1,721,077 research outputs found
Single-walled carbon nanotube saturable-absorber modelocked Tm:CLTGG laser
No full textWe report on the first passively mode-locked Tm3+-doped calcium tantalum gallium garnet laser generating 69-fs pulses at 2010.4 nm with an average power of 28 mW at a pulse repetition rate of ~87.7 MHz
Mode-locked self-starting Cr:forsterite laser using a single-walled carbon nanotube saturable absorber
No full textWe report what we believe to be the first passive mode-locking of Cr:forsterite laser using a single-walled carbon nanotube saturable absorber (SWCNT-SA). The dispersion-compensated Cr:forsterite laser in a self-starting configuration produces nearly Fourier transform-limited pulses as short as 120 fs near 1.25 mu m. The maximum average output power of 202 mW obtained with a 5% output coupler at a repetition rate of 79.1 MHz represents, to the best of our knowledge, the highest power level ever reported for SWCNT-SA mode locking of solid-state lasers. (C) 2008 Optical Society of Ameri
Yb:KYW planar waveguide laser Q-switched by evanescent-field interaction with carbon nanotubes
Full text linkWe report Q-switched operation of a planar waveguide laser by evanescent-field interaction with single-walled carbon nanotubes deposited on top of the waveguide. The saturable-absorber-integrated gain medium, which operates based on evanescent-field interaction, enables the realization of a diode-pumped 2.5-cm-long Q-switched Yb:KYW waveguide laser emitting at 1030 nm. With such a compact cavity design, we achieve maximumoutput powers of up to 30 mW, corresponding to a single-pulse energy of 124 nJ, at 241 kHz repetition rate. The shortest pulse duration of 433 ns is generated at a repetition rate of 231 kHz
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
Study of the characteristics of amplification of Tm and Tm-Ho doped fluorides for passive Q-switching applications
No full textThe great interest for diode pumped solids state laser sources (DPSSL), in the mid- and
far-infrared is principally related to the absorption spectrum of water. The latter shows
strong absorption for wavelength longer then 1.6μm, making these sources suitable for a
wide range of applications. Infrared lasers above 1.6μm in general, fall in the so called
eye safe region, for this reason pulsed 2μm sources are widely used in communication
technologies as well as in remote sensing, e.g. LIDAR systems and atmospheric physics.
The short penetration depth in biological tissues, due to the high absorption in water,
makes these sources suitable also as laser scalpel in medical applications. Moreover, they
are used as laser sources for Optical Parametric Oscillators for down conversion to the
mid-infrared by nonlinear crystals.
Rare Earths (REs), such as thulium and holmium, are commonly used in solid state gain
media thanks to their peculiar spectroscopic proprieties. They show sharp peaks in spectra
and metastable excited states, typical of free ions behaviour, also when inserted in bulk
hosts. These proprieties, clearly, are strongly related with their electronic configuration
and this thesis I give an idea of why REs have this configuration and of how to calculate
the free ions energy levels scheme. We describe also the characteristic ion-ion coupling
of the REs and the resulting non radiative energy transfer mechanism. Such phenomena,
when resonant, allow an efficient conversion of the radiation: halving the wavelength via
up-conversion, or doubling it by cross-relaxation.
Fluorides are widely used as crystal hosts in gain media REs based systems thanks to
their low maximum phonon energy. In this work I focus on BaY2F8(BYF) and YLiF4
(YLF) crystals, I introduce the concepts necessary to characterize laser crystals: ground
state absorption cross section, stimulated emission cross section and gain cross section.
The main goal is to investigate the characteristics of amplification of thulium and
thulium-holmium doped fluorides in the 2μm region for application in passive Q-switching,
PQS. This technique, simply obtained by intracavity insertion of a saturable absorber,
allow me to obtain short pulses (tens of ns long) and high peak power (up to tens of kW),
without the need to use active devices and thus considerably reducing costs and complexity.
The parts of a general DPSSL system, and in particular the details of the DPSSL used
in this work, are described. The role of the saturable absorber (SA) is to introduce time
modulated losses. It is engineered in order to prevent stimulated emission at the begin
the pumping phase and to be transparent when the fluence reaches the level of the SA
saturation fluence value allowing the pulse to be emitted. The saturable absorbers, used in this work, are large bandgap semiconductors (ZnS and ZnSe) doped with doubly ionized chromium ions. The bandgap ensures a wide transparency window and the chromium doping a
smooth absorption band from 1.5μm to 2.1μm.
The laser crystals have been tested in different configurations and in combination with
different absorbers in order to maximize the peak pulse and minimize the pulse duration.
Comparing the Tm:BYF performance obtained with the results in literature achieved with
other Tm-doped fluorides, we found out that the performance of BYF are lower. The
problem is the lower damaging threshold of BYF that makes it very difficult to work
without damage. Changing the setup in order to decrease the energy density on the
Tm:BYF crystals the damaging problem was still present. Instead working with- Tm-
Ho:YLF crystals was easier, mainly because in the co-doped system the energy densities
on the optical elements are several times lower. Comparing the data achieved with the
literature we demonstrated with this material for the first time sub μs pulse operation at a
room temperature (40ns of pulse duration), consequently improving also the peak power.
The improvement could be mainly imputed to the use of the Cr2+:ZnSe in place of the
Cr 2+ ZnS that shows a higher absorption cross section in the 2050μm region.
As regards Tm:BYF the next step could be the use BaYLuF8 instead of the BYF follow-
ing the same philosophy between YLF and LLF. Substituting the yttrium with lutetium
the thermo-mechanical proprieties could be improved and consequently the damage prob-
lem reduced. For Tm-Ho:YLF the next natural step is to perform the same experiments
with LLF that for single doped thulium system shows the best results to date
Multiphonon-Assisted Emission of Rare-Earth Ions: Towards Pulse Shortening in Mode-Locked Lasers
No full textVariations 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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