The continued growth of the communications industry has caused interest in mode-division multiplexing (MDM) techniques to flourish in recent years. These techniques allow individual waveguide modes to be used as distinct channels. However, as with any versatile technique, it should be also useful and beneficial to extend its application to other areas. This work concerns itself with an initial conceptual design of a mode-division multiplexing (MDM) enabled optical sensor network that can use modes to interrogate either specific sensors or sensor subsystems, and specifically with quanitizing and optimizing the injection and detection of the signal of interest. A hypothetical test setup is demonstrated, and the major issue of back reflection burying the intended signal is addressed, analyzed, and improved. Improvements in the signal-to-background contrast ratio (SBCR) of approximately 10dB were achieved depending on fibre type and proximal face. Suggestions for extensions to further improve the SBCR as well as for applications of this system are discussed. / Master of Science

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.

Tesis por compendio / [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. / [CA] 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]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/115938 / Compendio

La demande en bande passante des systèmes de communication optique ne cesse de croitre. Des débits de données de l’ordre de plusieurs centaines de TBit/s sont attendus dans un futur proche. La photonique silicium est une technologie majeure pour faire face à ces besoins croissants. Sa compatibilité avec les technologies CMOS permet naturellement une co-intégration photonique/électronique sur les mêmes circuits. A court terme, l’augmentation des débits de données dans les générations futures de système de communication optique passe par l’utilisation de formats de modulation avancés, et l’augmentation du nombre de bits par symbole transmis. A plus long terme, de nouvelles techniques de multiplexage sont nécessaires. Le multiplexage de modes est actuellement une solution attractive à l’étude dans ce but.Dans ce travail de thèse, différents moyens pour implémenter ces nouveaux systèmes de communication optiques sont étudiés au niveau de l’émetteur. Ces travaux incluent dans une première partie la modélisation, conception et caractérisation des modulateurs silicium. Dans une seconde partie, de nouveaux composants pour manipuler les modes sur circuits intégrés photoniques sont proposés, conçus et caractérisés, avec pour application le multiplexage de modes.Une nouvelle méthode a été proposée pour la modélisation des modulateurs optiques silicium. Cette méthode permet de réduire le temps de simulation de 2 ordres de grandeur, en maintenant un bon niveau de précision. En utilisant ce modèle, des modulateurs basés sur des diodes PN latérales et interdigitées ont été conçus pour fonctionner en bande O des communications optiques. Les résultats expérimentaux ont permis la mise en évidence de diagrammes de l’œil avec des taux d’extinction de 10 dB pour des modulations de type OOK (ON-OFF Keying) à 10Gbit/s. De plus des modulations de type BPSK (Binary Phase Shift Keying) ont également été démontrées à 10Gbit/s.De nouveaux convertisseurs de modes et multiplexeurs ont été proposés, conçus, fabriqués et caractérisés, pour être utilisés dans des systèmes de multiplexage modal. Les résultats expérimentaux ont permis de mettre en évidence des fonctionnements large bande passante avec de grands taux d’extinction. / Bandwidth demand in optical communication systems is continually growing. Data rate values in the order of several hundreds of TBps are expected in the near future. In order to cope with those expectations silicon based technologies are believed to be the best suited. Its naturally compatibility with CMOS easily enables the electronics and photonics co-integration. In the short-term the way increase data rates in next generation optical communication systems goes through using advanced modulation format and increase symbol rates. In the long-term view, new multiplexing techniques will be required. In this sense, mode division multiplexing is nowadays an attractive approach under consideration.In this Thesis work, the way to implement these new optical communication schemes is studied from the transmitter point of view. It includes, on a first part the modeling, design and characterization of silicon modulators. And in a second part, it includes the proposition, design and characterization of novel mode handling devices for mode division multiplexing.A new way of modeling silicon modulators has been developed. This new model permits to reduce the computation time of modulator analysis up to two orders of magnitude, while maintaining a good level of accuracy. Using the model, modulators based on lateral PN junctions and interdigitated PN junctions were designed to work in the O-Band of optical communications. Characterization work has been performed on these modulators with good results. Wide-open OOK (On Off Keying) eye diagrams with 10 dB extinction ratio were obtained at 10GBps. Furthermore, BPSK (Binary Phase Shift Keying) modulation was also demonstrated at 10GBps.New kind of mode converters and multiplexers, intended to work as mode division multiplexing subsystems have been proposed, designed, fabricated and characterized. Measured results show broad bandwidth operation with high extinction ratio.

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.

Few mode fibers and multimode fibers (MMFs) are traditionally regarded as unsuitable for important applications such as communications and sensing. A major challenge in using MMFs for aforementioned applications is how to precisely control the waveguide modes propagating within MMFs. In this thesis, we experimentally demonstrate a generic method for controlling the linearly polarized (LP) modes within a two-mode fiber (TMF). Our method is based on adaptive optics (AO), where one utilizes proper feedback signals to shape the wavefront of the input beam in order to achieve the desired LP mode composition. In the first part of this thesis, we demonstrate the feasibility of AO-based mode control by using the correlation between the experimentally measured field distribution and the desired mode profiles as feedback for wavefront optimization. Selectively excitation of pure LP modes or their combinations at the distal end of a TMF are shown. Furthermore, we demonstrate that selective mode excitation in the TMF can be achieved by using only 5×5 independent phase blocks. Afterwards, we extend our AO-based mode control method to more practical scenarios, where feedback signals are provided by all-fiber devices such as a directional fiber coupler or fiber Bragg gratings (FBGs). Using the coupling ratio of a directional coupler as feedback, we demonstrate adaptive control of LP modes at the two output ports of the directional coupler. With feedback determined by the relative magnitude of optical power reflected by a FBG and the transmitted power, selective excitations of the LP01 and the LP11 modes are experimentally shown. As the final component of this thesis, we experimentally combine the AO-based mode control with time-division-multiplexing. By choosing reflected pulses with appropriate arrival time for mode control, we can selectively excite the LP11 mode at different FBG locations within the TMF, based on the ratio of optical signals reflected by FBGs in the TMF and the transmitted signal. Using two lasers set at the two FBG peak reflection wavelengths associated with the LP01 and the LP11 modes, we can accomplish AO-based mode control within a TMF by using only the reflection signals from the FBG. By using the ratio of the reflected signals of two lasers as feedback, we demonstrate selective excitation of almost pure LP01 or LP11 mode at the FBG location within the TMF. The method developed in this thesis is generic and can be extended to many other applications using appropriately chosen feedback signals. It is possible to generalize the AO-based mode control method to MMF as well. This method may find important applications in MMF-based communication, sensing and imaging et. al. in the future. / Ph. D.