation order of such non-stationary schemes is discussed to quantify their approximation power. Based on these results, we see that the exponential B-spline generates exponential polynomials in the associated spaces, but it may not reproduce any exponential polynomials. Thus, we present {\em normalized} exponential B-splines that reproduce certain sets of exponential polynomials. One interesting feature is that depending on the normalization factor, the set of exponential polynomials to be reproduced is varied. This provides us with the necessary accuracy and flexibility in designing target curves and surfaces. Some numerical results are presented to support the advantages of the normalized scheme by comparing them to the results without normalization. In fact, the dimension of the space of exponential polynomials reproduced by the normalized exponential B-spline is limited to $2$, which leads to the limitation of the approximation order. By sacrificing the support of the subdivision mask, the defect can be overcome. We present the exponential {\em quasi}-spline with the unlimited reproduction capability by generalizing the exponential B-spline. Its built-in parameters enable us to control the tension of the limit curve so that the artifact of the interpolatory schemes occurred in highly irregular region of control points can be avoided. Depending on the asymptotic equivalence of (non-)stationary schemes, we analyze the smoothness of the exponential quasi-spline and provide the actual computation for it. Numerical examples which show fruitful benefits obtained from the exponential quasi-spline are...한국과학기술원 : 수리과학과

A family of non-uniform subdivision schemes with variable parameters for curve design

In this paper, we present non-uniform subdivision schemes with variable parameter sequences. A locally different tension parameter is set at each edge of the initial control polygon to control locally the shape of the resulting curve such that the scheme becomes non-uniform. Due to the variable parameters, the scheme can reproduce locally different analytic curves such as conics, Lissajous, trigonometric and catenary curves. Hence blending curves including such analytic components can be successfully generated. We discuss the convergence and smoothness of the proposed non-uniform schemes and present some numerical results to demonstrate their advantages in geometric modeling. Furthermore, as an application, we propose a chamfering algorithm which can be used in designing automobile and mechanical products. © 2017 Elsevier Inc