1,720,960 research outputs found
Scalar and vector modulational instability induced by parametric resonance in periodically tapered PCFs
No full textWe analyze the modulational instability process induced by periodic variations of the parameters of a PCF along the propagation direction, induced by an analogue of the parametric resonance in mechanics
Fourth-order dispersion mediated modulation instability in dispersion oscillating fibers
No full textWe investigate the role played by fourth-order dispersion on the modulation instability process in dispersion oscillating fibers. It not only leads to the appearance of instability sidebands in the normal dispersion regime (as in uniform fibers), but also to a new class of large detuned instability peaks that we ascribe to the variation of dispersion. All these theoretical predictions are experimentally confirmed. (C) 2013 Optical Society of America</p
Etude du processus d'instabilité modulationnelle dans les fibres optiques présentant un profil de dispersion périodique.
No full textThis thesis concerns the modulation instability (MI) process in optical fibers with a periodic dispersion landscape. MI is a well-known phenomenon but in the context of optical fibers most studies were performed in the presence of uniform dispersion. The aim of this work is to experimentally demonstrate the MI process in a special type of fiber: a photonic crystal fiber (PCF) presenting a sinusoidal dispersion profile. Part 1 focuses on the main characteristics of optical fibers, on the main physical processes occurring in fibers and on three numerical tools used. Part 2 starts with the context of the study. First, we introduce the quasi-phase matching (QPM) mechanism in longitudinal periodic systems. Then, we report an experimental demonstration of the MI process in this kind of fiber. The experimental MI spectrum is composed of ten MI lobes in a range of 10 THz. We propose a detailed description of MI lobes dynamics. We then study the impact of fourth-order dispersion on the MI process in DOFs. We demonstrate theoretically and experimentally the presence of a new MI windows due to a combination of oscillating second-order dispersion and fourth-order dispersion. Finally, this second part ends with the development of an analytical tool based on the truncated three-wave model. It allows simple interpretations of MI lobes dynamics during their propagation. We show that this tool is very accurate and allows controlling MI spectrum of a DOF.Ce travail de thèse concerne l'étude du processus d'instabilité modulationnelle (MI) dans des fibres optiques présentant un profil de dispersion périodique. La MI est un phénomène bien connu mais la majorité des études réalisées dans le contexte des fibres optiques concerne le cas de dispersion uniforme axialement. L'objectif de ces travaux est de démontrer expérimentalement l'établissement de la MI dans une fibre optique micro-structurée (PCF) dont la dispersion varie de façon sinusoïdale. La première partie présente les principales caractéristiques des fibres optiques et les principaux processus physiques mis en jeu, ainsi que les outils de simulations numériques employés. La seconde partie débute par une présentation du contexte de l'étude et est ensuite consacrée aux résultats. Tout d'abord, nous introduisons le concept du mécanisme de quasi-accord de phase (QPM) dans les systèmes périodiques longitudinaux. Ensuite, nous présentons la démonstration expérimentale du processus de MI dans ce type de fibre. Le spectre obtenu est composé de dix lobes de MI sur 10 THz. De plus, nous proposons une description détaillée de la dynamique de formation des différents lobes de MI. Nous démontrons théoriquement et expérimentalement la génération d'une nouvelle famille de lobes de MI due à une combinaison entre la dispersion oscillante et la dispersion d'ordre quatre. Enfin, cette seconde partie est clôturée par l'élaboration d'un outil analytique, basé sur le modèle à trois ondes tronqué. Il permet d'interpréter simplement la dynamique de formation des lobes de MI et de contrôler le spectre de MI d'une fibre à dispersion oscillante
Modulation instability in optical fibers with periodic dispersion
No full textCe travail de thèse concerne l’étude du processus d’instabilité modulationnelle (MI) dans des fibres optiques présentant un profil de dispersion périodique. La MI est un phénomène bien connu mais la majorité des études réalisées dans le contexte des fibres optiques concerne le cas de dispersion uniforme axialement. L’objectif de ces travaux est de démontrer expérimentalement l’établissement de la MI dans une fibre optique micro-structurée (PCF) dont la dispersion varie de façon sinusoïdale. La première partie présente les principales caractéristiques des fibres optiques et les principaux processus physiques mis en jeu, ainsi que les outils de simulations numériques employés. La seconde partie débute par une présentation du contexte de l’étude et est ensuite consacrée aux résultats. Tout d’abord, nous introduisons le concept du mécanisme de quasi-accord de phase (QPM) dans les systèmes périodiques longitudinaux. Ensuite, nous présentons la démonstration expérimentale du processus de MI dans ce type de fibre. Le spectre obtenu est composé de dix lobes de MI sur 10 THz. De plus, nous proposons une description détaillée de la dynamique de formation des différents lobes de MI. Nous démontrons théoriquement et expérimentalement la génération d’une nouvelle famille de lobes de MI due à une combinaison entre la dispersion oscillante et la dispersion d’ordre quatre. Enfin, cette seconde partie est clôturée par l’élaboration d’un outil analytique, basé sur le modèle à trois ondes tronqué. Il permet d’interpréter simplement la dynamique de formation des lobes de MI et de contrôler le spectre de MI d’une fibre à dispersion oscillante.This thesis concerns the modulation instability (MI) process in optical fibers with a periodic dispersion landscape. MI is a well-known phenomenon but in the context of optical fibers most studies were performed in the presence of uniform dispersion. The aim of this work is to experimentally demonstrate the MI process in a special type of fiber: a photonic crystal fiber (PCF) presenting a sinusoidal dispersion profile. Part 1 focuses on the main characteristics of optical fibers, on the main physical processes occurring in fibers and on three numerical tools used. Part 2 starts with the context of the study. First, we introduce the quasi-phase matching (QPM) mechanism in longitudinal periodic systems. Then, we report an experimental demonstration of the MI process in this kind of fiber. The experimental MI spectrum is composed of ten MI lobes in a range of 10 THz. We propose a detailed description of MI lobes dynamics. We then study the impact of fourth-order dispersion on the MI process in DOFs. We demonstrate theoretically and experimentally the presence of a new MI windows due to a combination of oscillating second-order dispersion and fourth-order dispersion. Finally, this second part ends with the development of an analytical tool based on the truncated three-wave model. It allows simple interpretations of MI lobes dynamics during their propagation. We show that this tool is very accurate and allows controlling MI spectrum of a DOF
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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