Caracterización elipsométrica de materiales dieléctricos de aplicación en el desarrollo de sensores evanescentes de fibra óptica para el sector aeroespacial

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Óptica, leída el 05-12-2002La caracterización elipsométrica de recubrimientos de TiO2 depositados por evaporación ha permitido establecer una relación entre la nanoestructura porosa del material y sus propiedades ópticas. Asimismo, ha quedado demostrada la influencia de la humedad relativa ambiental en el índice de refracción complejo de estos recubrimientos. Adicionalmente, mediante la medida elipsométrica de la adsorción de agua en el material se ha extraído información sobre la forma y la distribución del tamaño de los poros. La dependencia de las propiedades ópticas de los recubrimientos de TiO2 con la humedad relativa ambiental alerta sobre la posibilidad de inestabilidades en la respuesta de elementos ópticos que estén compuestos por estos recubrimientos ante cambios en las condiciones ambientales. Por otro lado, este comportamiento ha permitido seleccionar a este material para el desarrollo de un sensor óptico de humedad. El sensor de humedad presentado está basado en el acoplamiento del campo evanescente de una fibra óptica tipo D a una guía de ondas plana de TiO2 poroso depositado por evaporación. Se demuestra que es posible extraer información sobre la distribución del tamaño del poro del recubrimiento a partir de la respuesta óptica del sensor de fibra óptica a la adsorción de agua en la guía de ondas planas. El contraste de estos resultados con los obtenidos en las medidas elilpsométricas permiten obtener una completa caracterización óptica del recubrimiento. También se ha desarrollado un sensor de temperatura de fibra óptica basado en idénticos fundamentos utilizando como guía de ondas plana un material fotopolímero. Tanto la influencia de las condiciones ambientales en las propiedades ópticas de los materiales analizados en esta investigación, como los sensores de fibra óptica desarrollados tienen importantes repercusiones y resultan de especial interés para el sector aeroespacialDepto. de ÓpticaFac. de Ciencias FísicasTRUEpu

Comentarios sobre la flora rupestre del Moncayo.

EScUDERO, A.; PAYARON, 5.; HERRERO, A. & ÁLvAREZ FERNÁNDEZ, 1. 1994. Comentanos sobre La flora rupestre del Moncayo. Botanico Complutensis 19: 89-108. El Moncayo es un macizo montañoso de gran interés biogeográfico. El conocimiento de su flora y vegetación atrajo a los botánicos desde antiguo: Asso visil.ó la zona en 1778, y esta atracción ha continuado hasta la actualidad. Se presentan comentarios de interés corológico, ecológico y taxonómico sobre 37 táxones -ecolectados en medios rupestres. Entre ellos destacan: Sas-ifiaga níoncoyensis, Cerastium arvense subsp. arvense, Linaria a/pino, Coclileario pyí-eííaic-a,Silene saxifraga, etc

Analysis of nanostructured porous films by measurement of adsorption isotherms with optical fiber and ellipsometry

An optical method to determine the nanostructure and the morphology of porous thin films is presented. This procedure is based on the response of a side-polished optical fiber with the film under study, when an adsorption-desorption cycle is carried out. Spectroscopic ellipsometry provides additional information about the optical properties and adsorption behavior of the film. Pore size distribution, anisotropy, and inhomogeneity of films can be determined by use of these two complementary techniques. To check the performances and suitability of the optical method, we have characterized a typical porous material: a TiO_2 film deposited by evaporation. Water vapor has been used for the adsorption cycles. The well-known columnar structure of the evaporated TiO_2 has been evidenced, and the relation between the nanostructure and the optical properties of the film is showed

Nonideal optical response of liquid crystal variable retarders and its impact on their performance as polarization modulators

Liquid crystal variable retarders (LCVRs) will be used for the first time in a space instrument, the Solar Orbiter mission of the European Space Agency, as polarization states analyzers (PSAs). These devices will determine the Stokes parameters of the light coming from the Sun by temporal polarization modulation, using the so-called modulation matrix O. This is a matrix constituted by the first rows of properly selected PSA Mueller matrices. Calibrating a space instrument, in particular, finding O, is a critical point because in a spacecraft there is no possibility of physical access. Due to the huge difficulty in calibrating the complete instruments in all possible scenarios, a more complete calibration of the individual components has been done in ground in order to make extrapolations to obtain O in-flight. Nevertheless, apart from the individual calibrations, the experimental errors and nonideal effects that inhibit the system to reach the designed and theoretical values must be known. In this work, description and study of these effects have been done, focusing on the nonideal effects of the LCVRs and the azimuthal misalignments between the optical components of the PSA during the mechanical assembly. The Mueller matrix of a representative LCVR has been measured and mathematically decomposed by logarithm decomposition, looking for values of circular birefringence and fast axis angle variations as a function of voltage. These effects, in the absence of other nonidealities, affect the polarimetric performance, reducing the polarimetric efficiencies in some cases until 11%. Nevertheless, in this case, they are negligible if compared to the other nonideality studied, which are the azimuthal misalignments between the PSA optical components. The study presented in this work is key to extrapolate the PSA O matrix if the expected instrumental set-point temperatures are not reached in flight and can be used for the design and implementation of other polarimetric instruments

Optical inspection of liquid crystal variable retarder inhomogeneities

Liquid crystal variable retarders (LCVRs) are starting to be widely used in optical systems because of their capacity to provide a controlled variable optical retardance between two orthogonal components of incident polarized light or to introduce a known phase shifting (PS) between coherent waves, both by means of an applied voltage. Typically, the retardance or PS introduced by an LCVR is not homogeneous across the aperture. On the one hand, the LCVR glass substrates present a global bend that causes an overall variation of the retardance or PS. On the other hand, in the manufacturing process of an LCVR, there sometimes appears a set of micro-air bubbles that causes local retardance or PS inhomogeneities. In this work, we present an interferometric technique based on a Mach-Zehnder interferometer that is insensitive to vibrations and capable of inspecting and characterizing the LCVR's retardance or PS inhomogeneities. The feasibility of the proposed method is demonstrated in the experimental results, where the LCVR retardance is measured with an error of about 0:2 rad. The thickness of possible micro-air bubbles is obtained with a resolution of about 50 nm

Polarimetric performance of a polarization modulator based on liquid crystal variable retarders for wide acceptance angles

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Ellipsometric characterization of Bi and Al2O3 coatings for plasmon excitation in an optical fiber sensor

The authors present the results of the ellipsometric characterization of thin layers of bismuth and aluminum oxide deposited over the waist of a tapered optical fiber by pulsed laser deposition. The characteristics of the deposits are studied by spectroscopic ellipsometry. From the effective thicknesses determined by the ellipsometric characterization, it is shown by simulations that surface plasmon resonances (SPRs) can occur in the fiber device, and it is demonstrated experimentally. These results show the feasibility of employing bismuth as a plasmonic material in SPR fiber sensors based on doubly-deposited uniform-waist tapered optical fibers, which show excellent performance and versatility

Water adsorption in porous TiO_2–SiO_2 sol–gel films analyzed by spectroscopic ellipsometry

High refractive index TiO_2–SiO_2 thin films were prepared by the sol–gel method and the optical properties were characterized by spectroscopic ellipsometry. The results of the optical analysis were related with the TiO_2–SiO_2 molar ratio of the porous films. It was obtained that the higher is the TiO_2 molar ratio, the higher is the refractive index and the lower is the thickness under the same deposition conditions. In fact, when the TiO_2–SiO_2 molar ratio varies from 70–30% to 100–0%, the refractive index and the thickness vary from ~1.85 to ~1.95 λ=500 nm and from ~65 nm to ~40 nm, respectively. The water adsorption in the pores of the material produces a change in the refractive index of the film. The variation of the optical properties with the environmental relative humidity was also studied. In addition, information about the size and the shape of the pores was extracted from the water adsorption isotherms measured by ellipsometry