1,721,257 research outputs found
640 GHz real-time recording using temporal imaging
No full text640 GHz chirped beat waves are recorded on a real-time scope and 2.2 ps pulses are recorded on a single-shot streak camera with 1000:1 dynamic range after ~30× time magnification
Guided-wave temporal imaging based ultrafast recorders
No full textGuided-wave parametric temporal imaging is demonstrated with 1.8 ps resolution and 1000:1 dynamic range. Waveforms are - 30.1 x time magnified before recording single-shot on a streak camera, and on a real-time oscilloscope repeating at MHz rates
Frequency-doubling of a cladding-pumped Er<sup>3+</sup>/Yb<sup>3+</sup> femtosecond fiber laser system using a periodically-poled LiNbO<sub>3</sub>
No full textAs real-world ultra-fast optical devices proliferate, there is a growing need for highly reliable and compact sources of femtosecond pulses [1]. Currently most of these applications require moderate power sources operating around 800 nm, which is ideally compatible with frequency-doubling of femtosecond Er3+-fiber lasers. Previously integrated high-power fiber laser systems were developed based on chirped-pulse amplification schemes relying on chirped fiber gratings for pulse stretching and compression to minimize the nonlinearities of femtosecond fiber amplifiers [2]. The component count of such systems can be considerably reduced and the optical efficiency increased by implementing aperiodically poled lithium niobate [3] (APPLN), as APPLN allows a unique integration of chirped pulse amplification with frequency-doubling. Here we demonstrate the first system application of a APPLN frequency-doubler in conjunction with a high-power cladding-pumped Er3+/Yb3+ fiber laser. The experimental set-up is shown in Fig. 1. The fiber seed system is based on an environmentally stable fiber soliton laser [1] and generates bandwidth-limited 250 fsec pulses with pulse energies of 300 pJ at a repetition rate of 40 MHz at a wavelength of 1.56µm. To operate the cladding pumped power amplifier in saturation the pre-amplifier is used, which boosts the average signal power to 35 mW. Prior to amplification in the cladding-pumped power amplifier the pulses are stretched to ~ 1.7 psec in a 2.9 m length of positive dispersion fiber. Using a coupled pump power of ~6 W at 980 nm into the power amplifier, we obtain a signal power of 600 mW. After frequency-doubling in a length of 2 cm of APPLN an average power of 180 mW is obtained at 780 nm. The frequency-doubled pulse energy is 4.5 nJ. Note that the crystal was not AR-coated and the internal SH power was ~210 mW. The internal SH conversion efficiency was 40 %. An autocorrelation trace and the corresponding pulse spectra at the frequency-doubled wavelength are shown in Fig, 2. The pulse width is 290 fsec and assuming a gaussian pulse shape the time bandwidth product is 0.51, i.e. the pulses were within 20% of the bandwidth limit. Since currently APPLN allows the recompression of pulses up to 15 psec in width [3], we can expect that this technology may be upscaled to producing femtosecond pulses at Watt-level powers at 780 nm. Fig. 1: High-power frequency-doubled Er/Yb fiber laser. Fig. 2: Autocorrelation and spectrum of the pulses generated at 780 nm. The pulse width is 290 fsec and the time-bandwidth product is 0.51 assuming a Gaussian shape
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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