Organization of the inferotemporal cortex in the macaque monkey: Connections of areas PITv and CITvp

Visual cortex of macaque monkeys consists of a large number of cortical areas that span the occipital, parietal, temporal, and frontal lobes and occupy more than half of cortical surface. Although considerable progress has been made in understanding the contributions of many occipital areas to visual perceptual processing, much less is known concerning the specific functional contributions of higher areas in the temporal and frontal lobes. Previous behavioral and electrophysiological investigations have demonstrated that the inferotemporal cortex (IT) is essential to the animal's ability to recognize and remember visual objects. While it is generally recognized that IT consists of a number of anatomically and functionally distinct visual-processing areas, there remains considerable controversy concerning the precise number, size, and location of these areas. Therefore, the precise delineation of the cortical subdivisions of inferotemporal cortex is critical for any significant progress in the understanding of the specific contributions of inferotemporal areas to visual processing. In this study, anterograde and/or retrograde neuroanatomical tracers were injected into two visual areas in the ventral posterior and central portions of IT (areas PITv and CITvp) to elucidate the corticocortical connections of these areas with well known areas of occipital cortex and with less well understood regions of inferotemporal cortex. The locations of injection sites and the delineation of the borders of many occipital areas were aided by the pattern of interhemispheric connections, revealed following callosal transection and subsequent labeling with HRP. The resultant patterns of connections were represented on two-dimensional computational (CARET) and manual cortical maps and the laminar characteristics and density of the projection fields were quantified. The laminar and density features of these corticocortical connections demonstrate thirteen anatomically distinct subdivisions or areas distributed within the superior temporal sulcus and across the inferotemporal gyrus. These results serve to refine previous descriptions of inferotemporal areas, validate recently identified areas, and provide a new description of the hierarchical relationships among occipitotemporal cortical areas in macaques. ^

Composition of Carnian to Norian sandstones from the Wombat Plateau (Table 1)

The Carnian to Norian sediments, as much as 600 m in total thickness, recovered from ODP Sites 759 and 760 on the Wombat Plateau, are generally represented by fluvial-dominated deltaic successions. In general, the Carnian to Norian sandstones are quartzose. The average ratio of monocrystalline quartz grains, total feldspar grains, and total lithic fragments (i.e., Qm:F:Lt ratio) is 71:22:7. This indicates that they were derived mainly from the transitional continental and cratonic interior provenance terranes, such as the Pilbara Precambrian block to the south of the Wombat Plateau. The upper Carnian sediments, however, are characterized by more feldspathic sandstone petrofacies. They typically contain some volcanic rock fragments with trachytic texture and indicate the onset of the incipient rift-related tectonic movement, such as uplift and subsequent abrupt basin subsidence, together with volcanism in the Gondwana continental block. Mixed siliciclastic and carbonate cycles are typically intercalated in the prodelta to delta front deposits that developed mainly in a lagoon-like, restricted marine environment. The restricted marine environment developed during transgressions as the outflow of shallow water was restricted by depositional barriers. Around the barriers and/or delta lobes, carbonate shoals/banks were probably developed and the allochemical components of the neritic limestones may have been transported into the restricted marine environment by overwash processes and/or storm waves. Siliciclastic detritus, on the other hand, was mainly derived accompanied by delta progradation dominated by fluvial processes in the restricted marine environment. Therefore, we interpret the mixed siliciclastic and carbonate cycles in the deltaic successions to be a result of transgression-regression cycles in a deltaic system during the Late Triassic.

Trace-element geochemistry of tholeiitic basalts from DSDP Hole 55-433C

Hole 433C, a multiple re-entry hole drilled in 1862 meters of water on Suiko Seamount in the central Emperor Seamounts, penetrated 387.5 meters of lava flows overlain by 163.0 meters of sediments. The recovered volcanic rocks consist of three flow units (1-3) of alkalic basalt underlain by more than 105 flows or flow lobes (Flow Units 4-67) of tholeiitic basalt. This study reports trace-element, including rare-earth element (REE), data for 25 samples from 24 of the least altered tholeiitic flows. These data are used to evaluate the origin and evolution of tholeiitic basalts from Suiko Seamount and to evaluate changes in the mantle source between the time when Suiko Seamount formed, 64.7 ± 1.1 m.y. ago (see Dalrymple et al., 1980), and the present day. Stearns (1946), Macdonald and Katsura (1964) and Macdonald (1968) have established that chemically distinct lavas erupt during four eruptive stages of development of a Hawaiian volcano. These stages, from initial to final, are shield-building, caldera-filling, post-caldera, and post-erosional. The lavas of the shield-building stage are tholeiitic basalts, which erupt rapidly and in great volume. The shield-building stage is quickly followed by caldera collapse and by the caldera-filling stage, during which the caldera is filled by tholeiitic and alkalic lavas. During the post-caldera stage, a relatively thin veneer of alkalic basalts and associated differentiated lavas are erupted, sometimes accompanied by minor eruptions of tholeiitic lava. After a period of volcanic quiescence and erosion, lavas of the nephelinitic suite, which include both alkalic basalts and strongly SiO2-undersaturated nephelinitic basalts, may erupt from satellite vents during the post-erosional stage. Many Hawaiian volcanoes develop through all four stages; but individual volcanoes have become extinct before the cycle is complete. We interpret the tholeiitic lavas drilled on Suiko Seamount to have erupted during either the shield-building or the caldera-filling stage, and the overlying alkalic flows to have erupted during either the caldera-filling or the post-caldera stage (see Kirkpatrick et al., 1980).

Table I. Composition of till in north Greenland

From 1950 through 1900 studies on the glacial geology of northern Greenland have been made in cooperation with the U.S. Air Force Cambridge Research Laboratories. As a result of these studies four distinct phases of the latest glaciation have been recognized. The last glaciation extended over most of the land and removed traces of previous anes. Retreat of the ice mass began some time previous to 6000 years ago. This was followed by a rtse in sea level which deposited clay-silt succeeded by karne gravels around stagnant ice lobes in the large valleys. Marine terraces, up to 129 meters above present sea level, developed as readjustment occurred in the land free of ice. About 3700 years ago an advance of glaciers down major fjords took place followed by retreat to approximately the present position of the ice. Till in Peary Land, north of Frederick E. Hyde Fjord, contains only locally derived matertals indicating that the central Greenland ice cap did not cover the area.

Study of the fronts of Johnsons and Hurd Glaciers (Livingston Island, Antarctica) from 1957 to 2013, with links to shapefiles

The study of glacier fronts combines different geomatics measurement techniques as the classic survey using total station or theodolite, technical GNSS (Global Navigation Satellite System), using laser-scanner or using photogrammetry (air or ground). The measure by direct methods (classical surveying and GNSS) is useful and fast when accessibility to the glaciers fronts is easy, while it is practically impossible to realize, in the case of glacier fronts that end up in the sea (tide water glaciers). In this paper, a methodology that combines photogrammetric methods and other techniques for lifting the front of the glacier Johnsons, inaccessible is studied. The images obtained from the front, come from a non-metric digital camera; its georeferencing to a global coordinate system is performed by measuring points GNSS support in accessible areas of the glacier front side and applying methods of direct intersection in inaccessible points of the front, taking measurements with theodolite. The result of observations obtained were applied to study the temporal evolution (1957-2014) of the position of the Johnsons glacier front and the position of the Argentina, Las Palmas and Sally Rocks lobes front (Hurd glacier).

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 ?m or greater. A 20um thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200um thick SSC equipped with a planar back reflector.

3x3 Multibeam Network for a Triangular Array of Three Radiating Elements

A multibeam antenna study based on Butler network will be undertaken in this document. These antenna designs combines phase shift systems with multibeam networks to optimize multiple channel systems. The system will work at 1.7 GHz with circular polarization. Specifically, result simulations and measurements of 3 element triangular subarray will be shown. A 45 element triangular array will be formed by the subarrays. Using triangular subarrays, side lobes and crossing points are reduced.

Adaptive liquid-crystal phase-only passive devices

This PhD work is focused on liquid crystal based tunable phase devices with special emphasis on their design and manufacturing. In the course of the work a number of new manufacturing technologies have been implemented in the UPM clean room facilities, leading to an important improvement in the range of devices being manufactured in the laboratory. Furthermore, a number of novel phase devices have been developed, all of them including novel electrodes, and/or alignment layers. The most important manufacturing progress has been the introduction of reactive ion etching as a tool for achieving high resolution photolithography on indium-tin-oxide (ITO) coated glass and quartz substrates. Another important manufacturing result is the successful elaboration of a binding protocol of anisotropic conduction adhesives. These have been employed in high density interconnections between ITO-glass and flexible printed circuits. Regarding material characterization, the comparative study of nonstoichiometric silicon oxide (SiOx) and silica (SiO2) inorganic alignment layers, as well as the relationship between surface layer deposition, layer morphology and liquid crystal electrooptical response must be highlighted, together with the characterization of the degradation of liquid crystal devices in simulated space mission environment. A wide variety of phase devices have been developed, with special emphasis on beam steerers. One of these was developed within the framework of an ESA project, and consisted of a high density reconfigurable 1D blaze grating, with a spatial separation of the controlling microelectronics and the active, radiation exposed, area. The developed devices confirmed the assumption that liquid crystal devices with such a separation of components, are radiation hard, and can be designed to be both vibration and temperature sturdy. In parallel to the above, an evenly variable analog beam steering device was designed, manufactured and characterized, providing a narrow cone diffraction free beam steering. This steering device is characterized by a very limited number of electrodes necessary for the redirection of a light beam. As few as 4 different voltage levels were needed in order to redirect a light beam. Finally at the Wojskowa Akademia Techniczna (Military University of Technology) in Warsaw, Poland, a wedged analog tunable beam steering device was designed, manufactured and characterized. This beam steerer, like the former one, was designed to resist the harsh conditions both in space and in the context of the shuttle launch. Apart from the beam steering devices, reconfigurable vortices and modal lens devices have been manufactured and characterized. In summary, during this work a large number of liquid crystal devices and liquid crystal device manufacturing technologies have been developed. Besides their relevance in scientific publications and technical achievements, most of these new devices have demonstrated their usefulness in the actual work of the research group where this PhD has been completed. El presente trabajo de Tesis se ha centrado en el diseño, fabricación y caracterización de nuevos dispositivos de fase basados en cristal líquido. Actualmente se están desarrollando dispositivos basados en cristal líquido para aplicaciones diferentes a su uso habitual como displays. Poseen la ventaja de que los dispositivos pueden ser controlados por bajas tensiones y no necesitan elementos mecánicos para su funcionamiento. La fabricación de todos los dispositivos del presente trabajo se ha realizado en la cámara limpia del grupo. La cámara limpia ha sido diseñada por el grupo de investigación, es de dimensiones reducidas pero muy versátil. Está dividida en distintas áreas de trabajo dependiendo del tipo de proceso que se lleva a cabo. La cámara limpia está completamente cubierta de un material libre de polvo. Todas las entradas de suministro de gas y agua están selladas. El aire filtrado es constantemente bombeado dentro de la zona limpia, a fin de crear una sobrepresión evitando así la entrada de aire sin filtrar. Las personas que trabajan en esta zona siempre deben de estar protegidas con un traje especial. Se utilizan trajes especiales que constan de: mono, máscara, guantes de látex, gorro, patucos y gafas de protección UV, cuando sea necesario. Para introducir material dentro de la cámara limpia se debe limpiar con alcohol y paños especiales y posteriormente secarlos con nitrógeno a presión. La fabricación debe seguir estrictamente unos pasos determinados, que pueden cambiar dependiendo de los requerimientos de cada dispositivo. Por ello, la fabricación de dispositivos requiere la formulación de varios protocolos de fabricación. Estos protocolos deben ser estrictamente respetados a fin de obtener repetitividad en los experimentos, lo que lleva siempre asociado un proceso de fabricación fiable. Una célula de cristal líquido está compuesta (de forma general) por dos vidrios ensamblados (sándwich) y colocados a una distancia determinada. Los vidrios se han sometido a una serie de procesos para acondicionar las superficies internas. La célula se llena con cristal líquido. De forma resumida, el proceso de fabricación general es el siguiente: inicialmente, se cortan los vidrios (cuya cara interna es conductora) y se limpian. Después se imprimen las pistas sobre el vidrio formando los píxeles. Estas pistas conductoras provienen del vidrio con la capa conductora de ITO (óxido de indio y estaño). Esto se hace a través de un proceso de fotolitografía con una resina fotosensible, y un desarrollo y ataque posterior del ITO sin protección. Más tarde, las caras internas de los vidrios se acondicionan depositando una capa, que puede ser orgánica o inorgánica (un polímero o un óxido). Esta etapa es crucial para el funcionamiento del dispositivo: induce la orientación de las moléculas de cristal líquido. Una vez que las superficies están acondicionadas, se depositan espaciadores en las mismas: son pequeñas esferas o cilindros de tamaño calibrado (pocos micrómetros) para garantizar un espesor homogéneo del dispositivo. Después en uno de los sustratos se deposita un adhesivo (gasket). A continuación, los sustratos se ensamblan teniendo en cuenta que el gasket debe dejar una boca libre para que el cristal líquido se introduzca posteriormente dentro de la célula. El llenado de la célula se realiza en una cámara de vacío y después la boca se sella. Por último, la conexión de los cables a la célula y el montaje de los polarizadores se realizan fuera de la sala limpia (Figura 1). Dependiendo de la aplicación, el cristal líquido empleado y los demás componentes de la célula tendrán unas características particulares. Para el diseño de los dispositivos de este trabajo se ha realizado un estudio de superficies inorgánicas de alineamiento del cristal líquido, que será de gran importancia para la preparación de los dispositivos de fase, dependiendo de las condiciones ambientales en las que vayan a trabajar. Los materiales inorgánicos que se han estudiado han sido en este caso SiOx y SiO2. El estudio ha comprendido tanto los factores de preparación influyentes en el alineamiento, el comportamiento del cristal líquido al variar estos factores y un estudio de la morfología de las superficies obtenidas.

Simulation of the transmission performance for equalized holographic ROADM designs

A complete simulation of the transmission performance for Equalized Holographic ROADM (Reconfigurable Optical Add-Drop Multiplexer) designs is presented in this paper. These devices can address several wavelengths from the input to different output fibres, according to the holograms stored in a SLM (Spatial Light Modulator), where all the outputs are equalized in power. All combinations of the input wavelengths are possible at the different output fibres. To simulate the transmission performance of the EH-ROADM, a software program, from Optiwave, has been used. The correspondence between physical blocks of the device (grating, SLM, lens...) and those simulated in the program (filters, losses, splitters...) has been defined in order to obtain a close agreement between the theoretical transmission performance and the simulated one. To complete the review about Equalized Holographic ROADMs some guidelines about its design have been done.

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

Nanoimprinted diffraction gratings for crystalline silicon solar cells: implementation, characterization and simulation

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electro-optical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential.

Photon Management Structures for Absorption Enhancement in Intermediate Band Solar Cells and Crystalline Silicon Solar Cells

El objetivo de la tesis es investigar los beneficios que el atrapamiento de la luz mediante fenómenos difractivos puede suponer para las células solares de silicio cristalino y las de banda intermedia. Ambos tipos de células adolecen de una insuficiente absorción de fotones en alguna región del espectro solar. Las células solares de banda intermedia son teóricamente capaces de alcanzar eficiencias mucho mayores que los dispositivos convencionales (con una sola banda energética prohibida), pero los prototipos actuales se resienten de una absorción muy débil de los fotones con energías menores que la banda prohibida. Del mismo modo, las células solares de silicio cristalino absorben débilmente en el infrarrojo cercano debido al carácter indirecto de su banda prohibida. Se ha prestado mucha atención a este problema durante las últimas décadas, de modo que todas las células solares de silicio cristalino comerciales incorporan alguna forma de atrapamiento de luz. Por razones de economía, en la industria se persigue el uso de obleas cada vez más delgadas, con lo que el atrapamiento de la luz adquiere más importancia. Por tanto aumenta el interés en las estructuras difractivas, ya que podrían suponer una mejora sobre el estado del arte. Se comienza desarrollando un método de cálculo con el que simular células solares equipadas con redes de difracción. En este método, la red de difracción se analiza en el ámbito de la óptica física, mediante análisis riguroso con ondas acopladas (rigorous coupled wave analysis), y el sustrato de la célula solar, ópticamente grueso, se analiza en los términos de la óptica geométrica. El método se ha implementado en ordenador y se ha visto que es eficiente y da resultados en buen acuerdo con métodos diferentes descritos por otros autores. Utilizando el formalismo matricial así derivado, se calcula el límite teórico superior para el aumento de la absorción en células solares mediante el uso de redes de difracción. Este límite se compara con el llamado límite lambertiano del atrapamiento de la luz y con el límite absoluto en sustratos gruesos. Se encuentra que las redes biperiódicas (con geometría hexagonal o rectangular) pueden producir un atrapamiento mucho mejor que las redes uniperiódicas. El límite superior depende mucho del periodo de la red. Para periodos grandes, las redes son en teoría capaces de alcanzar el máximo atrapamiento, pero sólo si las eficiencias de difracción tienen una forma peculiar que parece inalcanzable con las herramientas actuales de diseño. Para periodos similares a la longitud de onda de la luz incidente, las redes de difracción pueden proporcionar atrapamiento por debajo del máximo teórico pero por encima del límite Lambertiano, sin imponer requisitos irrealizables a la forma de las eficiencias de difracción y en un margen de longitudes de onda razonablemente amplio. El método de cálculo desarrollado se usa también para diseñar y optimizar redes de difracción para el atrapamiento de la luz en células solares. La red propuesta consiste en un red hexagonal de pozos cilíndricos excavados en la cara posterior del sustrato absorbente de la célula solar. La red se encapsula en una capa dieléctrica y se cubre con un espejo posterior. Se simula esta estructura para una célula solar de silicio y para una de banda intermedia y puntos cuánticos. Numéricamente, se determinan los valores óptimos del periodo de la red y de la profundidad y las dimensiones laterales de los pozos para ambos tipos de células. Los valores se explican utilizando conceptos físicos sencillos, lo que nos permite extraer conclusiones generales que se pueden aplicar a células de otras tecnologías. Las texturas con redes de difracción se fabrican en sustratos de silicio cristalino mediante litografía por nanoimpresión y ataque con iones reactivos. De los cálculos precedentes, se conoce el periodo óptimo de la red que se toma como una constante de diseño. Los sustratos se procesan para obtener estructuras precursoras de células solares sobre las que se realizan medidas ópticas. Las medidas de reflexión en función de la longitud de onda confirman que las redes cuadradas biperiódicas consiguen mejor atrapamiento que las uniperiódicas. Las estructuras fabricadas se simulan con la herramienta de cálculo descrita en los párrafos precedentes y se obtiene un buen acuerdo entre la medida y los resultados de la simulación. Ésta revela que una fracción significativa de los fotones incidentes son absorbidos en el reflector posterior de aluminio, y por tanto desaprovechados, y que este efecto empeora por la rugosidad del espejo. Se desarrolla un método alternativo para crear la capa dieléctrica que consigue que el reflector se deposite sobre una superficie plana, encontrándose que en las muestras preparadas de esta manera la absorción parásita en el espejo es menor. La siguiente tarea descrita en la tesis es el estudio de la absorción de fotones en puntos cuánticos semiconductores. Con la aproximación de masa efectiva, se calculan los niveles de energía de los estados confinados en puntos cuánticos de InAs/GaAs. Se emplea un método de una y de cuatro bandas para el cálculo de la función de onda de electrones y huecos, respectivamente; en el último caso se utiliza un hamiltoniano empírico. La regla de oro de Fermi permite obtener la intensidad de las transiciones ópticas entre los estados confinados. Se investiga el efecto de las dimensiones del punto cuántico en los niveles de energía y la intensidad de las transiciones y se obtiene que, al disminuir la anchura del punto cuántico respecto a su valor en los prototipos actuales, se puede conseguir una transición más intensa entre el nivel intermedio fundamental y la banda de conducción. Tomando como datos de partida los niveles de energía y las intensidades de las transiciones calculados como se ha explicado, se desarrolla un modelo de equilibrio o balance detallado realista para células solares de puntos cuánticos. Con el modelo se calculan las diferentes corrientes debidas a transiciones ópticas entre los numerosos niveles intermedios y las bandas de conducción y de valencia bajo ciertas condiciones. Se distingue de modelos de equilibrio detallado previos, usados para calcular límites de eficiencia, en que se adoptan suposiciones realistas sobre la absorción de fotones para cada transición. Con este modelo se reproducen datos publicados de eficiencias cuánticas experimentales a diferentes temperaturas con un acuerdo muy bueno. Se muestra que el conocido fenómeno del escape térmico de los puntos cuánticos es de naturaleza fotónica; se debe a los fotones térmicos, que inducen transiciones entre los estados excitados que se encuentran escalonados en energía entre el estado intermedio fundamental y la banda de conducción. En el capítulo final, este modelo realista de equilibrio detallado se combina con el método de simulación de redes de difracción para predecir el efecto que tendría incorporar una red de difracción en una célula solar de banda intermedia y puntos cuánticos. Se ha de optimizar cuidadosamente el periodo de la red para equilibrar el aumento de las diferentes transiciones intermedias, que tienen lugar en serie. Debido a que la absorción en los puntos cuánticos es extremadamente débil, se deduce que el atrapamiento de la luz, por sí solo, no es suficiente para conseguir corrientes apreciables a partir de fotones con energía menor que la banda prohibida en las células con puntos cuánticos. Se requiere una combinación del atrapamiento de la luz con un incremento de la densidad de puntos cuánticos. En el límite radiativo y sin atrapamiento de la luz, se necesitaría que el número de puntos cuánticos de una célula solar se multiplicara por 1000 para superar la eficiencia de una célula de referencia con una sola banda prohibida. En cambio, una célula con red de difracción precisaría un incremento del número de puntos en un factor 10 a 100, dependiendo del nivel de la absorción parásita en el reflector posterior. Abstract The purpose of this thesis is to investigate the benefits that diffractive light trapping can offer to quantum dot intermediate band solar cells and crystalline silicon solar cells. Both solar cell technologies suffer from incomplete photon absorption in some part of the solar spectrum. Quantum dot intermediate band solar cells are theoretically capable of achieving much higher efficiencies than conventional single-gap devices. Present prototypes suffer from extremely weak absorption of subbandgap photons in the quantum dots. This problem has received little attention so far, yet it is a serious barrier to the technology approaching its theoretical efficiency limit. Crystalline silicon solar cells absorb weakly in the near infrared due to their indirect bandgap. This problem has received much attention over recent decades, and all commercial crystalline silicon solar cells employ some form of light trapping. With the industry moving toward thinner and thinner wafers, light trapping is becoming of greater importance and diffractive structures may offer an improvement over the state-of-the-art. We begin by constructing a computational method with which to simulate solar cells equipped with diffraction grating textures. The method employs a wave-optical treatment of the diffraction grating, via rigorous coupled wave analysis, with a geometric-optical treatment of the thick solar cell bulk. These are combined using a steady-state matrix formalism. The method has been implemented computationally, and is found to be efficient and to give results in good agreement with alternative methods from other authors. The theoretical upper limit to absorption enhancement in solar cells using diffractions gratings is calculated using the matrix formalism derived in the previous task. This limit is compared to the so-called Lambertian limit for light trapping with isotropic scatterers, and to the absolute upper limit to light trapping in bulk absorbers. It is found that bi-periodic gratings (square or hexagonal geometry) are capable of offering much better light trapping than uni-periodic line gratings. The upper limit depends strongly on the grating period. For large periods, diffraction gratings are theoretically able to offer light trapping at the absolute upper limit, but only if the scattering efficiencies have a particular form, which is deemed to be beyond present design capabilities. For periods similar to the incident wavelength, diffraction gratings can offer light trapping below the absolute limit but above the Lambertian limit without placing unrealistic demands on the exact form of the scattering efficiencies. This is possible for a reasonably broad wavelength range. The computational method is used to design and optimise diffraction gratings for light trapping in solar cells. The proposed diffraction grating consists of a hexagonal lattice of cylindrical wells etched into the rear of the bulk solar cell absorber. This is encapsulated in a dielectric buffer layer, and capped with a rear reflector. Simulations are made of this grating profile applied to a crystalline silicon solar cell and to a quantum dot intermediate band solar cell. The grating period, well depth, and lateral well dimensions are optimised numerically for both solar cell types. This yields the optimum parameters to be used in fabrication of grating equipped solar cells. The optimum parameters are explained using simple physical concepts, allowing us to make more general statements that can be applied to other solar cell technologies. Diffraction grating textures are fabricated on crystalline silicon substrates using nano-imprint lithography and reactive ion etching. The optimum grating period from the previous task has been used as a design parameter. The substrates have been processed into solar cell precursors for optical measurements. Reflection spectroscopy measurements confirm that bi-periodic square gratings offer better absorption enhancement than uni-periodic line gratings. The fabricated structures have been simulated with the previously developed computation tool, with good agreement between measurement and simulation results. The simulations reveal that a significant amount of the incident photons are absorbed parasitically in the rear reflector, and that this is exacerbated by the non-planarity of the rear reflector. An alternative method of depositing the dielectric buffer layer was developed, which leaves a planar surface onto which the reflector is deposited. It was found that samples prepared in this way suffered less from parasitic reflector absorption. The next task described in the thesis is the study of photon absorption in semiconductor quantum dots. The bound-state energy levels of in InAs/GaAs quantum dots is calculated using the effective mass approximation. A one- and four- band method is applied to the calculation of electron and hole wavefunctions respectively, with an empirical Hamiltonian being employed in the latter case. The strength of optical transitions between the bound states is calculated using the Fermi golden rule. The effect of the quantum dot dimensions on the energy levels and transition strengths is investigated. It is found that a strong direct transition between the ground intermediate state and the conduction band can be promoted by decreasing the quantum dot width from its value in present prototypes. This has the added benefit of reducing the ladder of excited states between the ground state and the conduction band, which may help to reduce thermal escape of electrons from quantum dots: an undesirable phenomenon from the point of view of the open circuit voltage of an intermediate band solar cell. A realistic detailed balance model is developed for quantum dot solar cells, which uses as input the energy levels and transition strengths calculated in the previous task. The model calculates the transition currents between the many intermediate levels and the valence and conduction bands under a given set of conditions. It is distinct from previous idealised detailed balance models, which are used to calculate limiting efficiencies, since it makes realistic assumptions about photon absorption by each transition. The model is used to reproduce published experimental quantum efficiency results at different temperatures, with quite good agreement. The much-studied phenomenon of thermal escape from quantum dots is found to be photonic; it is due to thermal photons, which induce transitions between the ladder of excited states between the ground intermediate state and the conduction band. In the final chapter, the realistic detailed balance model is combined with the diffraction grating simulation method to predict the effect of incorporating a diffraction grating into a quantum dot intermediate band solar cell. Careful optimisation of the grating period is made to balance the enhancement given to the different intermediate transitions, which occur in series. Due to the extremely weak absorption in the quantum dots, it is found that light trapping alone is not sufficient to achieve high subbandgap currents in quantum dot solar cells. Instead, a combination of light trapping and increased quantum dot density is required. Within the radiative limit, a quantum dot solar cell with no light trapping requires a 1000 fold increase in the number of quantum dots to supersede the efficiency of a single-gap reference cell. A quantum dot solar cell equipped with a diffraction grating requires between a 10 and 100 fold increase in the number of quantum dots, depending on the level of parasitic absorption in the rear reflector.

Estudio y desarrollo de sensores de fibra óptica para detección de vibraciones en ductos ascendentes submarinos

En esta tesis doctoral se describe el trabajo de investigación enfocado al estudio y desarrollo de sensores de fibra óptica para la detección de presión, flujo y vibraciones en ductos ascendentes submarinos utilizados en la extracción y transporte de hidrocarburos, con el objetivo de aplicarlos en los campos de explotación de aguas profundas en el Golfo de México pertenecientes a la Industria Petrolera Mexicana. El trabajo se ha enfocado al estudio y desarrollo de sensores ópticos cuasi distribuidos y distribuidos. En especial se ha profundizado en el uso y aplicación de las redes de Bragg (FBG) y de reflectómetros ópticos en el dominio del tiempo sensible a la fase (φ-OTDR). Los sensores de fibra óptica son especialmente interesantes para estas aplicaciones por sus ventajosas características como su inmunidad a interferencias electromagnéticas, capacidad de multiplexado, fiabilidad para trabajar en ambientes hostiles, altas temperaturas, altas presiones, ambientes salino-corrosivos, etc. Además, la fibra óptica no solo es un medio sensor sino que puede usarse como medio de transmisión. Se ha realizado un estudio del estado del arte y las ventajas que presentan los sensores ópticos puntuales, cuasi-distribuidos y distribuidos con respecto a los sensores convencionales. Se han estudiado y descrito los interrogadores de redes de Bragg y se ha desarrollado un método de calibración útil para los interrogadores existentes en el mercado, consiguiendo incertidumbres en la medida de la longitud de onda menores de ± 88 nm e incertidumbres relativas (la mas interesante en el campo de los sensores) menores de ±3 pm. Centrándose en la aplicación de las redes de Bragg en la industria del petróleo, se ha realizado un estudio en detalle del comportamiento que presentan las FBGs en un amplio rango de temperaturas de -40 ºC a 500 oC. Como resultado de este estudio se han evaluado las diferencias en los coeficientes de temperatura en diversos tramos de mas mismas, así como para diferentes recubrimientos protectores. En especial se ha encontrado y evaluado las diferencias de los diferentes recubrimientos en el intervalo de temperaturas entre -40 ºC y 60 ºC. En el caso del intervalo de altas temperaturas, entre 100 ºC y 500 ºC, se ha medido y comprobado el cambio uniforme del coeficiente de temperatura en 1pm/ºC por cada 100 ºC de aumento de temperatura, en redes independientemente del fabricante de las mismas. Se ha aplicado las FBG a la medición de manera no intrusiva de la presión interna en una tubería y a la medición del caudal de un fluido en una tubería, por la medida de diferencia de presión entre dos puntos de la misma. Además se ha realizado un estudio de detección de vibraciones en tuberías con fluidos. Finalmente, se ha implementado un sistema de detección distribuida de vibraciones aplicable a la detección de intrusos en las proximidades de los ductos, mediante un φ-OTDR. En este sistema se ha estudiado el efecto negativo de la inestabilidad de modulación que limita la detección de vibraciones distribuidas, su sensibilidad y su alcance. ABSTRACT This thesis describes the research work focused for the study and development of on optical fiber sensors for detecting pressure, flow and vibration in subsea pipes used in the extraction and transportation of hydrocarbons, in order to apply them in deepwater fields in the Gulf of Mexico belonging to the Mexican oil industry. The work has focused on the study and development of optical sensors distributed and quasi distributed. Especially was done on the use and application of fiber Bragg grating (FBG) and optical reflectometers time domain phase sensitive (φ-OTDR). The optical fiber sensors especially are interesting for these applications for their advantageous characteristics such as immunity to electromagnetic interference, multiplexing capability, reliability to work in harsh environments, high temperatures, high pressures, corrosive saline environments, etc. Furthermore, the optical fiber is not only a sensor means it can be used as transmission medium. We have performed a study of the state of the art and the advantages offered by optical sensors point, quasi-distributed and distributed over conventional sensors. Have studied and described interrogators Bragg grating and has developed a calibration method for interrogators useful for the existing interrogators in the market, resulting uncertainty in the measurement of the wavelength of less than ± 0.17 nm and uncertainties (the more interesting in the field of sensors) less than ± 3 pm. Focusing on the application of the Bragg gratings in the oil industry, has been studied in detail the behavior of the FBGs in a wide range of temperatures from -40 °C to 500 oC. As a result of this study we have evaluated the difference in temperature coefficients over various sections of the same, as well as different protective coatings. In particular evaluated and found the differences coatings in the range of temperatures between -40 º C and 60 º C. For the high temperature range between 20 ° C and 500 ° C, has been measured and verified the uniform change of the temperature coefficient at 1pm / ° C for each 100 ° C increase in temperature, in networks regardless of manufacturer thereof. FBG is applied to the non-intrusive measurement of internal pressure in a pipeline and measuring flow of a fluid in a pipe, by measuring the pressure difference between two points thereof. Therefore, has also made a study of detecting vibrations in pipes with fluids. Finally, we have implemented a distributed sensing system vibration applied to intrusion detection in the vicinity of the pipelines, by φ-OTDR. In this system we have studied the negative effect of modulation instability limits the distributed vibration detection, sensitivity and scope.

Packaging and testing of fiber Bragg gratings for use as strain sensor for rock specimens

This paper reports a packaging and calibration procedure for surface mounting of fiber Bragg grating (FBG) sensors to measure strain in rocks. The packaging of FBG sensors is performed with glass fiber and polyester resin, and then subjected to tensile loads in order to obtain strength and deformability parameters, necessaries to assess the mechanical performance of the sensor packaging. For a specific package, an optimal curing condition has been found, showing good repeatability and adaptability for non-planar surfaces, such as occurs in rock engineering. The successfully packaged sensors and electrical strain gages were attached to standard rock specimens of gabbro. Longitudinal and transversal strains under compression loads were measured with both techniques, showing that response of FBG sensors is linear and reliable. An analytical model is used to characterize the influences of rock substrate and FBG packaging in strain transmission. As a result, we obtained a sensor packaging for non-planar and complex natural material under acceptable sensitivity suitable for very small strains as occurs in hard rocks.

Detección de daño en materiales compuestos mediante fibra óptica

En este artículo se explora la aplicación del PCA (Principal Component Analysis), y mediciones estadísticas T 2 y Q para detectar daños en estructuras fabricadas en materiales compuestos mediante la utilización de FBGs (Fiber Bragg Grating). Un modelo PCA es construido usando datos de la estructura sin daños como un estado de referencia. Los defectos en la estructura son simulados causando pequeñas delaminaciones entre el panel y el rigidizador. Los datos de diferentes escenarios experimentales para la estructura sin daño y con daño son proyectados en el modelo PCA. Las proyecciones y los índices T 2 y Q son analizadas. Resultados de cada caso son presentados y discutidos demostrando la viabilidad y el potencial de usar esta formulación en SHM (Structural Health Monitoring)