1,720,957 research outputs found
Q-switched fibre lasers for distributed sensing applications
No full textThis thesis examines pulsed fibre sources for distributed sensing applications. A number of Q-switched fibre laser sources optimised for high peak powers, narrow linewidth and short pulse duration are described. The source specifications were dictated by the requirements of Raman and Brillouin distributed sensing systems. The spatial resolution of distributed sensors is related to the pulse width whereas the range is dependent on the power launched into the sensing fibre. Brillouin distributed sensors also require that the source linewidth is less than 10 GHz, the separation between the Rayleigh and Brillouin backscattered light. This constraint on laser linewidth leads to coherent Rayleigh noise on the Rayleigh backscattered trace. This noise can be reduced by a technique of frequency shift averaging. A Q-switched laser incorporating this technique was developed, which resulted in a Brillouin distributed temperature sensor with a temperature resolution of 1.4°C and a spatial resolution of 10metres over a range of 6.5km.The development of high power Q-switched fibre lasers leads to the possibility of generating Raman shifted pulses at wavelengths of l.64-1.65μm. Interest in this wavelength region stems from the increase in sensitivity to fibre micro-bend losses at these higher wavelengths and the ability to monitor the fibre whilst carrying out live data transmission. A diode pumped, pulsed source at 1.64μm producing 8Watt, 10ns pulses through a process of Raman generation was demonstrated. Q-switched laser technology was also used to increase the dynamic range of l.65μm OTDR. The technique utilised delayed Raman amplification of the l.65μm signal pulse by a co-propagating l.53μm pump pulse. Amplification occurs when the two pulses overlap. The position of the overlap is determined by the initial delay between the pulses and the fibre dispersion. An increase in dynamic range of 17.5dB has been observed and the 1.65μm OTDR range was extended to in excess of 100km
An extended range OTDR system at 1.65µm based on delayed Raman amplification
No full textThis paper describes both theoretical and experimental results obtained in an investigation of a new technique for increasing the dynamic range of 1.65µm OTDR systems. The technique utilises delayed Raman amplification of 1.65µm signal pulse by a 1.53µm pump pulse. Amplification occurs when the two pulses overlap and this position is determined by the initial delay between the pulses and the fibre dispersion. An increase in dynamic range of 17.5dB has been observed and the OTDR backscattered Rayleigh signal was detected up to 100km. No significant noise penalty is introduced due to the directionality of the Raman gain
Novel optical fibre distributed temperature sensor based on the Landau-Placzek ratio
Get PDFWe report our latest results on a compact, diode pumped optical fibre distributed temperature sensor based on Brillouin scattering. A high power, short pulse Q-switched Erbium/Ytterbium fibre laser and a double pass in-fibre Mach-Zehnder interferometer make this Brillouin distributed temperature sensor an attractive commercial device. A spatial and temperature resolution of 10 metres and + or - 1.7°C has been demonstrated
Towards active cable reburial monitoring using distributed fiber-optic sensing over 40km of a high voltage marine interconnector
No full textDistributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) measurements were acquired over a 40 km section of the 137 km long Skagerrak 4 (SK4) subsea cable before and during a retrenching operation. Five sections of the cable between 140 m and 280 m in length had been identified for reburial using conventional geophysical and video techniques. DTS data taken over a three week period prior to the reburial clearly showed cold spots at all of the planned reburial sites. Absolute temperature at these locations fluctuated by over 8 °C, which is interpreted as being in response to temporally and spatially varying bottom water temperatures. The reburial process itself was accurately monitored in real-time using DAS. After reburial, consistent DTS temperatures within ± 1 °C were measured at all reburial sites, comparable to the adjacent buried sections of the cable and thereby confirming reburial was successful
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
Recent advances in distributed optical fibre temperature sensing using the Landau-Placzek ratio
No full textDistributed optical fibre temperature sensing (DTS) provides an elegant way of monitoring the temperature at many points without necessitating the accurate positioning of individual discrete sensors. This paper demonstrates recent advances in distributed temperature sensing based on spontaneous Brillouin scattering. A high spatial and temperature resolution is achieved by exploiting a novel low cost detection system. The experimental configuration consists of two key components; a Q-switched laser source to generate the backscattered signal and the low cost detection system which comprises an in-fibre double pass Mach-Zehnder interferometer and a sensitive InGaAs APD connected to a computer based averaging system. It is the relative low cost of these components which has made the Brillouin based temperature sensor so attractive for commercial exploitation. A spatial resolution of 3.0 metres with a Brillouin temperature resolution of 0.9°C at a range of 16km has been achieved
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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