1,720,957 research outputs found
Study of thermal lensing in Nd:YVO<sub>4</sub> under intense diode-end-pumping
No full textWith its natural birefringence and high στf product, Nd:YVO4 is a popular choice as a lasing material for diode pumped solid-state lasers. The short absorption length for pump light at ~809nm relaxes the constraints on diode beam quality allowing tighter pump beam focusing and hence the use of relatively small laser mode sizes. However, in comparison to Nd:YAG it has poorer thermal properties, resulting in strong thermal-lensing, a knowledge of which is essential for optimum laser design. We present the results of a detailed study, considering the effect on thermal lensing in Nd:YVO4 of neodymium concentration, heat-sinking arrangement and laser configuration. The thermal lens powers in end-pumped Nd:YVO4 were determined by measuring the induced phase difference as a function of transverse position using a Mach-Zehnder interferometer, figure 1. The interferometer was also equipped with a set of mirrors to form a stable non-collinear resonator for the Nd:YVO4 crystal to allow thermal-lensing to be investigated under lasing and non-lasing conditions. Figure 1 The interferometer set-up and figure 2 example results. Figure 2 shows the results obtained for 1% and 0.3% Nd-doped YVO4 crystals with cooled faces perpendicular to the c-axis and probe beam polarization parallel to the c-axis. The thermal lens power is ~5 times greater under non-lasing than for the lasing conditions for the 1% doped crystal, which we attribute mainly to additional heating from ETU, further compounded by reduction of thermal conductivity at higher temperature. The much smaller corresponding value for the 0.3% doped crystal indicates the potential benefits to (~2) be achieved with lower Nd concentrations. The results for the perpendicular heat-sinking direction and the orthogonal probe beam polarization, which suggest that thermally-induced stresses play a role in thermal lensing behaviour will also be presented. The results provided by this study allow formulation of a design strategy for further power-scaling of Nd:YVO4 lasers and amplifiers whilst maintaining good output beam quality
Detailed study of thermal lensing in Nd:YVO<sub>4</sub> under intense diode end-pumping
No full textThermal lensing in diode-end-pumped Nd:YVO4 has been measured for different cooling arrangements and different neodymium concentrations under lasing and non-lasing conditions. The significant effect of energy-transfer-upconversion on lens aberration and beam quality is examined
The lightwave road show
No full textEducation and Training in Optics and Photonics 2003 (SPIE 9663) Tucson AZ 6 Oct In order to change the way that school children view physics, OSA’s University of Southampton Student Chapter has developed an optic roadshow. By taking simple optics experiments and demonstrations to primary schools we aim to dispel the negative image of classroom science and help teachers to convey the fascination of physics
Simple technique for measuring the energy-transfer-upconversion parameter in solid-state laser materials
Get PDFScaling the output power from solid-state lasers to higher power levels, whilst maintaining high efficiency and good beam quality is a challenging task. The chief difficulty is heat generation and its deleterious effects on laser performance. The main origin of heat generation is often considered to be quantum defect heating. However, many solid-state laser media also suffer from the problem of energy-transfer-upconversion, which can lead to a dramatic reduction in the effective upper laser level lifetime and a large increase in the fractional heat loading unless appropriate measures are taken. Thus, an accurate knowledge of the upconversion parameter and the impact of upconversion on laser performance is very important for power scaling of solid-state lasers, particularly when operating in pulsed (Q-switched) mode. The standard method for determining upconversion parameters, based on measurements of fluorescence decay times, can be quite time consuming and requires very careful attention to detail in the experimental set-up and pumping conditions if it is to yield accurate values
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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