Applications de résonateurs Fabry-Pérot pour l'imagerie par super-lentilles et pour les réseaux de Bragg à sauts de phase divisés

Le présent travail est divisé en deux sections, chacune étant consacrée à une application particulière des structures de Fabry-Perot dans un domaine spécifique de l'optique moderne. La Section I traite du design de structures d'imagerie par superlentilles, qui servent essentiellement à la génération d'images de champ proche de structures dont les caractères sont plus petits que la demi-longueur d'onde, en utilisant des approches originales et performantes. Ces structures sont constituées de couches de matériaux très minces, dont l'épaisseur est de l'ordre de la dizaine de nanometres, et dont les interfaces font office de miroirs à réflectivité complexe de telle sorte qu'on peut les associer à une cascade de cavités Fabry-Perot. La théorie fondamentale des structures de super-lentilles, qu'elles soient constituées du cas idéal utilisant une couche de matériau d'indice négatif ou de métal, est exposée. Une méthode de design basée sur une étude modale de la super-lentille métallique ainsi qu'une approche de design numérique de super-lentille basée sur un modèle raffiné de la super-lentille qui inclut l'objet à imager sont ensuite proposées. Dans les deux cas, les designs obtenus se comparent avantageusement avec d'autres présentés dans la littérature. La Section II étudie le problème de la division de sauts de phase dans les réseaux de Bragg écrits à l'aide d'un masque de phase dans lequel les sauts de phase sont inscrits. Dans les réseaux comprenant un seul saut de phase de à mi-longueur qu'on appelle filtres Fabry-Perot (FP) basés sur réseaux, la division en deux du saut de phase se manifeste physiquement par l'écriture d'une section de réseau très petite entre chacun des deux demi-sauts de phase causant des erreurs d'asymétrie dans sa réponse spectrale. On modélise les filtres FP à saut de phase divisé comme une succession de trois miroirs à réflectivité complexe variant avec la longueur d'onde séparés par les deux demi-sauts de phase. On constate que pour des filtres FP longs de plus de quelques millimètres, l'effet de la division d'un saut de phase est négligeable

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries

Design, Fabrication, and Characterization of a One-Dimensional Single-Material Polarizing Photonic Crystal

We examine a multilayered one-dimensional (1D) polarizing photonic crystal designed and fabricated out of a single material. This polarizer is designed for high reflection of the s polarization and low reflection of p polarization at the wavelength of 632.8 nm. This device is fabricated implementing the oblique angle deposition technique to produce six-bilayers of alternating high to low indices of titanium dioxide using e-beam PVD for depositing on top of a fused silica substrate. For modeling, we used transfer-matrix method and numerical finite-difference time-domain analysis to simulate behavior of the 1D photonic bandgap structure. Both model and simulation predict better than 98% reflection for s polarization and less than 1% reflection for p polarization at wavelength of 632.8 nm at an incident angle of 70°. The fabricated device exhibited 94% reflection for s polarization and less than 6% reflection for p polarization at 632.8 nm at an incident angle of 70°. The results are in good agreement with theoretical predictions

Photonic Crystal Directional Coupler Based Optomechanical Sensor

An extremely small ($6.5\times6.5\mu$m) optomechanical sensor is proposed that utilizes a photonic crystal (PC) etched onto silicon-on-insulator (SOI) using adapted complimentary metal-oxide-semiconductor fabrication technology. The destructive interference of light with the periodic structure can forbid its propagation inside the crystal across a range of frequencies and can be used to confine light near edge of a PC slab. By placing two PC edges near each other, a directional coupler is formed where light is periodically exchanged between the two waveguides. Wet-etching away the buried oxide residing beneath the photonic crystal directional coupler (PCDC), a membrane is formed. Exerting force on the PCDC alters the separation between the two PC edges and modulates the observed transmission at the coupler outputs. Buckle-mitigating structures are also demonstrated here which relieve the unpredictable compressive stress built into the top silicon layer of SOI during wafer fabrication. The PCDC sensors attempt to overcome some of the shortcomings of existing micromechanical sensors such as area constraints, material restrictions, stiction, and EM interference. PCDC sensors are also highly parallelizable due to their small size and wide optical bandwidth. PCDC sensors are envisaged to be used in microfluidic integration and are capable of 149kPa full scale pressure measurement ranges

DOC 2016-01 Creation of the Department of Electro-Optics and Photonics

Legislative Authority. DOC 2014-04: Actions Pertaining to Degree Programs and Academic Department

Non-destructive assessment and health monitoring of railway infrastructures

A continuous increase of the demand for high-speed traffic, freight tonnage as well as of the train operating frequency is worsening the decay conditions of many railway infrastructures. This occurrence affects economy-related business as well as it contributes to raise maintenance cost. It is known that a failure of a railway track may result in tremendous economic losses, law liabilities, service interruptions and, eventually, fatalities. Parallel to this, requirements to maintain acceptable operational standards are very demanding. In addition to the above, a main issue nowadays in railway engineering is a general lack of funds to allow safety and comfort of the operations as well as a proper maintenance of the infrastructures. This is mostly the result of a traditional approach that, on average, tends to invest on high-priority cost, such as safety-related cost, compromising lower-priority cost (e.g., quality and comfort of the operations). A solution to correct this trend can be to move from a reactive to a proactive action planning approach in order to limit more effectively the likelihood of progressive track decay. Within this context, this paper reports a review on the use of traditional and non-destructive testing (NDT) methods for assessment and health monitoring of railway infrastructures. State-of-the-art research on a stand-alone use of NDT methods or a combination of them for specific maintenance tasks in railways is discussed

Residual stress characterization in a single fibre composite specimen by using FBG sensor and the OLCR technique

The purpose of this thesis is to characterize internal strains in polymeric materials due to consolidation. In view of this, optical Fiber Bragg Grating sensors are an excellent non-destructive tool for internal strain characterization and damage detection in composite materials and structures. Fiber Bragg Gratings (FBG), have become increasingly used in engineering applications because of their inherent advantages with respect to traditional sensors. They can provide an important tool in experimental mechanics to perform key experiments that are difficult or impossible with other standard techniques. In this respect, they are ideally suited as strain measuring devices in composites, where they can be embedded non-invasively during fabrication. In view of this, the main goal of this thesis is the development of an experimental methodology to characterize the residual stresses that are generally present in many materials and is a complex problem to solve in micro-mechanics. The work is presented in three interrelated parts. Long-gauge-FBGs (Bragg grating of ∼ 24 mm) are introduced in cylindrical specimens of epoxy. In this configuration the fiber is simultaneously a reinforcement and a sensor in a single fiber composite. Because the epoxy matrix shrinks during the polymerization process, the optical sensor undergoes substantial non-uniform strain along the fiber. The response of the FBG to a non-uniform strain distribution is investigated by using an Optical Low-Coherence Reflectometry (OLCR) based technique which allows a direct reconstruction of the optical period along the grating without any a priori assumption about the strain field. A comparison with the most common reconstruction inverse technique T-Matrix is also proposed, showing that it generally introduces greater errors without ensuring the uniqueness of the solution. The OLCR permits in fact the direct measurement of the axial evolution of the residual strain along the core of the reinforcing fiber, thus providing important information on the internal state of stress of the specimen at a given stage of its preparation and, later on, during its service life. In addition, the measured strain distribution evolves along the fiber direction following a fourth-order function, which clearly presents a plateau over a 20 mm range at the center of the specimen. In particular, the maximum strain level reached after the matrix solidification is –2000 με which increases up to –6000 με at the end of the post-curing process of the resin. This value is consistent with the volume reduction of the free resin provided by the producer and equal to 2 %. This strain corresponds to -450 MPa axial compressive stress on the embedded reinforcing fiber. The implementation of FBG sensors to study the changes in the stress field when a crack is present in the sample is addressed next. Bragg wavelength distributions have been measured as a function of the depth of machined circular cracks in the radial direction of the cylinder. Three different crack depths (namely 7.5 mm, 11 mm and 12 mm) have been machined in the central section of the specimen. First, these measurements give an indication about the zone of influence of the reinforcing fiber on the residual stresses and, secondly, they permit the characterization of the effect of a mechanically induced crack on the initial residual stress state. In particular, only the stress relaxation due to the introduction of the deepest transversal crack significantly affects the FBG response with a related wavelength variation of 3 nm. These data are used as input to deduce the radial evolution of the stress field by adapting and improving the Crack Compliance (C.C.M) inverse Method to retrieve the stress field within a composite starting from a measurement of strain. A rigorous analytical approach to predict the residual stress field is described and verified numerically and experimentally. A very good agreement is found between experimental and numerical values, thus proving the reliability of the experimental approach. As last topic in this work, the response of a long FBG to the transverse crack propagation is monitored experimentally by using the OLCR and modeled numerically. Firstly, a Compact-Tension specimen submitted to a cyclic fatigue test is chosen with the FBG glued on its back-face and normal to the crack direction. A simple analytical model predict the FBG response as the crack advances. Secondly, long- FBG is embedded in a Compact-Tension specimen of the same dimensions. In particular when the natural crack overpasses the fiber, the grating can be used to measure the bridging forces between fracture surfaces and/or to measure the relative opening of the crack. Reconstruction of the FBG signal with T-matrix indicate problems associated to stress distributions due to highly non-uniform strain field. In this way, the FBG becomes the excellent candidate to study a number of interesting problems in the field of the fracture mechanics applications

Electrophysiologic assessment of (central) auditory processing disorder in children with non-syndromic cleft lip and/or palate

Session 5aPP - Psychological and Physiological Acoustics: Auditory Function, Mechanisms, and Models (Poster Session)Cleft of the lip and/or palate is a common congenital craniofacial malformation worldwide, particularly non-syndromic cleft lip and/or palate (NSCL/P). Though middle ear deficits in this population have been universally noted in numerous studies, other auditory problems including inner ear deficits or cortical dysfunction are rarely reported. A higher prevalence of educational problems has been noted in children with NSCL/P compared to craniofacially normal children. These high level cognitive difficulties cannot be entirely attributed to peripheral hearing loss. Recently it has been suggested that children with NSCLP may be more prone to abnormalities in the auditory cortex. The aim of the present study was to investigate whether school age children with (NSCL/P) have a higher prevalence of indications of (central) auditory processing disorder [(C)APD] compared to normal age matched controls when assessed using auditory event-related potential (ERP) techniques. School children (6 to 15 years) with NSCL/P and normal controls with matched age and gender were recruited. Auditory ERP recordings included auditory brainstem response and late event-related potentials, including the P1-N1-P2 complex and P300 waveforms. Initial findings from the present study are presented and their implications for further research in this area —and clinical intervention—are outlined. © 2012 Acoustical Society of Americapublished_or_final_versio