1,720,963 research outputs found
Interference of Intensity Noise in a Multimode Nd:YAG Laser
Get PDFWe investigate the behaviour of the intensity noise in a multi-longitudinal mode Nd:YAG laser. This type of laser is a nonlinear system which exhibits complicated dynamics within the intensity noise. For example, antiphase dynamics is where there is cancellation of one or more collective modes of oscillation, which are distinct from the longitudinal modes, in the total output.
Commonly lasers are studied experimentally to discriminate between models used to describe them. They are convenient since many external influences can be controlled and the oscillations of interest are low frequency (in the kHz regime) making their direct measurement relatively simple.
In our laser, the collective modes of oscillation are excited by broadband ambient noise. Because the phase of the excitation is unknown, we develop the cross spectral technique to measure the antiphase dynamics directly and form a picture of the intensity noise interference for two to five mode operation. For three mode operation, we measure the contributions of the longitudinal modes to the collective modes.
We also calculate power spectral densities of the individual longitudinal modes and the total intensity. We test relationships between these quantities, at the collective mode frequencies, which are derived from modal rate equation theory. For two mode operation, the theoretical relations are satisfied. For three mode operation, the relations are satisfied when the picture of interferences is taken into account. The cross spectral technique is therefore shown to be a sensitive test of the model developed by Pieroux and Mandel [T. Hill et al., Phys. Rev. A 66, 063803 (2002)].
The behaviour of the multimode laser operating near the threshold of a longitudinal mode is measured. Transitions in the cross spectrum are noted in some pairs of longitudinal modes, for an arbitrary but small pump rate above threshold of a longitudinal mode. It has been shown that longitudinal modes with a high threshold pump power may become more intense than those with a lower threshold [K. Otsuka et al., Opt. Lett. 23, 201 (1998), L. Stamatescu and M.W. Hamilton, (unpublished) (1999), N.B. Abraham et al., Phys. Rev. A 62, 013810 (2000), P.A. Khandokhin, E.A. Ovchinnikov and E.Yu. Shirokov, Phys. Rev. A 61, 053807 (2000)]. The AC noise component of the first two longitudinal modes to reach threshold, is found to exhibit similar properties to their intensity. The implications of the results of this thesis, on models used to describe the behaviour of solid state lasers, are also discussed.Thesis (Ph.D.)--Physics, 2003
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
A digital holographic imager for cloud microphysics studies
Get PDFClouds play a crucial role in regulating the climatic and meteorological systems of planetary atmospheres due to their impact on the radiative transfer of electromagnetic energy through the atmosphere and in governing the hydrological (or equivalent) cycles. The role of clouds is of particular interest within the Earth’s atmosphere where they cover approximately 70% of the Earth’s surface and also have an impact on aircraft safety considerations due to aircraft icing and associated hazards. To quantitatively understand the influence of clouds in an atmospheric system, the underlying physics of their formation, evolution and interaction with other atmospheric dynamical processes must be observed and modelled. These processes are governed by the underlying microphysical cloud properties such as the cloud particle shapes, sizes, spatial clustering and thermodynamic phase. Direct observations of these microphysical properties have historically proved challenging with large discrepancies seen between the outputs of climate models and direct observations, suggesting a lack of understanding of these processes. Digital holography is a three dimensional imaging technique that allows direct measurement of many microphysical observables, such as the particle size distribution, particle shape distribution and spatial distribution, making it an attractive solution to this observational challenge. Previous instruments have been expensive and heavy devices, limiting their use to ground based observations or on board expensive research aircraft flights, which has severely limited the amount of data obtained from these instruments. There is therefore a need for a low cost, light weight digital holographic instrument suitable for deployment on a tower structure or weather balloon to obtain these critically needed measurements in remote and widespread regions. The development of such an instrument is outlined in this thesis.Thesis (M.Phil.) -- University of Adelaide, School of Physical Sciences, 2017
A lidar for cirrus and mixed phase cloud studies
Get PDFCirrus and mixed phase clouds represent a major uncertainty in climate and weather models. This uncertainty can be reduced with a better understanding of the lifecycle and radiative properties of cirrus and mixed phase clouds, and by inputting local measurements into models. A cloud’s radiative properties are dependent on the thermodynamic phase of the cloud particles. Measurements made with a polarimetric lidar can be used to determine thermodynamic phase and improve our understanding of cirrus and mixed phase clouds. Few polarimetric lidar instruments are used in the southern hemisphere, representing a gap in understanding and measurements. An existing lidar instrument was upgraded and run for 6 months; 3 months with polarisation measurements. Important properties such as height, frequency of occurrence and thermodynamic phase have been measured up to heights of around 6 km. These measurements are consistent with ground and satellite based lidar, and with radiosonde measurements. Methods for determining additional properties of the clouds, such as the optical thickness and lidar ratio were researched. Sufficient measurements of cloud macrophysical properties allow for the determination of cloud microphysical properties, such as particle density and shape. To assist with determining these properties a polarimetric lidar simulation was written. Microphysical properties were not determined due to the lidar lacking sufficient range and resolution. Due to the low peak power of the laser used, increasing the range and resolution by increasing the peak power of the laser would be relatively easy.Thesis (M.Phil.) -- University of Adelaide, School of Physical Sciences, 2015
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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