Few-mode fibers (FMFs) and multimode fibers (MMFs) can provide much higher data-carrying capacities compared with single-mode fibers. But in order to achieve this goal, one must address the challenge of intermodal coupling and dispersion. Therefore the ability to accurately control the optical signal propagation in FMFs/MMFs can play a pivotal role in FMF/MMF applications. This thesis demonstrates the ability to excite, in FMFs and MMFs, the desired linearly polarized (LP) modes as well as their superpositions through adaptive optics (AO). Specifically, in the case of step-index FMFs, a phase-only spatial light modulator (SLM) is employed to manipulate the light at the fiber input end, driven by the feedback signal provided by the correlation between the charge coupled device (CCD) camera captured images at the fiber output end and the target light intensity profile. Through such an adaptive optical system, any arbitrarily selected LP modes can be excited at the distal end of the four-mode and seventeen-mode fibers, respectively. For a graded-index MMF with a uniform Bragg grating, we use a deformable mirror (DM) to perform the wavefront modulation at the fiber input end, where the feedback is based on the ratio of the grating-reflected signal power to the transmitted signal power. At the Bragg grating position of this highly multimode fiber, any desired principal mode groups can be successfully chosen. These experimental results suggest that adaptive control of optical wavefront in FMFs/MMFs is indeed feasible. / Master of Science / Optical fibers, in terms of the number of modes they support, can be generally divided into single-mode fibers (SMFs), and few-mode fibers/multimode fibers (FMFs/MMFs). FMFs/MMFs can provide much higher data-carrying capacities than SMFs. For example, an FMF/MMF that supports M modes can ideally increase the data transmission rate by a factor of M, where each mode can serve as a distinct communication channel. However, in order to achieve good performance, one must accurately control signal propagation in FMFs/MMFs, which are often degraded due to the multiple-mode nature. This thesis demonstrates the ability, using adaptive optics (AO), to control signal propagation in FMFs and a highly MMF, respectively. Specifically, in the case of FMFs, a phase-only spatial light modulator (SLM) is employed to manipulate the light at the fiber input, driven by AO feedback signal provided by the similarity between the real-time fiber output image and the target mode profile. Through such an adaptive optical system, any desired linearly-polarized (LP) modes can be excited at the output of the four-mode and seventeen-mode fibers, respectively. For the highly MMF with uniform Bragg grating, we use a deformable mirror (DM) to perform the wavefront modulation at the fiber input, where AO feedback is provided by the fiber Bragg grating (FBG) reflectivity. At the FBG position, any desired principal mode groups can be successfully chosen. These experimental results suggest that adaptive control of optical wavefront in FMFs/MMFs is indeed feasible, and may find a large number of applications in optical communication, sensing, and imaging.

[EN] Space-division multiplexing fibers emerged as a promising solution to overcome the imminent capacity crunch of conventional singlemode fiber networks. Despite these fibers were initially conceived as distribution media for long-haul high-capacity digital communications, they can be applied to a wide variety of scenarios including centralized radio access networks for wireless communications, data-center interconnects, Microwave Photonics signal processing and fiber sensing. Particular interest is raised by emerging communications paradigms, such as 5G and The Internet of Things, which require a full integration between the optical fiber and the wireless networks segments. Microwave Photonics, discipline that focuses on the generation, processing, control and distribution of radiofrequency signals by photonics means, is called to play a decisive role. One of the major challenges that Microwave Photonics has to overcome to satisfy next-generation communication demands relates to the reduction of size, weight and power consumption while assuring broadband seamless reconfigurability and stability. There is one revolutionary approach that has however been left untapped in finding innovative ways to address that challenge: exploiting space, the last available degree of freedom for optical multiplexing. In this Thesis, we propose to exploit the inherent parallelism of multicore and few-mode fibers to implement sampled discrete true time delay lines, providing, in a single optical fiber, a compact and efficient approach for both Microwave Photonics signal distribution and processing. For the multicore fiber approach, we study the influence of the refractive index profile of each heterogeneous core on the propagation characteristics as to feature specific group delay and chromatic dispersion values. We designed and fabricated two different heterogeneous trench-assisted 7-core fibers that behave as sampled true time delay lines. While one of them was fabricated by using 7 different preforms to feature a plenary performance, the other one employed a single preform with the aim of minimizing fabrication costs. In the case of few-mode fibers, we propose the implementation of a tunable true time delay line by means of a custom-designed fiber with a set of inscribed long period gratings that act as mode converters to properly tailor the sample group delays. We designed and fabricated a true time delay line on a 4-mode fiber by inscribing 3 long period gratings at specific positions along the fiber link. As a proof-of-concept validation, we experimentally demonstrated different Microwave Photonics signal processing functionalities implemented over both multicore and few-mode fiber approaches. This work opens the way towards the development of distributed signal processing for microwave and millimeter wave signals in a single optical fiber. These true time delay lines can be applied to a wide range of Information and Communication Technology paradigms besides fiber-wireless communications such as broadband satellite communications, distributed sensing, medical imaging, optical coherence tomography and quantum communications. / [ES] La multiplexación por división espacial en fibras ópticas surgió como una solución prometedora al inminente colapso en la capacidad de las redes de fibra monomodo convencionales. Aunque estas fibras fueron concebidas inicialmente como medio de distribución en comunicaciones digitales de larga distancia y alta capacidad, pueden emplearse en una amplia variedad de escenarios, incluyendo redes de acceso radio centralizadas para comunicaciones inalámbricas, interconexiones en centros de datos, así como procesado de señal en Fotónica de Microondas y sensado en fibra. Los paradigmas de comunicaciones emergentes despiertan un interés particular, como 5G y el Internet de las Cosas, que requieren una integración total entre el segmento de red de fibra óptica y el inalámbrico. La Fotónica de Microondas, disciplina que se focaliza en la generación, procesado, control y distribución de señales de radiofrecuencia por medio de la fotónica, está destinada a jugar un papel decisivo. Uno de los mayores desafíos que la Fotónica de Microondas debe superar para satisfacer los requisitos de las nuevas generaciones de comunicaciones se basa en la reducción de tamaño, peso y consumo de potencia, mientras se garantiza reconfiguración y estabilidad de banda ancha. Encontramos aquí un enfoque revolucionario capaz de abordar este desafío de una manera innovadora que, sin embargo, no ha sido aprovechado en este contexto: la explotación del espacio, el último grado de libertad para multiplexación óptica. En esta Tesis, proponemos explotar el paralelismo inherente de las fibras ópticas multinúcleo y de pocos modos para implementar líneas de retardo en tiempo real muestreadas que proporcionan, en una sola fibra óptica, una solución compacta y eficiente tanto para distribución como para procesado de señales de Fotónica de Microondas. En el caso de fibras multinúcleo, estudiamos la influencia del perfil de índice de refracción de cada núcleo heterogéneo en las características de propagación para que exhiba unos valores concretos de retardo de grupo y dispersión cromática. Diseñamos y fabricamos dos fibras distintas de 7 núcleos con zanjas que se comportan como líneas de retardo en tiempo real muestreadas. Mientras que una de ellas se fabricó utilizando 7 preformas diferentes para garantizar un funcionamiento completo, la segunda se fabricó utilizando una única preforma con el objetivo de minimizar costes de fabricación. En el caso de fibras de pocos modos, proponemos la implementación de líneas de retardo en tiempo real sintonizables mediante el uso de una fibra específicamente diseñada y la inscripción de un conjunto de redes de difracción de periodo largo que actúan como conversores de modos para ajustar adecuadamente el retardo de grupo de las muestras. Diseñamos y fabricamos una línea de retardo en tiempo real en una fibra de 4 modos mediante la inscripción de 3 redes de difracción de periodo largo en posiciones concretas a lo largo de enlace de fibra. Como validación de prueba de concepto, demostramos experimentalmente diferentes funcionalidades de procesado de señal de Fotónica de Microondas implementadas en fibras multinúcleo y de pocos modos. Este trabajo abre el camino hacia el desarrollo del procesado de señal distribuido para señales de microondas y ondas milimétricas en una única fibra óptica. Además, las líneas de retardo en tiempo real desarrolladas pueden aplicarse a una amplia variedad de paradigmas de Tecnologías de la Información y Comunicaciones más allá de las comunicaciones radio sobre fibra, como es el caso de las comunicaciones de banda ancha por satélite, el sensado distribuido, la imagen médica, la tomografía óptica coherente y las comunicaciones cuánticas. / [CA] La multiplexació per divisió espacial en fibres òptiques va sorgir com una solució prometedora a l'imminent col·lapse en la capacitat de les xarxes de fibra monomode convencionals. Encara que estes fibres foren concebudes inicialment com a mitjà de distribució en comunicacions digitals de llarga distància i alta capacitat, poden emprar-se en una àmplia varietat d'escenaris, incloent xarxes d'accés radio centralitzades per a comunicacions sense fils, interconnexions en centres de dades, així com processat de senyal en Fotònica de Microones i sensat en fibra. Els paradigmes de comunicacions emergents desperten un interès particular, com el 5G i la Internet de les Coses, que requereixen una integració total entre els segments de xarxa de fibra òptica i el de sense fils. La Fotònica de Microones, disciplina que es focalitza en la generació, processat, control i distribució de senyals de radiofreqüència per mitjà de la fotònica, està destinada a jugar un paper decisiu. Un dels majors desafiaments que la Fotònica de Microones ha de superar per satisfer els requisits de les noves generacions de comunicacions es basa en la reducció de grandària, pes i consum de potència, mentre es garanteix reconfiguració i estabilitat de banda ampla Trobem ací un enfocament revolucionari capaç d'abordar aquest desafiament d'una manera innovadora que, no obstant això, no ha sigut aprofitat encara en este context: la explotació de l'espai, l'últim grau de llibertat per a multiplexat òptic. En aquesta Tesi, proposem explotar el paral·lelisme inherent de les fibres òptiques multinucli i de pocs modes per a implementar línies de retard en temps real de mostres discretes que proporcionen, en una sola fibra òptica, una solució compacta i eficient tant per a distribució com per a processat de senyals de Fotònica de Microones. En el cas de fibres multinucli, estudiem la influència del perfil d'índex de refracció de cada nucli heterogeni en les característiques de propagació perquè exhibisca uns valors concrets de retard de grup i dispersió cromàtica. Dissenyem i fabriquem dues fibres distintes de 7 nuclis amb rases que es comporten com a línies de retard en temps real mostrejades. Mentre que una d'elles es va fabricar utilitzant 7 preformes diferents per a garantir un funcionament complet, la segona va fabricar-se utilitzant una única preforma amb l'objectiu de minimitzar costos de fabricació. En el cas de fibres de pocs modes, proposem la implementació de línies de retard en temps real sintonitzables mitjançant l'ús d'una fibra específicament dissenyada i la inscripció d'un conjunt de xarxes de difracció de període llarg que actuen com a convertidors de modes per tal d'ajustar adequadament el retard de grup de les mostres. Dissenyem i fabriquem una línia de retard en temps real en una fibra de 4 modes mitjançant la inscripció de 3 xarxes de difracció de període llarg en posicions concretes al llarg de l'enllaç de fibra. Com a validació de proba de concepte, demostrem experimentalment diferents funcionalitats de processat de senyal de Fotònica de Microones implementades en fibres multinucli i de pocs modes. Aquest treball obri el camí cap al desenvolupament del processat de senyal distribuït per a senyals de microones i ones mil·limètriques en una única fibra òptica. A més, aquestes línies de retard en temps real poden aplicar-se a una àmplia varietat de paradigmes de Tecnologies de la Informació i Comunicacions més enllà de les comunicacions radio sobre fibra, com es el cas de les comunicacions de banda ampla per satèl·lit, el sensat distribuït, la imatge mèdica, la tomografia òptica coherent i les comunicacions quàntiques. / Agradezco al Ministerio de Economía y Competitividad del Gobierno de España por la financiación recibida mediante la ayuda FPI. / García Cortijo, S. (2020). Distributed radiofrequency signal processing based on space-division multiplexing fibers [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/147858

Text plays a pivotal role in our daily lives, encompassing various forms such as social media posts, news articles, books, reports, and more. Consequently, Natural Language Processing (NLP) has garnered widespread attention. This technology empowers us to undertake tasks like text classification, entity recognition, and even crafting responses within a dialogue context. However, despite the expansive utility of NLP, it frequently necessitates a critical decision: whether to place trust in a model's predictions. To illustrate, consider a state-of-the-art (SOTA) model entrusted with diagnosing a disease or assessing the veracity of a rumor. An incorrect prediction in such scenarios can have dire consequences, impacting individuals' health or tarnishing their reputation. Consequently, it becomes imperative to establish a reliable method for evaluating the reliability of an NLP model's predictions, which is our focus-uncertainty estimation on NLP. Though many works have researched uncertainty estimation or NLP, the combination of these two domains is rare. This is because most NLP research emphasizes model prediction performance but tends to overlook the reliability of NLP model predictions. Additionally, current uncertainty estimation models may not be suitable for NLP due to the unique characteristics of NLP tasks, such as the need for more fine-grained information in named entity recognition. Therefore, this dissertation proposes novel uncertainty estimation methods for different NLP tasks by considering the NLP task's distinct characteristics. The NLP tasks are categorized into natural language understanding (NLU) and natural language generation (NLG, such as text summarization). Among the NLU tasks, the understanding could be on two views, global-view (e.g. text classification at document level) and local-view (e.g. natural language inference at sentence level and named entity recognition at token level). As a result, we research uncertainty estimation on three tasks: text classification, named entity recognition, and text summarization. Besides, because few-shot text classification has captured much attention recently, we also research the uncertainty estimation on few-shot text classification. For the first topic, uncertainty estimation on text classification, few uncertainty models focus on improving the performance of text classification where human resources are involved. In response to this gap, our research focuses on enhancing the accuracy of uncertainty scores by bolstering the confidence associated with winning scores. we introduce MSD, a novel model comprising three distinct components: 'mix-up,' 'self-ensembling,' and 'distinctiveness score.' The primary objective of MSD is to refine the accuracy of uncertainty scores by mitigating the issue of overconfidence in winning scores while simultaneously considering various categories of uncertainty. seamlessly integrate with different Deep Neural Networks. Extensive experiments with ablation settings are conducted on four real-world datasets, resulting in consistently competitive improvements. Our second topic focuses on uncertainty estimation on few-shot text classification (UEFTC), which has few or even only one available support sample for each class. UEFTC represents an underexplored research domain where, due to limited data samples, a UEFTC model predicts an uncertainty score to assess the likelihood of classification errors. However, traditional uncertainty estimation models in text classification are ill-suited for UEFTC since they demand extensive training data, while UEFTC operates in a few-shot scenario, typically providing just a few support samples, or even just one, per class. To tackle this challenge, we introduce Contrastive Learning from Uncertainty Relations (CLUR) as a solution tailored for UEFTC. CLUR exhibits the unique capability to be effectively trained with only one support sample per class, aided by pseudo uncertainty scores. A distinguishing feature of CLUR is its autonomous learning of these pseudo uncertainty scores, in contrast to previous approaches that relied on manual specification. Our investigation of CLUR encompasses four model structures, allowing us to evaluate the performance of three commonly employed contrastive learning components in the context of UEFTC. Our findings highlight the effectiveness of two of these components. Our third topic focuses on uncertainty estimation on sequential labeling. Sequential labeling involves the task of assigning labels to individual tokens in a sequence, exemplified by Named Entity Recognition (NER). Despite significant advancements in enhancing NER performance in prior research, the realm of uncertainty estimation for NER (UE-NER) remains relatively uncharted but is of paramount importance. This topic focuses on UE-NER, seeking to gauge uncertainty scores for NER predictions. Previous models for uncertainty estimation often overlook two distinctive attributes of NER: the interrelation among entities (where the learning of one entity's embedding depends on others) and the challenges posed by incorrect span predictions in entity extraction. To address these issues, we introduce the Sequential Labeling Posterior Network (SLPN), designed to estimate uncertainty scores for the extracted entities while considering uncertainty propagation from other tokens. Additionally, we have devised an evaluation methodology tailored to the specific nuances of wrong-span cases. Our fourth topic focuses on an overlooked question that persists regarding the evaluation reliability of uncertainty estimation in text summarization (UE-TS). Text summarization, a key task in natural language generation (NLG), holds significant importance, particularly in domains where inaccuracies can have serious consequences, such as healthcare. UE-TS has garnered attention due to the potential risks associated with erroneous summaries. However, the reliability of evaluating UE-TS methods raises concerns, stemming from the interdependence between uncertainty model metrics and the wide array of NLG metrics. To address these concerns, we introduce a comprehensive UE-TS benchmark incorporating twenty-six NLG metrics across four dimensions. This benchmark evaluates the uncertainty estimation capabilities of two large language models and one pre-trained language model across two datasets. Additionally, it assesses the effectiveness of fourteen common uncertainty estimation methods. Our study underscores the necessity of utilizing diverse, uncorrelated NLG metrics and uncertainty estimation techniques for a robust evaluation of UE-TS methods. / Doctor of Philosophy / Text is integral to our daily activities, appearing in various forms such as social media posts, news articles, books, and reports. We rely on text for communication, information dissemination, and decision-making. Given its ubiquity, the ability to process and understand text through Natural Language Processing (NLP) has become increasingly important. NLP technology enables us to perform tasks like text classification, which involves categorizing text into predefined labels, and named entity recognition (NER), which identifies specific entities such as names, dates, and locations within text. Additionally, NLP facilitates generating coherent and contextually appropriate responses in conversational agents, enhancing human-computer interaction. However, the reliability of NLP models is crucial, especially in sensitive applications like medical diagnoses, where errors can have severe consequences. This dissertation focuses on uncertainty estimation in NLP, a less explored but essential area. Uncertainty estimation helps evaluate the confidence of NLP model predictions. We propose new methods tailored to various NLP tasks, acknowledging their unique needs. NLP tasks are divided into natural language understanding (NLU) and natural language generation (NLG). Within NLU, we look at tasks from two perspectives: a global view (e.g., document-level text classification) and a local view (e.g., sentence-level inference and token-level entity recognition). Our research spans text classification, named entity recognition (NER), and text summarization, with a special focus on few-shot text classification due to its recent prominence. For text classification, we introduce the MSD model, which includes three components to enhance uncertainty score accuracy and address overconfidence issues. This model integrates seamlessly with different neural networks and shows consistent improvements in experiments. For few-shot text classification, we develop Contrastive Learning from Uncertainty Relations (CLUR), designed to work effectively with minimal support samples per class. CLUR autonomously learns pseudo uncertainty scores, demonstrating effectiveness with various contrastive learning components. In NER, we address the unique challenges of entity interrelation and span prediction errors. We propose the Sequential Labeling Posterior Network (SLPN) to estimate uncertainty scores while considering uncertainty propagation from other tokens. For text summarization, we create a benchmark with tens of metrics to evaluate uncertainty estimation methods across two datasets. This benchmark helps assess the reliability of these methods, highlighting the need for diverse, uncorrelated metrics. Overall, our work advances the understanding and implementation of uncertainty estimation in NLP, providing more reliable and accurate predictions across different tasks.

Making information stored in databases more accessible to users inexperienced in structured query language (SQL) by converting natural language to SQL queries has long been a prominent research area in both the database and natural language processing (NLP) communities. There have been numerous approaches proposed for this task, such as encoder-decoder frameworks, semantic grammars, and more recently with the use of large language models (LLMs). When training LLMs to successfully generate SQL queries from natural language questions there are three notable methods used, pretraining, transfer learning and in-context learning (ICL). ICL is particularly advantageous in scenarios where the hardware at hand is limited, time is of concern and large amounts of task specific labled data is nonexistent. This study seeks to evaluate two strategies in ICL, namely zero-shot and few-shot scenarios using the Mistral-7B-Instruct LLM. Evaluation of the few-shot scenarios was conducted using two techniques, random selection and Jaccard Similarity. The zero-shot scenarios served as a baseline for the few-shot scenarios to overcome, which ended as anticipated, with the few-shot scenarios using Jaccard similarity outperforming the other two methods, followed by few-shot scenarios using random selection coming in at second best, and the zero-shot scenarios performing the worst. Evaluation results acquired based on execution accuracy and exact matching accuracy confirm that leveraging similarity in demonstrating examples when prompting the LLM will enhance the models knowledge about the database schema and table names which is used during the inference phase leadning to more accurately generated SQL queries than leveraging diversity in demonstrating examples.

Doctor of Philosophy / Department of Physics / Brett D. Esry / The Efimov effect plays a central role in few-body systems at ultracold temperature and has thus accelerated a lot of studies on its manifestation in the collisional stability of the quantum degenerate gases. Near broad Feshbach resonances, Efimov physics has been studied both theoretically and experimentally through the zero-energy scattering observables. We have extended the theoretical studies of Efimov physics to a much broader extent. In particular, we have investigated the three-body Efimov physics near narrow Feshbach resonances and have also identified the Efimov features beyond the zero temperature limit. We have found, near a narrow Feshbach resonance, the non-trivial contribution from both of the resonance width and the short-range physics to the three-body recombination and vibrational dimer relaxation. Remarkably, the collisional stability of the Feshbach molecules are found to be opposite to that near the broad resonances: an increased stability for molecules made by bosons and a decreased stability for those made by fermions. The universal physics observed near the narrow Feshbach resonances is further found not to be limited to the zero temperature observables. We have found that the general features of Efimov physics and those pertaining to a narrow resonance are manifested in different energy ranges above zero temperature. This opens the opportunity to observe Efimov physics by changing the collisional energy while keeping the atomic interaction fixed. The landscape of the universal Efimov physics is thus delineated in both of the interaction and the energy domain. We have also investigated Efimov physics in heteronuclear four-body systems where the complexity can be reduced by approximations. In particular, we have proposed ways for controllable production of the Efimov tri-atomic molecules by three-body or four-body recombinations involving four atoms. We have also confirmed the existence of four-body Efimov effect in a system of three heavy particles and one light particle, which has resolved a decade-long controversy on this topic. Finally, we have studied the collisional properties of four identical bosons in 1D, which is important to the experiments on the quantum gases confined in the 1D optical lattices.

Bound state systems consisting of three nonrelativistic particles are numerically studied. Calculations are performed employing two-body and three-body forces as input in the Hamiltonian in order to study the role or contribution of three-body forces to the binding in these systems. The resulting differential Faddeev equations are solved as three-dimensional equations in the two Jacobi coordinates and the angle between them, as opposed to the usual partial wave expansion approach. By expanding the wave function as a sum of the products of spline functions in each of the three coordinates, and using the orthogonal collocation procedure, the equations are transformed into an eigenvalue problem. The matrices in the aforementioned eigenvalue equations are generally of large order. In order to solve these matrix equations with modest and optimal computer memory and storage, we employ the iterative Restarted Arnoldi Algorithm in conjunction with the so-called tensor trick method. Furthermore, we incorporate a polynomial accelerator in the algorithm to obtain rapid convergence. We applied the method to obtain the binding energies of Triton, Carbon-12, and Ozone molecule. / Physics / M.Sc (Physics)

Na análise de estruturas é comum a consideração da concentricidade entre vigas e placas. Estudos numéricos em pavimentos de laje nervurada, onde os espaçamentos das vigas são pequenos, mostram que a consideração da excentricidade entre placa e a viga resultam em redução dos deslocamentos na estrutura, entretanto não existem estudos sobre a influência da excentricidade em pavimentos com poucas vigas. Este trabalho apresenta a análise numérica de pavimentos com dimensões usuais considerando ou não a excentricidade com o objetivo de verificar a influência da excentricidade no comportamento da estrutura. As análises mostraram diferenças de deslocamentos e tensões nas estruturas estudadas. O efeito de excentricidade pode ser incluído através da modificação da matriz de rigidez do elemento finito de viga. Todos os pavimentos foram analisados com o software ANSYS com elementos de placa e viga que representassem os modelos do trabalho. / It\'s usual practice on structural analysis of buildings slabs to assume the concentricity between plates and beams. Numerical studies in waffle-slabs, where the distance between beams is small, showed that the eccentricity of beam to plate results in reduction of stresses and displacements of the system, however there is not similar study about the influence of eccentricity in slabs with few beams. This work presents numerical analysis results of at slabs with usual dimensions considering or not plate-beam eccentricity, aiming at evaluating its influence in structural behavior. The analysis showed differences of displacements and stress in studied structures. The effects of eccentricity can be included by modifying the stiffness matrix of the beam finite element. All slabs were analyzed with ANSYS program with suitable beam and plate elements.

Diese Arbeit beschäftigt sich mit der Carrier-Envelope Phasendrift modengekoppelter Laser sowie Maßnahmen zu deren aktiver und passiver Stabilisierung. Das restliche Phasenrauschen wird in verschiedenen Messaufbauten im Hinblick auf physikalische Ursachen und mögliche Optimierungen untersucht. Dazu werden einführend verschiedene Interferometeranordnungen zur Messung der CEP Drift von Ti:Saphir Oszillatoren systematisch auf ihr Eigenrauschen hin untersucht. In einem Vergleichsexperiment wird die Überlegenheit kompakter Interferometeraufbauten demonstriert. Zusätzlich wird eine einfache Anordnung vorgestellt, welche die Bestimmung der Phase eines spektralen Interferenzmusters. Das analoge Verfahren wird zur Einzelschussanalyse der CEP Drift von Verstärkerimpulsen verwendet. Hiermit werden erstmals schnelle Rauschbeiträge aufgedeckt. Ergänzend wird ein Konzept zur orthogonalen Kontrolle der CEP Drift mithilfe einer speziellen Kompensatoreinheit diskutiert. Der Kompensator besteht aus zwei Keilprismen, hergestellt aus unterschiedlichen optischen Materialien, die als Einheit verschoben werden. Durch geeignete Wahl der Materialien werden Effekte auf die Gruppenlaufzeit oder deren Dispersion gleichzeitig eliminiert. Darüber hinaus wird ebenfalls erstmalig ein lineares Messverfahren demonstriert, das die CEP Drift auch für ps-Oszillatoren erschließt. Zur linearen Detektion wird die spektrale Interferenz aufeinander folgender Impulse eines Impulszuges aufgelöst, die mit einem Ringresonator überlagert werden. Abschließend wird ein neues Verfahren präsentiert, das einen Impulszug generiert, dessen Einzelimpulse eine identische Feldstruktur aufweisen. Dazu das Messsignal direkt an ein externes Rückstellelement übergeben ohne eine zusätzliche Regelschleife zu benötigen. Auf diese Weise können Bandbreitenprobleme der Regelschleife vermieden werden. Da Rückstellelement und Laser voneinander getrennt sind, bleibt der Laser in seinem Betrieb ungestört. / This work discusses the carrier-envelope phase drift of mode-locked lasers as well as techniques for its active and passive stabilization. In order to reveal the physical origin of the drift, the investigations focus on the analysis of residual phase noise. From the analysis, potential improvements are developed. For this purpose, in a first experimental approach, different interferometer configurations for CEP drift detection of Ti:sapphire oscillators are compared. Comparative studies clearly reveal the superiority of compact interferometer set-ups in terms of noise. In a second series of experiments, a simple assembly is introduced for the direct extraction of the phase from spectral interference patterns. The analog method is demonstrated with single-shot measurements and utilized for stabilization of the CEP drift of an amplifier system, thereby enabling monitoring of additional fast noise contributions for the first time. Moreover, a concept for orthogonal control of the CEP drift by a specially designed compensator assembly is discussed. This assembly consists of two thin wedge prisms made from different optical materials. By choosing an appropriate material group delay and its dispersion are eliminated synchronously. Furthermore, a linear method is presented that provides access to the CEP drift of ps-oscillators for the first time. The newly introduced much more general linear approach relies on resolving the spectral interference of subsequent pulses from a pulse train, which are superimposed utilizing a ring resonator. Finally, a technique is demonstrated that generates a pulse train consisting of single pulses with identical field structure. Particularly, the heterodyne signal is directly fed forward to the external feedback element, replacing the classical servo loop. Therefore, servo bandwidth limitations are eliminated. Since feedback element and laser oscillator are decoupled, the laser performance is not corrupted by side effects from the feedback.

No presente trabalho, estudamos alguns aspectos da teoria de estruturas complexas em espaços de Banach. Demonstramos que se um espaço de Banach real $X$ tem a propriedade $P$, então todas as estruturas complexas em $X$ também satisfazem $P$, quando $P$ é qualquer uma das seguintes propriedades: propriedade de aproximação limitada, \\emph{G.L-l.u.st}, ser injetivo e ser complementado num espaço dual. Abordamos o problema da unicidade de estruturas complexas em espaços de Banach com base subsimétrica, provando que um espaço de Banach real $E$ com base subsimétrica e isomorfo ao espaço de sequências $E[E]$ admite estrutura complexa única. Por outro lado, apresentamos um exemplo de espaço de Banach com exatamente $\\omega$ estruturas complexas distintas. Também usamos a teoria de estruturas complexas para estudar o clássico problema dos hiperplanos no espaço $Z_2$ de Kalton-Peck. Com o propósito de distinguir $Z_2$ de seus hiperplanos nos perguntamos se os hiperplanos admitem estrutura complexa. Nesse sentido, provamos que os hiperplanos de $Z_2$ contendo a cópia canônica de $\\ell_2$ não admitem estruturas complexas que sejam extensões de estruturas complexas em $\\ell_2$. Também construímos uma estrutura complexa em $\\ell_2$ que não pode-se estender a nenhum operador em $Z_2$. / In this work, we study some aspects of the theory of complex structures in Banach spaces. We show that if a real Banach space $X$ has the property $P$, then all its complex structures also satisfy $P$, where $P$ is any of the following properties: bounded approximation property, \\emph{G.L-l.u.st}, being injective and being complemented in a dual space. We address the problem of uniqueness of complex structures in Banach spaces with subsymmetric basis by proving that a real Banach space $E$ with subsymmetric basis and isomorphic to the space of sequences $E [E]$ admits a unique complex structure. On the other hand, we show an example of Banach space with exactly $\\omega$ different complex structures. We also use the theory of complex structures to study the classical problem of hyperplanes in the Kalton-Peck space $Z_2$. In order to distinguish between $Z_2$ and its hyperplanes we wonder whether the hyperplanes admit complex structures. In this sense we prove that no complex structure on $\\ell_2$ can be extended to a complex structure on the hyperplanes of $Z_2$ containing the canonical copy $l_2$. We also constructed a complex structure on $l_2$ that can not be extended to any operator in $Z_2$.

No presente trabalho, estudamos alguns aspectos da teoria de estruturas complexas em espaços de Banach. Demonstramos que se um espaço de Banach real $X$ tem a propriedade $P$, então todas as estruturas complexas em $X$ também satisfazem $P$, quando $P$ é qualquer uma das seguintes propriedades: propriedade de aproximação limitada, \\emph{G.L-l.u.st}, ser injetivo e ser complementado num espaço dual. Abordamos o problema da unicidade de estruturas complexas em espaços de Banach com base subsimétrica, provando que um espaço de Banach real $E$ com base subsimétrica e isomorfo ao espaço de sequências $E[E]$ admite estrutura complexa única. Por outro lado, apresentamos um exemplo de espaço de Banach com exatamente $\\omega$ estruturas complexas distintas. Também usamos a teoria de estruturas complexas para estudar o clássico problema dos hiperplanos no espaço $Z_2$ de Kalton-Peck. Com o propósito de distinguir $Z_2$ de seus hiperplanos nos perguntamos se os hiperplanos admitem estrutura complexa. Nesse sentido, provamos que os hiperplanos de $Z_2$ contendo a cópia canônica de $\\ell_2$ não admitem estruturas complexas que sejam extensões de estruturas complexas em $\\ell_2$. Também construímos uma estrutura complexa em $\\ell_2$ que não pode-se estender a nenhum operador em $Z_2$. / In this work, we study some aspects of the theory of complex structures in Banach spaces. We show that if a real Banach space $X$ has the property $P$, then all its complex structures also satisfy $P$, where $P$ is any of the following properties: bounded approximation property, \\emph{G.L-l.u.st}, being injective and being complemented in a dual space. We address the problem of uniqueness of complex structures in Banach spaces with subsymmetric basis by proving that a real Banach space $E$ with subsymmetric basis and isomorphic to the space of sequences $E [E]$ admits a unique complex structure. On the other hand, we show an example of Banach space with exactly $\\omega$ different complex structures. We also use the theory of complex structures to study the classical problem of hyperplanes in the Kalton-Peck space $Z_2$. In order to distinguish between $Z_2$ and its hyperplanes we wonder whether the hyperplanes admit complex structures. In this sense we prove that no complex structure on $\\ell_2$ can be extended to a complex structure on the hyperplanes of $Z_2$ containing the canonical copy $l_2$. We also constructed a complex structure on $l_2$ that can not be extended to any operator in $Z_2$.