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1
Hempler, Nils. « Semiconductor disk laser pumped Cr²⁺:chalcogenide lasers ». Thesis, University of Strathclyde, 2010. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=15339.
Texte intégral
2
Zhang, Tao. « High power disk laser cutting ». Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609511.
Texte intégral
3
Butkus, Mantas. « Quantum dot based semiconductor disk lasers ». Thesis, University of Dundee, 2012. https://discovery.dundee.ac.uk/en/studentTheses/6b17df24-a721-4904-b49f-e35055990c16.
Texte intégralRésumé :
Since its first successful demonstration more than five decades ago [1], laser technology experienced a huge leap forward in terms of technological innovations and the understanding of underlying physical principles of operation. There were many efforts made by those in both the scientific and commercial communities who envisioned the potential of lasers. As a result, the laser now is a powerful scientific tool in many disciplines. It is widely used not only in physics, but also in chemistry, biology, medicine, engineering, environmental sciences, arts and their interdisciplinary fields. Moreover, it now has a vast number of applications in industry and everyday life whether it is light and matter interaction, communication and IT, healthcare and many other uses of this light source. By the same time, photonics comprises a market of multi-billion EUR value [2].At every stage of development, different laser parameters were engineered to suit those to specific application with some other parameters usually being sacrificed. Together with this, things like compactness and cost were always an issue to consider. A huge impact to the field of photonics was made by the development of semiconductor based structures that could be used as a light amplifying medium. Semiconductor lasers not only allowed the miniaturization of many devices, but also provided new opportunities for laser scientists due to ability to engineer their bandgap properties and to confine the carriers in different dimensions.The development of vertical external cavity surface emitting lasers (VECSELs), which are also known as optically pumped semiconductor lasers (OPSLs) or semiconductor disk lasers (SDLs) realized an important feature in semiconductor based lasers – high multi- Watt output power was combined with diffraction limited output beam profile.This work is devoted to the development of semiconductor disk lasers based on novel quantum dot (QD) structures. QD structures were embedded in this type of laser recently and allowed a number of advantages compared with the widely used quantum well (QW) structure. These included new spectral region coverage at 1-1.3 µm, enhanced wavelength tuneability and ultrafast carrier dynamics, which potentially will improve mode locked operation. QDs were also used as a base for semiconductor saturable absorbers in modelocking experiments.During the time of these studies, QD SDLs at new spectral regions and record output power were demonstrated. Power scaling up to 6 W was achieved for 1040 nm, 2.25 W for 1180 nm and 1.6 W for 1260 nm devices. Excited state transition in QDs was shown to be more efficient for high power QD SDLs as compared with ground state transition. New spectral regions were covered by QD SDLs using frequency doubling into the visible region with green, orange and red light emission with output powers of 2 W, 2.5 W and 0.34 W respectively. The broad gain bandwidth of the quantum dot material was explored and wavelength tuneability up to 60 nm around 1040 nm, 69 nm around 1180 nm, and 25 nm around 1260 nm was demonstrated.A QD based saturable absorber was used to mode-lock the quantum well SDL, resulting in the first such type of laser with sub-picosecond pulse widths. Pulses with duration of 870 fs at a repetition rate of 896 MHz and wavelength of 1028.5 nm were demonstrated. Pulses were 1.14 times Fourier limited and an average output power of 46 mW was achieved. Finally, quantum well based VECSELs with electrical pumping schemes were tested. The devices were first tested in the cw configuration. Highest output powers up to 60 mW were achieved from such devices. Devices were then tested in mode-locking experiments. Pulsed operation was observed and the measurements indicated 270 ps width pulses with 8 mW average output power at 1.9 GHz repetition rate. All devices operated at 980 nm.This thesis consists of six chapters. In the introductory part of this work, QD based SDLs and their development and applications will be reviewed together with their operational principles. Chapter two will describe the growth, fabrication and preparation of SDL samples. Continuous wave and mode-locked operation results will be presented in chapters three and four. Electrically pumped devices will be presented in chapter five along with experimental results. Conclusions and future prospects will be given at the end of this work. The list of publications which were generated during the studies is included at the beginning of this work.The work presented in thesis was done under the FAST-DOT project. This is a European FP 7 project targeted at the development of compact and low-cost novel quantum dot based laser sources for biophotonic applications.
4
Mastrocinque, Ernesto. « Laser welding of Ti6Al4V alloy by disk laser : analysis and optimization ». Doctoral thesis, Universita degli studi di Salerno, 2012. http://hdl.handle.net/10556/352.
Texte intégralRésumé :
2010 - 2011Titanium alloys have been successfully applied in many industrial fields because of their better performance and lighter weight than other commonly used structural materials. The conventional welding methods used for titanium alloys are tungsten inert gas (TIG) and plasma arc welding. In recent decades, autogenous processes with highly concentrated energy sources have become popular; these joining processes are laser and electron-beam welding. The power source can be concentrated in very small areas so as to achieve energy densities up to 10,000 times higher than those of the arc processes. Laser welding allows joints to be made with limited distortion. The fullyautomated process, ensures high productivity and high-quality joints. Laser technology is acquiring industrial interest because the electron-beam processes have limitations, such as the need to operate in vacuum, the increased costs and the emission of X-rays. Titanium alloys are widely used in the aircraft industry, because of their high strength-to-weight ratio, corrosion resistance, operating temperature and bonding with composite materials (electrochemical compatibility, similar coefficients of thermal expansion). The criteria for the design, manufacture and operation were changed to obtain structures that are lighter and more efficient than the ones made of aluminum. However, the structures in carbonfiber- reinforced-polymer require the use of metal structures, especially in areas of great concentration of loads. In spite of several advantages, these alloys lead to excessive manufacturing costs related to the cost of the raw materials, the high volumes of waste and the complex and expensive finishing. For these reasons, it is cheaper to produce semi-finished products by welding simpler parts, instead of casting and forming processes; therefore, laser welding can be used due to its high productivity and quality end-products. The aim of the thesis work is to find the better input process parameters values to weld 3 mm and 1 mm Ti6Al4V sheets using a 2 kW Yb:YAG disk laser. Both bead on plate and butt tests have been performed, and the beads quality is characterized in terms of geometric features, porosity content, microstructure, hardness and strength. This work is organized in five chapters. Chapter 1 discusses the principles of operation and the different types of laser including disk laser, used in the experimental part. Chapter 2 presents the properties of titanium and its alloys, highlighting the various fields of application. Chapter 3 presents a review of the different technologies used for welding of titanium alloys, focusing primarily on laser welding and its mechanisms. Chapter 4 describes the titanium alloy, equipment and methodologies used in the experimental work. Finally, Chapter 5 presents the results obtained. [edited by author]
X n.s.
5
Rodriguez-Valls, Omar. « Characterization and Modeling of a High Power Thin Disk Laster ». Master's thesis, University of Central Florida, 2010. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2099.
Texte intégralRésumé :
High power lasers have been adapted to material processing, energy, military and medical applications. In the Laser Plasma Laboratory at CREOL, UCF, high power lasers are used to produce highly ionized plasmas to generate EUV emission. This thesis examines the quality of a recently acquired high power thin disk laser through thermal modeling and beam parameter measurements. High power lasers suffer from thermally induced issues which degrade their operation. Thin disk lasers use an innovative heat extraction mechanism that eliminates the transverse thermal gradient within the gain medium associated with thermal lensing. A thorough review of current thin disk laser technology is described. Several measurement techniques were performed on a high power thin disk laser. The system efficiencies, spectrum, and temporal characteristics were examined. The laser was characterized in the far-field regime to determine the beam quality and intensity of the laser. Laser cavity simulations of the thin disk laser were performed using LASCAD. The induced thermal and stress effects are demonstrated. Simulated output power and efficiency is compared to those that have been quantified experimentally.M.S.E.E.
School of Electrical Engineering and Computer Science
Engineering and Computer Science
Electrical Engineering MSEE
6
Baker, Caleb W., et Caleb W. Baker. « Practical Design and Applications of Ultrafast Semiconductor Disk Lasers ». Diss., The University of Arizona, 2017. http://hdl.handle.net/10150/625900.
Texte intégralRésumé :
Vertical External Cavity Surface Emitting Lasers (VECSELs) have become well established in recent years for their design flexibility and promising power scalability. Recent efforts in VECSEL development have focused heavily on expanding the medium into the ultrafast regime of modelocked operation. Presented in this thesis is a detailed discussion regarding the development of ultrafast VECSEL devices. Achievements in continuous wave (CW) operation will be highlighted, followed by several chapters detailing the engineering challenges and design solutions which enable modelocked operation of VECSELs in the ultrafast regime, including the design of the saturable absorbers used to enforce modelocking, management of the net group delay dispersion (GDD) inside the cavity, and the design of the active region to support pulse durations on the order of 100 fs. Work involving specific applications - VECSELs emitting on multiple wavelengths simultaneously and the use of VECSEL seed oscillators for amplification and spectral broadening - will also be presented.
Key experimental results will include a novel multi-fold cavity design that produced record-setting peak powers of 6.3 kW from a modelocked VECSEL, an octave-spanning supercontinuum with an average power of 2 W generated using a VECSEL seed and a 2-stage Yb fiber amplifier, and two separate experiments where a VECSEL was made to emit on multiple wavelengths simultaneously in modelocked and highly stable CW operation, respectively. Further, many diagnostic and characterization measurements will be presented, most notably the in-situ probing of a VECSEL gain medium during stable modelocked operation with temporal resolution on the order of 100 fs, but also including characterization of the relaxation rates in different saturable absorber designs and the effectiveness of different methods for managing the net GDD of a device.
7
Scheller, Maik, Caleb W. Baker, Stephan W. Koch et Jerome V. Moloney. « Dual-Wavelength Passively Mode-Locked Semiconductor Disk Laser ». IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2016. http://hdl.handle.net/10150/621738.
Texte intégralRésumé :
A dual-wavelength mode-locked semiconductor
vertical-external-cavity-surface-emitting laser is demonstrated.
A semiconductor saturable absorber mirror allows for simultaneous
mode locking of pulses centered at two center wavelengths
with variable frequency spacing. The difference-frequency control
is achieved with an intracavity etalon. Changing the finesse of
the etalon enables the adjustment of the pulse duration between
6 and 35 ps. The emitted two-color pulses are modulated by a
beat frequency in the terahertz range. Self-starting mode-locking
with 0.8-W average output power is demonstrated.
8
Alfieri, Vittorio. « Disk laser welding of metal alloys for aerospace ». Doctoral thesis, Universita degli studi di Salerno, 2013. http://hdl.handle.net/10556/857.
Texte intégralRésumé :
2011 - 2012Laser welding is the logical processing solution to accomplish different needs. Improvements at the design stage are actually aimed to remove any mechanical fastening, thus moving towards a technology which would not increase the joint thickness; moreover, a number of benefits in comparison with conventional welding methods are provided when considering laser beams, since deep penetration is achieved and the energy is effectively used where needed, thus melting the interface to be joined rather than excessively heating up the base metal, which would suffer from thermal distortion and degradation of metallurgical properties otherwise. Further advantages are achieved in laser welding with thin disk sources, since high output power, high efficiency and good beam quality are simultaneously delivered, unlike traditional laser systems; costs are significantly reduced in comparison with lamp-pumped laser systems. As a consequence, specific interest is shown in aerospace where strict specifications apply. Nevertheless, a number of issues must be addressed, depending on the material to be welded, as many variables and sub processes concerning fusion and vaporization are involved in laser welding and a delicate balance between heating and cooling is in place within a spatially localized volume. Therefore, extensive studies are required to manage both the stability and the reproducibility of the overall process, before introducing any change in industrial environments. Methods, experimental results and discussions concerning laser welding of common metal alloys for aerospace are provided in this Ph.D. thesis. A general view of applications and basic advantages of laser welding is first given, with mention to diagnostics and safety. Hence, the principles of laser emission are examined, with respect to the architecture of the sources, beam geometry, quality and efficiency, in order to better portray the benefits of a thin disk laser concept. Processing dynamics of laser welding are explained afterward, referring to conduction and key-hole mode, instability, gas supply and leading governing parameters such as laser power, welding speed, defocusing and beam angle to be considered in the experimental work. Procedures are provided for proper bead characterization, from preliminary examinations including non destructive tests such as fluorescent penetrant inspections and radiographic tests, to sample preparation and eventual mechanical assessment in terms of tensile strength and Vickers micro hardness in the fused zone. A straightforward description of the design of experiment approach and the response surface methodology is given, so to introduce the testing method to be taken, as well as the steps for data elaboration via statistical tools. Hence, four case studies about metal aerospace alloys are presented and discussed in their common seam configuration: autogenous butt and overlapping welding of aluminum alloy 2024; autogenous butt welding of titanium alloy Ti-6Al-4V; dissimilar butt welding of Haynes 188 and Inconel 718; dissimilar overlapping welding of Hastelloy X and René 80. All of the welding tests were conducted at the Department of Industrial Engineering at the University of Salerno; a Trumpf Tru-Disk 2002 Yb:YAG disk-laser source with a BEO D70 focusing optics, moved by an ABB IRB 2004/16 robot was employed. When needed, additional tests for the purpose of specific bead characterization were conducted by Avio and Europea Microfusioni Aerospaziali. As general procedure for each topic, the operating ranges to be examined are found via preliminary trials in combination with the existing literature on the subject. Then, special consideration is given to the processing set-up, the resulting bead profile, possible imperfections, defects and overall features; consistent constraint criteria for optimization of the responses are chosen on a case-by-case basis depending on materials and seam geometry and referring to international standards as well as customer specifications for quality compliance. Optimal combinations of the input welding parameters for actual industrial applications are eventually suggested, based on statistical tools of analysis. Convincing reasons are provided to give grounds to improvements in real applications. Moreover, based on the results, a proper device for bead shielding, to be conveniently adjusted depending on both geometry and materials to be welded has been designed, produced and patented (SA2012A000016). As concerning aluminum welding, a comprehensive description is given for laserrelated issues: reflectivity and thermal conductivity influence on the material response is illustrated; the porosity evolution is discussed with respect to thermal input and defocusing; a theory for softening in the fused zone is provided through energy dispersive spectrometry and estimations of magnesium content in the crosssection. Optimization is performed for butt configuration of 1.25 mm thick sheets; the discussion about the interactions among the governing factors is deepen with reference to overlapping welding. With respect to titanium welding, optimization is performed for 3 mm thick butt welding; the resulting micro structure in the weld is discussed since it is thought to be closely related to the mechanical properties. In particular, special care is taken of the grain size as a function of the governing factors. Dissimilar welding of super alloys is considered for gas turbine components; for this specific purpose, laser welding is expected to offer a valid alternative to arc and electron beam welding, whose weaknesses are pointed out. Given their actual application in the engine, Haynes 188 and Inconel 718 are examined in butt welding configuration, whilst an overlapping geometry is preferred for Hastelloy X and René 80. Considerable tolerances are matched, thus promoting the suggested range of the operating variables. [edited by author]
XI n.s.
9
Sickinger, Daniel. « Development of a Thulium Germanate Thin Disk Laser Prototype ». Thesis, The University of Arizona, 2016. http://hdl.handle.net/10150/613444.
Texte intégralRésumé :
A Thulium Germanate thin disk laser prototype is developed and its potential applications are discussed. Unfortunately, the thin disk gain material for the CW prototype was unable to lase due to thermal limitations within the disk. However, a CW output power model and a physical pump chamber module have been developed, along with the supporting Zemax models and alignment procedures so other gain materials and future improvements can be tested.
10
Innerhofer, Edith. « High average power Yb:YAG thin disk laser and its application for an RGB laser source / ». Zürich, 2005. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=16108.
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11
Marchese, Sergio Vincenzo. « Towards high field physics with high power thin disk laser oscillators ». Konstanz Hartung-Gorre, 2007. http://d-nb.info/987877879/04.
Texte intégral
12
Willis-Ott, Christina. « Ytterbium-doped fiber-seeded thin-disk master oscillator power amplifier laser system ». Doctoral diss., University of Central Florida, 2013. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/5890.
Texte intégralRésumé :
Lasers which operate at both high average power and energy are in demand for a wide range of applications such as materials processing, directed energy and EUV generation. Presented in this dissertation is a high-power 1 ?m ytterbium-based hybrid laser system with temporally tailored pulse shaping capability and up to 62 mJ pulses, with the expectation the system can scale to higher pulse energies. This hybrid system consists of a low power fiber seed and pre-amplifier, and a solid state thin-disk regenerative amplifier. This system has been designed to generate high power temporally tailored pulses on the nanosecond time scale. Temporal tailoring and spectral control are performed in the low power fiber portion of the system with the high pulse energy being generated in the regenerative amplifier. The seed system consists of a 1030 nm fiber-coupled diode, which is transmitted through a Mach-Zehnder-type modulator in order to temporally vary the pulse shape. Typical pulses are 20-30 ns in duration and have energies of ~0.2 nJ from the modulator. These are amplified in a fiber pre-amplifier stage to ~100 nJ before being used to seed the free-space Yb:YAG thin-disk regenerative amplifier. Output pulses have maximum demonstrated pulse energies of 62 mJ with 20 ns pulse after ~250 passes in the cavity. The effects of thermal distortion in laser and passive optical materials are also. Generally the development of high power and high energy lasers is limited by thermal management strategies, as thermally-induced distortions can degrade laser performance and potentially cause catastrophic damage. Novel materials, such as optical ceramics, can be used to mitigate thermal distortions; however, thorough analysis is required to optimize their fabrication and minimize thermal distortions. Using a Shack-Hartmann wavefront sensor (SHWFS), it is possible to analyze the distortion induced in passive and doped optical elements by high power lasers. For example, the thin-disk used in the regenerative amplifier is examined in-situ during CW operation (up to 2 kW CW pump power). Additionally, passive oxide-based optical materials and Yb:YAG optical ceramics are also examined by pumping at 2 and 1 ?m respectively to induce thermal distortions which are analyzed with the SHWFS. This method has been developed as a diagnostic for the relative assessment of material quality, and to grade differences in ceramic laser materials associated with differences in manufacturing processes and/or the presence of impurities. In summation, this dissertation presents a high energy 1 ?m laser system which is novel in its combination of energy level and temporal tailoring, and an analysis of thermal distortions relevant to the development of high power laser systems.Ph.D.
Doctorate
Optics and Photonics
Optics and Photonics
Optics
13
Wolters, Ulrike [Verfasser]. « Inversion Dependent Losses in Yb:YAG and their Effects on Thin-Disk Laser Operation / Ulrike Wolters ». Aachen : Shaker, 2014. http://d-nb.info/1053903987/34.
Texte intégral
14
Haq, Mohammad Ashraful. « Continuum and molecular dynamics analyses of lubricant evaporation and flow due to laser heating in heat-assisted magnetic recording ». University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1531140850318054.
Texte intégral
15
Schättiger, Farina [Verfasser]. « Characterization and optimization of mode-locking of a high-power Yb:YAG thin-disk laser / Farina Schättiger ». Konstanz : Bibliothek der Universität Konstanz, 2014. http://d-nb.info/1099293677/34.
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16
Deppe, Bastian [Verfasser], et Christian [Akademischer Betreuer] Kränkel. « High-intracavity-power thin-disk laser for the alignment of molecules / Bastian Deppe ; Betreuer : Christian Kränkel ». Hamburg : Staats- und Universitätsbibliothek Hamburg, 2019. http://d-nb.info/1186891238/34.
Texte intégral
17
Kahle, Hermann [Verfasser]. « AlGaInP-based high-performance semiconductor disk lasers for the red spectral range : Gain-chip design, harmonic generation and a new laser concept / Hermann Kahle ». München : Verlag Dr. Hut, 2017. http://d-nb.info/1126296317/34.
Texte intégral
18
Prinz, Stephan [Verfasser], Reinhard [Akademischer Betreuer] Kienberger, Reinhard [Gutachter] Kienberger et Franz [Gutachter] Pfeiffer. « Thin-disk laser pumped high-energy few-cycle OPCPA / Stephan Prinz ; Gutachter : Reinhard Kienberger, Franz Pfeiffer ; Betreuer : Reinhard Kienberger ». München : Universitätsbibliothek der TU München, 2018. http://d-nb.info/1173322760/34.
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19
Yeh, Wei-Hung. « Interaction of the focused laser beam with the grooved surface of optical disk : Evanescent coupling and vector diffraction effects ». Diss., The University of Arizona, 1999. http://hdl.handle.net/10150/284281.
Texte intégralRésumé :
The primary objective of this dissertation is to present a clear physical picture and useful insights of polarization effects in the diffraction of focused beams by grooved, multilayer-coated disks. The reading process of optical disk systems significantly relies on the reaction of the incident focused beam to the disk structure, may it be the groove profile or coating materials. The resulting complex-amplitude from diffraction is the main source for the readout signal. In the presence of the periodic pattern and the focused beam, however, different polarization states usually result in different complex-amplitudes. A good understanding of polarization effects in grooved multilayer disks is thus required for the optimum design of optical data storage systems. The pursuit of high-density recording inevitably drives the optical data storage industry to reduce the wavelength of light sources, decrease the track pitch of optical disks, and increase the numerical aperture of objective lenses. The track pitch and the size of the focused spot gradually approach the optical wavelength. Under these circumstances, the analysis of the interaction of focused beams with this type of high-frequency periodic disk using conventional scalar diffraction theory is no longer adequate. Only through vector diffraction study of polarization effects in the interaction of the focused beam with the periodic pattern can the characteristics of an optical disk system be fully understood and improved. Starting from the introduction of various polarization effects in optical disk systems and basic concepts of both scalar and vector diffraction theory, we then focus on the studies of diffraction patterns at the exit pupil of the objective lens and on the disk surface. Different behavior on the baseball pattern and in the effective groove depth is observed for the two polarization states. The use of the solid immersion lens to extensively increase the area density of optical disk systems prompts us to investigate the influence of evanescent-wave coupling on the near-field optical disk system. Finally, we study the feasibility of using a novel differential polarization microscope to reduce polarization effects and to increase the image contrast of magnetic domains on magneto-optical disks.
20
Weber, Michael A. « An Investigation of the Feasibility of Microscale Adaptive Passive Vibration Neutralizers ». Thesis, Virginia Tech, 2001. http://hdl.handle.net/10919/32296.
Texte intégralRésumé :
This thesis concerns the control of an adaptive passive vibration neutralizer and the feasibility of miniaturizing this type of tunable vibration neutralizer for small-scale applications.
An analytical model for the adaptive passive vibration neutralizer is derived and compared to experimental results. A tuning algorithm is derived from a curve-fit of experimental tests on the specific neutralizer. A more generic tuning algorithm is also developed, which does not require testing of the neutralizer for optimal control. Both tuning algorithms are tested using a chirp forcing function to simulate drift in the excitation frequency of a host structure. Computer simulation and experimental results are given for these tests.
A novel low-cost, small-scale vibration neutralizer is constructed from packing bubble-wrap. Analytical models for the stiffness are calculated, and experimental data is used to derive a damped mass-spring model.
Miniaturization of vibration neutralizers is described, and many of the pitfalls in design are discussed. Theoretical tuning frequencies of possible adaptive passive vibration neutralizers at different scales are included. The goal for these miniaturized vibration neutralizers is vibration control in computer hard drives.
A hard drive is analyzed for vibration problems. Included are plots of the velocities of the read-write head and spindle. Limitations of the measurement equipment are discussed, and directions for future work on small-scale tunable vibration neutralizers are outlined.
Master of Science
21
Salhi, Mohamed. « Spectroscopie et caractérisation laser de cristaux massifs et de couches minces cristallines de fluorures dopés terres rares autour de 2um ». Thesis, Normandie, 2017. http://www.theses.fr/2017NORMC270/document.
Texte intégralRésumé :
Depuis les années 1970, le développement des sources lasers de puissance fonctionnant autour de 2 µm basées sur des cristaux dopés Thulium Tm3+ et/ou Holmium Ho3+est un domaine de recherche actif du fait de ses multiples applications. L’émission autour de 1.9 µm correspond à une transition du niveau excité 3F4 vers le niveau fondamental 3H6 dans un ion trivalent Tm3+, alors que l’émission à 2 µm et au-delà correspond à la transition entre le niveau 5I7 et le niveau fondamental 5I8 de l’ion Ho3+. Ces transitions laser font aujourd’hui l’objet de nombreuses applications dans domaines très variés. nous avons étudié les propriétés spectroscopiques de cristaux de fluorures dopés terres rares en vue d’une émission autour de 2 µm. À partir des spectres d’absorption obtenus, l’analyse de Judd-Ofelt a permis de calculer les durées de vie radiatives ainsi que les rapports de branchement dans les ions Tm3+ et Ho3+. En comparaison avec d’autres cristaux tels que les oxydes, les durées de vie des niveau 3F4(Tm3+) et 5I7(Ho3+) dans les cristaux de fluorures sont plus longues ce qui favorise un fonctionnement laser en régime impulsionnel. Une étude sur la dynamique de fluorescence a été réalisée afin de caractériser l’émission autour de 1.9µm de l’ion Tm3+ et celle autour de 2µm de l’ion Ho3+. Les spectres d’émission obtenus offrent de larges bandes, assez structurées dans la matrice LiYF4 et plus larges et moins structurées dans la matrice CaF2. Dans un second temps, en mesurant les durées de vie du niveau émetteur 3F4 pour différentes concentrations en ions Tm3+, nous avons calculé les paramètres de transferts. Les résultats obtenus ont montré que le processus de relaxation croisée est beaucoup plus efficace dans le cristal CaF2 que dans le LiYF4. Nous avons calculé également les coefficients de transfert et de transfert inverse entre les ions Tm3+ et Ho3+ suite à une excitation de l’ion Tm3+. Nous avons montré que la matrice LiYF4 offre une meilleure efficacité de transfert en vue d’une émission autour de 2µm. Le fonctionnement laser de couches minces en configuration « disque mince » a été étudié. Une première démonstration d’un laser en disque mince LiYF4 : Tm émettant à 1.9 µm réalisé à partir d’une couche épitaxiée par LPE a été faite. En simple passge, une puissance de sortie de 306 mW et une efficacité laser de 36% en puissance absorbée sont atteintes pour une couche de 240 μm d'épaisseur.nous avons étudié un oscillateur laser en guide d’onde LiYF4 :Tm en géométrie planaire et linéaire. Ces guides sont caractérisés par de faibles pertes de propagation grâce à la qualité du procédé de fabrication. Une puissance maximale de 470 mW et un rendement de 14% en puissance incidente ont été obtenus. Une modélisation numérique de l’amplification optique montre que le guidage des faisceaux pompe et sonde permet d’atteindre des gains importants. Le modèle est ensuite adapté pour le régime de saturation afin de calculer le rendement d’extraction et de déterminer le potentiel des guides LiYF4 :Tm3+ en tant comme amplificateur autour de 1.9 µmSince the 1970s, the development of power laser sources operating around 2 μm based on Thulium Tm3 + and / or Holmium Ho3 + doped crystals is an active field of research because of its multiple applications. The emission around 1.9 μm corresponds to a transition from the excited level 3F4 to the fundamental level 3H6 in a trivalent ion Tm3 +, whereas the emission at 2 μm and beyond corresponds to the transition between the level 5I7 and the fundamental level 5I8 of the Ho3 + ion. These laser transitions are today the subject of many applications in a wide variety of fields. We have studied the spectroscopic properties of rare earth doped fluoride crystals for emission around 2 μm. From the absorption spectra obtained, the Judd-Ofelt analysis made it possible to calculate the radiative lifetimes as well as the branching ratios in the Tm3 + and Ho3 + ions. In comparison with other crystals such as oxides, the lifetimes of the 3F4 (Tm3 +) and 5I7 (Ho3 +) levels in the fluoride crystals are longer, which favors pulse-mode laser operation. A study on fluorescence dynamics was carried out in order to characterize the emission around 1.9 μm of the Tm3 + ion and that around 2 μm of the Ho3 + ion. The emission spectra obtained have broad bands, fairly structured in the LiYF4 matrix and wider and less structured in the CaF2 matrix. In a second step, by measuring the lifetimes of the 3F4 emitter level for different Tm3 + ion concentrations, we calculated the transfer parameters. The results obtained showed that the process of cross relaxation is much more efficient in the CaF2 crystal than in LiYF4. We also calculated the transfer and inverse transfer coefficients between Tm3 + and Ho3 + ions following excitation of the Tm3 + ion. We have shown that the LiYF4 matrix offers a better transfer efficiency with a view to emission around 2 μm. The laser operation of thin layers in "thin disc" configuration has been studied. A first demonstration of a thin-film LiYF4: Tm laser emitting at 1.9 μm made from an epitaxial layer with LPE was made. In single passge, an output power of 306 mW and a laser efficiency of 36% in absorbed power are reached for a 240 μm thick layer. We have studied a laser oscillator in LiYF4: Tm waveguide in planar geometry. and linear. These guides are characterized by low propagation losses thanks to the quality of the manufacturing process. A maximum power of 470 mW and a yield of 14% in incident power were obtained. A numerical modeling of the optical amplification shows that the guidance of the pump and probe beams makes it possible to achieve significant gains. The model is then adapted for the saturation regime in order to calculate the extraction efficiency and to determine the potential of the LiYF4: Tm3 + guides as an amplifier around 1.9 μm
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Aleknavičius, Aidas. « Investigation of composite laser active elements with thin doped layers ». Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2013. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2013~D_20130919_161151-74601.
Texte intégralRésumé :
The aim of this PhD thesis was to investigate the concept and application abilities of a proposed composite laser element finding factors that limit performance of such laser element. In this work a composite laser element was proposed which combines decent properties of various laser elements. Nevertheless, experiments showed that such element has no advantage over a widely used rod type active laser elements. In order to explain the nature of induced optical aberrations, theoretical models describing optical, thermo-optical and thermo-mechanical properties of active medium were developed and applied. An original method to rate induced optical aberrations which allow unambiguous comparison of different aberrations was presented. A simple model to calculate the variation of intensity distribution as the beam propagates was presented also. All models were confirmed experimentally. Using developed models it was found that the main cause of optical aberrations in the proposed element is the undoped medium between thin doped layers. Generally, abilities of proposed active laser elements are limited by thermal effects in the undoped medium.Disertacijos darbo tikslas buvo, modeliuojant bei vykdant eksperimentus, išanalizuoti siūlomo sudėtinio aktyvaus lazerio elemento koncepcijos prielaidas ir taikymų galimybes, bei surasti veiksnius, ribojančius pasiūlyto sudėtinio aktyvaus lazerio elemento galimybes. Šiame darbe buvo pasiūlytas lazerio aktyvusis elementas, kuris apjungia įvairių tipų lazerinių aktyviųjų elementų gerąsias savybes. Tačiau eksperimentai parodė, jog toks sudėtinis elementas neturi pranašumo prieš plačiai naudojamus strypo formos lazerinius aktyviuosius elementus. Siekiant paaiškinti susidarančių optinių iškraipymų priežastis, buvo sukurti ir pritaikyti teoriniai modeliai, aprašantys aktyvios terpės optines, termo-optines bei termo-mechanines savybes. Sukurtas optinių iškraipymų vertinimo metodas, leidžiantis palyginti skirtingų konfigūracijų elementus šiluminių reiškinių sąlygotų optinių iškraipymų atžvilgiu. Taip pat pateiktas paprastas modelis, gerai aprašantis iš daugiamodžio šviesolaidžio išėjusios kaupinimo spinduliuotės intensyvumo skirstinio kitimą. Visi pateikti teoriniai modeliai buvo patvirtinti eksperimentiniais matavimais. Naudojantis sukurtais teoriniais modeliais buvo nustatyta, jog pagrindinė optinių iškraipymų priežastis siūlomame sudėtiniame lazeriniame elemente yra nelegiruota terpė tarp plonų legiruotų sluoksnių, kurioje vykstantys šiluminiai reiškiniai riboja tokio lazerinio aktyvaus elemento galimybes.
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Olafsson, Jonas. « Antimicrobial Susceptibility Testing Directly from Urine Samples : a Comparison between Standardised and Direct Disk Diffusion Testing together with Direct Species Identification using Matrix Assisted Laser Desorption/Ionisation Time of Flight ». Thesis, Linnéuniversitetet, Institutionen för naturvetenskap, NV, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-27645.
Texte intégralRésumé :
Urinary tract infection (UTI) is a very common infection in humans and a majority is caused by Escherichia coli. UTI are commonly treated empirically. However, empiric treatment has become more problematic due to increased antibiotic resistance to commonly used antibiotic agents. It is therefore desirable with short turnover times for antimicrobial susceptibility testing and species identification to improve antibiotic treatment at an early stage. Matrix Assisted Laser Desorption/Ionisation Time of Flight (MALDI-TOF) can provide species identification faster than former routine methods. This study compared direct and standard susceptibility testing using disk diffusion on Enterobacteriaceae (EB) from urine samples. The possibility to standardise the inoculum for direct susceptibility testing via a pellet obtained by a series of centrifugations was also evaluated, as well as direct species identification with MALDI-TOF from the pellet. Results from direct susceptibility testing from urine samples with EB, performed either directly from the urine or with a standardised inoculum, correlated well to those obtained with standardised susceptibility testing using EUCAST disk diffusion methodology with few errors, of which most were associated with Proteus mirabilis. The concept of standardising the inoculum for direct susceptibility testing to 0.5 McFarland was labour intensive and did not improve the results further. However, direct species identification from the urine pellet using MALDI-TOF showed good correlation to routine identification. Of 238 samples, an EB was correctly identified in 148 samples using MALDI-TOF.
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Parrotta, Daniele Carmine. « Raman lasers intracavity-pumped by semiconductor disk lasers ». Thesis, University of Strathclyde, 2012. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=24895.
Texte intégralRésumé :
Semiconductor disk lasers (SDLs) are efficient and commercially attractive devices as they can emit several Watts in continuous-wave (cw) operation, with good beam quality, low noise and great wavelength flexibility. SDL fundamental emission ranges from red to mid-infrared, while ultraviolet and other visible wavelength can be obtained via harmonic generation. This research shows that Raman conversion is an efficient way to extend the spectral coverage of well-established SDLs. The first experimental work consisted in the demonstration of a KGW Raman laser intracavity-pumped by a 1055 nm InGaAs SDL for laser emission at ~1.14 µm. This work represents the first Raman conversion of an SDL ever reported. Output power up to 0.8 W, broad wavelength tunability and cascaded Raman conversion (with low output coupling) were observed. The following experiment consisted in using synthetic single-crystal diamond as the Raman medium. In the last few years diamond has become a prominent Raman crystal as it provides high Raman gain, large Stokes shift and unrivalled thermal conductivity. A diamond Raman laser intracavity-pumped by an InGaAs SDL emitted up to 4.4 W at 1228 nm and was tuned from 1209-1256 nm. With an optical conversion efficiency exceeding 14%, this laser rivals the optical efficiencies of other cw Raman lasers and, perhaps more importantly, SDLs with direct emission at ~1.2 µm. Orange emission, with maximum output power of 1.5 W at 614 nm and wavelength tuning from 604.5-619.5 nm, was obtained via intracavity second harmonic generation in the Raman laser cavity. Raman conversion of a red-emitting GaInP SDL using a synthetic diamond crystal is also reported. As GaInP SDLs are less efficient than InGaAs SDLs, Raman threshold was more difficult to achieve, despite the Raman gain increasing for decreasing wavelengths. Nonetheless, Raman conversion in the deep red was observed, with output power of few tens of ~W, due to the low output coupling for the Raman laser, and tunable emission from 738-748 nm. This work is still at an early stage, so higher output power and enhanced laser efficiency may be achieved in future experiments.
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Hönninger, Clemens. « Ultrafast ytterbium-doped bulk lasers and laser amplifiers / ». [S.l.] : [s.n.], 1998. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=12952.
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Vetter, Sharon Louise. « Novel applications of semiconductor disk lasers ». Thesis, University of Strathclyde, 2011. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=16777.
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Imayama, Shintaro. « Studies of the rotating-disk boundary-layer flow ». Doctoral thesis, KTH, Strömningsfysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-158973.
Texte intégralRésumé :
The rotating-disk boundary layer is not only a simpler model for the study of cross-flow instability than swept-wing boundary layers but also a useful simplification of many industrial-flow applications where rotating configurations are present. For the rotating disk, it has been suggested that a local absolute instability, leading to a global instability, is responsible for the small variation in the observed laminar-turbulent transition Reynolds number however the exact nature of the transition is still not fully understood. This thesis aims to clarify certain aspects of the transition process. Furthermore, the thesis considers the turbulent rotating-disk boundary layer, as an example of a class of three-dimensional turbulent boundary-layer flows. The rotating-disk boundary layer has been investigated in an experimental apparatus designed for low vibration levels and with a polished glass disk that gave a smooth surface. The apparatus provided a low-disturbance environment and velocity measurements of the azimuthal component were made with a single hot-wire probe. A new way to present data in the form of a probability density function (PDF) map of the azimuthal fluctuation velocity, which gives clear insights into the laminar-turbulent transition region, has been proposed. Measurements performed with various disk-edge conditions and edge Reynolds numbers showed that neither of these conditions a↵ect the transition process significantly, and the Reynolds number for the onset of transition was observed to be highly reproducible. Laminar-turbulent transition for a ‘clean’ disk was compared with that for a disk with roughness elements located upstream of the critical Reynolds number for absolute instability. This showed that, even with minute surface roughness elements, strong convectively unstable stationary disturbances were excited. In this case, breakdown of the flow occurred before reaching the absolutely unstable region, i.e. through a convectively unstable route. For the rough disk, the breakdown location was shown to depend on the amplitude of individual stationary vortices. In contrast, for the smooth (clean-disk) condition, the amplitude of the stationary vortices did not fix the breakdown location, which instead was fixed by a well-defined Reynolds number. Furthermore, for the clean-disk case, travelling disturbances have been observed at the onset of nonlinearity, and the associated disturbance profile is in good agreement with the eigenfunction of the critical absolute instability. Finally, the turbulent boundary layer on the rotating disk has been investigated. The azimuthal friction velocity was directly measured from the azimuthal velocity profile in the viscous sublayer and the velocity statistics, normalized by the inner scale, are presented. The characteristics of this three-dimensional turbulent boundary-layer flow have been compared with those for the two-dimensional flow over a flat plate and close to the wall they are found to be quite similar but with rather large differences in the outer region.QC 20150119
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Turnbull, Andrew. « Design and characterisation of ultrafast semiconductor disk lasers ». Thesis, University of Southampton, 2015. https://eprints.soton.ac.uk/381283/.
Texte intégralRésumé :
In this thesis I present my work on improving the design of semiconductor gain and saturable absorber structures towards the goal of a Watt-level, sub-200 fs mode-locked vertical-external cavity surface-emitting laser (ML-VECSEL). The performance of MLVECSELs has increased significantly in recent years with sub-500 fs pulse durations demonstrated at Watt-level average output powers. However, ML-VECSELs with sub-200 fs pulse durations have only been reported with milliwatt average output powers. Absence of detailed knowledge of the dynamic response of both gain and saturable absorber structures makes it di�fficult to develop new designs for improved laser performance. It is, therefore, critical to fully characterise the macroscopic parameters of the semiconductor laser structures used in ML-VECSELs. Here, I present the characterisation of ultrafast surface recombination semiconductor saturable absorbers, with a design that has given the shortest pulse durations from a ML-VECSEL to date. I demonstrate a ML-VECSEL utilising a cavity design based on the extracted absorber parameters that would be suitable for high average mode-locked output powers using surface-recombination absorbers. An understanding of fundamental mode-locking mechanisms present in ML-VECSELs is vital for optimising structures as pulse durations approach 100 fs. The evolution of the spectral components present from lasing onset and during pulse formation in a ML-VECSEL is measured; identifying three distinct regimes in the spectral evolution. I interpret the measured transients to extract a value for pulse-shortening per cavity round-trip, allowing a qualitative description of pulse formation in ML-VECSELs to be developed. I present construction and characterisation of a ML-VECSEL containing new "shortmicrocavity" gain structures that have demonstrated FWHM pulse durations of 193 fs with 1 KW peak output powers; a benchmark result for peak output power from a ML-VECSEL with sub-200 fs pulse duration.
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Narvaez, Javier A. « Thermal Conductivity of Poly-Alpha-Olefin (PAO)-Based Nanofluids ». University of Dayton / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1282162148.
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Gabalis, Martynas. « Šiluminių-optinių reiškinių plono disko ir plokščioje kompozitinėje YAG lazerio aktyviojoje terpėje modeliavimas ». Master's thesis, Lithuanian Academic Libraries Network (LABT), 2011. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2011~D_20110224_090151-48870.
Texte intégralRésumé :
Pirmoje šio darbo dalyje pristatomi temperatūros pasiskirstymo aktyviojoje lazerio terpėje kompiuterinio modeliavimo rezultatai. Skaičiavimai atlikti dviem aktyviosios terpės geometrijoms – plono disko ir plokščiojo kompozitinio elemento. Apžvelgiama skirtingų aktyvios terpės ir kaupinimo parametrų įtaka temperatūros pasiskirstymui. Antroje darbo dalyje pristatomi lazerio pluošto bangos fronto iškraipymų, atsirandančių dėl nevienodo temperatūros pasiskirstymo aktyvioje terpėje ir paviršiaus deformacijų, skaičiavimo rezultatai.This work presents results of numerical simulation of temperature distribution inside laser active medium. Calculations were performed for two different active medium geometries. First one thin disk geometry and second - composite slab. Influence of different pump and active medium parameters on temperature distribution are presented. Second part of this work presents results of wave front aberrations calculations. Those aberrations were resulted by irregular temperature distribution and surface deformations.
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Imayama, Shintaro. « Experimental study of the rotating-disk boundary-layer flow ». Licentiate thesis, KTH, Mekanik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-95147.
Texte intégralRésumé :
Rotating-disk flow has been investigated not only as a simple model of cross flow instability to compare with swept-wing flow but also for industrial flow applications with rotating configurations. However the exact nature of laminar-turbulent transi- tion on the rotating-disk flow is still major problem and further research is required for it to be fully understood, in particular, the laminar-turbulent transition process with absolute instability. In addition the studies of the rotating-disk turbulent boundary- layer flow are inadequate to understand the physics of three-dimensional turbulent boundary-layer flow. In present thesis, a rotating-rotating disk boundary-layer flow has been inves- tigated experimentally using hot-wire anemometry. A glass disk with a flat surface has been prepared to archieve low disturbance rotating-disk environment. Azimuthal velocity measurements using a hot-wire probe have been taken for various conditions. To get a better insight into the laminar-turbulent transition region, a new way to describe the process is proposed using the probability density function (PDF) map of azimuthal fluctuation velocity. The effect of the edge of the disk on the laminar-turbulent transition process has been investigated. The disturbance growth of azimuthal fluctuation velocity as a function of Reynolds number has a similar trend irrespective of the various edge conditions. The behaviour of secondary instability and turbulent breakdown has been in- vestigated. It has been found that the kinked azimuthal velocity associated with secondary instability just before turbulent breakdown became less apparent at a cer- tain wall normal heights. Furthermore the turbulent breakdown of the stationary mode seems not to be triggered by its amplitude, however, depend on the appearance of the travelling secondary instability. Finally, the turbulent boundary layer on a rotating disk has been investigated. An azimuthal friction velocity has been directly measured from the azimuthal velocity profile in the viscous sub-layer. The turbulent statistics normalized by the inner and outer sclaes are presented.QC 20120529
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MacLean, Alexander James. « Power scaling and wavelength control of semiconductor disk lasers ». Thesis, University of Strathclyde, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487871.
Texte intégralRésumé :
The semiconductor disk laser (SDL) takes advantage of the potential for band-gap engineering in semiconductor multilayers to give a wavelength flexible laser source; the free-space cavity not only gives the freedom of design to achieve good beam quality and power scaling, but also to insert wavelength control elements into the cavity for spectral narrowing and tuning, and for second harmonic generation (SHG). Thermal modelling using finite element analysis (FEA) was used to compare the different thermal management techniques used to dissipate the heat generated by the optical pumping process, which ultimately limits the output power achievable in these devices: an intracavity diamond heatspreader was chosen as the most appropriate technique where wavelength diversity is required.
33
Iorio, Giuliano. « The thickness of the gaseous layer in disk galaxies ». Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7411/.
Texte intégralRésumé :
il profilo verticale del gas dipende sia dal potenziale galattico, e quindi dalla distribuzione di materia, sia dalla dispersione di velocit`a del gas
La nostra analisi teorica ha mostrato che l’altezza scala dell’idrogeno gassoso ad un certo raggio `e fortemente legata alla componente di massa dominate: nelle parti centrali, dove domina la materia barionica, il disco gassoso incrementa il suo spessore esponenzialmente con una lunghezza scala pari al doppio di quella del disco stellare; dalle zone intermedie fino alla fine del disco, il profilo verticale del gas `e influenzato dalla distribuzione di DM. In queste zone lo spessore del disco gassoso cresce con il raggio in maniera circa lineare. Tale fenomeno viene definito gas flaring.
Lo scopo principale di questa tesi `e la ricerca di una nuova tecnica per poter stimare il profilo vertical del gas in galassie con inclinazione intermedia.
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Appelquist, Ellinor. « The rotating-disk boundary-layer flow studied through numerical simulations ». Doctoral thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-200827.
Texte intégralRésumé :
This thesis deals with the instabilities of the incompressible boundary-layer flow thatis induced by a disk rotating in otherwise still fluid. The results presented include bothwork in the linear and nonlinear regime and are derived from direct numerical sim-ulations (DNS). Comparisons are made both to theoretical and experimental resultsproviding new insights into the transition route to turbulence. The simulation codeNek5000 has been chosen for the DNS using a spectral-element method (SEM) witha high-order discretization, and the results were obtained through large-scale paral-lel simulations. The known similarity solution of the Navier–Stokes equations for therotating-disk flow, also called the von K ́arm ́an rotating-disk flow, is reproduced by theDNS. With the addition of modelled small simulated roughnesses on the disk surface,convective instabilities appear and data from the linear region in the DNS are anal-ysed and compared with experimental and theoretical data, all corresponding verywell. A theoretical analysis is also presented using a local linear-stability approach,where two stability solvers have been developed based on earlier work. Furthermore,the impulse response of the rotating-disk boundary layer is investigated using DNS.The local response is known to be absolutely unstable and the global response, onthe contrary, is stable if the edge of the disk is assumed to be at radius infinity. Herecomparisons with a finite domain using various boundary conditions give a globalbehaviour that can be both linearly stable and unstable, however always nonlinearlyunstable. The global frequency of the flow is found to be determined by the Rey-nolds number at the confinement of the domain, either by the edge (linear case) or bythe turbulence appearance (nonlinear case). Moreover, secondary instabilities on topof the convective instabilities induced by roughness elements were investigated andfound to be globally unstable. This behaviour agrees well with the experimental flowand acts at a smaller radial distance than the primary global instability. The sharpline corresponding to transition to turbulence seen in experiments of the rotating diskcan thus be explained by the secondary global instability. Finally, turbulence datawere compared with experiments and investigated thoroughly.QC 20170203
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Appelquist, Ellinor. « Direct numerical simulations of the rotating-disk boundary-layer flow ». Licentiate thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-146087.
Texte intégralRésumé :
This thesis deals with the instabilities of the incompressible boundary-layer flow that is induced by a disk rotating in otherwise still fluid. The results presented are mostly limited to linear instabilities derived from direct numerical simulations (DNS) but with the objective that further work will focus on the nonlinear regime, providing greater insights into the transition route to turbulence. The numerical code Nek5000 has been chosen for the DNS using a spectral-element method in an effort to reduce spurious effects from low-order discretizations. Large-scale parallel simulations have been used to obtain the present results. The known similarity solution of the Navier–Stokes equation for the rotating-disk flow, also called the von Karman flow, is investigated and can be reproduced with good accuracy by the DNS. With the addition of small roughnesses on the disk surface, convective instabilities appear and data from the DNS are analysed and compared with experimental and theoretical data. A theoretical analysis is also presented using a local linear-stability approach, where two stability solvers have been developedbased on earlier work. A good correspondence between DNS and theory is found and the DNS results are found to explain well the behaviour of the experimental boundary layer within the range of Reynolds numbers for small amplitude (linear) disturbances. The comparison between the DNS and experimental results, presented for the first time here, shows that the DNS allows (for large azimuthal domains) a range of unstable azimuthal wavenumbers β to exist simultaneously with the dominantβ varying, which is not accounted for in local theory, where β is usually fixed for each Reynolds number at which the stability analysis is applied. Furthermore, the linear impulse response of the rotating-disk boundary layer is investigated using DNS. The local response is known to be absolutely unstable. The global response is found to be stable if the edge of the disk is assumed to be at infinity, and unstable if the domain is finite and the edge of the domain is placed such that there is a large enough pocket region for the absolute instability to develop. The global frequency of the flow is found to be determined by the edge Reynolds number.QC 20140708
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Ozkan, Musa. « Boundary layer transition over rotating disks ». Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/87170/.
Texte intégralRésumé :
This thesis summarizes results of a combined numerical and experimental study investigating the effects of surface roughness, and of the geometry of the ow domain (confinement) on the boundary{layer transition over rotating disks. Numerically, a three{dimensional enclosed cavity ow in a rotor{stator flow configuration is simulated. The effects induced by surface roughness of the rotor disk and the effects induced by the stator geometry enclosing the ow domain are investigated. The steady{state velocity pro les of the boundary{layer ow on the rotating disk are obtained, subjected to a linear stability analysis and compared to relevant data from the literature. Experimentally, the ow over rotating disks is studied for smooth disk surfaces and for disks with concentric grooves representing distributed roughness. The disks are mounted submerged inside a water {filled tank. Due to the surrounding perimeter wall of the tank and the liquid surface this arrangement resembles the classic rotor{stator flow configuration. Comprehensive measurements of the boundary{layer ow and its laminar{turbulent transition were performed by means of an hot{ lm anemometer. The computational results suggest that, for the rotor{stator ow investigated, the roughness{induced effects are very similar to the geometry{induced effects, both in nature and magnitude. This suggests that it may be di cult to distinguish between both effects in experiments where the ow domain is restricted. Nevertheless, in comparison to previous hot{ lm measurements employing the same experimental facility, the data of the current study have been significantly improved by means of introducing a new calibration technique. The new experimental data discussed here confirm recent theoretical results of our research group in that they corroborate that an increase in the roughness level can reduce the number of stationary vortices and also stabilize the Type{I (cross{ ow) instability mode. However, the detailed analysis of the experimental data, in comparison to the theoretically predicted magnitude of the roughness{induced and the geometry{induced effects, reveal that future studies would greatly bene t from the availability of a new air{based rotating{disk apparatus.
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Aleknavičius, Aidas. « Sudėtinių aktyvių lazerinių elementų su plonais legiruotais sluoksniais tyrimas ». Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2013. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2013~D_20130919_161139-43417.
Texte intégralRésumé :
Disertacijos darbo tikslas buvo, modeliuojant bei vykdant eksperimentus, išanalizuoti siūlomo sudėtinio aktyvaus lazerio elemento koncepcijos prielaidas ir taikymų galimybes, bei surasti veiksnius, ribojančius pasiūlyto sudėtinio aktyvaus lazerio elemento galimybes. Šiame darbe buvo pasiūlytas lazerio aktyvusis elementas, kuris apjungia įvairių tipų lazerinių aktyviųjų elementų gerąsias savybes. Tačiau eksperimentai parodė, jog toks sudėtinis elementas neturi pranašumo prieš plačiai naudojamus strypo formos lazerinius aktyviuosius elementus. Siekiant paaiškinti susidarančių optinių iškraipymų priežastis, buvo sukurti ir pritaikyti teoriniai modeliai, aprašantys aktyvios terpės optines, termo-optines bei termo-mechanines savybes. Sukurtas optinių iškraipymų vertinimo metodas, leidžiantis palyginti skirtingų konfigūracijų elementus šiluminių reiškinių sąlygotų optinių iškraipymų atžvilgiu. Taip pat pateiktas paprastas modelis, gerai aprašantis iš daugiamodžio šviesolaidžio išėjusios kaupinimo spinduliuotės intensyvumo skirstinio kitimą. Visi pateikti teoriniai modeliai buvo patvirtinti eksperimentiniais matavimais. Naudojantis sukurtais teoriniais modeliais buvo nustatyta, jog pagrindinė optinių iškraipymų priežastis siūlomame sudėtiniame lazeriniame elemente yra nelegiruota terpė tarp plonų legiruotų sluoksnių, kurioje vykstantys šiluminiai reiškiniai riboja tokio lazerinio aktyvaus elemento galimybes.The aim of this PhD thesis was to investigate the concept and application abilities of a proposed composite laser element finding factors that limit performance of such laser element. In this work a composite laser element was proposed which combines decent properties of various laser elements. Nevertheless, experiments showed that such element has no advantage over a widely used rod type active laser elements. In order to explain the nature of induced optical aberrations, theoretical models describing optical, thermo-optical and thermo-mechanical properties of active medium were developed and applied. An original method to rate induced optical aberrations which allow unambiguous comparison of different aberrations was presented. A simple model to calculate the variation of intensity distribution as the beam propagates was presented also. All models were confirmed experimentally. Using developed models it was found that the main cause of optical aberrations in the proposed element is the undoped medium between thin doped layers. Generally, abilities of proposed active laser elements are limited by thermal effects in the undoped medium.
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Brunken, Marco. « Entwicklung eines Autokorrelators zur Messung der Mikropulsdauer und Aufbau eines Er:YAG-Lasers zur Verstärkung der Mikropulse am Freie-Elektronen-Laser ». [S.l.] : [s.n.], 2002. http://elib.tu-darmstadt.de/diss/000239.
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Rye, Ryan Patrick. « Investigation of disc brake squeal via sound intensity and laser vibrometry ». Thesis, Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/19045.
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Lingwood, R. J. « Stability and transition of the boundary layer on a rotating disk ». Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307947.
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Chen-Sverre, Theo. « Ultrafast mode-locked semiconductor disk lasers for frequency domain measurement systems ». Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/417999/.
Texte intégralRésumé :
Ultrafast pulses can now be generated in mode-locked vertical external-cavity surfaceemitting lasers (ML-VECSELs) at high powers and GHz repetition rates. This laser platform has also demonstrated the addition of intracavity elements and flexibility of cavity geometries. In this thesis, we report the demonstration of two versions of repetition rate tunable VECSELs. The first, produced 215-fs pulse trains of up to 20 mW for a continuously tunable repetition rate between 0.880-1.880 GHz, directly emitted by the VECSEL. The second, produced 300-fs pulses at average powers of 50 mW, and able to tune the pulse repetition rate from 0.960-1.720 GHz. These VECSELs pave the way to performing frequency domain analysis in the GHz by resonantly exciting a sample under study. Here we have designed time-domain and frequency-domain experiments to characterise the acoustic behaviour of mechanically responsive nanostructures. The VECSEL also provides a stable platform for fs pulse generation, with a high Q-factor cavity making it possible to harness the large intracavity power. In this thesis we describe the first demonstration of a semiconductor saturable absorbing mirror (SESAM) coated with a patterned mask addition of an intracavity photoconductive antenna (PCA).
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Rodriguez-Garcia, Julio M. « Tunable, continuous-wave semiconductor disk lasers with emission in the deep ultraviolet ». Thesis, University of Strathclyde, 2017. http://digitool.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=30464.
Texte intégralRésumé :
The work here presented is focused around the design, characterization and ultimately first demonstration of a deep ultraviolet, frequency tripled semiconductor disk laser (SDL). The construction of such a laser is described, together with a review of the relevant theory and investigation of the underlying processes in order to improve the system. SDLs based on gallium indium phosphide have attracted intense investigation over the last ten years for their fundamental emission in the 660nm-690nm (visible red emission), a region previously only available to SDLs by means of nonlinear frequency conversion. Such frequency conversion applied to red SDLs provides access to the highly energetic ultraviolet region of the electromagnetic spectum but, although its conversion to the near ultraviolet has been rather successful, further extension into the deep UV presents a new set of challenges that must be addressed in order to achieve efficient laser emission. 78 μW of deep ultraviolet emission at 225nm have been achieved in continuous-wave operation. The output wavelength is tunable for 350 cm-1. This is the shortest wavelength emitted from an SDL to date and is the first implementation of intracavity frequency tripling in a visible SDL.
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Combrinck, Madeleine Lelon. « Boundary layer response to arbitrary accelerating flow ». Thesis, University of Pretoria, 2016. http://hdl.handle.net/2263/61287.
Texte intégralRésumé :
This thesis was aimed developing a fundamental understanding of the boundary layer response
to arbitrary motion. In this context arbitrary motion was defined as the unsteady translation and
rotation of an object.
Research objectives were developed from the gaps in knowledge as defined during the literature survey.
The objectives were divided into three main activities; mathematical formulations for non-inertial
bulk flow and boundary layer equations, implementation of said formulations in a numerical solver and
simulations for various applications in arbitrary motion.
Mathematical formulations were developed for the bulk flow and boundary layer equations in arbitrary
motion. It was shown that the conservation of momentum and energy equations remains invariant
in the non-inertial forms. The conservations of momentum equation can at most have six fictitious terms
for unsteady arbitrary motion. The origin of the terms were found to be from transformation of the material
derivative to the non-inertial frame. All fictitious terms were found to be present in the boundary
layer equations, none could be eliminated during an order of magnitude analysis.
The vector form of the non-inertial equations were implemented in a novel OpenFOAM solver. The
non-inertial solver requires prescribed motion input and operate on a stationary mesh. Validation of the
solver was done using analytical solutions of a steady, laminar flat plate and rotating disk respectively.
Numerical simulation were done for laminar flow on a translating plate, rotating disk and rotating
cone in axial flow. A test matrix was executed to investigated various cases of acceleration and deceleration
over a range of 70 g to 700 000g. The boundary layer profiles, boundary layer parameters and
skin friction coefficients were reported.
Three types of boundary layer responses to arbitrary motion were defined. Response Type I is viscous
dominant and mimics the steady state velocity profile. In Response Type II certain regions of the
boundary layer are dominated by viscosity and others by momentum. Response Type III is dominated
by momentum. In acceleration the near-wall velocity gradient increases with increasing acceleration. In
deceleration separation occurs at a result of momentum changes in the flow.
The mechanism that causes these responses have been identified using the developed boundary layer
equations. In acceleration the relative frame fictitious terms become a momentum source which results
in an increase in velocity gradient at the wall. In deceleration the relative frame fictitious terms become
a momentum sink that induced an adverse pressure gradient and subsequently laminar separation.Hierdie tesis is gerig op die ontwikkeling van 'n fundamentele begrip aangaande die grenslaag reaksie op arbitrêre beweging. In hierdie konteks word arbitrêre beweging gedefinieer as die ongestadigde translasie en rotasie van 'n voorwerp. Navorsingsdoelwitte is ontwikkel uit die gapings soos omskryf in die literatuuroorsig. Die doelwitte is verdeel in drie hoof aktiwiteite; wiskundige formulerings vir ongestadigde vloei en grenslaag vergelykings, implementering van hierdie formulerings in 'n numeriese kode en simulasies vir verskeie gevalle van arbitrêre beweging. Wiskundige formulerings is ontwikkel vir die vloei en grenslaag vergelykings in arbitrêre beweging. Daar is bewys dat die behoud van massa en energie vergelykings onveranderd in die nie-inertiële vorms bly. Die behoud van momentum vergelyking kan hoogstens ses fiktiewe terme vir ongestadigde, arbitrêre beweging hê. Die oorsprong van die terme is vanuit die transformasie van die ongestadigde en adveksie terme (aan die linker kant van die momentum vergelyking) na die nie-inertiële raam. Alle fiktiewe terme is teenwoordig in die grenslaag vergelykings. Die vektor vorm van die nie-inertiële vergelykings is in 'n nuwe OpenFOAM oplosser geïmplementeer. Die nie-inertiële oplosser vereis voorgeskrewe beweging insette en werk op 'n stilstaande rooster. Die oplosser is getoets teen analitiese oplossings van 'n gestadigde, laminêre plaat plaat en 'n roterende skyf, onderskeidelik. Numeriese simulasies is gedoen vir laminêre vloei op 'n translerende plaat, roterende skyf en roterende konus in aksiale vloei. 'n Toets matriks is gebruik om ondersoek in te stel na gevalle van versnelling en vertraging oor 'n verskeidenheid van 70 g tot 700 000 g. Die grenslaag profiele, grenslaag parameters en oppervlak wrywingskoëffisiënte is aangemeld nie. Drie tipes grenslaag reaksies op arbitrêre beweging is gedefinieer. Reaksie Tipe I is viskeus dominant en boots die bestendige snelheidsprofiel na. In reaksie Tipe II sekere dele van die grenslaag is oorheers deur viskositeit en ander deur momentum. Reaksie Tipe III word in totaliteit oorheers deur momentum. In versnelling die snelheid helling teen die objek neem toe met toenemende versnelling. In vertraging is 'n negatiewe snelheidsprofiel waargeneem as gevolg van momentum veranderinge in die vloei. Die meganisme wat hierdie reaksies veroorsaak is geïdentifiseer deur die grenslaag vergelykings. In versnelling word die fiktiewe terme 'n bron van momentum. Dit lei tot 'n toename in snelheid helling op die objek. In vertraging word die fiktiewe terme 'n momentum gebruiker wat 'n negatiewe drukgradiënt veroorsaak en gevolglik laminêre vloei wegbreking veroorsaak.
Thesis (PhD)--University of Pretoria, 2016.
Mechanical and Aeronautical Engineering
PhD
Unrestricted
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Al, Nakdali Dalia [Verfasser], et Martin [Akademischer Betreuer] Koch. « High-Power Operation of Semiconductor Disk Lasers / Dalia Al Nakdali. Betreuer : Martin Koch ». Marburg : Philipps-Universität Marburg, 2015. http://d-nb.info/1081215615/34.
Texte intégral
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Lomenzo, Richard Allan Jr. « Static Misalignment Effects is a Self-Tracking Laser Vibrometry System for Rotating Bladed Disks ». Diss., Virginia Tech, 1998. http://hdl.handle.net/10919/29398.
Texte intégralRésumé :
The application of laser Doppler vibrometry to high speed rotating structures has been hampered by technical limitations. Whereas full-field three-dimensional velocity measurements can be made on stationary structures, the capability on rotating structures is limited to low speed, one-dimensional, steady state operation. This work describes the implementation of a self-tracking laser vibrometry system which overcomes many of the limitations of current techniques for vibration measurements on rotating structures. A model of the self-tracker is developed and used to predict the effects of static misalignments on the position and velocity errors. These predictions are supported by experimental results and simplified models of the self-tracker.
NOTE: (02/2011) An updated copy of this ETD was added after there were patron reports of problems with the file.Ph. D.
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Guha, Biswarup. « Surface-enhanced optomechanical disk resonators and force sensing ». Thesis, Sorbonne Paris Cité, 2017. http://www.theses.fr/2017USPCC154/document.
Texte intégralRésumé :
L'optomécanique est la science des interactions entre la lumière et les mouvements mécaniques. Ce rapport de thèse décrit des expériences réalisées avec des microdisques fabriqué dans différents résonateurs semi-conducteurs III-V: l'Arséniure de Gallium (GaAs), l'Arséniure d'Aluminium Gallium (AlGaAs) et l'Arséniure d'Indium Phosphide (InGaP). Ces matériaux sont compatibles avec les fonctionnalités de l’optoélectronique et procurent un couplage optomécanique géant. Pour améliorer les performances des résonateurs en GaAs, nous avons développé des méthodes de traitement de surface permettant de réduire la dissipation optique par un facteur dix et ainsi d'atteindre un facteur de qualité de six millions. En plus de ces études sur le GaAs, nous avons réalisés une étude comparative des interactions optomecaniques dans des microdisques d'InGaP et d'AlGaAs, et nous avons mis en évidences leurs résonances optomécaniques. Finalement, nous avons réalisé des mesures de force avec des résonateurs en GaAs, démontrant un nouveau principe de détection basé sur notre étude de leur la trajectoire dans l'espace de phase et leur bruit de phaseOptomechanics studies the interaction between light and mechanical motion. This PhD thesis reports on optomechanical experiments carried with miniature disk resonators fabricated out of distinct III-V semiconductors: Gallium Arsenide (GaAs), Aluminium Gallium Arsenide (AlGaAs) and Indium Gallium Phosphide (InGaP). These materials are compliant with optoelectronics functionalities and provide giant optomechanical coupling. In order to boost performances of GaAs resonators, we implemented surface control techniques and obtained a ten-fold reduction of optical dissipation, attaining a Q of six million. On top of GaAs, we performed a comparative investigation of optomechanical interactions in InGaP and AlGaAs disk resonators, and demonstrated their operation as optomechanical oscillators. Finally, we carried out optomechanical force sensing experiments with GaAs resonators, analyzing a new sensing principle in light of the phase space trajectory and phase noise of the corresponding oscillators
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Kahle, Michael. « Laser-Carburisierung von Eisen und Edelstahl ». Doctoral thesis, [S.l.] : [s.n.], 2006. http://webdoc.sub.gwdg.de/diss/2006/kahle.
Texte intégral
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De, Beer N., et A. I. Odendaal. « The geometrical accuracy of a custom artificial intervertebral disc implant manufactured using Computed Tomography and Direct Metal Laser Sintering ». Journal for New Generation Sciences, Vol 10, Issue 3 : Central University of Technology, Free State, Bloemfontein, 2012. http://hdl.handle.net/11462/613.
Texte intégralRésumé :
Published ArticleRapid Manufacturing (RM) has emerged over the past few years as a potential technology to successfully produce patient-specific implants for maxilla/facial and cranial reconstructive surgeries. However, in the area of spinal implants, customization has not yet come to the forefront and with growing capabilities in both software and manufacturing technologies, these opportunities need to be investigated and developed wherever possible. The possibility of using Computed Tomography (CT) and Rapid Manufacturing (RM) technologies to design and manufacture a customized, patient-specific intervertebral implant, is investigated. Customized implants could aid in the efforts to reduce the risk of implant subsidence, which is a concern with existing standard implants. This article investigates how accurately the geometry of a customized artificial intervertebral disc (CAID) can represent the inverse geometry of a patient's vertebral endplates. The results indicate that the endplates of a customized disc implant can be manufactured to a calculated average error of 0.01mm within a confidence interval of 0.022mm, with 95% confidence, when using Direct Metal Laser Sintering.
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Al-Sheikh, Mayss [Verfasser], Burkhard [Gutachter] Dick et Mike P. [Gutachter] Holzer. « Evaluation der Sicherheit und Effektivität der Kataraktoperation unter Verwendung des Femtosekunden Lasers Catalys Precision Laser System / Mayss Al-Sheikh ; Gutachter : Burkhard Dick, Mike P. Holzer ». Bochum : Ruhr-Universität Bochum, 2014. http://d-nb.info/1133361471/34.
Texte intégral
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Fredrich-Thornton, Susanne T. [Verfasser]. « Nonlinear Losses in Single Crystalline and Ceramic Yb:YAG Thin-Disk Lasers / Susanne T. Fredrich-Thornton ». Aachen : Shaker, 2011. http://d-nb.info/1070152757/34.
Texte intégral