1,720,959 research outputs found
High energy nanosecond laser systems for nonlinear optics and remote sensing applications
Get PDFDuring my PhD research activity carried out at the Laser Source Laboratory of the University of Pavia, I had the opportunity to study and develop complex high-energy (from several mJ to several tens of mJ) and high spectral purity, even single longitudinal mode (SLM), nanosecond laser systems operating at 1 μm and 2 μm. High spectral purity, high energy nanosecond pulses are requested in many applications ranging from nonlinear optics frequency up- and down-conversion, high resolution spectroscopy, LIDAR and remote sensing.
The first laser system I had the opportunity to developed is a table-top pulsed Master Oscillator Power Amplifier (MOPA) laser originally intended as a part of a more complex laser system aimed to work as a transmitter for High Spectral Resolution LIDAR (HSRL) applications.
Given the very demanding specifications in terms of both spectral and spatial quality, and high pulse energy, we opted for a MOPA architecture based on a high quality, compact, low-energy seeder and a suitable number of amplifiers. As a seeder we designed and built a Nd:YVO4 microchip laser operating in the gain switching regime. The gain switching technique has the main drawback of producing low-energy pulses. This required a careful design of a multi-stage, high-gain amplification chain.
This MOPA laser was employed as a pump source for a nonlinear optics experiment consisting in the demonstration of Raman conversion in the visible with a SrWO4 crystal.
During the second year of my PhD, I took part in a 6-month research activity at KTH Royal Institute of Technology in Stockholm, Sweden. During this research activity I had the opportunity to take part in a project aimed to build a tunable double-wavelength nanosecond excitation source, with pulse energies in the range of tens of mJ. This rather complex MOPA laser system was intended to be a universal laboratory source for testing different THz platforms which were developed in parallel by other colleagues at KTH.
In order to achieve higher power conversion efficiency in THz generation the tunable source was designed to operate in the spectral range around 2 μm providing two wavelengths close enough to permit THz generation through difference frequency generation in proper nonlinear materials.
The architecture chosen for the 2-μm laser source was an Optical Parametric Oscillator (OPO) followed by an Optical Parametric Amplifier (OPA) both pumped at 1 μm, since this was the most practical solution for achieving the required pulse energies in the spectral range of interest.
During the final year of my PhD I focused my research activity on high-energy (multi-mJ), Q-switched nanosecond laser oscillators architecture suitable to be employed in remote sensing and military applications. This kind of applications are usually very demanding for what concerns the robustness of the laser, since it is supposed to be working in harsh conditions.
To this purpose, Porro prisms have been widely employed in substitution of usual flat or concave mirrors in Fabry-Perot resonators. Porro prisms are retro-reflecting prisms with 90◦-cut reflecting surfaces that allow the laser beam to be reflected into the same direction where it comes from, up to a small angle of misalignment from the normal of the input plane face of the prism.
To this purpose, I investigated both a Passively Q-switched (PQS) and an Electro-Optical Modulator (EOM) Actively Q-Switched (AQS) resonator configurations. The interest in these two different Q-switching techniques is motivated by the trade-off required from the specific application, i.e., generally AQS can provide higher energies as well as a higher pulse-to-pulse jitter stabilization, whereas PQS architectures generally are far simpler and therefore more cost-effective and reliable.During my PhD research activity carried out at the Laser Source Laboratory of the University of Pavia, I had the opportunity to study and develop complex high-energy (from several mJ to several tens of mJ) and high spectral purity, even single longitudinal mode (SLM), nanosecond laser systems operating at 1 μm and 2 μm. High spectral purity, high energy nanosecond pulses are requested in many applications ranging from nonlinear optics frequency up- and down-conversion, high resolution spectroscopy, LIDAR and remote sensing.
The first laser system I had the opportunity to developed is a table-top pulsed Master Oscillator Power Amplifier (MOPA) laser originally intended as a part of a more complex laser system aimed to work as a transmitter for High Spectral Resolution LIDAR (HSRL) applications.
Given the very demanding specifications in terms of both spectral and spatial quality, and high pulse energy, we opted for a MOPA architecture based on a high quality, compact, low-energy seeder and a suitable number of amplifiers. As a seeder we designed and built a Nd:YVO4 microchip laser operating in the gain switching regime. The gain switching technique has the main drawback of producing low-energy pulses. This required a careful design of a multi-stage, high-gain amplification chain.
This MOPA laser was employed as a pump source for a nonlinear optics experiment consisting in the demonstration of Raman conversion in the visible with a SrWO4 crystal.
During the second year of my PhD, I took part in a 6-month research activity at KTH Royal Institute of Technology in Stockholm, Sweden. During this research activity I had the opportunity to take part in a project aimed to build a tunable double-wavelength nanosecond excitation source, with pulse energies in the range of tens of mJ. This rather complex MOPA laser system was intended to be a universal laboratory source for testing different THz platforms which were developed in parallel by other colleagues at KTH.
In order to achieve higher power conversion efficiency in THz generation the tunable source was designed to operate in the spectral range around 2 μm providing two wavelengths close enough to permit THz generation through difference frequency generation in proper nonlinear materials.
The architecture chosen for the 2-μm laser source was an Optical Parametric Oscillator (OPO) followed by an Optical Parametric Amplifier (OPA) both pumped at 1 μm, since this was the most practical solution for achieving the required pulse energies in the spectral range of interest.
During the final year of my PhD I focused my research activity on high-energy (multi-mJ), Q-switched nanosecond laser oscillators architecture suitable to be employed in remote sensing and military applications. This kind of applications are usually very demanding for what concerns the robustness of the laser, since it is supposed to be working in harsh conditions.
To this purpose, Porro prisms have been widely employed in substitution of usual flat or concave mirrors in Fabry-Perot resonators. Porro prisms are retro-reflecting prisms with 90◦-cut reflecting surfaces that allow the laser beam to be reflected into the same direction where it comes from, up to a small angle of misalignment from the normal of the input plane face of the prism.
To this purpose, I investigated both a Passively Q-switched (PQS) and an Electro-Optical Modulator (EOM) Actively Q-Switched (AQS) resonator configurations. The interest in these two different Q-switching techniques is motivated by the trade-off required from the specific application, i.e., generally AQS can provide higher energies as well as a higher pulse-to-pulse jitter stabilization, whereas PQS architectures generally are far simpler and therefore more cost-effective and reliable
Threshold reduction and mode selection with uncoated Raman crystal acting as a low-finesse cavity
No full textWe report a stimulated Raman scattering threshold reduction by a factor of ∼4 using a simple uncoated SrWO4
crystal with plane/parallel faces pumped by 1.5 mJ, 11 ns single-frequency pulses at 532 nm. The weak etalon
effect is sufficient to shape both the spatial and spectral characteristics of the output Raman beam. The Raman
signal tends to be single-frequency close to threshold, which might be an interesting way to further seed Raman
amplifiers for high-energy Fourier-limited pulses at particular visible or near-infrared wavelengths
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
87 fs Pulse Generation in a Diode-Pumped SESAM Mode-Locked Yb:YLF Laser
No full textWe present for the first time sub-100-fs pulse generation in a SESAM mode-locked, Yb:YLF laser pumped with low-power single-mode fiber-coupled lasers at 976 nm. Almost Fourier transform limited 87-fs pulses at 1052 nm were obtained
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
87 fs pulse generation in a diode-pumped semiconductor saturable absorber mirror mode-locked Yb:YLF laser
No full textA Yb:YLF crystal has been investigated in a femtosecond oscillator pumped by two 400 mW single-mode fiber-coupled diodes emitting at 976 nm and mode locked with a semiconductor saturable absorber mirror. Almost Fourier transform-limited pulses with durations of 87 and 107 fs were demonstrated for extraordinary and ordinary polarizations, respectively. This is, to the best of our knowledge, the first demonstration of sub-100 fs pulses with Yb:YLF, and it proves the potential for ultrashort pulse generation and amplification with this material
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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