Advanced optical modulation and format conversion

Tese de Doutoramento em Engenharia Eletrotécnica apresentada à Universidade de Aveiro.Nos últimos anos, a crescente procura e troca de informação tem levado ao aumento de tráfego nas redes de comunicação óticas actuais. As comunicações coerentes, com recurso à amplitude e fase do sinal, ressurgem como uma das técnicas de transmissão capazes de aumentar a eficiência espectral e o rendimento dos canais óticos. Nesse âmbito, este trabalho apresenta um estudo sobre a conversão de formatos de modulação de sinais, usando técnicas exclusivamente no domínio ótico, através de conversão de comprimento de onda, com base no MZI-SOA. Esta técnica, aplicada em nós óticos que interligam redes óticas com débitos binàrios distintos, permite uma maior escalabilidade e eficiência da rede. A tese começa por apresentar uma caracterização experimental detalhada das propriedades estáticas e dinámicas do MZI-SOA. É depois proposto um modelo semi-analítico que descreve a evolução da amplitude e fase do sinal ótico à saída do MZI-SOA. Os coeficientes do modelo são obtidos recorrendo a um algoritmo genético multiobjectivo. O modelo é validado experimentalmente, explorando a dependência do sinal ótico com os parâmetros operacionais do MZISOA. Segue-se a proposta de uma técnica de conversão de formato de modulação de amplitude para modulação de fase contínua. Finalmente, é feito um estudo das potencialidades do MZI-SOA para conversão de formato de modulação de amplitude para modulação QPSK e QAM. Mostra-se a depedência da constelação do sinal com o desvio dos parâmetros operacionais, em torno do valor ótimo. A técnica é validada experimentalmente para modulação QPSK.ABSTRACT: Over the years, the increased search and exchange of information lead to an increase of traffic intensity in todays optical communication networks. Coherent communications, using the amplitude and phase of the signal, reappears as one of the transmission techniques to increase the spectral efficiency and throughput of optical channels. In this context, this work present a study on format conversion of modulated signals using MZI-SOAs, based exclusively on alloptical techniques through wavelength conversion. This approach, when applied in interconnection nodes between optical networks with different bit rates and modulation formats, allow a better efficiency and scalability of the network. We start with an experimental characterization of the static and dynamic properties of the MZI-SOA. Then, we propose a semi-analytical model to describe the evolution of phase and amplitude at the output of the MZI-SOA. The model’s coefficients are obtained using a multi-objective genetic algorithm. We validate the model experimentally, by exploring the dependency of the optical signal with the operational parameters of the MZI-SOA. We also propose an all-optical technique for the conversion of amplitude modulation signals to a continuous phase modulation format. Finally, we study the potential of MZI-SOAs for the conversion of amplitude signals to QPSK and QAM signals. We show the dependency of the conversion process with the operational parameters deviation from the optimal values. The technique is experimentally validated for QPSK modulation.Apoio financeiro da Fundação para a Ciência e Tecnologia — FCT através da bolsa SFRH / PROTEC / 50015 / 2009

Next generation passive optical networks based on orthogonal frequency division multiplexing techniques

In recent decades, the industry of communications has acquired huge significance, and nowadays constitutes an essential tool for the society information. Thus, the exponential growth in demand of broadband services and the increasing amount of information to be transmitted have spurred the evolution of the access network infrastructure to effectively meet the user needs in an effective way in terms of costs of both installation and maintenance. Passive optical networks (PON) are currently considered the most efficient and least costly alternative to deploy fiber to the home environment. In order to allow many users simultaneously coexist PONs based on time multiplexing (TDMA) have been developed. Looking ahead, however, it is expected that these techniques do not meet the requirements on access networks. In consequence, other multiple access techniques such as Wavelength Division Multiplexing Access (WDMA) or Orthogonal Frequency Division Multiplexing Access (OFDMA) are currently under study and development for use in the next generation of PONs. Particularly, in recent years OFDM has stood out among the scientific community to be considered a solution with great potential on future implementation of PONs. This is especially true due to the capacity of OFDM to work with multilevel modulations, its high tolerance to chromatic dispersion, and its high flexibility and granularity in terms of bandwidth management. Given the above, the aim of this Thesis is to study deeply the advantages and challenges of implementing the standard OFDM as an access network solution; likewise, it offers solutions to improve its performance. In order to evaluate the main structures and strategies for OFDM-based PON, a comparative analysis of all of them is performed firstly, highlighting their sensitivity levels, maximum range and number of users. A key aspect for network providers is the cost of operation, deployment and maintenance of networks. As a low-cost solution, this Thesis proposes a network model called Statistical-OFDMA-PON based on intensity modulation and direct detection. In addition, dynamic bandwidth management strategies are applied into this model getting an improvement in the power balance which in turn, allows to increase the maximum range and the scalability in number of users. One of the main OFDM problems is the Peak-to-Average Power Ratio (PAPR) which increases with the number of carriers. This thesis proposes a new algorithm based on folding the signal and transmitting auxiliary information in order to compensate the PAPR effect and thus increase the sensitivity of the optical system. On the other hand, OFDMA requires a large number of operations in the digital domain resulting in a high computational effort, which in turn results in an increased cost. For this reason, this Thesis presents a study on the optimization of the required resolution in the Digital-to-Analog / Analog-to-Digital Converters (DAC/ADCs) maintaining the transmission quality. The optimization of the computation time may make the OFDMA-based optical network more attractive for future PONs. Finally, another problem concerning the OFDM optical networks is their sensitivity to Phase Noise (PN). In this regard, this Thesis presents a study of the effect of the laser linewidth and its dependence on signal bandwidth. A mitigation technique based on pilot tones is implemented and the limiting values for the laser linewidth are found to be within the reach of present low-cost light sources.En estas últimas décadas, la industria de las comunicaciones ha adquirido gran importancia y hoy en día, constituye una herramienta imprescindible para el funcionamiento en la sociedad de la información. Así pues, el crecimiento exponencial en la demanda de servicios de banda ancha y la carga de información cada vez mayor que se necesita transmitir ha estimulado la evolución de las infraestructuras del tramo de acceso a la red para poder satisfacer las necesidades del usuario de forma efectiva en términos de costes de instalación y de mantenimiento. Las redes ópticas pasivas (Passive Optical Networks, PON) son actualmente consideradas la alternativa más eficiente y de menor coste para desplegar fibra hasta los hogares. Con el fin de permitir que muchos usuarios coexistan simultáneamente se han desarrollado PONs basadas en multiplexación en tiempo (Time Division Multiplexing Access, TDMA). De cara al futuro, sin embargo, se prevé que estas técnicas no permitan cubrir las exigencias sobre las redes de acceso. En consecuencia, otras técnicas de acceso múltiple al medio como el acceso múltiple por división de longitud de onda (Wavelength Division Multiplexing Access, WDMA) o el acceso múltiple por división de frecuencia ortogonal (Orthogonal Frequency Division Multiplexing Access, OFDMA) se encuentran actualmente en proceso de estudio y desarrollo para su uso en la futura generación de PONs. En concreto, en los últimos años OFDM se ha destacado entre la comunidad científica al considerarse una solución con gran potencial para su futura implantación en redes de acceso pasivas. Esto es especialmente cierto debido a la capacidad que el OFDM para trabajar con modulaciones multinivel, así como su alta tolerancia a la dispersión cromática y a la gran flexibilidad y granularidad que posibilita en términos de gestión del ancho de banda. Por todo lo anterior, el objetivo de esta Tesis es estudiar con profundidad las ventajas y los retos de aplicar el estándar OFDM como solución de red de acceso; del mismo modo, ofrece soluciones para mejorar su rendimiento. Con el objetivo de evaluar las principales estructuras basadas en OFDM-PON, en primer lugar se realiza un análisis comparativo de todas ellas destacando sus niveles de sensibilidad, máximo alcance y número de usuarios. Un aspecto fundamental para los proveedores de red es el coste de operación, despliegue y mantenimiento de las redes. Como solución de bajo coste, esta Tesis propone un modelo de red llamado Statistical-OFDMA-PON que se basa en modulación de intensidad y detección directa. Además, este modelo se completa con estrategias de gestión dinámica del ancho de banda de los usuarios que conforman la estructura de red propuesta consiguiendo una mejora en el balance de potencias que permite aumentar distancia y número de usuarios. Uno de los principales problemas del OFDM es el alto nivel de la relación de potencia de pico a potencia media (Peak-to-Average Power Ratio, PAPR) creciente con el número de portadoras. Esta Tesis propone un nuevo algoritmo basado en el pliegue de la señal y la transmisión de información auxiliar para compensar el efecto del PAPR aumentando así la sensibilidad del sistema óptico. Por otro lado, OFDMA requiere un número elevado de operaciones en el dominio digital resultando en un alto esfuerzo computacional que a su vez se traduce en un aumento del coste. Por esta razón, esta Tesis presenta un estudio sobre la optimización de la resolución requerida en los conversores analógicodigital (Digital-to-Analog/Analog-to-Digital Converters, DAC/ADCs) manteniendo la calidad de transmisión. La optimización del tiempo de cómputo requerido puede dotar de un mayor atractivo la solución de red óptica basada en OFDMA. Finalmente, otro de los problemas que presentan las redes ópticas OFDM es su sensibilidad frente al ruido de fase (Phase Noise, PN). En este aspecto, esta Tesis presenta un estudio del efecto del ancho de línea del láser y su dependencia con el ancho de banda de la señal. Técnicas de mitigación basadas en tonos piloto han sido implementadas y se han encontrado los valores limitantes del ancho de línea dentro del alcance de los láseres de bajo coste

Kerr nonlinearity in optical communication systems

Single-mode fibers (SMFs) are approaching their nonlinear capacity limit soon, andthe new technology for increasing the capacity per fiber like space-division multiplexing(SDM) is not ready yet. Therefore, mitigating the nonlinearity in SMFs becomesan important aspect of the current research. Optical phase conjugation (OPC) comesas a promising method for simultaneous compensation of dispersion and nonlinearityin optical fiber link for a broadband signal and in real-time. However, it is limitedby the power and dispersion symmetry requirements around the mid-link OPC device.The power symmetry almost has been achieved by using Raman amplification.Another scheme for achieving the power symmetry was by achieving that for thenonlinear effective region instead of the total link through shifting those regions byadding a dispersive element collocated with the OPC. In this thesis, the two methodswill be investigated in improving the symmetry around the mid-link OPC eitherthrough simulation or experimentally. A mathematical analysis for the latter methodwill be performed by estimating the four-wave mixing (FWM) power from the interactionsof three tones propagating through the optical fiber. Then a closed-formformula for the nonlinear noise power from the transmission of Nyquist-shaped wavedivision multiplexed (WDM) signal will be driven. The closed-form will be used inpredicting the performance of a system employing mid-link OPC with lumped amplification.In order to verify the mathematical results, simulations were run and give a good agreement with the theory. The closed-form formula is verified experimentally through the transmission of 4.08Tb/s WDM signal over 600km with mid-link OPC and 75km span is added after the OPC to improve the symmetry. The Raman amplification scheme in improving the power symmetry around the mid-link OPC has been tested with a real-time transceiver which proves the potential application of mid-link OPC in a real commercial system. The dispersion slope effect on the nonlinearity modeling is studied and a figure of merit is developed to predict when the dispersion slope needs to be considered in the models to give accurate results.The different OPC designs based on optical fiber are discussed and a wavelength shift-free OPC design is presented

Proceedings of the Mobile Satellite Conference

A satellite-based mobile communications system provides voice and data communications to mobile users over a vast geographic area. The technical and service characteristics of mobile satellite systems (MSSs) are presented and form an in-depth view of the current MSS status at the system and subsystem levels. Major emphasis is placed on developments, current and future, in the following critical MSS technology areas: vehicle antennas, networking, modulation and coding, speech compression, channel characterization, space segment technology and MSS experiments. Also, the mobile satellite communications needs of government agencies are addressed, as is the MSS potential to fulfill them