Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Lasers are commonly used in the industry for various applications such as laser cutting, laser drilling, lithography, medical applications, surface cleaning and a myriad of other applications. In any application of a laser the beam properties are significant. In the paint removal application discussed in this thesis, the beam properties of the laser beam can have a large impact on the efficiency of the paint removal process. The pulse energy or the average output power of the laser is normally an important parameter in laser materials processing applications. The spatial profile or intensity distribution of the beam also has an influence on the process. The propagation of the laser beam from the laser to the working point is also significant in applying the laser beam to the material. In the ideal scenario one would like to control all the parameters of the laser in terms of the output, in energy or output power, the propagation of the laser beam and the intensity distribution of the beam. The process of laser-based paint removal is no different to this. In this process a TEA CO2 laser is used for the ablation of paint from a substrate. In this application high pulse energy is required from the laser together with good beam propagation properties for delivery of the beam over a long distance. For this application the multimode beam of the TEA CO2 laser can be applied for the paint removal. The multimode beam has sufficiently high pulse energy for the paint removal process, but is not suitable for propagating over long distances through a beam path with a finite aperture. Furthermore the multimode beam does not have a uniform energy intensity distribution. It would therefore be ideal if the TEA CO2 laser could be designed with a custom beam that has a uniform intensity distribution, high pulse energy and good beam propagation. These requirements lead to the study of flattened irradiance profile laser beams. In this thesis flattened irradiance profile beams in the form of Flattened Gaussian beams are investigated. The theory of the Flattened Gaussian profile as well as the propagation of the beam is investigated. Furthermore the generation of such a beam internally to the laser resonator is studied. In succession to this a custom laser resonator was designed and implemented on the TEA CO2 laser. The resulting Flattened Gaussian Beam was characterised and applied to the application of laser paint removal. It was finally shown that the Flattened Gaussian Beam could be successfully generated and applied with equal success in the application of laser paint removal. / AFRIKAANSE OPSOMMING: Lasers word algemeen in die industrie gebruik vir toepassings soos laser snywerk, laser boorwerk, litografie, mediese toepassings, oppervlakreiniging en verskeie ander. In enige toepassing van 'n laser is die eienskappe van die laserbundel van groot belang vir die proses. In die verf verwydering toepassing wat bespreek word in hierdie tesis het die bundel eienskappe 'n groot invloed op die effektiwiteit van die verf stropings proses. Die pulsenergie of uitset drywing van die laser is gewoonlik 'n belangrike parameter in 'n materiaalverwerkings toepassing. Die ruimtelike profiel of energie intensiteitprofiel van die bundel het ook 'n invloed op die proses. Die voortplanting van die bundel vanaf die laser na die werkspunt het ook 'n beduidende invloed op die toepassing van die laserbundel op die materiaal. In die ideale geval sal mens graag al die parameters van die laserbundel soos pulsenergie of drywing, die bundel voortplanting en energie intensiteitprofiel wil beheer. Die toepassing van die laser vir verfverwydering vereis ook die beheer van hierdie unieke parameters wat reeds genoem is. In hierdie proses is 'n TEA CO2 laser gebruik vir die verwydering van verf van 'n substraat. Die toepassing vereis hoë pulsenergie saam met goeie bundel voortplantingseienskappe vir lewering van die bundel oor lang afstande. Die multimode bundel van die laser kan gebruik word vir hierdie toepassing. Die multimode bundel bevat genoegsame energie vir die verfstropings proses maar is nie geskik vir voortplanting oor lang afstande deur 'n bundelpad wat 'n beperking op die bundel grootte het nie. Verder het die multimode bundel ook nie 'n uniforme energie intensiteitprofiel nie. Dit sou ideal wees as die TEA CO2 laser toegerus kon word met 'n toepassingsgerigte bundel wat hoë puls energie, goeie bundel voortplanting en 'n uniforme intensiteitprofiel het. Hierdie vereiste het gelei tot die studie van laserbundels met 'n uniforme plat energie intensiteitprofiel. In hierdie tesis word plat intensiteit bundels in die vorm van plat Gaussiese bundels ondersoek. Die teorie van plat Gaussiese bundels sowel as die voortplanting van hierdie bundels word hier ondersoek. Verder word die opwekking van hierdie bundels intern tot die laserresonator ook ondersoek. Na die ondersoek is daar oorgegaan in die ontwerp en implementering van 'n doelgemaakte resonator op 'n TEA CO2 laser. Die resonator het 'n plat Gaussiese bundel as uitset gelewer. Die bundel was gevolglik gekarakteriseer en aangewend in 'n verfstropings toepassing. Ten einde is daar getoon dat 'n plat Gaussiese bundel suksesvol opgewek en toegepas kon word.

Deformation of a Gaussian beam is observed when it propagates through a plasma. Self-focusing of the beam may be observed when the intensity of the laser increases the index of refraction of plasma gas. Due to the difficulties in solving the nonlinear partial differential equation in Maxwell's wave equation, a numerical technique has been developed in favor of the traditional analytical method. Result of numerical solution shows consistency with the analytical method. This further suggests the validity of the numerical technique employed. A three dimensional graphics package was used to depict the numerical data obtained from the calculation. Plots from the data further show the deformation of the Gaussian beam as it propagates through the plasma gas.

Ocean-bottom seismometers (OBS) allow wider angle recording and therefore, they have the potential to significantly enhance imaging of deep subsurface structures. Currently, conventional OBS data analysis still uses first arrival traveltime tomography and prestack Kirchhoff depth migration method. However, using first arrival traveltimes to build a velocity model has its limitations. In the Taiwan region, subduction and collision cause very complex subsurface structures and generate extensive basalt-like anomalies. Since the velocity beneath basalt-like anomalies is lower than that of high velocity anomalies, no first-arrival refractions for the target areas occur. Thus, conventional traveltime tomography is not accurate and amplitude constrained traveltime tomography can be dangerous. Here, a new first-arrival stochastic tomography method for automatic background velocity estimation is proposed. Our method uses the local beam semblance of each common-shot or common-receiver gathers instead of first-arrival picking. Both the ray parameter and traveltime information are utilized. The use of Very Fast Simulated Annealing (VFSA) method also allows for easier implementation of the uncertainty analysis. Synthetic and real data benchmark tests demonstrate that this new method is robust, efficient, and accurate. In addition, migrated images of low-fold data or data with limited observation geometry like OBS are often corrupted by migration aliasing. Incorporation of prestack instantaneous-slowness information into the imaging condition can significantly reduce migration artifacts and noise and improve the image quality in areas of poor illumination. Here I combine slowness information with Gaussian beam depth migration and implement a new slowness driven Gaussian beam prestack depth migration. The prestack instantaneous slowness information, denoted by ray parameter gathers p(x,t), is extracted from the original OBS or shot gathers using local slant stacking and subsequent localsemblance analysis. In migration, we propagate both the seismic energy and the principal instantaneous slowness information backward. At a specific image location, the beam summation is localized in the resolution-dependent Fresnel zone where the instantaneousslowness-information-related weights are used to control the beams. The effectiveness of the new method is illustrated using two synthetic data examples: a simple model and a more realistic complicated sub-basalt model. / text

Orientadores: Stefano De Leo, Luis Eduardo Evangelista de Araújo / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-27T16:22:41Z (GMT). No. of bitstreams: 1 Araujo_ManoelPedrode_D.pdf: 1955745 bytes, checksum: d09312f15b1d058162aef061fcfc0682 (MD5) Previous issue date: 2015 / Resumo: Esta tese apresenta uma análise sobre o deslocamento de Goos-Hänchen, o desvio angular da lei de Snell e o efeito de interferência entre feixes ópticos gaussianos. Em nosso estudo o deslocamento de Goos-Hänchen foi obtido por meio do método da fase estacionária. No regime de incidência crítica, tal deslocamento apresenta uma forte dependência com a largura do feixe, em contraste com as expressões clássicas de Artmann, que predizem um deslocamento infinito. Também na incidência crítica, observamos que, dependendo da magnitude da largura da cintura do feixe, ocorre uma quebra de simetria na distribuição de momento. A maximização da quebra de simetria leva ao desvio angular da lei de Snell. Mostramos como reproduzir a máxima quebra de simetria por uma estrutura dielétrica. Como resultado, obtivemos uma nova fórmula analítica para o desvio angular. Ademais, foi possível estimar o deslocamento de Goos-Hänchen por meio do efeito de interferência entre feixes. Nesta análise, observamos que, na incidência crítica, a estimativa usada na literatura para o deslocamento de Goos-Hänchen não é válida. Portanto, uma nova fórmula foi introduzida para estimar tal deslocamento / Abstract: This thesis presents some of the main phenomena associated with Goos-Hanchen shift, the angular deviation of the Snell¿s law and the interference effect among Gaussian optical beams. In our study the Goos-Hänchen shift was obtained by using the stationary phase method. In the case of incidence at critical angle, such displacement shows a strong dependence on the beam width in contrast with the classical expressions of Artmann, which predict an infinite displacement. Also in the critical incidence we observed that, depending on the magnitude of the beam waist, there is a symmetry breaking in the momentum distribution. The maximization of symmetry breaking leads to the angular deviations of the Snell¿s law. In this analysis we showed how to maximize this breaking of the symmetry by a dielectric structure. As a result, we obtained an analytical formula to the Snell¿s law angular deviation. Furthermore, we could estimate the displacement of Goos- Hänchen through the interference effect among beams. The results of this analysis reveal that, for the critical incidence, the estimative used in the literature for the Goss-Hänchen shift is not valid. Therefore, a new formula was introduced to estimate such displacement. / Doutorado / Física / Doutor em Ciências

Cette thèse porte sur la conception, la réalisation et l'étude expérimentale d'une source laser bifréquence de haute cohérence émettant à 1 µm en vue d'obtenir par photomélange un émetteur THz. Nous nous intéressons plus particulièrement aux lasers à semiconducteur émettant par la surface en cavité externe verticale (VeCSEL), l'objectif étant d'obtenir un fonctionnement laser bifréquence robuste en régime continu, basé sur la coexistence simultanée de deux modes transverses de Laguerre-Gauss. La sélection de seulement deux modes transverses est réalisée grâce à des masques de pertes insérés intracavité dans le plan transverses. Les caractéristiques du laser bifréquence, telles que l'équilibre entre les puissances des deux modes, le caractère monofréquence de chacun des deux modes, l'accordabilité de l'écart de fréquence, la simultanéité de l'émission et la cohérence du battement THz obtenu, sont étudiées. Enfin, la génération THz par photomélange est effectuée grâce au VeCSEL bifréquence et à une photodiode UTC commerciale. / This work focuses on the design, realization and experimental study of highly coherent dual-frequency laser sources emitting at 1 µm for THz radiation generation by photomixing. We are particularly interested in vertical-external-cavity surface-emitting laser (VeCSEL), the aim being to obtain a robust dual-frequency continuous wave operation, based on simultaneous coexistence of two Laguerre-Gaussian transverse modes. We design intracavity transverse selective losses mask to select only the two Laguerre-Gaussian modes. The stable and simultaneous dual-frequency operation, the beat-frequency tunability range and the temporal coherence was specifically studied. We demonstrated THz emission by seeding a uni-travelling-carrier photodiode by an optically-pumped dual-frequency vertical-external-cavity surface-emitting.

We study numerical methods for time-dependent partial differential equations describing wave propagation, primarily applied to problems in quantum dynamics governed by the time-dependent Schrödinger equation (TDSE). We consider both methods for spatial approximation and for time stepping. In most settings, numerical solution of the TDSE is more challenging than solving a hyperbolic wave equation. This is mainly because the dispersion relation of the TDSE makes it very sensitive to dispersion error, and infers a stringent time step restriction for standard explicit time stepping schemes. The TDSE is also often posed in high dimensions, where standard methods are intractable. The sensitivity to dispersion error makes spectral methods advantageous for the TDSE. We use spectral or pseudospectral methods in all except one of the included papers. In Paper III we improve and analyse the accuracy of the Fourier pseudospectral method applied to a problem with limited regularity, and in Paper V we construct a matrix-free spectral method for problems with non-trivial boundary conditions. Due to its stiffness, the TDSE is most often solved using exponential time integration. In this thesis we use exponential operator splitting and Krylov subspace methods. We rigorously prove convergence for force-gradient operator splitting methods in Paper IV. One way of making high-dimensional problems computationally tractable is low-rank approximation. In Paper VI we prove that a splitting method for dynamical low-rank approximation is robust to singular values in the approximation approaching zero, a situation which is difficult to handle since it implies strong curvature of the approximation space. / eSSENCE

Proposed are novel sensors for extreme environment power plants, laser beam analysis and biomedicine. A hybrid wireless-wired extreme environment temperature sensor using a thick single-crystal Silicon Carbide (SiC) chip embedded inside a sintered SiC probe design is investigated and experimentally demonstrated. The sensor probe employs the SiC chip as a Fabry-Perot (FP) interferometer to measure the change in refractive index and thickness of SiC with temperature. A novel temperature sensing method that combines wavelength-tuned signal processing for coarse measurements and classical FP etalon peak shift for fine measurements is proposed and demonstrated. This method gives direct unambiguous temperature measurements with a high temperature resolution over a wide temperature range. An alternative method using blackbody radiation from a SiC chip in a two-color pyrometer configuration for coarse temperature measurement and classical FP laser interferometry via the same chip for fine temperature measurement is also proposed and demonstrated. The sensor design is successfully deployed in an industrial test rig environment with gas temperatures exceeding 1200 C. This sensor is proposed as an alternate to all-electrical thermocouples that are susceptible to severe reliability and lifetime issues in such extreme environments. A few components non-contact thickness measurement system for optical quality semi-transparent samples such as Silicon (Si) and 6H SiC optical chips such as the one used in the design of this sensor is proposed and demonstrated. The proposed system is self-calibrating and ensures a true thickness measurement by taking into account material dispersion in the wavelength band of operation. For the first time, a 100% repeatable all-digital electronically-controlled pinhole laser beam profiling system using a Texas Instruments (TI) Digital Micro-mirror Device (DMD) commonly used in projectors is experimentally demonstrated using a unique liquid crystal image generation system with non-invasive qualities. Also proposed and demonstrated is the first motion-free electronically-controlled beam propagation analyzer system using a TI DMD and a variable focus liquid lens. The system can be used to find all the parameters of a laser beam including minimum waist size, minimum waist location and the beam propagation parameter M2. Given the all-digital nature of DMD-based profiling and all-analog motion-free nature of the Electronically Controlled Variable Focus Lens (ECVFL) beam focus control, the proposed analyzer versus prior-art promises better repeatability, speed and reliability. For the first time, Three Dimensional (3-D) imaging is demonstrated using an electronically controlled Liquid Crystal (LC) optical lens to accomplish a no-moving parts depth section scanning in a modified commercial 3-D confocal microscope. The proposed microscopy system within aberration limits has the potential to eliminate the sample or objective motion-caused mechanical forces that can distort the original sample structure and lead to imaging errors. A signal processing method for realizing high resolution three dimensional (3-D) optical imaging using diffraction limited low resolution optical signals is also proposed.

[pt] A optomecânica é um campo em crescimento que estuda sistemas nos quais luz e movimento mecãnico estão acoplados por meio de pressão de radiação. Neste trabalho apresentamos a teoria básica acerca de cavidades ópticas e pinças ópticas, duas importantes ferramentes frequentemente utilizadas em experimentos de optomecânica, bem como suas implementações práticas. No que diz respeito a cavidades ópticas, nós apresentamos a implementação de cavidades de Fabry Pérot formadas por um espelho plano e um espelho esférico e de cavidades formadas por dois espelhos esféricos, tanto na configuração confocal como na configuração não confocal, e comparamos a performance dessas diferentes cavidades. No que diz respeito a pinças ópticas, nós apresentamos uma pinça óptica capaz de aprisionar esferas micrométricas em um meio aquoso e a usamos para estudar o movimento de partículas aprisionadas. / [en] Optomechanics is a growing field that studies systems where light and mechanical motion are coupled via radiation pressure. In this work, we present the basic theory regarding optical cavities and optical tweezers, two important tools that are often used in optomechanical setups, as well as their experimental implementations. On the subject of optical cavities, we present the implementation of Fabry Pérot cavities formed by one plane mirror and one spherical mirror and cavities formed by two spherical mirrors, both on the confocal and on the non-confocal configuration, and compare the performance of these different cavities. On the subject of optical tweezers, we present an optical tweezer capable of trapping micro-spheres in a water medium and use it to study the movement of trapped particles.

La thèse, dont le sujet est "Modélisation de la propagation électromagnétique en milieux à gradient d'indice basée sur les faisceaux gaussiens - Application à la propagation en atmosphère réaliste et à la radio-occultation entre satellites" a été commencée le 2 décembre 2013, au Département ÉlectroMagnétisme et Radar (DEMR) de l'Onera de Toulouse et avec le laboratoire LAPLACE de l'Université Paul Sabatier. Elle est co-financée par l'ONERA et par la Région Midi-Pyrénées. L'encadrement a été assuré par Jérôme Sokoloff (Laplace/UPS, directeur de thèse), Alexandre Chabory (ENAC, co-directeur) et Vincent Fabbro (ONERA). L'École Doctorale est l' "École Doctorale Génie Électrique, Électronique, Télécommunications : du système au nanosystème". Le faisceau gaussien a été principalement utilisé dans la recherche scientifique afin d'étudier les systèmes optiques tels que les lasers. Des études plus rares et plus récentes ont proposé de l'utiliser pour modéliser la propagation des ondes sismiques. Ses propriétés spatiales et spectrales ont amené certains auteurs à étudier son utilisation dans des modèles de propagation atmosphériques. Cette thèse a consisté à développer un modèle, appelé GBAR (Gaussian Beam for Atmospheric Refraction), de propagation troposphérique réaliste et déterministe en utilisant le formalisme des faisceaux gaussiens. La démarche adoptée a consisté à reprendre les équations fondamentales introduites par Cerveny et Popov décrivant de façon itérative la propagation d'un faisceau gaussien en milieu inhomogène, sous hypothèse de haute fréquence (modèle asymptotique). De nouvelles équations ont été développées à partir d'elles pour obtenir une description analytique de la propagation d'un faisceau gaussien dans un milieu troposphérique décrit par les variations spatiales de l'indice de réfraction. L'hypothèse de base pour l'obtention de la formulation analytique est que le gradient de l'indice de réfraction peut être considéré vertical et constant au voisinage du faisceau. Les équations analytiques pour la description de la propagation d'un seul faisceau ont ensuite été étendues à la modélisation d'un champ quelconque dans un milieu troposphérique pouvant contenir de fortes variations du gradient d'indice, y compris des inversions de gradient. Ceci a été réalisé en couplant les équations analytiques avec la procédure de décomposition multi-faisceaux développée dans sa thèse pas Alexandre Chabory. Le modèle GBAR a été validé dans des milieux troposphériques réalistes issus de simulations du modèle météo méso-échelle WRF (Weather Research and Forecasting). Dans un troisième temps, le modèle a été utilisé pour simuler des inversions de données de radio-occultation. Des outils existent pour fournir un modèle d'interprétation de ces données pour estimer les propriétés physiques de l'atmosphère à partir des mesures en phase, amplitude, Doppler et délai des signaux GNSS transmis entre satellites en orbite autour de la Terre / The subject of this PhD thesis is " Electromagnetic propagation modeling in inhomogeneous media with refractive index gradients based on Gaussian beams - Application to realistic atmospheric propagation and radio occultation between satellites ". The study started on december 2nd, 2013 at the DEMR (Département Électromagnétisme et Radar) department of the ONERA research laboratory, in Toulouse, France. It was funded both by the ONERA and Région Midi-Pyrénées. It was supervised by Jérôme Sokoloff (LAPLACE/UPS, thesis director), Alexandre Chabory (ENAC, thesis co-director) and Vincent Fabbro (ONERA). The doctoral school was "École Doctorale Génie Électrique, Électronique, Télécommunications : du système au nanosystème ". The Gaussian beam was mostly used in scientific investigations to study optical systems such as lasers. Rarer and more recent works suggested the use of the Gaussian beam formalism in order to model the propagation of seismic waves. The properties of the Gaussian beam also led some authors to develop models for atmospheric propagation. In this thesis a model based on Gaussian beams called GBAR (Gaussian Beam for Atmospheric Refraction) was developped for tropospheric propagation in realistic and deterministic conditions. The scientific approach consisted in rewritting the fundamental equations introduced by Cerveny and Popov describing iteratively the propagation of a Gaussian beam in inhomogeneous media, under the high-frequency assumption (asymptotic model). New equations were derived from them in order to get analytical equations of the propagation of a Gaussian beam in inhomogeneous media described by the variations of the refractive index. The basic assumption under to get the analytical equations is to consider that the refractive index gradient is vertical and constant around the beam axis. The analytical equations that describe the propagation of a Gaussian beam were extended to model the propagation of an arbitrary field in a tropospheric medium with strong variations and inversions of the refractive index. This was done by coupling the analytical equations with the multibeam expansion procedure developped by Alexandre Chabory in his PhD thesis. The GBAR model was validated in tropospheric conditions, using refractive index grids from the WRF (Weather Research and Forecasting) mesoscale meteorological model. In the third and final phase, the GBAR model was used to simulate Radio Occultation data inversions. Tools exist to allow for interpretations of Radio Occultation data in order to estimate the physical properties of the atmosphere from measured phased, amplitude, Doppler shift and delay of GNSS signals transmitted between satellites orbiting around the Earth

Notre travail concerne l’étude par spectroscopie à haute résolution de molécules polyatomiques en phase gazeuse fortement excitées vibrationnellement. L’excitation de molécules dans leurs hauts niveaux de vibration est typiquement produite par absorption de lumière infrarouge proche, voire visible, d’où la dénomination de « vibrations colorées » ou « overtones ». L’excitation de degrés de liberté de vibration à des énergies aussi inhabituelles place la molécule dans un régime tellement anharmonique que de nouvelles approches théoriques deviennent nécessaires pour en modéliser le comportement. L’observation spectrale des ces transitions requiert une technique expérimentale ultra-sensible pour compenser les faibles intensités des raies d’absorption. Dans ce contexte, nous avons mis au point un dispositif ICLAS qui a pour objectif de surmultiplier le parcours de la lumière dans la cellule d’absorption en insérant celle-ci dans la cavité résonante d’un laser saphir-titane. Le chemin d’absorption ainsi obtenu est équivalent à plusieurs dizaines de kilomètres en fonction de la durée des impulsions lasers.<p>Nous avons ensuite couplé le dispositif ICLAS avec un spectromètre à transformée de Fourier en synchronisant l’acquisition des données avec les impulsions. L’avantage de ce couplage réside dans l’obtention d’une technique à la fois très sensible grâce à l’ICLAS mais aussi possédant une haute résolution et une large couverture spectrale grâce au spectromètre. La réalisation de ce dispositif est décrite dans la première partie de ce travail.<p>Nous avons ensuite appliqué ce dispositif instrumental à l'étude des molécules C2HD et 13C12CH2. L’analyse des spectres enregistrés est détaillée dans la seconde partie de ce travail. Celle-ci inclut également une introduction à la description théorique des niveaux d’énergie de vibration-rotation adaptée au cas de la molécule d’acétylène. L’analyse est axée principalement sur la structure rotationnelle et l’identification vibrationnelle des nouvelles bandes observées grâce aux performances du nouveau spectromètre. Les premières étapes vers la construction d’un modèle rovibrationnel global pour les deux molécules étudiées sont également décrites. / Doctorat en sciences, Spécialisation physique / info:eu-repo/semantics/nonPublished