This thesis work deals with the development of three different categories of interrogation techniques for Fiber Bragg Grating based sensor networks. Such networks are used for structural health monitoring and other applications. A bulk grating based interrogation system is described first, which includes an optical source, switch, circulator, embedded controller, and software. The center wavelength determination technique employed is detailed and is shown to be highly accurate from test data. The comparison with resistance strain gauges is presented which shows that the system developed provides an accurate strain reconstruction. The system is also compared with a commercial optical spectrum analyzer and is found to exhibit good accuracy and fidelity. The system has been field tested on an aircraft structure with 14 sensors spread over 4 channels. Strain data reconstruction from these tests is shown to accurately reproduce the loading conditions. A second system developed is based on the matched filter technique using a mechanical fiber stretcher; the details of this system are presented with a mathematical treatment of the technique. The design of the fiber stretcher is also described. This design is regarded to be novel since it tries to provide large interrogation bandwidths using a parallel topology. The results of tests have shown good resolution and comparative tests with resistance strain gauges have shown accurate reproduction of strain. Finally, an interrogation system based on a wavelength tunable source is presented. This system is a precursor to a time division multiplexed interrogation system, which has also been described. Three laser configurations have been set up and characterized. The laser sweep tests have been performed on two configurations and a sensor grating reconstruction test has also been carried out.

The task of the project was to evaluate a differential phase shift keying demodulation technique by replacing a Mach-Zehnder interferometer receiver with an optical filter (Fiber Bragg Grating). Computer simulations were made with single optical transmission, multi channel systems and transmission with combined angle/intensity modulated optical signals. The simulations showed good results at both 10 and 40 Gbit/s. Laboratory experiments were made at 10 Gbit/s to verify the simulation results. It was found that the demodulation technique worked, but not with satisfactory experimental results. The work was performed at Eindhoven University of Technology, Holland, within the framework of the STOLAS project at the department of Electro-optical communication.

The objective of this thesis is to study of optical effects, arising in the form of non-uniform waveguide structure, complicated refractive index profiles or due to pulse propagation in dense wavelength division multiplexing (DWDM) optical communication systems. These effects are important and critically influence the performance of DWDM optical systems. A comprehensive survey of current literature on optical effects due to nonuniform optical structure and nonlinear optical effects is first done, showing their advantages and disadvantage in optical communication systems. A survey on methods of optical waveguide analysis is also done. The main contribution has been made to three main aspects of the problem : Accurate analysis of uniform/non-uniform optical waveguides with arbitary refractive index profiles Pulse propagation and distortion in DWDM Raman amplification systems. Use of non-uniform FBG to compensate for pulse distortion We study several existing analytical techniques developed so far for analyzing the mode of non-uniform optical waveguide structures. Later, we verify the analytical results by finite element method (FEM). The convergence study is also carried out. A new computational technique is proposed modifying the finite element method to analyze complex refractive index profiles required for the analysis, namely single mode step index profile, multi clad fiber, W -profile, chirp profile etc. An accuracy of 10−4 in the calculation of propagation constant/eigen-value is demonstrated. Dispersion characteristics of optical fibers w.r.t. different profile parameters is evaluated. A modification to scalar BPM is proposed and applied to study the effects of inhomogeneities along the propagation direction. The applicability and accuracy of the method is tested using integrated optic waveguide devices, namely, graded index slab waveguide. The proposed BPM uses Fourier decomposition of the transverse field. Coupled mode theory (CMT) of optical waveguides in non-homogeneous optical medium is applied to study the interaction of lightwaves propagation together such as in a DWDM system. The BPM results is verified by CMT. The inhomogeneous waveguide theory is extended to study pulse propagation in DWDM optical communication system. Nonlinear optical effects are an important aspects of DWDM systems with fiber Raman amplifier. Finite difference time domain (FDTD) method is necessary to study these nonlinear optical effects as other conventional methods are not suitable here. Here, we discuss DWDM optical communication systems due to nonlinearity in the form of SRS effect. In case of FRA, we study the various kinds of fiber profile design parameters, for the purpose to achieve and extend the flat gain bandwidth over the EDFA window. We also propose and study, a new bi-directional optical fiber transmission scheme with various constraints, using Raman amplification process with and without pump depletion. Our scheme, provides an advantage like high SNR, low pump induced noise, for long-haul communication link. We find that, there is a quite significant crosstalk and power coupling among the dense DWDM channels but earlier discussed BPM fails to account for possible interference effects among the channels. To reduce the harmful nonlinear optical effects like four wave mixing (FWM), we need to deploy a high chromatic dispersion fiber, which will ultimately lead to high pulse walk-off rate among the DWDM channels; hence for high bit rate long haul systems, walk-off effect can not be ignored. Application of FDTD provided an improved insight into the effect of GVD on stimulated Raman scattering crosstalk than different modulation techniques and line codes. It is shown through analysis that pulse walk-off phenomena may distort the data asymmetrically; especially for case of wide-band DWDM transmission system. Hence, the pulse walk-off effect should be considered in future systems containing optical amplifier. It is shown, that large walk-off rate may reduce the crosstalk among DWDM channels but tends to increase the asymmetric pulse distortion. Data may lose due to high walk-off effect. We also investigate channel addition/removal process in DWDM fiber Raman amplifier. We also demonstrate that the pulse walk-off effect tends to lead significantly to positive chirp for higher frequency channels. This feature can be exploited to overcome the chromatic dispersion effects in DWDM transmission systems. Pulse walk-off induced chirp, can be compensated by using the nonuniform fiber Bragg grating (NUFBG). The CMT due to periodic perturbation of the circular cylindrical waveguide structures is applied here. Here, we discuss the function of fiber Bragg grating as a transmission versus reflecting grating filter. We also discuss, FBG application to gain flattening of an EDFA window as well as how the group velocity dispersion (GVD) will be affected with bandwidth and coupling coefficient. We develop a new analytical technique to estimate the bandwidth of FBG based optical system. Finally, we investigate the dispersion compensation properties, pulse distortion, peak reflectivity analysis in uniform/non-uniform FBG due to an uniform/non-uniform incoming signal. More complicated refractive index profile can significantly reduce the GVD as well as side lobes intensity. Dispersion characteristic due to an arbitrary refractive index profile is discussed in details for the case of non-uniform FBG. Thus, we concluded that wide band DWDM optical communication system need to closely take into account various inhomogeneities and nonlinearities of optical fibers w.r.t. wave and pulse propagation.

No description available.

When azobenzene-modified photosensitive polymer films are irradiated with light interference patterns, topographic variations in the film develop that follow the electric field vector distribution resulting in the formation of surface relief grating (SRG). The exact correspondence of the electric field vector orientation in interference pattern in relation to the presence of local topographic minima or maxima of SRG is in general difficult to determine. In my thesis, we have established a systematic procedure to accomplish the correlation between different interference patterns and the topography of SRG. For this, we devise a new setup combining an atomic force microscope and a two-beam interferometer (IIAFM). With this set-up, it is possible to track the topography change in-situ, while at the same time changing polarization and phase of the impinging interference pattern. To validate our results, we have compared two photosensitive materials named in short as PAZO and trimer. This is the first time that an absolute correspondence between the local distribution of electric field vectors of interference pattern and the local topography of the relief grating could be established exhaustively. In addition, using our IIAFM we found that for a certain polarization combination of two orthogonally polarized interfering beams namely SP (↕, ↔) interference pattern, the topography forms SRG with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures below diffraction limit with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We have also probed for the stresses induced during the polymer mass transport by placing an ultra-thin gold film on top (5–30 nm). During irradiation, the metal film not only deforms along with the SRG formation, but ruptures in regular and complex manner. The morphology of the cracks differs strongly depending on the electric field distribution in the interference pattern even when the magnitude and the kinetic of the strain are kept constant. This implies a complex local distribution of the opto-mechanical stress along the topography grating. The neutron reflectivity measurements of the metal/polymer interface indicate the penetration of metal layer within the polymer resulting in the formation of bonding layer that confirms the transduction of light induced stresses in the polymer layer to a metal film. / Azobenzolhaltige Polymere gehören zu einer Klasse funktionaler Materialien, bei denen durch ein äußeres Strahlungsfeld eine starke mechanische Reaktion ausgelöst werden kann. Durch die Bindung an das Polymerrückgrat können die Azobenzole, die unter UV-Belichtung eine Photoisomerisierung ausführen, was zum Teil drastische Effekte zur Folge hat. Unter Belichtung mit Intensitätsmustern, d.h. mit räumlich variierender Verteilung der Polarisation oder der Intensität des einfallenden Lichts verändert sich die Topographie der azobenzolhaltigen Filme, was zur Bildung von Oberflächengittern (engl. Surface Relief Gratings, SRG) führt. In dieser Arbeit wurde eine neue Methode vorgeschlagen, bei der das Verhalten elastischer/morphologischer Eigenschaften unter verschiedenen Belichtungsbedingungen, d.h. mit unterschiedlicher Verteilung der Polarisation und der Intensität in situ lokal als Funktion der Position entlang der SRG aufgenommen werden kann. Außerdem wurde hier vorgeschlagen, opto-mechanische Spannungen, die innerhalb der photosensitiven Polymerfilme während der Belichtung entstehen, mit Hilfe dünner aufgebrachter metallischen Schichten abzubilden und zu analysieren.

Nowadays, silicon photonics is a widely studied research topic. Its high-index-contrast and compatibility with the complementary metal-oxide-semiconductor technology make it a promising platform for low cost high density integration. Several general problems have been brought up, including the lack of silicon active devices, the difficulty of light coupling, the polarization dependence, etc. This thesis aims to give new attempts to novel solutions for some of these problems. Both theoretical modeling and experimental work have been done. Several numerical methods are reviewed first. The semi-vectorial finite-difference mode solver in cylindrical coordinate system is developed and it is mainly used for calculating the eigenmodes of the waveguide structures employed in this thesis. The finite-difference time-domain method and beam propagation method are also used to analyze the light propagation in complex structures. The fabrication and characterization technologies are studied. The fabrication is mainly based on clean room facilities, including plasma assisted film deposition, electron beam lithography and dry etching. The vertical coupling system is mainly used for characterization in this thesis. Compared with conventional butt-coupling system, it can provide much higher coupling efficiency and larger alignment tolerance. Two novel couplers related to silicon photonic wires are studied. In order to improve the coupling efficiency of a grating coupler, a nonuniform grating is theoretically designed to maximize the overlap between the radiated light profile and the optical fiber mode. Over 60% coupling efficiency is obtained experimentally. Another coupler facilitating the light coupling between silicon photonic wires and slot waveguides is demonstrated, both theoretically and experimentally. Almost lossless coupling is achieved in experiments. Two approaches are studied to realize polarization insensitive devices based on silicon photonic wires. The first one is the use of a sandwich waveguide structure to eliminate the polarization dependent wavelength of a microring resonator. By optimizing the multilayer structure, we successfully eliminate the large birefringence in an ultrasmall ring resonator. Another approach is to use polarization diversity scheme. Two key components of the scheme are studied. An efficient polarization beam splitter based on a one-dimensional grating coupler is theoretically designed and experimentally demonstrated. This polarization beam splitter can also serve as an efficient light coupler between silicon-on-insulator waveguides and optical fibers. Over 50% coupling efficiency for both polarizations and -20dB extinction ratio between them are experimentally obtained. A compact polarization rotator based on silicon photonic wire is theoretically analyzed. 100% polarization conversion is achievable and the fabrication tolerance is relatively large by using a compensation method. A novel integration platform based on nano-epitaxial lateral overgrowth technology is investigated to realize monolithic integration of III-V materials on silicon. A silica mask is used to block the threading dislocations from the InP seed layer on silicon. Technologies such as hydride vapor phase epitaxy and chemical-mechanical polishing are developed. A thin dislocation free InP layer on silicon is obtained experimentally. / QC20100705

Decades of efforts have pushed the replacement of electrical interconnects by optical links to the interconnects between computers, racks and circuit boards. It may be expected that optical solutions will further be used for inter-chip and intra-chip interconnects with potential benefits in bandwidth, capacity, delay, power consumption and crosstalk. Silicon integration is emerging to be the best candidate nowadays due to not only the dominant status of silicon in microelectronics but also the great advantages brought to the photonic integrated circuits (PICs). Regarding the recent breakthroughs concerning active devices on silicon substrate, the question left is no longer the feasibility of the optical interconnects based on silicon but the competitiveness of the silicon device compared with other alternatives. This thesis focuses on the study of two key components for the optical interconnects, both especially designed and fabricated for silicon platform. One is a high speed electroabsorption modulator (EAM), realized by transferring an InP-based segmented design to the hybrid silicon evanescent platform. The purpose here is to increase the speed of the silicon PICs to over 50  Gb/s or more. The other one is a high performance grating coupler, with the purpose to improve the optical interface between the silicon PICs and the outside fiber-based communication system. An general approach based on the transmission line analysis has been developed to evaluate the modulation response of an EAM with a lumped, traveling-wave, segmented or capacitively-loaded configuration. A genetic algorithm is used to optimize its configuration. This method has been applied to the design of the EAMs on hybrid silicon evanescent platform. Based on the comparison of various electrode design, segmented configuration is adopted for the target of a bandwidth over 40 GHz with as low as possible voltage and high extinction ratio. In addition to the common periodic analysis, the grating coupler is analyzed by the antenna theory assisted with an improved volume-current method, where the directionality of a grating coupler can be obtained analytically. In order to improve the performance of the grating coupler, a direct way is to address its shortcoming by e.g. increasing the coupling efficiency. For this reason, a nonuniform grating coupler with apodized grooves has been developed with a coupling efficiency of 64%, nearly a double of a standard one. Another way is to add more functionalities to the grating coupler. To do this, a polarization beam splitter (PBS) based on a bidirectional grating coupler has been proposed and experimentally demonstrated. An extinction ratio of around -20 dB, as well as a maximum coupling efficiency of over 50% for both polarizations, is achieved by such a PBS with a Bragg reflector underneath. / QC 20100906

La corrosion des armatures de renforcement des structures en béton représente un enjeu socio-économique majeur. Sa détection et le suivi de son évolution constituent un défi pour la recherche appliquée. Les techniques standards non destructives de détection de corrosion mettent en œuvre des procédés indirects tels la mesure d’impédance, de potentiels, ou par ultrasons. Leurs capacités d’auscultation sont limitées dans l’espace (notamment en profondeur), leur coût reste élevé dans un contexte de maintenance périodique et elles conduisent à des paramètres d’interprétation complexe. Des progrès sont nécessaires dans la détection et l’analyse fiable de la progression des processus de corrosion. Dans ce travail, nous présentons une nouvelle méthode pour détecter la corrosion et le suivi de son évolution, basée sur l’observation directe des changements intervenant à l’interface fer-béton par Capteur à Fibre Optique (CFO). L'attaque par corrosion de la surface de l'armature dépend de plusieurs paramètres électrochimiques (température, pH, carbonatation, présence de chlorures, contamination biologique, etc.). Deux comportements mécaniques à l'interface fer-béton sont distingués. Dans le premier cas (carbonatation), le produit d'oxydation du métal reste à l'interface et augmente la pression interne, pouvant conduire à la fissuration de la couche de béton extérieure. Dans le second cas (piqures), les ions métalliques sont évacués hors de la structure avec comme conséquence une réduction de section des barres d'armature (affaiblissement du renforcement). Un CFO innovant est proposé dans le but de localiser et quantifier les deux types de corrosion précités. Le principe consiste à observer l’impact direct de la corrosion sur l’état de déformation d’une fibre optique préalablement précontrainte par construction. Deux procédés métrologiques sont étudiés : Bragg et réflectométrie fréquentielle (Optical Frequency-Domain Reflectometry - OFDR). Des tests de corrosion accélérée montrent la faisabilité du procédé. Une procédure de fabrication simplifiée et à coût optimisé est proposée pour la surveillance in situ et répartie des structures de génie civil, dans une perspective future de maintenance conditionnée. / Corrosion of reinforced bars (rebars) in concrete structures remains a major issue in civil engineering works, being its detection and evolution a challenge for the applied research. Usual non-destructive corrosion detection methods involve impedance, potential or ultra-sonic indirect measurements of complex interpretation. Besides, they are restricted to near-surface examinations and the maintenance cost is still high (scheduled maintenance). Many efforts remain to be done to survey the onset and progression of corrosion processes in a reliable way. In this work, we present a new methodology to detect the onset of corrosion and to monitor its evolution, based on the direct observation of rebar–concrete interface changes by the use of an Optical Fiber Sensor (OFS). The corrosion attack over rebar surface depends on several physical, chemical and electrochemical parameters (temperature, pH, presence of chlorides/CO2, biological contamination, etc.). Two types of mechanical behavior and described. In the first case (carbonation), metal oxidation products stay at the interface and increase internal pressure, potentially leading to a crack of the external concrete layer. In the second case (pitting), metal ions are evacuated out of the structure, leading to a reduction of the rebar section (structural weakness). An innovative sensor design is proposed with the purpose of localizing and quantifying the amount of both corrosion types. The basic principle consists in measuring the impact of corrosion over the state of strain of a prestressed optical fiber. Two metrological techniques are used: Fiber Bragg Grating (FBG) and Optical Frequency-Domain Reflectometry (OFDR). Accelerated corrosion tests were performed in electrolytic solutions for both kinds of corrosion types (pitting and carbonation) and provide a proof-of-concept for the technique. A low-cost, simplified manufacturing procedure is proposed with the aim to provide distributed and in situ Structural Health Monitoring (SHM), suitable for future Condition-Based Maintenance (CBM) of civil engineering concrete structures.

Aujourd’hui le monde des télécommunications est en plein essor et le nombre de services proposés aux consommateurs augmente d’année en année. Les technologies employées font appel à l’optique. Le simple constat du nombre d’applications "sans fils" qui se développent permet de se rendre compte de l’importance des microondes dans l’ensemble des technologies modernes de communication. Cette thèse se situe dans ce cadre. Elle présente des techniques de modélisation de base pour concevoir et optimiser un guide d’onde de surface fonctionnant dans le domaine des microondes. Dans le premier chapitre, des outils de simulation permettant de calculer la réponse de diffraction par les réseaux de strips métalliques ont été développés. Les méthodes rigoureuses qui ont été retenues sont la méthode MMFE, CBCM et la méthode C qui prennent en compte la nature vectorielle de la lumière. La difficulté de convergence des calculs numériques et le problème de discontinuité des champs sur les strips métalliques est mise en évidence. Pour traiter ces problèmes, un changement de coordonnées est proposé, c’est le système de " Coordonnées adaptative " qui permet de resserrer les lignes de coordonnées au voisinage des points des discontinuités. Il en résulte une diminution du saut de discontinuité en ces points et une amélioration de la convergence des calculs numériques. Dans le second chapitre, ces méthodes sont étendues au problème de la diffraction en incidence oblique encore appelé diffraction conique. Dans le troisième chapitre, nous avons appliqué ces méthodes pour étudier et caractériser les réseaux de strips métalliques déposés sur une couche diélectrique. Nous avons mis en évidence le phénomène de résonance sur les facteurs de réflexion et nous avons pu montré que ces effets des résonances sont dus au couplage de l’onde de surface et de l’onde plane incidente. Afin d’analyser les couplages résultants qui existent entre le mode, nous avons étudié les pôles de la matrice S et déterminé la sensibilité et l’influence des paramètres optoélectroniques sur le pic de résonance. Cette étude a permis de déterminer le triplet (hauteur, facteur de forme, l’angle de polarisation ) relatif à la structure pour que le guide soit optimisé. / Today, the world of telecommunications is booming and the number of services offered to the consumers rises from year to year. Current Technologies call for optics. The simple report of the number of " wireless " which is increasing allows bewaring of the importance of the microwaves in the whole world of modern communication technologies. This thesis is mainly in the same area. It presents tools of basic modeling to design and optimize surface waves guide that works in the field of microwaves. In the first chapter, simulation tools allowing to calculate the answer of diffraction by the metallic strip grating were developed. The rigorous methods which were adopted are the MMFE, CBCM and the C method which take into account the vector nature of the light.The difficulty in convergence of numerical calculations and the problem of discontinuity of the fields on the metallic strips are highlighted. In order to deal with these problems, the change of co-ordinates is proposed, it is the system of " adaptive co-ordinates " which makes it possible to tighten the lines of co-ordinates in the vicinity of the point of discontinuities. It results from it a reduction from the jump of discontinuity in these points and an improvement of the convergence of numerical calculations. In the second chapter, these methods are extended to the problem of diffraction in oblique,also called conical diffraction. In the third chapter, we have applied these methods to study and characterize the metallic strip grating deposited to a dielectric layer. We highlighted the phenomenon of resonance on the factors of reflexion and we could shown that these effects of resonances are due to the coupling of the surface of waves and of the incidence plane waves. So as to analyze the coupling results which exist between the modes we have studied the poles of the matrix S and determined the sensitivity and the influence of the optoelectronic parameters on the peak of resonance. This study made it possible to determine the triplet (height, fill Factor, the angle of polarization) relating to the structure so that the guide will be optimized.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Grades de Bragg nÃo-lineares tÃm sido consideradas desde o final do sÃculo passado para aplicaÃÃes em sistemas de comunicaÃÃes Ãpticas e sensoriamento. O estudo de pulsos ultra-curtos em grades de Bragg lineares, entretanto, sà tem sido considerado nos Ãltimos anos, devido ao desenvolvimento de tÃcnicas numÃricas especÃficas para se resolver o problema. Neste trabalho, foi realizado um estudo analÃtico-numÃrico das caracterÃsticas de transmissÃo e reflexÃo das grades de Bragg nÃo-lineares. Pela primeira vez, foram consideradas variaÃÃes periÃdicas da nÃo-linearidade no dispositivo operando no regime de onda continua, levando a uma nova classe de grades nÃo-uniformes. CaracterÃsticas dos estados bi- e multi-estÃveis foram extensamente investigados nas grades de Bragg nÃo-lineares. TambÃm, pela primeira vez, foi realizado o estudo numÃrico de pulsos ultracurtos ($sim$1 ps) incidindo em grades nÃo-lineares. O enfoque foi dado para a dependÃncia da intensidade de um pulso ultracurto ao passar por tal grade. Foram estudadas, ainda, as dependÃncias na forma temporal da profundidade de modulaÃÃo da grade e do Ãndice nÃo-linear. Grades apodizadas foram consideradas, jà que estas sÃo de importÃncia fundamental nos sitemas de comunicaÃÃes modernos. / Nonlinear fiber Bragg gratings has been considered since the end of last century for applications in optical communications and sensor techniques. The investigation of ultrashort pulses in linear Bragg gratings, however has been considered in the last few years due the development of specifical numerical techniques to solve this problem. In the present work an analytical and numerical study of the reflection and transmission characteristics of nonlinear Bragg gratings was done. For the first time, it has been considered periodic variations of the nonlinearity in that devices operating in the continuous wave regime, leading to a new class of nonuniform gratings. It was extensively investigated the bi- and multistable characteristics in these nonlinear fiber Bragg gratings. Also, for the first time, the numerical study of ultrahsort pulses ($sim$1 ps) incident in nonlinear gratings was done. The focus was the input pulse intensity dependence on that gratings. Also, the depedences in the time shapes of grating index modulation depth and nonlinear index were studied. Apodized gratings were considered since they are of fundamental importance in modern communications systems.