Digital imaging techniques have found several medical applications in the development of computer aided detection systems, especially in neuroimaging. Recent advances in Diffusion Tensor Imaging (DTI) aim to discover biological markers for the early diagnosis of Alzheimer’s disease (AD), one of the most widespread neurodegenerative disorders. We explore here how different supervised classification models provide a robust support to the diagnosis of AD patients. We use DTI measures, assessing the structural integrity of white matter (WM) fiber tracts, to reveal patterns of disrupted brain connectivity. In particular, we provide a voxel-wise measure of fractional anisotropy (FA) and mean diffusivity (MD), thus identifying the regions of the brain mostly affected by neurodegeneration, and then computing intensity features to feed supervised classification algorithms. In particular, we evaluate the accuracy of discrimination of AD patients from healthy controls (HC) with a dataset of 80 subjects (40 HC, 40 AD), from the Alzheimer’s Disease Neurodegenerative Initiative (ADNI). In this study, we compare three state-of-the-art classification models: Random Forests, Naive Bayes and Support Vector Machines (SVMs). We use a repeated five-fold cross validation framework with nested feature selection to perform a fair comparison between these algorithms and evaluate the information content they provide. Results show that AD patterns are well localized within the brain, thus DTI features can support the AD diagnosis.
We study pattern formation in a paraxial model for an unidirectional ring resonator filled with a semiconductor
sample and driven by a coherent injected field beyond the mean field limit (MFL). We perform numerical
simulations to describe the three-dimensional dynamics of the coherent field profile. For fast media spontaneous
self-confinement leads to the formation of 3D dissipative addressable spatial solitons, we show that for carrier
dynamics compatible with GaAs/GaAlAs MQW devices longitudinal self-confinement is hindered by the slow carrier interband dynamics.
We develop a model that describes the optical response of a semiconductor quantum dot microcavity pumped above
transparency but kept slightly below threshold. The model takes into account the inhomogeneous broadening of the dot
emission, the coupling mechanisms between quantum dots and the wetting layer and incorporates gain asymmetry
factors in the thermo-emission and capture coefficients. The role of asymmetries with respect to alpha factor and pattern
formation is investigated. We then study the conditions for the onset of bistability and modulational instability and
characterize the patterns formed.
We consider the paraxial model for a nonlinear resonator with a saturable absorber beyond the mean-field limit. We introduce a general stability analysis to evidence modulational-instabilities leading to the destabilization of a homogeneous field profile, eventually causing the formation of 3D structures. Further on, for accessible parametric domains, we show in simulations the phenomenon of total radiation confinement leading to the formation of 3D localized bright structures. Such structures are a direct generalization of 2D Cavity Solitons, recently observed in broad-area VCSELs, but they are confined also in the propagation dimension. At difference from freely propagating light bullets, here the self-organization proceeds from the resonator feedback/dissipation, combined with diffraction and nonlinearity.
We show that such cavity light bullets can be independently excited and erased by appropriate pulses. They can be addressed to form arrays in the transverse field profile as well as serial trains in the longitudinal direction of the resonator thus combining serial and parallel encoding in the same device. Once created, they endlessly travel the cavity roundtrip.
We report the experimental observation of transverse optical patterns in optically injected phase-coupled VCSEL arrays. The devices consist of a single VCSEL with a metallic top contact defining a 8x8 pixels array. The different semiconductor/air - metal/air index step determines the periodic variation of the cavity transmission, that enables the selection of a single laser supermode above threshold. The patterns were observed in the near field at driving currents below the laser threshold, while injecting a nearly resonant optical field. At large frequency detuning hexagonal patterns were observed, whereas rolls appeared closer to the cavity resonance. The experimental observations are explained according to a well established model (Spinelli, Tissoni, Brambilla, Prati, Lugiato, Phys.Rev.A 58, (1998) 2542) accounting for diffraction and nonlinear effects in optically injected VCSELs, and modified to allow for the spatially modulated cavity transmission.
Cavity solitons are stationary self-organized bright intensity peaks which form over a homogeneous background in the section of broad area radiation beams. They are generated by shining a writing/erasing laser pulse into a nonlinear optical cavity, driven by a holding beam. The ability to control their location and their motion by introducing phase or amplitude gradients in the holding beam makes them interesting as mobile pixels for all-optical processing units. We show the generation of a number of cavity solitons in broad area vertical cavity semiconductor microresonators electrically pumped above transparency but slightly below threshold. The observed spots can be written, erased and manipulated as independent objects. We analyze experimentally the cavity solitons domain of existence in the parameter space and how their characteristics are affected by inhomogeneities and impurities of the vertical cavity devices. A theoretical model, keeping into account the devices characteristics, reproduces numerically the experimental observations with good agreement.
In this work we present an innovative optical sensor for on- line and non-intrusive welding process monitoring. It is based on the spectroscopic analysis of the optical VIS emission of the welding plasma plume generated in the laser- metal interaction zone. Plasma electron temperature has been measured for different chemical species composing the plume. Temperature signal evolution has been recorded and analyzed during several CO<SUB>2</SUB>-laser welding processes, under variable operating conditions. We have developed a suitable software able to real time detect a wide range of weld defects like crater formation, lack of fusion, excessive penetration, seam oxidation. The same spectroscopic approach has been applied for electric arc welding process monitoring. We assembled our optical sensor in a torch for manual Gas Tungsten Arc Welding procedures and tested the prototype in a manufacturing industry production line. Even in this case we found a clear correlation between the signal behavior and the welded joint quality.
Cavity solitons (CS) appear as self-confined light peaks embedded in the transverse profile of a homogeneous coherent field propagating in a nonlinear cavity. They have recently been predicted for GaAs semiconductor micro cavities for which we have developed a microscopic model that describes the field and the carrier dynamics inside the active region. Here we improve our previous model by adding the temperature dynamics. A detailed study of the instabilities affecting the homogeneous stationary state of the output field is performed. In this way we can address the numerical research of patterns and CS. We then show how it is possible to study intrinsic stability properties of CS by means of semi- analytical techniques that allow to describe the destabilizing mechanisms for solitons, mutual interaction properties and their response to perturbations; possible conceptual schemes for optical information treatment and logic gates are investigated.
We study the formation of self organized light peaks, in GaAs microcavities. By means of analytical and numerical techniques, experimentally accessible parametric domains can be found, where stable and robust CS can be addressed, shifted and brought to interaction ranges, thus realizing some basic schemes for optical information treatment. A Fourier-Newton approach is applied to gain quantitative information on CS's dynamical response to external control fields or on CS pair interaction.
Cavity solitons appear as stationary, isolated peaks of light superimposed onto a homogeneous background field in the transverse profile of the coherent field transmitted or reflected by a non-linear resonator. These self-organized structures are theoretically predicted and simulated in a broad area multi-quantum-well vertical microresonator. We develop models suited to describe the macroscopic properties of the medium and the nonlinear interaction with the coherent field. Parametric domains and operational regimes for stable solitons are investigated along with some quantitative appreciation of their characteristics. Intrinsic stability properties of solitons are investigated by means of semi-analytical techniques and this allows to describe the destabilizing mechanisms for solitons, mutual interaction properties, their response to perturbations and some of their dynamical features.
Cavity solitons appear as bright spots in the transverse intensity profile. They are similar to spatial solitons, but arise in dissipative systems. Here we consider a broad area vertical cavity resonator, driven by an external coherent field, at room temperature. The active material is constituted either by bulk GaAs, or by a Multiple Quantum Well GaAs/AlGaAs structure (MQW). A general model valid for both configurations is presented and a set of nonlinear dynamical equations is derived. The linear stability analysis of the homogeneous steady states is performed in a general form, holding for the two cases. Then, the nonlinear susceptibilities are specified: in the bulk case, we basically work in the free-carrier approximation, with some phenomenological corrections, such as the Urbach tail and the band-gap renormalization. For the bulk case, some numerical results concerning spatial pattern formation and cavity solitons are given. In the MQW case, on the contrary, we derive a full many-body theory, with the Coulomb enhancement treated in the Pade approximation.