1,721,214 research outputs found
Mid-IR frequency comb with sub-Hertz relative line width based on Raman soliton self-frequency shift
No full textOptical frequency combs in the mid-infrared spectral region became an invaluable tool for molecular spectroscopy. This spectral region is filled with highly specific vibrational transitions which allow for a precise identification of the composition of gas mixtures and fundamental research of phenomena in physics and chemistry with highest precision. Fre- quency combs in the mid-infrared are often generated via nonlinear frequency conversion of near-infrared lasers. In this work, two frequency components of the same Yb:fiber comb are converted to the mid-infrared at 4 μm via difference frequency generation (DFG) inside a nonlinear crystal. The long-wavelength component is generated from the shorter wavelength driving laser via Raman soliton self-frequency shift in a highly nonlinear fiber.In the past, not much attention has been paid to the coherence properties and optical line widths of similar DFG frequency combs. To study these properties, the generated mid-infrared light is interfered with a reference frequency comb of known properties. The phase noise of the generated heterodyne beat is characterized to determine the optical line width of the mid-infrared frequency comb. The results show sub-Hz relative line widths relative between the two combs. This method also allows to characterize optical sidebands generated by a dither-and-lock technique, which was used in a previous work for long-term stabilization of the source for minimal intensity noise. In this work, this stabilization mechanism is replaced with an interferometric method which does not require modulation. Paired with the low comb line width this allows for the use of the presented mid-infrared frequency comb for molecular spectroscopy at high resolution with high long-term stability
Mitt-Infrarot-Frequenzkamm mit sub-Hz relativer Linienbreite basierend auf Raman-Soliton-Selbstfrequenzverschiebung
No full textOptical frequency combs in the mid-infrared spectral region became an invaluable tool for molecular spectroscopy. This spectral region is filled with highly specific vibrational transitions which allow for a precise identification of the composition of gas mixtures and fundamental research of phenomena in physics and chemistry with highest precision. Frequency combs in the mid-infrared are often generated via nonlinear frequency conversion of near-infrared lasers. In this work, two frequency components of the same Yb:fiber comb are converted to the mid-infrared at 4 μm via difference frequency generation (DFG) inside a nonlinear crystal. The long-wavelength component is generated from the shorter wavelength driving laser via Raman soliton self-frequency shift in a highly nonlinear fiber. In the past, not much attention has been paid to the coherence properties and optical line widths of similar DFG frequency combs. To study these properties, the generated mid-infrared light is interfered with a reference frequency comb of known properties. The phase noise of the generated heterodyne beat is characterized to determine the optical line width of the mid-infrared frequency comb. The results show sub-Hz relative line widths relative between the two combs. This method also allows to characterize optical sidebands generated by a dither-and-lock technique, which was used in a previous work for long-term stabilization of the source for minimal intensity noise. In this work, this stabilization mechanism is replaced with an interferometric method which does not require modulation. Paired with the low comb line width this allows for the use of the presented mid-infrared frequency comb for molecular spectroscopy at high resolution with high long-term stability.Optische Frequenzkämme im Spektralbereich des mittleren Infrarots sind zu einem wichtigen Werkzeug der molekularen Spektroskopie geworden. Jener Spektralbereich ist reich an hochspezifischen Vibrationsübergängen, die sowohl eine genaue Bestimmung der Zusammensetzung von Gasgemischen als auch die Erforschung von grundlegenden Phänomenen der Physik und Chemie mit höchster Präzision erlauben. Frequenzkämme im mittleren Infrarot werden für gewöhnlich durch nichtlineare Frequenzkonversionsprozesse von Lasern im nahen Infrarot erzeugt. Für diese Arbeit werden zwei Frequenzkomponenten des selben Yb:Faser-Frequenzkammes in einem nichtlinearen Kristall mittels Differenzfrequenzmischung (DFG) in den mittleren Infrarot bei 4 μm konvertiert. Dabei entsteht die langwellige Komponente mittels Raman-Soliton-Selbstfrequenzverschiebung in einer hochnichtlinearen Faser aus dem kurzwelligerem Licht des treibenden Lasersystems. Für ähnliche DFG-Frequenzkämme ist in der Vergangenheit wenig Augenmerk auf deren Kohärenzeigenschaften und optischen Linienbreiten gelegt worden. Um diese zu vermessen, wird das erzeugte Mitt-Infrarot-Licht mit einem Referenzfrequenzkamm mit bekannten Eigenschaften interferiert. Das Phasenrauschen der erzeugten heterodynen Schwebungsnoten wird charakterisiert, um die optische Linienbreite des Mitt-Infrarot-Kamms zu bestimmen. Die Ergebnisse zeigen Sub-Hz relative Linienbreiten zwischen den beiden Kämmen.
Diese Methode erlaubt ebenfalls die Charakterisierung optischer
Seitenbänder, die von einer Dither-and-Lock-Technik erzeugt wurden, die in einer Vorarbeit zur Langzeitstabilisierung der Quelle für minimales Intensitätsrauschen genutzt wurde. In dieser Arbeit wird jene Stabilisierungsmethode durch eine neue interferometrische Methode ersetzt, die ohne Modulation auskommt. Dies gepaart mit der geringen Kamm-Linienbreite erlaubt den Einsatz des präsentierten Mitt-Infrarot-Frequenzkamms für molekulare Spektroskopie mit hoher Auflösung und hoher Langzeitstabilität
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
Recent progress on monolithic fiber amplifiers for next generation of gravitational wave detectors
Full text linkSingle-frequency fiber amplifiers in MOPA configuration operating at 1064 nm (Yb3+) and around 1550 nm (Er3+ or Er3+:Yb3+) are promising candidates to fulfill the challenging requirements of laser sources of the next generation of interferometric gravitational wave detectors (GWDs). Most probably, the next generation of GWDs is going to operate not only at 1064 nm but also at 1550 nm to cover a broader range of frequencies in which gravitational waves are detectable. We developed an engineering fiber amplifier prototype at 1064 nm emitting 215 W of linearly-polarized light in the TEM00 mode. The system consists of three modules: the seed source, the pre-amplifier, and the main amplifier. The modular design ensures reliable long-term operation, decreases system complexity and simplifies repairing and maintenance procedures. It also allows for the future integration of upgraded fiber amplifier systems without excessive downtimes. We also developed and characterized a fiber amplifier prototype at around 1550 nm that emits 100 W of linearly-polarized light in the TEM00 mode. This prototype uses an Er3+:Yb3+ codoped fiber that is pumped off-resonant at 940 nm. The off-resonant pumping scheme improves the Yb3+-to-Er3+ energy transfer and prevents excessive generation of Yb3+-ASE
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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