This dissertation presents the research that was focused on the study of solitons in nonintegrable systems governed by effective models of partial differential equations (PDEs) of (1+1) dimensions, that is, one spatial and one time dimension. In the main research line, we considered a model of two fields given by nonlinear Schrödinger equations with logarithmic nonlinearity and linear coupling, the main aspects of the collision dynamics of two Gaussian solitons (gaussons) were investigated, addressing the stability of the solutions, the interaction mechanisms, and the regularity of the scattering. The study of gaussons collisions in this model was performed via two approaches: semianalytic (variational/reduced model) and direct numerical simulation. For the system considered in this work, both approaches provided similar results which revealed the existence of a chaotic and fractal dynamics involving the initial conditions and the post-collisional properties of the solitons, which were verified by the analysis of the correlations between the input and output parameters. The fractal patterns are constituted by regularity windows that are positioned accordingly to a well defined rule, which was obtained via a linearization model that was found to be very similar in both approaches (i.e., the windows arrangement was well described by the variational model). In such windows the solitons collision dynamic turns out to be not chaotic (regular collision), it becomes predictable and very well defined, hence the width of these windows provides the conditions in which the control of the interaction is possible. Besides, by analyzing the dynamics of the variational parameters of the reduced model and also the dynamics of the most relevant physical quantities in the exact approach via PDEs (namely the total kinetic and potential energies), we attested that these windows arise due to a resonant mechanism of energy exchange between translational and vibrational soliton modes. By studying the linear stability of the solitary waves, we proved that the vibrational modes originate from shape oscillations induced by internal modes and also, possibly, by instability modes excited during the collisions. We verified that these oscillations are well defined in all regular collisions. The viability of the variational method in the system studied is due to the suitable description of the vibrational modes by the reduced model, and also the low influence of the emitted nonlinear radiation in the regular collision processes found in the direct simulations.

Este trabalho apresenta uma proposta para estudo das características de um canal de transmissão de dados feito com fibras ópticas modificadas eletroopticamente pela técnica de poling. Essa técnica é realizada para poder criar um coeficiente de não-linearidade não nulo aplicando simultaneamente um campo elétrico intenso e uma temperatura alta. Esse coeficiente é determinado pela mudança de fase interferência causada pela alta tensão no braço ativo do interferômetro, ou seja, a aplicação de tensão modula a fase da onda propagante. A primeira tentativa de realizar este estudo foi usando uma configuração experimental de um interferômetro Mach-Zehnder (MZI). Antes dos testes, um modulador comercial (OC-192, da JDS Uniphase) foi utilizado para calibrar o sistema. Em seguida, o modulador comercial foi substituído por uma fibra óptica modificada eletroopticamente. Devido à falta de equipamentos importantes, por exemplo, a falta de um amplificador elétrico que atinja 200 V e uma fonte de luz ASE (Emissão Espontânea Amplificada), a tentativa de estudar as propriedades desse sistema de modulação foi realizada através de simulações usando a plataforma COMSOL Multiphysics®. Foram utilizadas as características geométricas e ópticas de dois tipos de fibras reais (F051007-1C e SMF28) para construir o sistema MZI. Os resultados obtidos mostraram que não são esperadas alterações ou distorções nas ondas senoidais se a frequência delas for inferior a 10 GHz, o que é próximo ao valor teórico calculado para este sistema (9,1 GHz). Com base nesses resultados, concluiu-se que o modelo feito para a simulação do sistema experimental tem o potencial de ser uma ferramenta poderosa para realizar a estimativa de canal nesse tipo de sistema.

Monitoring multiple water quality parameters, such as nitrites (NO⁻₂) concentration, is vital to provide well-being and growth in fish. Optical fibers show great potential in aquaculture, due to the possibility of multiplexing sensor elements along the fiber, enabling multi-parametric in-situ measurements. On the other hand, electrochemical sensors are equally interesting in this context due to their low-cost and high sensitivity in the detection of various biochemical parameters, such as organic or inorganic contaminants, and even pathogens. The combination of electrochemical with optical detection can be extremely useful towards the development of lab-on-fiber concepts miniaturized for a wide monitoring of the water quality. This way, the goal of the present work consisted in developing and testing electrochemical sensors, based on laser-induced graphene (LIG) electrodes directly scribed on optical fibers. The sensors were applied for the detection of nitrites in concentrations relevant to aquaculture. First, the production of LIG was studied by photothermally converting polyimide with laser irradiating powers of 2.5 W and 4 W. In addition, two types of geometries were tried, one where LIG was produced in all the 360◦ of the fiber, and other where LIG was produced with only one laser passage. The sensors were coated with gold nanoparticles (AuNPs), by dropcasting technique, in order to enhance the sensitivity to nitrites. Through SEM and Raman analysis, 2.5 W produced LIG appears to be near the LIG formation threshold, presenting a heterogeneous morphology. In contrast, 4 W LIG presents a homogeneous and porous morphology, whose influence on its electrochemical response was evident in cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) characterizations. Specifically, EIS studies clearly demonstrated the impact of LIG’s porosity in its electrochemical response, making it necessary to use equivalent transmission line models, which consider porosity effects, in order to correctly model the impedimetric response of LIG produced at 4 W. The 3D porous structure of LIG produced at 4 W increases significantly the effective biodetection area, resulting in superior performance when detecting nitrites, compared to LIG produced at 2.5 W. Detection tests were performed in PBS for different concentrations of NO⁻₂ with differential pulse voltammetry (DPV) technique, and it was found that 4 W LIGs of one passage with AuNPs present a measured LOD, of 20 μM, a linear range of 20-200 μM and sensitivity of (1.93 ± 0.13) nA/μM. The sensor was calibrated from 6.6-8 pH and the current response remains similar, with sensitivities between 1.88 nA/μM and 1.77 nA/μM, slightly decreasing with pH. The LIG sensors were tested in real aquaculture water samples, before and after entering RAS (pH 7.6 and 7.0), and showed similar responses to those of PBS, with sensitivities of (1.86 ± 0.09) nA/μM and (1.71 ± 0.07) nA/μM, respectively, also reporting a linear range of 20-200 μM, which covers the relevant concentrations in aquaculture, and a measured LOD of 20 μM which is lower than the threshold concentration considered toxic for fish (43 μM). The produced LIG sensor was tested together with an optical pH sensor in optical fiber, as proof-of-concept of a hybrid platform.

The aim of this paper is to propose and evaluate multiuser linear precoding techniques for downlink of multicell MIMO based networks. We consider a high-speed backhaul network where the base stations are transparently linked by optical fiber to a central unit. This architecture allows an efficient joint multicell multiuser processing. The proposed precoder is designed in two phases: first the intercell interference is removed by applying a block diagonalization algorithm. Then the system is further optimized by proposing three power allocation algorithms with per-BS power constraint and different complexity tradeoffs: one optimal to minimize the BER and two suboptimal. The performance of the proposed schemes is evaluated, considering typical pedestrian scenarios based on LTE specifications. Numerical results show that the proposed suboptimal power allocation schemes achieve a performance close to the optimal. © 2011 IEEE.

This paper deals with a new multi-user linear precoding technique for downlink of distributed MIMO OFDM systems. We consider a distributed broadband wireless system where some base stations are transparently linked by optical fiber to a central unit. We further assume that the user terminals are equipped with an antenna array and the distributed base stations equipped with either single or an antenna array. This architecture provides a high speed backhaul channel allowing an efficient joint multi-cell processing. The proposed power allocation algorithm is based on minimization of the sum of inverse signal-to-noise ratio (SNR-1) on each user terminal over the available subcarriers. The motivation to minimize the sum of SNR-1 instead of bit error rate (BER) is in the fact that the first criterion achieves a closed-form solution, which is more interesting from practical point of view. The aim is to propose a practical distributed precoding technique to remove the inter-cell interference and improve the user's fairness at the cell-edges. The performance of the proposed scheme is evaluated, considering typical pedestrian scenarios based on LTE specifications.

In this work we demonstrate the use of optical fiber sensors to measure temperature in thin metallic bonding wires. Temperature was measured in copper wires with diameter of 0.10, 0.28, 0.60 and 0.70 mm and for different values of the driven electrical current (0.75-10.00 A). A theoretical model for the system, which takes into account the relevant heat exchange mechanism, was developed. The results demonstrate the feasibility of the optical sensors application for the measurement of temperature in thin metallic bonding wires. (C) 2010 Elsevier Ltd. All rights reserved.

This work has shown that by using simple analysis techniques, such as the average Q-factor, one may employ asynchronous histograms to derive information that would otherwise require a complete optical receiver as well as a BER counter to estimate. More complex techniques, such as the decomposition of asynchronous histograms, allow handling signals that suffer from distortion to the point where the original signal format is unrecognizable. In addition, it has been shown that histogram comparison techniques either using theoretical histograms or reference histograms can be used to extract information on the signal under analysis. Some practical aspects on the design of asynchronous sampling systems for the acquisition of asynchronous histograms have been considered. In particular, the most demanding aspect has been shown to be the sampling aperture time. It has been shown that the aperture time of the sampling system conditions the performed measurement by restricting the noise or distorting the signal under analysis. As a consequence, compensation methods have to be used either by adjusting the performed measurement or by using reference histograms. In summary, the analysis of asynchronous histograms has survived the evolution of optical communications systems as an effective proposal for advanced optical performance monitoring thanks to its remarkable flexibility. Asynchronous histogram acquisition systems can be easily designed for very wide ranges of bit rates and arbitrary signal formats. An extensive set of simple and robust methods to analyze asynchronous histograms has been developed over the years, and application to novel modulation formats can be carried out with relatively little effort.

In this work we demonstrate the use of optical fiber sensors to measure temperature in thin metallic bonding wires. Temperature was measured in copper wires with diameter of 0.10, 0.28, 0.60 and 0.70 mm and for different values of the driven electrical current (0.75-10.00 A). A theoretical model for the system, which takes into account the relevant heat exchange mechanism, was developed. The results demonstrate the feasibility of the optical sensors application for the measurement of temperature in thin metallic bonding wires. (C) 2010 Elsevier Ltd. All rights reserved.

In this paper, we reduce a partial differential equation that models the optical fiber fuse to an ordinary differential equation (ODE) using a traveling variable. This similarity reduction neglects the radiation loss term; however, the obtained fuse speeds are in good agreement with the ones obtained with the full propagation equation. The ODE results reveal the dependence of the fuse speed with the thermal parameters of silica and with the optical power density. We have also combined those results with experimental ones in order to adjust fiber absorption parameters.

A single-photon source based on the stimulated four-wave mixing (SFWM) process in optical fibers is presented. At the output of the source, the state of polarization (SOP) of the photons can be adjusted in order to obtain any linear polarization. A theoretical model to describe the average photon counts recorded in the avalanche photodiodes (APDs) is presented. The experimental results show an accurate detection of two non-orthogonal linear SOPs after propagation through a 60 km quantum channel, and good agreement with theory. This source, operating in a low power regime, can be used for quantum key distribution (QKD) using polarization-encoding in quantum communications.