Le moment orbital angulaire (OAM) de la lumière est une grandeur quantifiée associée à la phase d’un vortex optique et est actuellement une des variables explorées pour les technologies quantiques.Dans ce contexte, cette thèse étudie expérimentalement la conversion de vortex optiques par une vapeur de rubidium, via la transition Raman stimulée à deux photons 5S₁/₂ − 5D₅/₂. Quand les atomes sont soumis à deux lasers respectivement à 780 nm et 776 nm, ils génèrent des rayonnements cohérents, infrarouge à 5,23 μm et bleu à 420 nm. On examine le rayonnement bleu lorsque l’un des lasers ou les deux sont des vortex, en particulier des modes de Laguerre-Gauss. Dans une première partie nous montrons que si l’OAM est porté par le laser à 776 nm, alors le rayonnement bleu émis porte un OAM qui respecte l’accord de phase azimutale et de phase de Gouy. Nous montrons aussi que la conversion est efficace sur une grande plage d’OAM allant de -50 à +50, que l’efficacité est gouvernée par le produit des intensités des lasers incidents et que le rayonnement bleu se comporte comme un mode de Laguerre-Gauss pur. Dans une deuxième partie nous montrons qu’il est possible de convertir une superposition de vortex ou une paire de vortex coaxiaux et que l’OAM du rayonnement bleu émis obéit à la règle de somme des OAM incidents. Pour les cas étudiés, nous proposons un modèle de mélange à quatre ondes qui établit les règles de sélection du processus de conversion d’OAM. Ce travail ouvre la voie vers la conversion d’OAM utilisant des transitions vers des niveaux atomiques plus élevés. / The orbital angular momentum of light (OAM) is a quantized quantity arising from the azimuthal phase carried by optical vortices and is well-known for quantum technology applications. Its set of values is theoretically infinite.In this context this thesis experimentally study the conversion of optical vortices in a rubidium vapor through the 5S₁/₂ − 5D₅/₂ stimulated Raman transition. When the atoms are illuminated with laser beams at 780 nm and 776 nm they generate two coherent light beams at 5,23 μm and 420 nm. We investigate the blue light when one laser or both are optical vortices, in particular Laguerre-Gaussian modes. In a first part we show that if the laser at 776 nm carries an OAM the blue light is an optical vortex with an OAM which respects azimutal and Gouy phase matchings. We further show that the conversion is efficient on a large set of OAM from -50 to +50, that the efficiency is governed by the product of the input laser intensities and that the blue light behaves like a pure Laguerre-Gaussian mode. In a second part we demonstrate the conversion of a vortex superposition or a pair of coaxial vortices and that the OAM of the emitted light obeys the conservation rule of total OAM. For each studied case we propose a four wave mixing model establishing selection rules for the conversion process. This work opens possibilities towards OAM conversion using higher atomic levels.

Tribology has been recognized as a very important discipline in different branches of industry because almost every mechanical system has some moving parts. Due to the relative motion between these different mechanical parts, a variety of contacts are formed, and they are very often lubricated with oil. Environmental protection and ecological awareness are becoming increasingly important, which in turn has resulted in the shift to a low-carbon society, making water more interesting as a possible lubricant. On the one hand, water is less environmentally damaging as a lubricant than oil, but on the other hand, water has very poor lubrication properties, as its viscosity is 100 times lower than the viscosity of oil. These limitations might be overcome by appropriate surface engineering (e.g., diamond-like carbon, DLC). Tribological tests were performed in oil and water for two different contacts. Steel/steel and steel/DLC were investigated. DLC was recognized as a very promising solution, which ensures low friction and low wear. DLC was deposited on a real hydraulic part in an orbital hydraulic motor and tested under real industrial conditions. The overall efficiency of the hydraulic motor was measured.

La génération d’harmoniques d’ordre élevé est un processus d’interaction lumière-matière hautement non-linéaire permettant la synthèse d’impulsions sub-femtosecondes, dites attosecondes (1 as = 10⁻¹⁸ s). Mes travaux de thèse portent sur l’étude du transfert de moment angulaire lors de ce processus, afin de contrôler les caractéristiques spatiales et de polarisation du rayonnement émis dans l’extrême ultraviolet. Comme pour la matière, le moment angulaire de la lumière peut être séparé en une composante de spin, associée à l’état de polarisation du faisceau, et une composante orbitale, reliée à la forme du front d’onde. La maitrise complète du moment angulaire des harmoniques nécessite de recourir à des schémas de génération à deux faisceaux non-colinéaires, créant un réseau de diffraction dans le milieu générateur. Nous avons montré que, bien que les règles de transfert obéissent à des lois de conservation du moment angulaire, la description fine du phénomène requiert une analyse précise du champ laser dans le milieu de génération. Ces travaux ouvrent des perspectives de mise en forme avancée des impulsions attosecondes. / High-order harmonic generation is a highly nonlinear laser-matter interaction process which allows the synthesis of sub-femtosecond pulses, also called attosecond (1 as = 10⁻¹⁸ s) pulses. My PhD is centered around the study of angular momentum transfer during this process, in order to control spatial and polarization features of the radiation which is emitted in the extreme ultraviolet. As for matter, the angular momentum of light can be divided into a spin component, associated with the beam’s polarization, and an orbital component, related to the shape of the wavefront. The control of high harmonics’ angular momentum requires generating schemes involving two crossing beams, thus creating a diffraction grating in the generating medium.We have shown that, although the transfer rules obey conservation laws of the angular momentum, the fine description of the phenomenon requires an accurate analysis of the laser field in the generation medium. This work opens the road for advanced shaping of attosecond pulses.

No description available.

In this dissertation, we discuss the Distributed Spacecraft Attitude Control System Simulator (DSACSS) testbed developed at Virginia Polytechnic Institute and State University for the purpose of investigating various control techniques for single and multiple spacecraft. DSACSS is comprised of two independent hardware-in-the-loop simulators and one software spacecraft simulator. The two hardware-in-the-loop spacecraft simulators have similar subsystems as flight-ready spacecraft (e.g. command and data handling; communications; attitude determination and control; power; payload; and guidance and navigation). The DSACSS framework is a flexible testbed for investigating a variety of spacecraft control techniques, especially control scenarios involving coupled attitude and orbital motion. The attitude hardware simulators along with numerical simulations assist in the development and evaluation of Lyapunov based asymptotically stable, nonlinear attitude controllers with three reaction wheels as the control device. The angular rate controller successfully tracks a time varying attitude trajectory. The Modified Rodrigues Parmater (MRP) attitude controller results in successfully tracking the angular rates and MRP attitude vector for a time-varying attitude trajectory. The attitude controllers successfully track the reference attitude in real-time with hardware similar to flight-ready spacecraft. Numerical simulations and the attitude hardware simulators assist in the development and evaluation of a robust, asymptotically stable, nonlinear attitude controller with three reaction wheels as the actuator for attitude control. The MRPs are chosen to represent the attitude in the development of the controller. The robust spacecraft attitude controller successfully tracks a time-varying reference attitude trajectory while bounding system uncertainties. The results of a Global Positioning System (GPS) hardware-in-the-loop simulation of two spacecraft flying in formation are presented. The simulations involve a chief spacecraft in a low Earth orbit (LEO), while a deputy spacecraft maintains an orbit position relative to the chief spacecraft. In order to maintain the formation an orbit correction maneuver (OCM) for the deputy spacecraft is required. The control of the OCM is accomplished using a classical orbital element (COE) feedback controller and simulating continual impulsive thrusting for the deputy spacecraft. The COE controller requires the relative position of the six orbital elements. The deputy communicates with the chief spacecraft to obtain the current orbit position of the chief spacecraft, which is determined by a numerical orbit propagator. The position of the deputy spacecraft is determined from a GPS receiver that is connected to a GPS hardware-in-the-loop simulator. The GPS simulator creates a radio frequency (RF) signal based on a simulated trajectory, which results in the GPS receiver calculating the navigation solution for the simulated trajectory. From the relative positions of the spacecraft the COE controller calculates the OCM for the deputy spacecraft. The formation flying simulation successfully demonstrates the closed-loop hardware-in-the-loop GPS simulator. This dissertation focuses on the development of the DSACSS facility including the development and implementation of a closed-loop GPS simulator and evaluation of nonlinear feedback attitude and orbit control laws using real-time hardware-in-the-loop simulators. / Ph. D.

The physical-layer security of a line-of-sight (LOS) free-space optical (FSO) link using orbital angular momentum (OAM) multiplexing is studied. We discuss the effect of atmospheric turbulence to OAM-multiplexed FSO channels. We numerically simulate the propagation of OAM-multiplexed beam and study the secrecy capacity. We show that, under certain conditions, the OAM multiplexing technique provides higher security over a single-mode transmission channel in terms of the total secrecy capacity and the probability of achieving a secure communication. We also study the power cost effect at the transmitter side for both fixed system power and equal channel power scenarios.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Stellenbosch University and the Katholieke Universiteit Leuven has a joint undertaking to develop a satellite communications payload. The goals of the project are: to undertake research and expand knowledge in the area of dynamically configurable antenna beam forming, to prove the viability of this research for space purposes and to demonstrate the feasibility of the development in a practical application. The practical application is low Earth orbit satellite communication system for applications in remote monitoring. Sensor data will be uploaded to the satellite, stored and forwarded to a central processing ground station as the satellite passes over these ground stations. The system will utilise many low-cost ground sensor stations to collect data and distribute it to high-end ground stations for processing. Applications of remote monitoring systems are maritime- and climate change monitoring- and tracking. Climate change monitoring allows inter alia, for the monitoring of the effects and causes of global warming. The Katholieke Universiteit Leuven is developing a steerable antenna to be mounted on the satellite. Stellenbosch University is developing the communications payload to steer and use the antenna. The development of the communications protocol stack is part of the project. The focus of this work is to implement the application layer protocol, which handles all file level communications and also implements the communications strategy. The application layer protocol is called the Satellite Communications Software System (SCSS). It handles all high level requests from ground stations, including requests to store data, download data, download log files and upload configuration information. The design is based on a client-server model, with a Station Server and Station Handler. The Station Server schedules ground stations for communication and creates a Station Handler for each ground station to handle all ground station requests. During the design, all file formats were defined for efficient ground station-satellite communications and system administration. All valid ground station requests and handler responses were also defined. It was also found that the system may be made more efficient by scheduling ground stations for communications, rather than polling each ground station until one responds. To be able to schedule ground station communications, the times when ground stations will come into view of the satellite have to be predicted. This is done by calculating the positions of the Satellite and ground stations as functions of time. A simple orbit propagator was developed to predict the satellite distance and to ease testing and integration with the communications system. The times when a ground station will be within range of the satellite were then predicted and a scheduling algorithm developed to minimise the number of ground stations not able to communicate. All systems were implemented and tested. The SCSS executing on the Satellite was developed and tested on the satellite on-board computer. Embedded implementations possess strict resource limitations, which were taken into account during the development process. The SCSS is a multi-threaded system that makes use of thread cancellation to improve responsiveness. / AFRIKAANSE OPSOMMING: Die Universiteit van Stellenbosch ontwerp tans ’n satelliet kommunikasieloonvrag in samewerking met die Katolieke Universiteit van Leuven. Die doel van die projek is om navorsing te doen oor die lewensvatbaarheid van dinamies verstelbare antenna bundelvorming vir ruimte toepassings, asook om die haalbaarheid van hierdie navorsing in die praktyk te demonstreer. Die praktiese toepassing is ’n satellietkommunikasiestelsel vir afstandsmonitering, wat in ’n Lae-Aarde wentelbaan verkeer. Soos die satelliet in sy wentelbaan beweeg, sal sensor data na die satelliet toe gestuur, gestoor en weer aangestuur word. Die stelsel gebruik goedkoop sensorgrondstasies om data te versamel en aan te stuur na kragtiger grondstasies vir verwerking. Afstandsmoniteringstelsels kan gebruik word om klimaatsverandering, sowel as die posisie van skepe en voertuie, te monitor. Deur oa. klimaatsveranderinge te dokumenteer, kan gevolge en oorsake van globale verhitting gemonitor word. Die Katholieke Universiteit van Leuven is verantwoordelik vir die ontwerp en vervaardiging van die satelliet antenna, terwyl die Universiteit van Stellenbosch verantwoordelik is vir die ontwerp en bou van die kommunikasie loonvrag. ’n Gedeelte van hierdie ontwikkeling sluit die ontwerp en implementasie van al die protokolle van die kommunikasieprotokolstapel in. Dit fokus op die toepassingsvlak protokol van die protokolstapel, wat alle leêrvlak kommunikasie hanteer en die kommunikasiestrategie implementeer. Die toepassingsvlaksagteware word die Satellietkommunikasie sagtewarestelsel (SKSS) genoem. Die SKSS is daarvoor verantwoordelik om alle navrae vanaf grondstasies te hanteer. Hierdie navrae sluit die oplaai en stoor van data, die aflaai van data, die aflaai van logs en die oplaai van konfigurasie inligting in. Die ontwerp is op die standaard kliënt-bediener model gebasseer, met ’n stasiebediener en ’n stasiehanteerder. Die stasiebediener skeduleer die tye wanneer grondstasies toegelaat sal word om te kommunikeer en skep stasiehanteerders om alle navrae vanaf die stasies te hanteer. Gedurende die ontwerp is alle leêrformate gedefinieer om doeltreffende adminstrasie van die stelsel, asook kommunikasie tussen grondstasies en die satelliet te ondersteun. Alle geldige boodskappe tussen die satelliet en grondstasies is ook gedefnieer. Daar is gevind dat die doeltreffendheid van die stelsel verhoog kan word deur die grondstasies wat wil kommunikeer te skeduleer, eerder as om alle stasies te pols totdat een reageer. Om so ’n skedule op te stel, moet die tye wanneer grondstasies binne bereik van die satelliet gaan wees voorspel word. Hierdie voorspelling is gedoen deur die posisies van die satelliet en die grondstasies as funksies van tyd te voorspel. ’n Eenvoudige satelliet posisievoorspeller is ontwikkel om toetsing en integrasie met die SKSS te vergemaklik. ’n Skeduleringsalgoritme is toe ontwikkel om die hoeveelheid grondstasies wat nie toegelaat word om te kommunikeer nie, te minimeer. Alle stelsels is geimplementeer en getoets. Die SKSS, wat op die satelliet loop, is ontwikkel en getoets op die satelliet se aanboord rekenaar. Die feit dat ingebedde stelsels oor baie min hulpbronne beskik, is in aanmerking geneem gedurende die ontwikkeling en implementasie van die SKSS. Angesien die SKSS ’n multidraadverwerkingsstelsel is, word daar van draadkansellasie gebruik gemaak om die stelsel se reaksietyd te verbeter.

Cette thèse présente une étude du système à deux corps contrôlé en poussée faible, et en particulier, des problèmes de transferts orbitaux. Après une étude de contrôlabilité, nous nous focalisons sur le lien entre la commande optimale en temps minimum et les contrôles en boucle fermée construits à partir de la méthode de Jurdjevic-Quinn. Des simulations numériques montrent que les commandes Jurdjevic-Quinn peuvent être proches de la commande temps minimum. Pour comprendre cette propriété nous étudions un système contrôlé moyen dont les trajectoires approchent celles des systèmes à faible poussée. Cette technique nous permet de répondre à une conjecture concernant le comportement asymptotique du temps minimum quand la poussée tends vers zéro. D'autre part elle constitue une piste prometteuse pour construire une fonction de Lyapunov associée la méthode de Jurdjevic-Quinn efficace: un contrôle en boucle fermée proche de la commande optimale.

<p>Density functional theory is a formally exact theory to describe ground state properties due to the existence of the exact functional. In practice, the usefulness of density functional theory relies on the accuracy of density functional approximations. After decades of effort of functional developments, the present-day state-of-the-art density functional approximations have achieved reasonably good accuracy for small systems. However, the error grows with system size. One of the dominant errors intrinsic in the mainstream density functional approximations is the delocalization error, which arises because of the violation of Perdew-Parr-Levy-Balduz (PPLB) linearity condition. The PPLB condition governs the formulation of the density functional theory for fractional-charge systems, for which the ground state energy for the exact functional, as a function of the fractional electron number, should yield a series of line-segments across the integer points. In this dissertation, by imposing the PPLB condition in a local, size-consistent way, we develop the local scaling correction (LSC) and its updated version, the localized orbital scaling correction (LOSC), which largely improve upon the mainstream density functional approximations across system sizes. With the LOSC, we open a door towards a systematic elimination of delocalization error. Besides the ground state functional development, we also develop a gentlest ascent dynamics approach for accessing the excited states via time-independent ground state density functionals. This is also useful for exploring Kohn-Sham energy landscapes of approximate density functionals. I will also review the PPLB formulation of density functional theory for fractionally charged systems, and show that it is equivalent to the formulation normally used for fractional system calculations under certain assumptions. Furthermore, I will examine the behavior of the fractional system energy as a function of the fractional number of electrons for different mainstream functionals, and relate it to their errors for integer systems.</p> / Dissertation

Ce mémoire est une partie d’un programme de recherche qui étudie la superintégrabilité des systèmes avec spin. Plus particulièrement, nous nous intéressons à un hamiltonien avec interaction spin-orbite en trois dimensions admettant une intégrale du mouvement qui est un polynôme matriciel d’ordre deux dans l’impulsion. Puisque nous considérons un hamiltonien invariant sous rotation et sous parité, nous classifions les intégrales du mouvement selon des multiplets irréductibles de O(3). Nous calculons le commutateur entre l’hamiltonien et un opérateur général d’ordre deux dans l’impulsion scalaire, pseudoscalaire, vecteur et pseudovecteur. Nous donnons la classification complète des systèmes admettant des intégrales du mouvement scalaire et vectorielle. Nous trouvons une condition nécessaire à remplir pour le potentiel sous forme d’une équation différentielle pour les cas pseudo-scalaire et pseudo-vectoriel. Nous utilisons la réduction par symétrie pour obtenir des solutions particulières de ces équations. / This thesis is part of a research program studying superintegrable systems with spin. In particular, we consider a Hamiltonian with a spin-orbital interaction in three dimensions admitting an integral of motion that is a matrix polynomial second order in the momenta. Since we are considering a Hamiltonian which is invariant under rotation and parity, we classify the integrals of motion into irreducible O(3) multiplets. We obtain the commutator of the Hamiltonian with the scalar, pseudoscalar, vector and axial vector operators. We provide a complete classification for the scalar and vector cases. We find the necessary condition for superintegrability on the potential as a differential equation. We use symmetry reduction methods to obtain particular solutions of this equation.