Subcarrier intensity modulated free-space optical communication systems

This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel

Optical Wireless Data Center Networks

Bandwidth and computation-intensive Big Data applications in disciplines like social media, bio- and nano-informatics, Internet-of-Things (IoT), and real-time analytics, are pushing existing access and core (backbone) networks as well as Data Center Networks (DCNs) to their limits. Next generation DCNs must support continuously increasing network traffic while satisfying minimum performance requirements of latency, reliability, flexibility and scalability. Therefore, a larger number of cables (i.e., copper-cables and fiber optics) may be required in conventional wired DCNs. In addition to limiting the possible topologies, large number of cables may result into design and development problems related to wire ducting and maintenance, heat dissipation, and power consumption. To address the cabling complexity in wired DCNs, we propose OWCells, a class of optical wireless cellular data center network architectures in which fixed line of sight (LOS) optical wireless communication (OWC) links are used to connect the racks arranged in regular polygonal topologies. We present the OWCell DCN architecture, develop its theoretical underpinnings, and investigate routing protocols and OWC transceiver design. To realize a fully wireless DCN, servers in racks must also be connected using OWC links. There is, however, a difficulty of connecting multiple adjacent network components, such as servers in a rack, using point-to-point LOS links. To overcome this problem, we propose and validate the feasibility of an FSO-Bus to connect multiple adjacent network components using NLOS point-to-point OWC links. Finally, to complete the design of the OWC transceiver, we develop a new class of strictly and rearrangeably non-blocking multicast optical switches in which multicast is performed efficiently at the physical optical (lower) layer rather than upper layers (e.g., application layer). Advisors: Jitender S. Deogun and Dennis R. Alexande

Free Space Optical Communications with High Intensity Laser Power Beaming

This research demonstrates the feasibility of utilizing high intensity laser power beaming (HILPB) systems as a conduit for robust free-space optical communications over large distances and in challenging atmospheric conditions. The uniqueness of vertical multi-junction (VMJ) photovoltaic cells used in HILPB systems in their ability to receive and to convert at high efficiency, very high intensity laser light of over 200 W/cm2, presents a unique opportunity for the development of the robust free space optical communication system by modulating information signals onto the transmitted high intensity photonic energy. Experiments were conducted to investigate and validate several optical communications concepts. A laser modulator was implemented to exhibit the excellent transient response of the VMJ technology at very high illumination intensities, and thus show its applicability to optical communications. In addition, beam polarization optic stages were employed to demonstrate a secure multi-channel communications scheme. The off-axis response of the receiver and the beam profile were characterized in order to evaluate the feasibility of developing acceptable pointing and tracking geometries. Finally, the impact of signal modulation on the total converted energy was evaluated and shown to have minimal effect on the overall power transmission efficiency. Other aspects of the proposed communication system are studied including: quantifying beamwidth and directivity, signal-to-noise-ratio, information bandwidth, privacy, modulation and detection schemes, transmission channel attenuation and disturbances (atmospheric turbulence, scintillation from index of refraction fluctuations, absorption and scattering from thermal and moisture variation) and beam acquisition tracking and pointing influence on the performance metrics of optical transmission technologies. The result of this research demonstrates the feasibility of, and serves as a comprehensive design guide for the implementation of a HILPB communication system. S

Comunicações óticas de alto débito em espaço livre

Signal distribution through high capacity links is nowadays a requirement that is becoming more crucial than ever. Therefore, the introduction of fifth generation mobile networks lead to the need for high capacity fronthaul and backhaul. Fiber distributions are commonly available in developed countries, however there are some areas that do not sustain this type of resource, such as in areas with low residential user density. To overcome the idea that resources are allocated only in these areas, free space optics (FSO) present a suitable and lower cost solution and can be used in temporary events (e.g. when the fiber is disrupted). Free space optics is an optical type of communication that requires line of sight, where the atmosphere is the transmission medium. Due to changes in the atmospheric conditions, free space optics systems suffer from variable attenuation. In the scope of this work, the atmospheric interaction is the subject of study. To accomplish link optimization, several alternatives are studied, being based on channel prediction along side with probabilistic constellation shaping. A concave mirror together with a positioning system is also used to overcome structure sway and other sources of pointing error. In the lab, the estimation processes are used to produce 64-QAM connection with variable transmission rates between 400 Gbit/s and 500 Gbit/s and the gain from using adaptive modulation is evaluated. In the last stage, an automatic gimbal positioning system is also tested for several control algorithms.Atualmente, a distribuição de sinais por meio de canais de alta capacidade é um requisito cada vez mais crucial. Nesse sentido, a progressiva introdução de novas tecnologias de acesso móvel de quinta geração desempenha um papel fundamental, fomentando a necessidade de desenvolver novas técnicas de transmissão de alto débito para redes de fronthaul e/ou backhaul. As distribuições de fibra estão bem disseminadas entre os países desenvolvidos, no entanto, existem algumas áreas que não sustentam esse tipo de recurso, o que é comum em regiões de baixa densidade de clientes residenciais. Para combater a ideia de que os recursos são alocados apenas nas grandes áreas, a ótica de espaço livre apresenta uma solução adequada. Alternativamente, devido à facilidade e rapidez de instalação, as comunicações óticas de espaço livre podem ser utilizadas como alternativa à fibra em casos de destruição da mesma. A comunicação ótica em espaço livre é um tipo de comunicação direcional que requer linha de vista, sendo a atmosfera o meio de propagação. Devido às variações atmosféricas, as comunicações oticas de espaço livre sofrem atenuação variável. No decorrer deste trabalho, a interação atmosférica é objeto de estudo. Para realizar a otimização do canal de transmissão, várias alternativas são estudadas, baseando-se na previsão de canais em tempo real em conjunto com modulação probabilística da constelação. Um espelho côncavo equipado com sistema de posicionamento é também usado para superar as vibrações e outras fontes de erro posicional. No laboratório, os processos de estimativa são usados para adaptar uma conexão 64-QAM com taxas de transmissão entre 400 Gbit/s e 500 Gbit/s. O ganho do uso da modulação adaptativa é avaliado face a formatos de modulação fixos. No último estágio, o sistema de posicionadores é também testado, com o intuito de avaliar o seu impacto na estabilidade do canal.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

WDM/TDM PON bidirectional networks single-fiber/wavelength RSOA-based ONUs layer 1/2 optimization

This Thesis proposes the design and the optimization of a hybrid WDM/TDM PON at the L1 (PHY) and L2 (MAC) layers, in terms of minimum deployment cost and enhanced performance for Greenfield NGPON. The particular case of RSOA-based ONUs and ODN using a single-fibre/single-wavelength is deeply analysed. In this WDM/TDM PON relevant parameters are optimized. Special attention has been given at the main noise impairment in this type of networks: the Rayleigh Backscattering effect, which cannot be prevented. To understand its behaviour and mitigate its effects, a novel mathematical model for the Rayleigh Backscattering in burst mode transmission is presented for the first time, and it has been used to optimize the WDM/TDM RSOA based PON. Also, a cost-effective, simple design SCM WDM/TDM PON with rSOA-based ONU, was optimized and implemented. This prototype was successfully tested showing high performance, robustness, versatility and reliability. So, the system is able to give coverage up to 1280 users at 2.5 Gb/s / 1.25 Gb/s downstream/upstream, over 20 Km, and being compatible with the GPON ITU-T recommendation. This precedent has enabled the SARDANA network to extend the design, architecture and capabilities of a WDM/TDM PON for a long reach metro-access network (100 km). A proposal for an agile Transmission Convergence sub-layer is presented as another relevant contribution of this work. It is based on the optimization of the standards GPON and XG-PON (for compatibility), but applied to a long reach metro-access TDM/WDM PON rSOA-based network with higher client count. Finally, a proposal of physical implementation for the SARDANA layer 2 and possible configurations for SARDANA internetworking, with the metro network and core transport network, are presented

On-chip mode-locked microwave photonic integrated circuits

Mención Internacional en el título de doctorLa generación de frecuencias cada vez más elevadas, llegando al rango de ondas Milimétricas (mmW, de 30 a 300 GHz) y ondas de Terahertz (THz, 300-3000 GHz) es una forma de hacer frente a la creciente necesidad de ancho de banda en las comunicaciones inalámbricas que utilizan técnicas de modulación simples. Los resultados experimentales recientes han informado tasas de datos de hasta 48 Gbps en una frecuencia portadora de 300 GHz que demuestra que una solución rentable para hacer frente a los anchos de banda requeridos en las comunicaciones inalámbricas es aumentar la frecuencia de la onda portadora en la región de ondas milimétricas y más allá. Las dificultades para generar, amplificar y modular las señales en estas frecuencias se han superado mediante la combinación de lo mejor de dos mundos, electrónica y fotónica, resultando en un nuevo campo comúnmente referido hoy en día como fotónica de microondas. Un sistema de fotónica de microondas usualmente incluye un sintetizador de frecuencia óptica (OFS) y un convertidor opto-electrónico (OEC). El OFS es especialmente diseñado para entregar una señal óptica que cuando es convertida al dominio eléctrico por el OEC genera una señal eléctrica de alta frecuencia. Actualmente, la mayoría si no son todos los reportes de enlaces de comunicación inalámbrica que operan por encima de 100 GHz emplean generación fotónica de la frecuencia portadora. Generalmente, hay dos principales técnicas de generación de señal fotónica comúnmente usadas, fuentes pulsadas y fuentes heterodinas. Uno de los méritos de las fuentes pulsadas sobre las heterodinas es que para la misma potencia óptica se alcanza mayor potencia eléctrica emitida (aproximadamente 7 dBm por encima). Por lo tanto, las fuentes pulsadas son una opción atractiva para ser implementada como un generador de portadora con el nuevo enfoque de la integración fotónica. En el marco de este trabajo, hemos desarrollado un nuevo tipo de fuentes fotónicas basadas en la técnica de generación de señal pulsada, la cual puede ser fabricada mediante una fundición genérica en un circuito integrado fotónico. Nuestras soluciones son estructuras de láser mode-locked integradas en chip, diseñadas usando bloques de construcción genéricos de una plataforma de tecnología de integración fotónica en indio-fósforo. Nosotros hemos diseñado y caracterizado nuevas estructuras de láser mode-locked integradas en chip usando reflectores de interferencia multimodo, los cuales nos permiten ubicar los dispositivos en cualquier lugar dentro del chip, haciendo la señal óptica disponible para el resto de componentes en el chip, y permitiendo las subsecuentes operaciones de procesamiento de señal fotónica dentro del chip (modulación, filtrado óptico, multiplicación de la tasa de repetición, etc.). Nosotros hemos publicado un láser ―colliding pulse mode-locked‖ integrado en chip trabajando a una tasa de repetición de 70 GHz y un nuevo láser ―multiple colliding pulse mode-locked‖ integrado en chip operando a una tasa de repetición de 100 GHz. Como valor añadido, las características de rendimiento de las nuevas estructuras de láser mode-locked integradas en chip permiten ser utilizadas como una fuente pulsada en enlaces inalámbricos en banda-E y banda-F. La estructura de láser ―colliding pulse mode-locked‖ integrada en chip trabajando a una taza de repetición de 70 GHz se ha utilizado para demostrar el enlace inalámbrico en banda-E, mientras que la estructura de láser ―multiple colliding pulse mode-locked‖ integrada en chip funcionando a una frecuencia de 100 GHz fue usada para demostrar el enlace inalámbrico en banda-F.The generation of increasingly higher frequencies, reaching the Millimeter (MMW, 30 to 300 GHz) and Terahertz-wave (THz, 300 to 3000 GHz) range is one way to address the increasing need for bandwidth in wireless communications using simple modulation techniques. Recent experimental results have reported data rates up to 48 Gbps on a 300 GHz carrier frequency demonstrating that a cost effective solution to cope with the required bandwidths in wireless communications is to increase the carrier wave frequency into the millimeter wave region and beyond. The difficulties to generate, amplify and modulate signals at these frequencies have been overcome by combining the best of two worlds, electronics and photonics, arising a new field commonly referred nowadays as microwave photonics. A microwave photonic system usually involves an optical frequency synthesizer (OFS) and an opto-electronic converter (OEC). The OFS is specifically designed to deliver an optical signal that when is converted to the electrical domain by the OEC generates a high frequency electrical signal. Currently, most if not all of the reported wireless communication links operating above 100 GHz employ photonic generation of the carrier frequency. Generally, there are two main photonic signal generation techniques commonly used, pulsed sources and heterodyning sources. One of the merits of pulsed sources over heterodyning ones is that for the same optical power achieves a higher electrical emitted power (about 7 dBm above). Thus, pulsed sources are an attractive option to be implemented as a carrier generator with the novel approach of photonic integration. In the frame of this work, we have developed a new kind of photonic sources based on the pulsed signal generation technique, which can be fabricated within a generic foundry in a photonic integrated circuit. Our solutions are on-chip mode-locked laser structures, designed using generic building blocks from an indium-phosphide photonic integration technology platform. We have designed and characterized novel on-chip mode-locked laser structures using multimode interference reflectors, which enable us to locate the devices anywhere on the chip, making the optical signal available to the rest of the components on the chip, and allowing subsequent photonic signal processing operations within the chip (modulation, optical filtering, repetition rate multiplication and so on). We have reported an on-chip colliding pulse mode-locked laser working at 70 GHz repetition rate and a novel on-chip multiple colliding pulse mode-locked laser operating at 100 GHz repetition rate. As an added value, the performance characteristics of the novel on-chip mode-locked laser structures allow them to be used as a pulsed source in E-band and F-Band wireless link demonstrations. The on-chip colliding pulse mode-locked laser working at 70 GHz repetition rate has been used to demonstrate E-band wireless link while the on-chip multiple colliding pulse mode-locked laser operating at 100 GHz repetition rate was used to demonstrate the F-Band wireless link.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: John Mcinerney.- Secretario: Horacio Lamela Rivera.- Vocal: Erwin Bent