1,720,968 research outputs found
Thermally-guided Yb-doped fiber-rod amplifier and laser
No full textIn this paper we describe a thermally-guided fiber-rod amplifier and laser oscillator, which exploit thermal guiding to achieve mode control. This power scalable approach operates with mode sizes that are typically an order of magnitude larger than traditional single-mode fibers. Operating as an amplifier we achieve a small-signal gain of 5dB at 1030nm, whilst maintaining excellent beam quality (M2 < 1.1). In a laser configuration we obtained 13.1W of diffraction-limited (M2 < 1.1) output power at 1032.5nm, with a slope efficiency 53% with respect to absorbed power. A model predicting beam evolution through the thermally-guided fiber-rod gain medium is presented and validated
Amplification of a radially polarized beam in a thermally guiding ytterbium-doped fiber rod
No full textAmplification of a radially polarized beam from an Yb:YAG seed laser at 1030 nm in a very-large-core triple-clad ytterbium-doped fiber configured as a thermally guiding fiber rod amplifier is demonstrated. The amplifier yielded a maximum output power of 10.7 W (limited by available pump power) with corresponding single-pass gain of 6.4 dB, while the radial polarization purity was maintained over the full range of pump power with a measured polarization extinction ratio of 100:1. The amplified output beam displayed the characteristic donut-shaped intensity profile with negligible beam distortion as evidenced by a measured beam propagation factor (M2) of 2.1 ± 0.1. A simplified model for propagation of a radially polarized beam in a thermally guided fiber rod is developed to provide insight into the underlying design principles and to establish a strategy for scaling output power. The power scaling limitations of this approach are considered
Amplification of a radially polarized beam in a thermally-guiding fiber-rod amplifier
No full textLaser and optical amplifier geometries may be split into categories such as rod and fiber. Rod gain media are susceptible to thermal effects at high power, whereas fiber suffer from detrimental non-linear effects due to their long length and small mode areas. Here we present an application of a hybrid architecture between the two geometries – the Thermally-Guiding Fiber-Rod (TGFR). The TGFR inherits the large mode area of the rod amplifier, the high surface area of a fiber, and exploits thermal lensing to guide modes.We present a successful demonstration of amplification of a radially polarized mode using the TGFR. A 1030 nm continuous-wave radially polarized seed source of high purity and beam quality (M2=1.9±0.1) was constructed using thermal bifocussing in a Yb:YAG crystal to provide mode selection. This seed source was carefully focussed into the 300 µm core of a 10 cm long sample of commercially available triple-clad Yb-doped silica fiber in order to satisfy the thermal guidance condition and avoid waveguiding due to the refractive index step. The TGFR was pumped using a high power 915 nm diode laser.The radially polarized mode was preserved through transmission of the TGFR. The output beam polarization was maintained at 99.1% purity while the M2 factor was measured to be 2.1±0.1. The maximum output power was 12.6 W of radially polarized light, corresponding to a gain of 7.0 dB limited by available pump power. This promising geometry the potential for further power scaling of radially-polarized beams for application in laser processing
Thermally guided ytterbium-doped fiber-rod laser (Conference Presentation)
No full textFiber and bulk lasers form two distinct classes of solid-state laser, both of which have achieved tremendous success in various arenas, but they are not without their limitations. In this paper, we investigate an alternative laser geometry occupying a domain that lies between traditional fiber and bulk laser systems. This geometry comprises a fiber-based thin-rod structure, with a diameter on the order of several hundred microns, and with length on the order of several centimetres. The motivation is to combine the advantages of the fiber geometry for excellent thermal management and the bulk geometry for greater immunity to non-linear effects and optical damage, whilst elegantly controlling the laser mode profile using thermally-induced waveguiding. Rare earth ion-doped silica is an excellent candidate to demonstrate the thermally-guided fiber-rod laser (TGFRL) due to its high fracture limit, positive thermo-optic coefficient and well-established fabrication which can produce high purity material with exceptionally low loss. A 300μm core diameter, triple-clad Yb-doped fiber is used to demonstrate the TGFRL. Thermally-induced waveguides can be tailored to have significantly larger transverse dimensions than conventional ‘engineered’ waveguides yielding potential performance benefits, especially in pulsed mode. We will present results covering thermally-induced waveguiding, amplification performance and cw laser performance at 976nm and 1030nm, with >10W achieved at 1030nm with excellent beam quality, M2 < 1.1, and slope efficiencies approaching 50% with respect to absorbed pump power. We will also present preliminary results for amplification of radially-polarised beams, highlighting the potential of these devices in a range of applications
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
- …
