Side-polished fiber allows access to the evanescent field propagating in the cladding of a few-mode fiber. This cladding mode is analyzed and experimentally validated to further the design of a novel class of fiber optic devices. To do this, specific modes are excited in the polished fiber using a phase-only spatial light modulator to determine spatial mode distribution. Each mode is excited and compared to the expected field distribution and to confirm that higher order modes can propagate through side-polished fiber. Based on each mode’s distribution, a side-polished fiber can be designed so that perturbations on the polished portion of the fiber effect each mode independently. By carefully analyzing the effects of identical perturbations on each mode, it is determined that each mode can be isolated based on the geometry of the polished fiber and careful alignment of the mode field. This research has the potential to advance the development of novel fiber-based sensors and communications devices utilizing mode-based interferometry and mode multiplexing. / M.S. / Fiber optic devices have seen significant advancement since the realization of the laser and low-loss optical fiber. Modern day fiber optics are commonly utilized for high-bandwidth communications and specialized sensing applications. Utilizing multiple modes, or wave distributions, in a fiber provides significant advantages towards increasing bandwidth for communications and provides potential for more accurate sensing techniques. Significant research has been conducted in both the sensing and communication field, but mode-domain devices have the capability to significantly advance the field of fiber optic devices. This thesis demonstrates the potential for side-polished fiber geometry to effect each mode independently, thus allowing side-polished fiber to be utilized for realizing novel devices such as multiplexing devices and fiber optic sensors.

Long-period fiber gratings (LPGs) with a period ranging from several hundred micrometers to a few millimeters can couple a core mode to discrete co-propagating cladding modes when the phase matching condition is satisfied. The rapid attenuation of cladding modes results in loss bands in the transmission spectrum. As the attenuation bands are sensitive to the LPG period and the fiber surrounding environment such as temperature, strain and ambient refractive index, LPGs can be used for sensing. However, traditional LPGs with gratings inscribed in the fibers can only sense a single point and cannot be used for distributed sensing. Although new ideas were proposed to use traveling LPG formed by a pulsed acoustic wave, the large attenuation of the acoustic wave in the fiber greatly limits the sensing range to only several meters. In this thesis, we proposed to use a traveling LPG formed by the interference of two high power co-propagating core modes, usually LP01 and LP11. The beating of the two modes will induce a refractive index grating due to the optical Kerr effect, and the grating is called optically induced long-period fiber grating (OLPG). Compared to the grating induced by acoustic waves, OLPG is able to travel for a long distance due to the small attenuation of the guided core modes. Mode conversion in the OLPG is numerically simulated and analyzed using the finite-difference beam propagation method (FD-BPM). The result shows full conversion for both core-core and core-cladding mode coupling under phase matching condition. Moreover, the sensitivity of OLPG to temperature, axial strain and ambient refractive index is investigated and analyzed. It is seen that the sensitivities of temperature and axial strain with OLPG are different from the traditional LPGs since the period variation in OLPG is caused by the effective index difference of the two core modes at the writing wavelength, while in the traditional LPGs it is directly induced by temperature or strain. For the refractive index sensitivity with a large cladding, OLPG behaves the same as a traditional LPG with only material contributions since the grating period remains unchanged. / Master of Science

[ES] En esta Tesis Doctoral, se propone diferentes técnicas de acoplo y conversión modal destinadas a aumentar la capacidad de transporte en sistemas de telecomunicaciones sobre fibra óptica. En particular, el objetivo principal es el desarrollo de la tecnología necesaria para conseguir una multiplexación modal utilizando un número limitado de modos, de manera controlada. Para ello, se estudian dos escenarios MDM con dos longitudes de onda distinta. Por un lado, usando la longitud de onda de 850 nm sobre SSMF favoreciendo la utilización de componentes ópticos y electro-ópticos de coste mucho menor que sus equivalentes en la banda C+L. Esta novedosa tecnología de transmisión permitirá una nueva generación de interconexiones ópticas de muy alta capacidad aplicable a enlaces chip-a-chip, a backplanes ópticos y también a clústeres de computación de altas prestaciones y centros de conmutación de red. Por otro lado, usando la longitud de onda de 1550 nm sobre guías ópticas basadas en SOI, es decir, Si (silicio) sobre sustrato de SiO2 (óxido de silicio) favoreciendo la utilización de dispositivos basados en tecnología integrada que ofrecen un menor tamaño, mejor repetibilidad y robustez que los dispositivos basados en fibra óptica. Para ello, se propone el uso de acopladores ópticos fusionados siendo un elemento indispensable a la hora de multiplexar y demultiplexar los distintos modos ópticos en un enlace MDM a 850 nm. Esta técnica permite multiplexar/demultiplexar los modos ópticos cuando el tipo de acoplador óptico utilizado es simétrico (DC, del inglés directional coupler), siendo necesario la utilización de un conversor de modos. También se estudia la posibilidad de convertir el modo óptico mediante la utilización de un acoplador óptico asimétrico (ADC, del inglés asymmetrical directional coupler), no siendo necesario utilizar un conversor de modos y simplificando el esquema MDM. Además, en esta tesis doctoral también se propone y evalúa el diseño de un conversor de modos mecánico basado en SSMF. Esta técnica permite obtener el primer modo de orden superior con una alta calidad y sin la necesidad de utilizar un ADC. Después de esto, se propone y evalúa la posibilidad de utilizar acopladores comerciales (diseñados a 1550 nm) a la longitud de onda de 850 nm permitiendo de esta forma reducir la necesidad de utilizar acopladores ópticos y conversores modales específicamente diseñados en dicha longitud de onda. Esta técnica reduciría los costes del sistema al necesitar un menor número de dispositivos y aprovechar los dispositivos diseñados a 1550 nm, siendo más económicos que los diseñados a 850 nm. En esta Tesis también se propone el uso de ADCs en guías strip basadas en SOI para la conversión y multiplexación de los modos ópticos desde la guia fundamental a la guia de dos modos, a la longitud de onda de 1550 nm. Para ello se estudia y demuestra experimentalmente diferentes diseños con el fin de obtener el diseño más robusto frente a las tolerancias de fabricación consiguiendo un rendimiento óptimo. Además, el uso de DCs sobre guías ridge es comúnmente utilizado y ofrece mejores prestaciones que el basado en guías strip, por ese motivo esta Tesis estudia y evalúa el uso de ADCs sobre guías ridge mediante el método de análisis de los índices efectivos de los supermodos par e impar. De esta forma se realiza una comparación entre los diseños óptimos de ambas estructuras (strip y ridge) con el objetivo de averiguar qué diseño ofrece mejores prestaciones. Por último, se propone y estudia el diseño de un acoplador grating capaz de multiplexar y demultiplexar los modos ópticos del modo fundamental y del primer orden superior desde la guia óptica a la fibra óptica y viceversa. Para ello se proponen diferentes diseños con el objetivo de conseguir un diseño más tolerante y eficiente frente a los errores por desalineamiento obteniendo un acoplo óptimo. / [CAT] En aquesta Tesi Doctoral, es proposen diferents tècniques d'acoblament i conversió modal destinades a augmentar la capacitat de transport en sistemes de telecomunicacions sobre fibra òptica. En particular, l'objectiu principal és el desenrotllament de la tecnologia necessària per a aconseguir una multiplexació modal utilitzant un número limitat de modes, de manera controlada. Per a això, s'estudien dos escenaris MDM amb dos longituds d'onda distinta. D'una banda, usant la longitud d'ona de 850 nm sobre SSMF afavorint la utilització de components òptics i electro-òptics de cost molt menor que els seus equivalents en la banda C+L. Aquesta nova tecnologia de transmissió permetrà una nova generació d'interconnexions òptiques de molt alta capacitat aplicable a enllaços chip-a-chip, a backplanes òptics i també a clústers de computació d'altes prestacions i centres de commutació de xarxa. D'altra banda, usant la longitud d'ona de 1550 nm sobre guies òptiques basades en SOI, és a dir, Si (silici) sobre substrat de SiO2 (òxid de silici) afavorint la utilització de dispositius basats en tecnologia integrada que ofereixen una menor grandària, millor repetibilitat i robustesa que els dispositius basats en fibra òptica. Per a això, es proposa l'ús d'acobladors òptics fusionats sent un element indispensable a l'hora de multiplexar i demultiplexar els distints modes òptics en un enllaç MDM. Aquesta tècnica permet multiplexar/demultiplexar els modes òptics quan el tipus d'acoblador òptic utilitzat és simètric (DC, de l'anglès directional coupler), sent necessari la utilització d'un convertidor de modes. També s'estudia la possibilitat de convertir el mode òptic per mitjà de la utilització d'un acoblador òptic asimètric (ADC, de l'anglès asymmetrical directional coupler), no sent necessari utilitzar un convertidor de modes i simplificant l'esquema MDM. Es mes, en aquesta tesi doctoral també es proposa i avalua el disseny d'un convertidor de modes mecànic basat en SSMF. Aquesta tècnica permet obtindre el primer mode d'orde superior amb una alta qualitat sense la necessitat d'utilitzar un ADC. Després d'açò, es proposa i avalua la possibilitat d'utilitzar acobladors comercials (dissenyats a 1550 nm) a la longitud d'ona de 850 nm permetent d'esta manera reduir la necessitat d'utilitzar acobladors òptics i convertidors modals específicament dissenyats en la dita longitud d'ona. Aquesta tècnica reduiria els costos del sistema al necessitar un menor nombre de dispositius i aprofitant els dispositius dissenyats a 1550 nm, sent més econòmics que els dissenyats a 850 nm. En aquesta Tesi també es proposa l'ús de ADCs en guies strip basades en SOI per a la conversió i multiplexació dels modes òptics des de la guia fonamental a la guia de dos modes, a la longitud d'ona de 1550 nm. Per a això s'estudia i demostra experimentalment diferents dissenys a fi de obtindré el disseny més robust enfront les toleràncies de fabricació aconseguint un rendiment òptim. A més, l'ús de DCs sobre guies ridge és comunament utilitzat i ofereix millors prestacions que el basat en guies strip, per eixe motiu aquesta Tesi estudia i avalua l'ús de ADCs sobre guies ridge per mitjà del mètode d'anàlisi dels índexs efectius dels supermodes parell i imparell. D'aquesta manera es realitza una comparació entre els dissenys òptims de les dos estructures (strip i ridge) amb l'objectiu d'esbrinar quin disseny ofereix millors prestacions. Finalment, es proposa i estudia el disseny d'un acoblador grating capaç de multiplexar i demultiplexar els modes òptics del mode fonamental i del primer orde superior des de la guia òptica a la fibra òptica i viceversa. Per a això es proposen diferents dissenys amb l'objectiu d'aconseguir un disseny més tolerant i eficient enfront dels errors per desalineament obtenint un acoblament òptim. / [EN] In this Ph.D. thesis, different mode coupling and mode conversion techniques with the aim to increase the transport capacity in telecommunications systems over optical fiber are proposed. Concretely, the main aim is the development of the technology to achieve MDM using a limited controlled number of modes. Two different MDM scenarios based on two distinct wavelengths have been considered. On one hand, using the 850 nm wavelength over SSMF favors the use of optical and electro-optical devices with costs much lower than their equivalent in the C+L band. This novel transmission technology enables a new generation of very high capacity optical interconnections applicable to chip-to-chip links, to optical backplanes, and also to high-performance computing clusters and network switching centre interconnections. On the other hand, using the 1550 nm wavelength over optical waveguides based on SOI, i.e., Si (Silicon) above SiO2 substrate (silicon oxide), allows the use of integrated devices offering a less size, better repeatability and robustness in comparison with the optical fiber devices. Fused fiber couplers are proposed as key elements to (de)multiplex different fiber modes in a MDM link at 850 nm. The use of a symmetric directional coupler (DC) as a (de)multiplexer requires the use of an additional mode converter. The use of an asymmetrical directional coupler (ADC) as optical (de)multiplexer and mode converter is proposed, avoiding the necessity of an additional mode converter and simplifying the MDM scheme. Furthermore, in this Ph.D. thesis it is also proposed and evaluated the design of a mechanical mode converter at 850 nm using a SSMF. This technique permits to obtain the first high order mode with high quality and without the necessity of using an ADC. After that, it is analyzed and investigated the employment of commercial optical couplers (designed at 1550 nm) at 850 nm wavelength operation, thus avoiding the use of optical couplers and mode converters specifically designed at 850 nm wavelength. The MDM system costs are reduced as fewer devices are required and commercial components designed at 1550 nm are cheaper than the counterparts at 850 nm. In this Ph.D. thesis it is also considered the employment of ADCs over strip waveguides based on SOI technology for the conversion and multiplexing of the optical modes, from single-mode waveguide to high order mode waveguide at the 1550 nm wavelength. Thus, it has been studied and experimentally investigated different designs aimed to achieve the most robust configuration, in which the yield is less affected by the fabrication tolerances. Furthermore, the use of DCs over ridge waveguides is commonly employed and it offers better performance than strip waveguides. For this reason, the Ph.D. thesis studies and evaluates the use of ADCs with ridge waveguides by considering the effective refractive indexes of the even and odd supermodes analysis. In this way, a comparison between strip and ridge structures is done in order to find the optimum design that offer the best features. Finally, it is analyzed the design of a grating coupler capable of multiplexing and demultiplexing the fundamental and the high order mode from the waveguide to the optical fiber and vice versa. Thus, different designs are evaluated in order to achieve a design more robust and efficient to the coupling misalignments. / García Rodríguez, D. (2018). Few-Mode Transmission Technology for Ultra-High Capacity Optical Networks [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/115938 / TESIS

The higher-order modes (HOMs) of an optical fiber has been demonstrated as a new dimension to transmitting signals with the development of mode-division multiplexing (MDM) technique. This dissertation aims to explore the HOMs as an extra degree of freedom for device innovation. In particular, with femtosecond (FS) laser point-by-point (PbP) inscription technique which opens up a unique possibility to explore the HOMs for device innovation, we design, fabricate, and characterize novel-structured fiber Bragg gratings (FBGs) written in the step-index two-mode fibers. We also develop a numerical model for the PbP gratings which has the potential for inverse design problem. Chapter 2 begins with a general framework of MDM with adaptive wavefront shaping in few-mode fibers (FMFs) and multimode fibers (MMFs), followed by two examples in slightly more detail. The fabrication setup and an short overview of the FS laser system will also be covered. In Chapter 3, we show the design, fabrication, and characterization of off-axis Bragg gratings in a step-index two-mode fiber (TMF). Through measuring the transmission and reflection spectra along with the associated reflected mode intensity profiles under different input polarization, we experimentally investigate the off-axis TM-FBGs (FBGs in a TMF) with multiple characteristics reported for the first time to our best knowledge. To highlight, we report the laser-induced birefringence exhibits strong offset dependence, the reflectivity heavily depends on the offset and polarization, and particularly the mode pattern can be controlled solely through polarization. The design and characterization of cross-axis TM-FBGs are presented in Chapter 4. Specifically, these gratings show six primary reflection peaks, which are identified through mode-decomposition based on the intensity profiles through nonlinear optimization problem. We also show in this chapter the development of a numerical model for the general PbP gratings, implementation of this model into standard coupled-wave analysis shows reasonable agreement to the experimental findings. In Chapter 5, discussions and suggestions for future studies are given. / Doctor of Philosophy / The higher-order modes (HOMs) of an optical fiber has been demonstrated as a new space for signal transmission, in the ``mode space'' one can use the modes as distinct multiplexing channel and therefore increase the data capacity of a single fiber. This work aims to explore if the the higher-order modes can also add some extra degree of freedom for device innovation. In particular, we use femtosecond (FS) laser point-by-point (PbP) technique for device fabrication, since the structural change induced by this fabrication methods is highly localized, typically ranging from a few hundred nanometers to a few micrometers. Hence this particular fabrication technique offers a unique possibility of exploiting the HOMs for device innovation. In this work, we fabricate, and characterize fiber Bragg gratings (FBGs) with novel structural designs written within the step-index two-mode fibers, with multiple characteristics reported for the first time as far as we know. We also develop a numerical model for the PbP gratings which has the potential for inverse design problem.

The Internet has fundamentally changed the way of modern communication. Current trends indicate that high-capacity demands are not going to be saturated anytime soon. From Shannon's theory, we know that information capacity is a logarithmic function of signal-to-noise ratio (SNR), but a linear function of the number of dimensions. Ideally, we can increase the capacity by increasing the launch power, however, due to the nonlinear characteristics of silica optical fibers that imposes a constraint on the maximum achievable optical-signal-to-noise ratio (OSNR). So there exists a nonlinear capacity limit on the standard single mode fiber (SSMF). In order to satisfy never ending capacity demands, there are several attempts to employ additional degrees of freedom in transmission system, such as few-mode fibers (FMFs), which can dramatically improve the spectral efficiency. On the other hand, for the given physical links and network equipment, an effective solution to relax the OSNR requirement is based on forward error correction (FEC), as the response to the demands of high speed reliable transmission. In this dissertation, we first discuss the model of FMF with nonlinear effects considered. Secondly, we simulate the FMF based OFDM system with various compensation and modulation schemes. Thirdly, we propose tandem-turbo-product nonbinary byte-interleaved coded modulation (BICM) for next-generation high-speed optical transmission systems. Fourthly, we study the Q factor and mutual information as threshold in BICM scheme. Lastly, an experimental study of the limits of nonlinearity compensation with digital signal processing has been conducted.

In this thesis, I have developed a range of theoretical and numerical techniques for modelling the behaviour of partially coherent optical systems and multi-mode detectors. The numerical simulations were carried out for the ultra-low-noise Transition Edge Sensors (TESs) being proposed for use on the SAFARI instrument on the cooled aperture infrared space telescope SPICA (34 - 210 μm). The optical behaviour of the SAFARI system is described in terms of the optical modes of the telescope, as distinct from the optical modes of the detector. The performance of the TESs were assessed in terms of signal power, background power and photon noise. To establish a method for precisely characterising and calibrating ultra-low-noise TESs, a cryogenic test system was designed and engineered to measure the optical efficiencies of the SAFARI TESs. The multi-mode, partially coherent illumination conditions of the measurement system were engineered to be precisely the same as those of the telescope. A major difference between the test system and the telescope’s optics is that the telescope will have focusing elements, but the test system was designed to avoid focusing elements in order to keep the optical path as clean as possible. The theoretical formalism and numerical models were adapted accordingly to address this difference. The numerical simulations show that the test system could provide near identical optical performance as that of the telescope system even though the focusing elements were absent. I also performed experimental measurements to investigate the optical efficiencies of the multi-mode TESs. The detectors worked exceedingly well in all respects with satisfactory optical efficiencies. In addition, it has been shown that the optical model provides a good description of the optical behaviour of the test system and detectors. Further modal analysis was developed to study losses in the multi-mode horns. The optical behaviour of the waveguide-mounted thin absorbing films in the far-infrared was modelled using a mode-matching method.

[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 no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/147858 / TESIS

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.

A MBS (Multi-port beamsplitter) for higher dimensional quantum communication has been designed and constructed and the theory and method for this is presented in this thesis. It uses optical fibers in a heterogeneous structure with a single-mode fiber spliced to a multi-mode fiber and then spliced to a few-mode fiber. Three MBS:s were constructed and tested to see if superpositions between spatial modes could be generated. One with 5.65cm multi-mode fiber, one with 9cm of multi-mode fiber and one with just the single-mode fiber spliced to the few-mode fiber. The optical modes that where focused on for the superposition were the linear polarized LP01, LP11a and LP11b modes. Simulations of superpositions between these modes were performed and experiments were done to see if these simulations could be realised. The shapes of these superpositions could be seen with a camera and the stability of the different modal powers and the stability of the phases between the modes where also tested. The last experiment tested the tunability of the modes by finding their maximum and minimum output power for each individual mode. The results of these experiments show that the stability of power and relative phases are high and testing of the tunability shows that the 9cm MBS is the most tunable, the 5.65cm MBS the second best and the SMF-FMF MBS the worst. Even though the shapes of the superpositions, the stability and tunability shows very positive results, the conclusion is that more experiments are required in order to identify the superpositions and for this to be used in a quantum communication system. / En Multi-port stråldelare (MSD) för kvantkommunikation med hjälp av rumsliga optiska moder har blivit designad och konstruerad. Teorin, metoden och resultatet av detta arbete presenteras i denna uppsats. Denna konstruktion använder sig av optiska fiber i heterogena strukturer med en single-mode fiber svetsad till en multi-mode fiber som i sin tur är svetsad till en few-mode fiber. Tre stycken MSD blev konstruerade och testade för att se om superpositioner mellan rumsliga moder kunde bli genererade, en med 5.65cm multi-mode fiber, en med 9cm multi-mode fiber och en med bara en single-mode fiber svetsad till en few-mode fiber. De moder som fokuserades på för superpositionerna var de linjärpolariserade moderna LP01, LP11a och LP11b. Superpositionerna simulerades och sen genomfördes experiment för att se ifall de kunde bli genererade. Formerna av dessa superpositioner kunde hittas och synliggöras med en kamera. Stabiliteten av modernas energi och stabiliteten av faserna mellan moderna testades också. Det sista experimentet som gjordes testade justerbarheten av moderna genom att hitta den minimala samt maximala intensiteten för varje mod. Experimenten visar att intensiteterna och de relativa faserna har hög stabilitet för alla konstruerade MSD, men i justerbarhets experimentet visar det sig att 9cm MSD:en presterar bäst, 5.65cm MSD:en presterar näst bäst och SMF-FMF strukturen presterar sämst. Trots att formerna av superpositionerna kunde hittas för alla tre konstruktioner, och att testen i stabiliteten visar goda resultat så krävs mer experiment för att identifiera superpositionerna mellan moderna och ifall denna konstruktion går att implementera i ett kvantkommunikationssystem.

With the growing need for secure and high-capacity communications, innovative solutions are needed to meet the demands of tomorrow. One such innovation is to make use of the still unutilized spatial dimension of light in communications, which has promising applications in both enabling higher data traffic as well as the security protocols of the future in quantum communications. The perhaps most promising way of realizing this technology is through spatial division multiplexing (SDM) in optical fibers. There are many challenges and open questions in implementing this, such as how perturbations to the signal should be kept under control and which type of optical fiber to use. Consequently, this thesis focuses on the implementation of SDM in few-mode fibers where the perturbation effects on the spatial distribution have been investigated. Following this investigation, an implementation of adaptive spatial mode control using a motorized polarization controller has been implemented. The mode control has been done with the focus on having relevance for quantum technology applications such as Quantum Key Distribution (QKD) and quantum random number generation (QRNG) but also for spatial division multiplexing (SDM) for general communications. For this reason, two evaluation metrics have been optimized for: extinction ratio and equal amplitude. The control algorithm used is an adaptation of the optimization algorithm Stochastic Parallel Gradient Descent (SPGD). Control has been achieved in stabilizing the extinction ratio of LP11a and LP11b over 12 hours with an average extinction ratio of 98 %. Additionally, equal amplitude between LP11a and LP11b has been achieved over 1 hour with an average relative difference of 0.42 % and 0.45 %. Out of the perturbation effects investigated; temperature caused large disturbances to the signal which later is corrected for with the implemented algorithm.