In this thesis I describe experimental work and present data on the guiding of Rubidium atoms along free-space propagating light beams as well as within hollow core glass fibres, namely photonic crystal fibres. I describe experiments, laser systems and vacuum trap assemblies designed to facilitate this guiding. These experiments are intended to aid progression within the field of cold atom guidance wherein narrow diameter, long distance hollow-fibre guides are a current goal. Realisation of these guides could lead to promising applications such as atom interferometers and spatially accurate, multi-source, atom depositors. Herein, guided fluxes are observed in free-space guiding experiments for distances up to 50mm and up to 10GHz red-detuning from resonance. Additionally hollow-core, Kagome structured, quasi- and true-photonic crystal fibres are characterised. Finally a number of detailed fibre-guiding magneto-optic traps are developed. Both cold atomic-beams and cold atomic clouds are reliably positioned above fibre entrance facets in conjunction with a guiding laser beam coupled into the fibre core. Issues regarding optical flux detection outwith fibre confinement appear to have hindered observation of guided atoms. A far more sensitive detection system has been developed for use in current, ongoing fibre-guide experiments.

Examensarbetet Binary surfaces - ljusemitterande textiler för inredningssammanhang är ett undersökande arbete om hur fiberoptik kan användas i vävda strukturer. I arbetet har de parametrar som påverkar mötet mellan fiberoptik och väv, samt de möjligheter de tillsammans skapar utforskats. Resultatet är två exempel på tyger vävda med fiberoptik i kombination med mer traditionella textila material. Exemplen är framtagna för inredningssammanhang och visas som en rumsavdelare och som yttertyg på sittmöbler. / The thesis Binary surfaces - light-emitting textiles for an interior design context is an investigative work on how optical fibres can be used in woven structures. The work explores the different parameters that affect the interaction between optical fibres and weaving, and the possibilities they create together. The result is two examples of fabrics woven with optical fibres in combination with more traditional textile materials. The examples are designed for an interior context and are shown as a room divider and as outer fabric on seating furnishing.

This work encompasses a study of fibre optic devices and micro-resonator cavities and their use as strain sensors, interferometers, and eventually as light sources. We demonstrate the use of a pi-shifted fibre Bragg grating as a strain sensor and quantify its performance through several metrics relevant to its operation in a frequency-locked system. We then discuss the use of waveguide devices as refractive index sensors in a detailed and critical analysis of most of the literature on fibre optic-based interferometry where we define new metrics of assessment that are aimed to be device-independent. We then categorize and study the performance of the literature on fiber-optic refractive index sensors and highlight few key devices of each category based on their performance or design. We relate the performance of these interferometers to various design parameters to outline a way for future researchers to create high-performance devices. Finally, we study light emission in dye-loaded droplet micro-resonators, which are a promising new platform to generate low-threshold lasing action excited by lasers coupled to the whispering gallery modes of the resonator. We characterize multiple types of emissions in these cavities and pave the way for future microdroplet-based lasers. / Graduate / 2022-08-10

Tesis por compendio / [ES] La necesidad de confinar y manipular micro-objetos tiene aplicaciones en múltiples áreas de la ciencia y tecnología. Actualmente, existen diversas técnicas para lograr este objetivo, y una de las más destacadas es el uso de las llamadas pinzas ópticas, que se han convertido en una herramienta ampliamente utilizada en laboratorios de todo el mundo. Este trabajo de investigación se centra en el fascinante campo del atrapamiento y manipulación de micro-objetos, con un enfoque destacado en la combinación de elementos ópticos difractivos y la técnica de pinzas ópticas. Esta combinación permite un aumento de la versatilidad de los sistemas experimentales de pinzas ópticas. Los avances presentados en esta tesis tienen aplicaciones en una amplia gama de campos, desde la nanotecnología hasta la biología celular. Como lentes difractivas implementadas en los sistemas de pinzas ópticas, se introducen las lentes difractivas Kinoform basadas en la secuencia aperiódica m-Bonacci. Estas lentes permiten atrapar múltiples partículas simultáneamente y manipularlas tridimensionalmente en dos planos focales diferentes, lo que amplía significativamente las posibilidades de investigación y desarrollo en diversas disciplinas. Además, se aborda la generación de múltiples trampas ópticas mediante lentes Kinoform cuadrifocales basadas en otra secuencia aperiódica conocida como Silver Mean, permitiendo atrapar partículas en cuatro planos focales de manera simultánea. Este avance mejora significativamente la versatilidad de los sistemas de pinzas ópticas. Adicionalmente el uso de vórtices multiplexados en un sistema de pinzas ópticas, permite atrapar de manera independiente múltiples partículas y transferir momento angular. Estos avances abren nuevas posibilidades en la construcción de micromotores y aplicaciones de micro-ensamblaje. Un efecto asociado a las trampas ópticas es la generación de microburbujas, en la actualidad estas se han convertido en objeto de estudio debido a la facilidad de generación y a sus posibles aplicaciones como agentes de transporte de partículas o micro-objetos. Aprovechando este efecto en esta tesis se implementa una técnica de atrapamiento que emplea fuerzas termoforéticas en la captura y manipulación de microburbujas en líquidos. Esto constituye otro avance importante en el campo del atrapamiento tridimensional. Por último, se desarrolla un laboratorio virtual utilizando COMSOL Multiphysics para simular el atrapamiento acústico, lo que permite a los estudiantes interactuar con el sistema y comprender mejor este fenómeno. Este enfoque educativo proporciona herramientas valiosas para la comprensión y análisis de la manipulación de partículas, lo que beneficia a los estudiantes de pre-grado y grado interesados en este campo. En conjunto, todos estos avances representan contribuciones significativas en el campo del atrapamiento y manipulación de partículas, en particular a través de las pinzas ópticas, promoviendo el progreso tecnológico y científico en diversas disciplinas y brindando oportunidades educativas para futuras generaciones de investigadores y científicos. A lo largo del desarrollo de esta tesis, se han creado nuevos elementos difractivos que superan ciertas limitaciones y aumentan las capacidades de las pinzas ópticas, abriendo nuevas perspectivas de aplicación para tecnologías preexistentes. / [CA] La necessitat de confinar i manipular microobjectes té aplicacions a múltiples àrees de la ciència i la tecnologia. En l'actualitat, hi ha diverses tècniques per assolir aquest objectiu, i una de les més destacades és l'ús de les anomenades pinces òptiques, que han esdevingut una eina molt utilitzada en laboratoris de tot el món. Aquest treball de recerca se centra en el fascinant camp de la captura i la manipulació de microobjectes, destacant la combinació d'elements òptics difractius i la tècnica de les pinces òptiques. Aquesta combinació permet augmentar la versatilitat dels sistemes experimentals de pinces òptiques. Els avenços presentats en aquesta Tesi tenen aplicacions en una àmplia gamma de camps, des de la nanotecnologia a la biologia cel·lular. Com a lents difractives implementades en sistemes de pinces òptiques, es presenten les lents difractives Kinoform basades en la seqüència aperiòdica m-Bonacci. Aquestes lents permeten atrapar simultàniament múltiples partícules i manipular-les tridimensionalment en dos plans focals diferents, fet que amplia significativament les possibilitats de recerca i desenvolupament en diverses disciplines. A més, s'aborda la generació de múltiples trampes òptiques utilitzant lents Kinoform quadrifocals basades en una altra seqüència aperiòdica coneguda com a Silver Mean, que permet atrapar partícules en quatre plans focals simultàniament. Aquest avenç millora significativament la versatilitat dels sistemes de pinces òptiques. A més, l'ús de vòrtexs multiplexats en un sistema de pinces òptiques permet atrapar múltiples partícules de manera independent i transferir el moment angular. Aquests avenços obren noves possibilitats en la construcció de micromotors i aplicacions de microassemblatge. Un efecte associat a les trampes òptiques és la generació de microbombolles, actualment aquestes s'han convertit en objecte d'estudi a causa de la facilitat de generació i de les seves potencials aplicacions com a agents de transport de partícules o microobjectes. Aprofitant aquest efecte, aquesta Tesi implementa una tècnica d'atrapament que utilitza forces termoforètiques en la captura i manipulació de microbombolles en líquids. Això constitueix un altre avenç important en el camp de l'atrapament tridimensional. Finalment, es desenvolupa un laboratori virtual utilitzant COMSOL Multiphysics per simular l'atrapament acústic, cosa que permet als estudiants interactuar amb el sistema i comprendre millor aquest fenomen. Aquest enfocament educatiu proporciona eines valuoses per a la comprensió i l'anàlisi de la manipulació de partícules, cosa que beneficia els estudiants de grau i postgrau interessats en aquest camp. En conjunt, tots aquests avenços representen contribucions significatives al camp de l'atrapament i la manipulació de partícules, particularment a través de pinces òptiques, promovent el progrés tecnològic i científic en diverses disciplines i proporcionant oportunitats educatives per a futures generacions d'investigadors i científics. Al llarg del desenvolupament d'aquesta Tesi, s'han creat elements difractius nous que superen certes limitacions i augmenten les capacitats de les pinces òptiques, obrint noves perspectives d'aplicació per a tecnologies preexistents. / [EN] The need to confine and manipulate micro-objects has applications in multiple areas of science and technology. Currently, there are several techniques to achieve this goal, and one of the most prominent is the use of the so-called optical tweezers, which have become a widely used tool in laboratories around the world. This research work focuses on the fascinating field of micro-object capture and manipulation, highlighting the combination of diffractive optical elements and the optical tweezers technique. This combination allows to increase the versatility of the experimental optical tweezers systems. The advances presented in this thesis have applications in a wide range of fields, from nanotechnology to cell biology. As diffractive lenses implemented in optical tweezers systems, Kinoform diffractive lenses based on the aperiodic m-Bonacci sequence are presented. These lenses allow multiple particles to be trapped simultaneously and manipulated three-dimensionally in two different focal planes, which significantly expands the possibilities for research and development in various disciplines. In addition, the generation of multiple optical traps is addressed using quadrifocal Kinoform lenses based on another aperiodic sequence known as Silver Mean, which allows particles to be trapped in four focal planes simultaneously. This advance significantly improves the versatility of optical tweezer systems. In addition, the use of multiplexed vortices in an optical tweezer system allows multiple particles to be trapped independently and angular momentum to be transferred. These advances open up new possibilities in micromotor construction and microassembly applications. One effect associated with optical traps is the generation of microbubbles, currently these have become an object of study due to the ease of generation and their potential applications as transport agents for particles or micro-objects. Taking advantage of this effect, this thesis implements an trapping technique that employs thermophoretic forces in the capture and manipulation of microbubbles in liquids. This constitutes another important advance in the field of three-dimensional trapping. Finally, a virtual laboratory is developed using COMSOL Multiphysics to simulate acoustic trapping, allowing students to interact with the system and better understand this phenomenon. This educational approach provides valuable tools for the understanding and analysis of particle manipulation, benefiting undergraduate and graduate students interested in this field. Taken together, all of these advances represent significant contributions to the field of particle trapping and manipulation, particularly through optical tweezers, promoting technological and scientific progress in various disciplines and providing educational opportunities for future generations of researchers and scientists. Throughout the development of this thesis, new diffractive elements have been created that overcome certain limitations and increase the capabilities of optical tweezers, opening new application perspectives for pre-existing technologies. / We acknowledge the financial support from Ministerio de Ciencia e Innovación (grants PID2019-107391RB-I00 and PID2022-1142407NB-I00), Generalitat Valenciana (grant PROMETEO/2019/048 and CI-PROM/2022/30), and Universitat Politècnica de València (PAID-01-20-25), Spain. We acknowledge the financial support from CONACyT (grant A1-S-28440). / Muñoz Pérez, FM. (2024). Diseño y caracterización experimental de sistemas de atrapamiento y manipulación de micro-objetos mediante técnicas ópticas, térmicas y acústicas [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/203436 / Compendio

Gallium nitride (GaN)-based light-emitting diodes (LEDs) with cylindrical nanostructures have been the subject of significant research interest in the past decade, due to the potential of such structures to increase light extraction efficiency and deliver highly directional light emission. Nanorod LEDs, where the light emission is within the nanocylinder, have the additional potential to increase internal quantum efficiency and emit in colours previously thought impractical with GaN-based LEDs. Optical modelling of nanostructured LEDs is usually carried out using finite-difference time-domain methods, which are computationally intensive and do not always provide sufficient insight into the physics underlying the simulation results. This thesis proposes an intuitive, quasi-analytic method based on modal expansion. It is found that it is possible to calculate the far field diffraction patterns of all guided modes supported by a single nanorod, with full consideration of Fabry-Perot effects, in minutes using a standard office desktop computer. Focus is placed on the case of a nanorod of radius 140 nm, for which angular photoluminescence measurements were available to provide a means of validating the model. Consideration of the guided modes alone provides a compelling explanation for gross features in the measured data where none previously existed. It is shown that, using a standard equation from a textbook, it is possible to calculate how much each of the guided and radiation modes of a single nanorod is excited by a Hertzian dipole of known position and orientation with respect to the nanorod geometry. When interference between these modes is considered, it is possible to calculate the total far field angular emission pattern due to that dipole. Comparing these patterns with photoluminescence measurements allows one to infer the locations and orientations of dipole current sources; the results are found to be consistent with those of cathodoluminescence studies.

Denna rapport beskriver ett examensarbete där optiska fibrer av plast integrerades i varpled i en väv. Syftet med examensarbetet var att undersöka om det är möjligt att industriellt producera ljusemitterande vävar med optiska fibrer. Arbetet delades upp i olika delar, både teoretiska och praktiska. Den teoretiska delen innebar informationssökning inom olika medier, såsom vetenskapliga artiklar och intervjuer med insatta människor inom de berörda områdena. Den praktiska delen i examensarbetet bestod av flera olika moment. Först utfördes en provvävning i en handvävstol, för att finna en lämplig bindning och lära känna materialen. Därefter konstruerades en metod för att testa olika slipmaterial att slipa den optiska fibern med, och utifrån resultaten valdes två slipmaterial ut. Sedan sattes en väv med optiska fibrer upp i en vävmaskin och ett flertal provvävar vävdes. I ett optiklaboratorium skedde sedan mätningar på provvävarnas ljusemittering.Resultatet av mätningarna visade att de optiska fibrerna inte klarade av krökningarna i väven. För mycket ljus läckte ut i krökningarna så väven lyste inte mer än 50 cm. Slutsatsen är att det är fullt möjligt att väva med optisk fiber i varpled, men en bindning där de optiska fibrerna ligger rakt i väven måste konstrueras. In this bachelor thesis has plastic optical fibres been integrated in the warp of a weave. The purpose of the thesis was to make a research if it is possible to produce light emitting weaves with optical fibres in an industrial production. The thesis was divided into several parts, both theoretical and practical. The theoretical part involved taking part of different types of information, such as scientific articles and interviews with knowledgeable people within the related areas.The practical part in this thesis was composed of several different phases. A test weave in a handloom was first performed to find out which weave to use and to understand the materials behaviour. Then a method for testing different materials to grind the optical fibre with was constructed, and out of the results two materials were chosen. After that, a warp with optical fibres was placed into a loom and several prototypes were woven. In an optic laboratory were measurements made on the prototypes’ light emitting ability. The results of the measurements showed that the optical fibres did not cope with the bends in the weave. Too much light emitted out of the bends so the weave only emitted light for 50 cm. The conclusion is that it is possible to weave with optical fibres in the warp, but a new weave where the optical fibres lie straight in the fabric must be constructed. / Program: Textilingenjörsutbildningen

Kunststofflichtwellenleiter (POFs) stellen ein verhältnismäßig neues Medium zur optische Datenkommunikation über kurzen Strecken dar. Während ihrer Einsatzdauer unterliegen POFs unterschiedlichen Arten von Umweltbeanspruchungen, hauptsächlich durch hohe Temperatur, hohe Feuchtigkeit und mechanischen Belastungen. Zahlreiche experimentelle Forschungen beschäftigten sich mit der standardisierten Prüfung der Zuverlässigkeit von im Handel erhältlichen Fasern. Jedoch gab es bisher wenig Erfolg bei der Bemühung, zwei grundlegende optische Erscheinungen, Absorption und Streuung, die die Lichtausbreitung in Fasern stark beeinflussen, zu verstehen und praktisch zu modellieren: Diese beiden Effekte beschreiben nicht nur die Qualität neuer Fasern, sondern sie werden auch stark durch die Alterungsprozess beeinflusst.<br><br> Der Hauptzweck dieser Doktorarbeit war es, ein praktisch verwendbares und theoretisch gut fundiertes Modell der Lichtausbreitung in nicht gealterten und gealterten POFs zu entwickeln und es durch optische Experimente zu verifizieren. Dabei wurden anwendungsorientierte Aspekte mit theoretischer POF-Modellierung kombiniert. Die Arbeit enthält die erste bekannte Anwendung der Wellenanalyse zur Untersuchung der winkelabhängigen Eigenschaften der Streuung in Lichtwellenleitern.<br><br> Für die praktischen Experimente wurden mehrere POF-Proben unterschiedlicher Hersteller künstlich gealtert, indem sie bis 4500 Stunden bei 100 °C gelagert wurden (ohne Feuchtekontrolle). Die Parameter der jeweiligen Simulationen wurden mittels einer systematischen Optimierung an die gemessen optischen Eigenschaften der gealterten Proben angeglichen. Die Resultate deuten an, dass der Übertragungsverlust der gealterten Fasern in den ersten Tagen und Wochen der Alterung am stärksten durch eine wesentliche physikalische Verschlechterung der Kern-Mantel-Grenzfläche verursacht wird. Chemische Effekte des Alterungsprozesses scheinen im Faserkernmaterial zuerst nach einigen Monaten aufzutreten. / This thesis discusses theoretical and practical aspects of modelling of light propagation in non-aged and aged step-index polymer optical fibres (POFs). Special attention has been paid in describing optical characteristics of non-ideal fibres, scattering and attenuation, and in combining application-oriented and theoretical approaches. The precedence has been given to practical issues, but much effort has been also spent on the theoretical analysis of basic mechanisms governing light propagation in cylindrical waveguides.<br><br>As a result a practically usable general POF model based on the raytracing approach has been developed and implemented. A systematic numerical optimisation of its parameters has been performed to obtain the best fit between simulated and measured optical characteristics of numerous non-aged and aged fibre samples. The model was verified by providing good agreement, especially for the non-aged fibres. The relations found between aging time and optimal values of model parameters contribute to a better understanding of the aging mechanisms of POFs.

Kunststofflichtwellenleiter (POFs) stellen ein verhältnismäßig neues Medium zur optische Datenkommunikation über kurzen Strecken dar. Während ihrer Einsatzdauer unterliegen POFs unterschiedlichen Arten von Umweltbeanspruchungen, hauptsächlich durch hohe Temperatur, hohe Feuchtigkeit und mechanischen Belastungen. Zahlreiche experimentelle Forschungen beschäftigten sich mit der standardisierten Prüfung der Zuverlässigkeit von im Handel erhältlichen Fasern. Jedoch gab es bisher wenig Erfolg bei der Bemühung, zwei grundlegende optische Erscheinungen, Absorption und Streuung, die die Lichtausbreitung in Fasern stark beeinflussen, zu verstehen und praktisch zu modellieren: Diese beiden Effekte beschreiben nicht nur die Qualität neuer Fasern, sondern sie werden auch stark durch die Alterungsprozess beeinflusst. <br><br> Der Hauptzweck dieser Doktorarbeit war es, ein praktisch verwendbares und theoretisch gut fundiertes Modell der Lichtausbreitung in nicht gealterten und gealterten POFs zu entwickeln und es durch optische Experimente zu verifizieren. Dabei wurden anwendungsorientierte Aspekte mit theoretischer POF-Modellierung kombiniert. Die Arbeit enthält die erste bekannte Anwendung der Wellenanalyse zur Untersuchung der winkelabhängigen Eigenschaften der Streuung in Lichtwellenleitern. <br><br> Für die praktischen Experimente wurden mehrere POF-Proben unterschiedlicher Hersteller künstlich gealtert, indem sie bis 4500 Stunden bei 100 °C gelagert wurden (ohne Feuchtekontrolle). Die Parameter der jeweiligen Simulationen wurden mittels einer systematischen Optimierung an die gemessen optischen Eigenschaften der gealterten Proben angeglichen. Die Resultate deuten an, dass der Übertragungsverlust der gealterten Fasern in den ersten Tagen und Wochen der Alterung am stärksten durch eine wesentliche physikalische Verschlechterung der Kern-Mantel-Grenzfläche verursacht wird. Chemische Effekte des Alterungsprozesses scheinen im Faserkernmaterial zuerst nach einigen Monaten aufzutreten. / This thesis discusses theoretical and practical aspects of modelling of light propagation in non-aged and aged step-index polymer optical fibres (POFs). Special attention has been paid in describing optical characteristics of non-ideal fibres, scattering and attenuation, and in combining application-oriented and theoretical approaches. The precedence has been given to practical issues, but much effort has been also spent on the theoretical analysis of basic mechanisms governing light propagation in cylindrical waveguides. <br><br> As a result a practically usable general POF model based on the raytracing approach has been developed and implemented. A systematic numerical optimisation of its parameters has been performed to obtain the best fit between simulated and measured optical characteristics of numerous non-aged and aged fibre samples. The model was verified by providing good agreement, especially for the non-aged fibres. The relations found between aging time and optimal values of model parameters contribute to a better understanding of the aging mechanisms of POFs.<br><br>

Des fibres optiques en verres de chalcogénure transmettant dans le moyen infrarouge (MIR) ont été développées par le laboratoire Verres et Céramiques de l’université de Rennes 1. Des travaux ont ensuite montré les potentialités de ces fibres comme outil diagnostic pour la spectroscopie MIR appliquée aux biofluides. Le spectre moyen infrarouge d’un échantillon complexe est le reflet de sa composition moléculaire qui, lorsque celui-ci est un biofluide, peut être assimilé à une image métabolique instantanée d’un individu. Cette technique constitue donc un outil intéressant pour le diagnostic médical. La société Diafir fut crée à la suite du programme ANR émergence FIR-MED pour développer les potentialités de la spectroscopie par fibre optique. L’objectif de l’entreprise est de développer un système composé d’un spectromètre spécifiquement adapté au capteur à fibre optique et d’un algorithme associé pour une réponse diagnostique ne nécessitant pas de connaissance particulière en spectroscopie infrarouge. Dans le cadre de ces travaux de thèse associant l’IRDL et la société Diafir, des projets variés d’applications médicales de la technologie ont été abordés avec pour objectif de développer une méthodologie d’analyse des spectres MIR. Pour cela, il a été nécessaire dans un premier temps de caractériser le signal obtenu à partir des capteurs et d’optimiser et valider des protocoles de mesure applicables aux différents biofluides de manière à réduire au mieux les sources de variabilité d’ordre physique et environnemental. La construction d’un diagnostic médical à partir de spectres infrarouge qui sont des données en grande dimension implique de sélectionner un petit nombre de variables spectrales explicatives. Des méthodes de sélection de variables ont été sélectionnées afin d’éviter l’effet « boite noire » en établissant le lien avec les fonctions biochimiques impactées par les pathologies. Cette approche spectrale infrarouge a notamment permis de mettre en évidence, au cours du développement de pathologies, l’existence de transitions a priori non détectées par les dosages biochimiques classiques. / Chalcogenide glass optical fibres exhibiting unique properties of mid-infrared (MIR) transparency have been developed by the Laboratoire Verres et Céramiques of Rennes 1 University. Our studies investigate the potential of such fibres as a tool for MIR spectroscopy applied to biofluids based diagnostic. The MIR spectrum of complex samples features its molecular composition which, when a biofluid is considered, can be assimilate to an instant metabolic imaging of an individual. The Diafir Company was created following the ANR emergence FIR-MED program to develop the potential of optical fibre biomedical spectroscopy. The company's goal is to develop a system that is composed of the optical fibre sensor, a spectrometer specifically designed to these sensors and an associated algorithm for a diagnostic response without specific knowledge of infrared spectroscopy. As part of this thesis work, linking the IRDL and the Diafir Company, various projects of biomedical applications were driven with the aim of developing a MIR spectra analysis methodology. Accordingly, it was necessary initially to characterize the signal obtained from the sensors and to optimize and validate robust measurement protocols for each biofluid tested with the aim to reduce the physical and environmental sources of variability. The medical diagnosis construction from infrared spectra, that are high dimensional data, involves selecting from the whole spectral data set a small number of explanatory variables. Purposely, particular variables selection algorithms were selected to avoid the effect of "black box" by establishing the link with the biochemical functions affected by the disease. This infrared spectral approach allowed identifying previously unrevealed transitions in the time course of pathologies which were not detected from conventional biochemical markers

This thesis presents the first research on polymer-segmented-cladding-fibre (PSCF), an emerging class of microstructured- optical-fibres (MOFs), which allows single-mode operation with ultra-large-core area. This research covers the modelling, design, fabrication and experiment of the polymer optical cross-fibre (4-period-SCF) whose cross-sectional view resembles a cross. A new wedge waveguide model has been formulated and applied to demonstrate that for any given parameters, the cross fibre gives the same performance for single-mode operation as the N-period-SCFs (for N = 2, 6 and 8). These fibres behave identically if the high-index segment angle, θ1, is the same and the low-index segment angular width, θ2, is sufficiently large for negligible adjacent mode coupling effects. This remarkable finding has significant ramifications for SCF fabrication, design and performance. Theoretical predictions confirmed by experiments demonstrated that a cross-fibre is all that needed to fabricate a large-core single-mode-fibre with no geometry-induced birefringence. The high-index outer ring effects on the cross fibre single-mode performance have been systematically investigated for the first time. The study reveals that the ring index value higher than its core index has very strong effects on single-mode performance. Within a narrow range of θ1, the minimum fibre length required for single-mode operation is reduced but outside this angle range, longer single-mode length is required. Furthermore, the fibre can be anti-guiding if θ1 exceeds the cutoff angle. Incorporating the fabrication constraints, the optimal cross-fibre design with high-index ring is achieved by optimising the relative index difference, high-index segment angle and core-cladding diameter ratio. Two preform-making techniques developed for the cross-fibres fabrication include the cladding-segment-in-tube method and the core-cladding-segment-in-tube method. The innovative approach in these methods overcomes the problems of bubble formation and fractures, which are related to the fibre structure complexity and the polymer intrinsic properties and their processing. It enables the successful drawing of single-mode fibres. This thesis reports the first experimental demonstration of single-mode operation of large-core cross-fibre. Three experimental studies with different cross-fibre designs have demonstrated (i) large-core single-mode operation, (ii) high-index ring effects on fibre performance and (iii) cross-fibre optimal design trial. Apart from this, the 8-period-SCF fibre performance has been demonstrated experimentally.