• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 194
  • 81
  • 43
  • 22
  • 13
  • 11
  • 4
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 2062
  • 627
  • 593
  • 516
  • 512
  • 187
  • 175
  • 129
  • 126
  • 122
  • 106
  • 100
  • 95
  • 92
  • 89
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
101

Micro-fabricated diffractive optics for quantum sensors and atomic clocks

McGilligan, James P. January 2017 (has links)
This thesis describes the design, construction and experimental realisationof a cold-atom, atomic clock. Diffractive optics are used to address the size constraints of the typical laser cooling apparatus to facilitate a portable device. The design and measurement of a wide range of grating parameters, including period, duty cycle, etch depth, and coating are characterised for optimum performance in a cold-atom system. The grating magneto-optical trap (GMOT) demonstrates an atom number and temperature competitive with state-of-the-art experiments, while greatly simplifying the optical footprint. The compact nature of the GMOT facilitates proof-of-principle quantum sensing for future miniaturised atomic devices and instruments. These initial measurements include in situ magnetometry from compensating effects of homogeneous and inhomogeneous magnetic fields. Coherent population trapping was studied on the D1 line of rubidium to realise a high contrast superposition state for clock measurements. These initial clock measurements lead to the development of a microwave regime atomic clock, based on a Raman-Ramsey interrogation of the clock states. The theory and construction of this clock are discussed, with an outlook to the development and demonstration of a second generation experiment.
102

Design, microfabrication and characterisation of Photonic Integrated Circuits

Cantarella, Giuseppe January 2017 (has links)
This doctoral dissertation deals with the design, fabrication and characterization of state-of-the-art Photonic Integrated Circuits (PICs) for non-linear applications. Silicon PICs is a technology mainly used for application in telecommunications and quantum optics. The strong third order non-linearity of silicon makes it also attractive for non-linear PIC design. In FWM applications, SOI technology can be used not only for non-linear generation but also to fabricate photonic filters to remove the residual pump. This thesis deals with three requirements for the realisation of on-chip FWM optical devices, the dual polarisation rejection of the pump on-chip and the integration and stabilisation of the FWM source and optical filter. In this work two of the most used SOI photonic integrated filters, ring resonators and Bragg gratings, are presented. These devices present two different solutions for high extinction(≈ 60 dB) dual polarisation filtering. An integrated structure of non-linear source and filter is presented. The device used for non-linear generation is then monolithically integrated with a novel ring resonators cascade filter technology. FWM experiments were carried out obtaining an on chip pump high dual polarisation extinction of 62 dB with a low insertion loss for the propagating signal and idler of only 1.8 dB.The realisation of a microprocessor feedback loop stabilisation system integrated with SOI non-linear structures is also demonstrated. The system is based on a local thermal heater element on-chip used to stabilise the PICs against thermal refractive index variations. Using this method, a silicon π-phase shifted grating with a cavity Q-factor of 40k is demonstrated to operate over an ambient temperature detuning range of 40 oC and injection wavelength range of 1.5 nm, nearly 3 orders of magnitude greater than the resonant cavity line width. The last part of this work is dedicated to the description of a custom made laser photolitography system for rapid prototyping of PIC designs, a tool designed to overcome the costs of the typical lithography systems and drastically decrease the time required for multiple micro-fabrications. The hardware and the software created for this tool are presented together with the first results on the fabrication of SU-8 Photoresist (SU − 8) on Silicon Dioxide (SiO2) waveguides, bends, Mach Zehnder interferometers and ring resonators.
103

Cerenkov emission in radiotherapy

Helo, Y. January 2015 (has links)
A new potential quality assurance (QA) method is explored for clinical electron beams and clinical proton beams based on imaging and measuring Cerenkov light. A simulation was performed of the deposited energy and of Cerenkov production in water using Geant4. Monte Carlo simulation was used to predict the measured light distribution around the water phantom, to reproduce Cerenkov images and to find the relation between deposited energy and Cerenkov production. The camera was modelled as a pinhole camera in Geant4, to attempt to reproduce Cerenkov images. The potential of using a standard commercial camera to image Cerenkov light generated from electrons in water for fast QA measurement of a clinical electron beam was explored and compared to ionization chamber measurements. The new method was found to be linear with dose and independent of dose rate (to within 3%). The uncorrected practical range measured using Cerenkov images was found to overestimate the actual value by 3 mm in the worst case. The field size measurements underestimated the field sizes at the edges by 5% without applying any correction factor. Still, the measured field size could be used to monitor relative changes in the beam profile. Finally, the beam-direction profile measurements were independent of the field size within 2%. We found that imaging Cerenkov emission from a breast phantom during electron irradiation could be a suitable tool to monitor the dose and the dose rate consistency with high precision and short-term repeatability better than 3% except when measuring very low doses. Cerenkov light measurements were linear with dose and independent of dose rate. The maximum light intensity occurred at an angle of 45.0°. We were unable to identify the regions of the phantom with higher scattering and absorption properties, designed to mimic diseased tissues using images of Cerenkov emission of an optical breast phantom. We found that the Cerenkov light emissions in proton therapy can be divided into two distinct mechanisms: a fast component due to prompt gamma interactions (99.13%) and neutron interactions (0.87%), and a slow component due to radioactive decay. The simulated depth distribution of the Cerenkov emission shows a strong relation with the depth distribution of the induced radioactive isotopes, which emit positrons. The fast component was found to be linear with dose and independent of dose rate, while the slow component increases non- linearly with dose and is highly dependent on dose rate. Imaging Cerenkov light during electron radiotherapy or proton therapy could be used as a very quick routine QA tool.
104

Studies of the action of ionising radiation on aqueous solutions

Wilkinson, J. January 1952 (has links)
No description available.
105

Studies of the detection of optical coherence using a coincidence counting technique

Haig, Nigel Douglas January 1969 (has links)
No description available.
106

Nonlinear optics in a thermal Rb vapour at high magnetic fields

Whiting, Daniel James January 2017 (has links)
We present investigations of nonlinear optical phenomena using a ⁸⁷Rb thermal vapour in a high magnetic field. The magnetic field (of strength 0.6~T) separates the optical transitions by more than their Doppler-broadened linewidths. This technique provides a high level of control over thermal-vapour systems, which can be used as a simpler alternative to conventional state-preparation methods such as optical pumping. Using this approach, we study electromagnetically induced transparency in a non-degenerate 3-level ladder system. Finding excellent agreement with the numerically modelled transmission spectra, we are able to directly infer the dipole moment of the 5P₃/₂→5D₅/₂ transition |< 5P₃/₂||er||5D₅/₂ >| = (2.27±0.002_stat±0.04_syst) ea₀. Changing the coupling field to a standing-wave geometry, we observe resonant enhanced absorption: an effect that previously had only been observed in systems involving 4 or more atomic states. We also consider four-wave mixing (FWM) in a diamond level-scheme. Compared to the zero magnetic field case, we find good agreement with the FWM spectra using a simple model, even in the regime of strong laser dressing. Finally, we investigate heralded single-photon generation by spontaneous FWM. With strong laser dressing we observe the appearance of collective quantum beats, a single-photon interference effect due to the relative motion of atoms in a collective spin excitation. A violation of the Cauchy-Schwarz inequality by 6.7 standard deviations is reported.
107

Poliment de vidre amb làser de CO2: tractament del feix i aplicació a grans superfícies

Armengol Cebrian, Jesús 14 December 2000 (has links)
En nombrosos processos de modificació induïda per làser de les propietats d'una superfície (enduriment de metalls, formació d'aleacions, cristalització de semiconductors, etc...) és necessari que la temperatura de la superfície assoleixi durant el temps adequat, uns valors acotats en un interval molt estret. Per tant l'èxit de l'aplicació es fonamenta en aconseguir que la distribució d'intensitat del feix làser aplicada làser indueixi sobre la superfície de la mostra una distribució uniforme de temperatura.El treball que es presenta en aquesta tesis incideix sobre aquest aspecte clau en les aplicacions amb feixos làser d'alta potència: com obtenir distribucions d'intensitat que permetin generar tractaments tèrmics uniformes. És convenient destacar que l'estudi dels processos d'uniformització/integració del feix que es realitza, té com objectiu desenvolupar una nova aplicació en el camp del tractament superficial dels materials: el poliment làser de grans superfícies de vidre. En aquest cas, la necessitat d'uniformitat en la distribució de temperatura encara és més gran degut al menor valor de la conductivitat tèrmica dels vidres en comparació amb la dels metalls o semiconductors.Pel que fa als aspectes concrets, es realitza un estudi teòric i experimental de les distribucions d'intensitat que s'obtenen amb els sistemes de integració de feixos làser de CO2 d'alta potència: miralls facetats i calidoscopis. Aquests sistemes es basen en la superposició de diferents porcions del front d'ona ("beamlets") en el pla de millor integració per obtenir així una distribució d'intensitat uniforme. Tot i així i de la manera com s'analitza en el treball, si s'utilitzen làsers de cavitats inestables que proporcionen feixos monomode de coherència espacial i temporal elevada, la superposició dels beamlets origina un patró interferencial que modula la distribució de intensitat. Una aportació especialment rellevant que s'inclou en el treball ha estat desenvolupar un nou sistema "d'integració activa" que permet reduir extraordinàriament la contribució no uniforme lligada al patró interferencial. La combinació d'aquesta nova tècnica d'integració i un sistema òptic ha permès processar superfícies de vidre de grans dimensions (mostres de 80mm de diàmetre).Respecte als resultats concrets obtinguts en el poliment làser de vidre, el treball demostra que aquest procés és factible en vidres termorresistents amb coeficients de dilatació linial compressos entre 80 i 100 x 10-7 ºC-1. Aquest resultat és una altra de les aportacions originals de treball i es basa en el pre-escalfament de les mostres de vidre abans del procesament làser, a una temperatura lleugerament superior a la corresponent al punt de transformació.Un altre aspecte destacable és que s'acota la viabilitat de la tècnica discutint per a tots els mètodes d'uniformització/ integració del feix, quin és l'efecte de la irradiació amb làser sobre la textura de la superfície (rugositat i ondulacions) i la seva forma geomètrica. Així es demostra que la tècnica de poliment làser és extraordinàriament eficaç per eliminar les components d'alta freqüència espacial de la rugositat mentre que no és adequada per eliminar o reduir les ondulacions de període espacial major. Pel que fa a la forma geomètrica es demostra que sempre es modifica després del procesament làser, però que aquesta modificació és parametritzable i repetitiva de manera que pot controlar-se amb el disseny correcte dels experiments i de les mostres.Per acabar senyalar que la qualitat científica de la tesis ve avalada per quatre publicacions[*] en revistes indexades i que el treball s'ha realitzat en el marc d'un ampli projecte científic-tecnològic dirigit a desenvolupar un prototip preindustrial pel poliment de superfícies òptiques amb làser de CO2, la financiació del qual ha corregut a càrrec majoritàriament d'empreses privades vinculades al sector de l'òptica oftàlmica.
108

Determinació del factor de cobertura en teixits de calada per processament de la imatge digital i espectroscòpia de reflexió

Tàpias Anton, Montserrat 08 June 2012 (has links)
In this thesis we applied, in a complementary way, techniques of digital image processing and reflectance spectroscopy to perform cover factor (CF) measurements in monochrome openwork fabrics. To evaluate the results provided by both techniques, we used the average CF obtained by a group of 18 expert observers as a reference. IMAGE PROCESSING PART: We developed a new, automatic and objective method for measuring both total and partial CF, and the width of the threads, from the digital image of the fabric. Two algorithms were developed for total and partial CF determinations. The image processing technique used to measure the total CF was the segmentation of the holes from the fabric threads. Conventional methods of segmentation did not lead to similar results than those of the total visual CF. Therefore, we designed a new algorithm (MSSQ) to measure the total CF of fabrics, independently of their degree of coverage. All the CF obtained in 81 typified textile samples were correlated with the CF obtained visually. The best fit to the Stevens' psychophysical law was achieved with the method MSSQ. These results were also consistent with the construction parameters of fabrics. The determination of partial CF in the warp and weft directions was performed by filtering in the Fourier space the warp and weft threads of the sample. For the design of these filters we applied the Radon transform to the power spectrum of the image of the fabric, in order to determine the direction of the warp and weft harmonics. The values of partial CF were consistent with the parameters that characterize the threads and were logical, expected and confirmed the robustness of the method. The values of the diameters of the threads were also consistent with the parameters of the yarn manufacturing and with the manual measurement performed on the image of the same fabric. This method is easily implementable in a laboratory of textile analysis. REFLECTANCE PART: We modelled the reflectance of a flat layer of fabric arranged on a support of known reflectance using the Kubelka-Munk theory. For this purpose we assumed that the optical behaviour of a layer of fabric could be determined by the superposition of two ideal elements: a neutral density filter with no thickness and with different values of transmittance depending on both the direction of the beam (input or output) and whether the beam passes through the covered part of the yarns or through the holes, plus a uniform and homogeneous layer of a material that meets the assumptions of the theory of Kubelka-Munk. The final model contained CF as a parameter. For a set of different samples of fabrics, the integrated spectral reflectance of a stack of increasing number of layers was experimentally measured with a spectroradiometer, up to stabilize the measure. The experimental reflectances were introduced in the generalized model to calculate the values of the parameters involved, particularly the CF. We solved the resulting system of 101 ¿ (m - 1) equations using the least squares method, where m is the total number of stacked layers of fabric, with 103 unknowns. This yielded a couple of estimates of CF. We established correlations between the measured CF (obtained as the solution of these equations) and the visual CF and analyzed the effect of perturbations in the reflectance measurements to explain the discrepancies between these two estimates of CF. Even if we observed problems of precision in one of the estimates and inaccuracy in the other, arising from an unknown parameter in the model, the intervention of the CF in the described model has been proven.
109

Nuevas evidencias sobre la anisotropía del espacio visual y la influencia del entorno en el rendimiento visual

Torrents Gómez, Aurora 22 December 2011 (has links)
Desde hace años se han realizado múltiples investigaciones que tratan sobre la fiabilidad de la percepción de las distancias en profundidad y la influencia del entorno. En ocasiones, las pistas presentes en el espacio visual entran en conflicto y/o su interpretación conduce a sesgos y errores, produciendo efectos ilusorios al no corresponder las medidas físicas con las percibidas. El objetivo global de esta tesis doctoral ha sido poner de manifiesto nuevas evidencias de la anisotropía del espacio visual y de la influencia del entorno en relación al juicio de distancias entre objetos. Para ello, se han diseñado y ejecutado tres grupos de experimentos cuyos objetivos parciales fueron: Analizar el rol de las disparidades verticales en tareas de juicio de distancias relativas entre objetos situados en diferentes planos de profundidad, en función de la orientación del estímulo. Determinar la influencia del fondo y de la orientación del estímulo en tareas de juicio de distancias relativas entre objetos, situados en un mismo plano frontoparalelo o en diferentes planos de profundidad. Verificar la naturaleza neural de la anisotropía del espacio visual desde un enfoque psicofísico no invasivo mediante el uso de SIRDS. Los resultados obtenidos en esta tesis contribuyen a la comprensión sobre la integración de claves en Visión Binocular y los sesgos visuales perceptivos en la percepción de distancias. / Recent decades have witnessed multiple studies investigating the accuracy of our visual system in depth perception, as well as the influence of environmental factors during depth judgment tasks. It is not uncommon for Virtual Space cues to offer contradictory or conflicting information, thus leading to bias and error, which in turn originate illusory effects resulting from discrepancies between real and perceived dimensions. Our research in the integration of cues in Binocular Vision aimed at determining the influence of background characteristics (curved, flat, etc) on the perception of visual stimuli presented over it. Therefore, three different experimental settings were designed and conducted, with partial objectives defined as follows: To evaluate the influence of vertical disparities in depth judgment tasks when stimuli were located at different depth planes and presented different orientations. To determine the influence of background configuration and stimulus orientation on depth judgment tasks, both with stimuli at the same frontoparallel plane or at different depth planes. To evidence the neural origin of the Visual Space anisotropy from a non invasive psychophysical approach with the use of SIRDS. The findings of the present PhD thesis contribute to our understanding of the integration of cues in Binocular Vision, as well as of the nature of visual bias in depth perception.
110

Medida de la respuesta acomodativa con un sistema de doble paso: aplicación al estudio de la acomodación en función de la edad.

Aldaba Arevalo, Mikel 22 October 2012 (has links)
Accommodation, defined as the dioptric change of the eye, enables clear vision at different distances. Its study is interesting due to different aspects: the progressive loss of amplitude of accommodation known as presbyopia, the recent interest in accommodative restauration techniques in pseudophakic patients, its possible relation to myopia development and the associated pathologies. The most widely used techniques to measure accommodation are the subjectives, although it has been shown that they overestimate accommodation due to pseudoaccommodation. There are also several objective techniques as retinoscopy, autorefractometers or aberrometers. In the last years it has been suggested that a more realistic accommodative measurement is done when all the aberrations of the eye are taken into acount, and not just defocus. The goal of this thesis has been to develop a method based on the double pass technique for the accommodative response measurement and its application to the study of accommodation with age. Double pass technique is based on projecting a point source on the retina and recording images after retinal reflection and double pass through the ocular media. This technique provides information about the optical quality of the eye and takes into acount all the factors influencing the retinal image quality, thus it is ideal for being applied in the measurement of tha accommodative response. In the first part of the thesis the method for the accommodative response measurement based on double pass technique has been developed and it has been validated by comparison with Hartmann-Shack measurements. The developed experimental setup combines both techniques, double pass and Hartmann-Shack, and it is specifically designed for accommodative response measurements, including elements like the open field fixation test that allows a more natural accommodative stimulation. The method developed is based on finding the vergence that maximizes the quality of the double pass image. Several double pass images are recorded by means of an axial scanning and the vergence of the best double pass image is related to accommodative response. Double pass measurements showed a good agreement with Hartmann-Shack measurements and no statistically significant differences, thus validating our method. In the second part of the thesis, the method has been applied for the study of the accommoditve response in a wide age range population. The accommodative response measured in young patients was higher than the published in the literature, due to our measurements being based on the retinal image quality and not on defocus. While in older patients, the accommodative response measurements are in good agreement with previous works. The accommodative stimulus-response curve and its slope also showed a worsening with age, with a more noticeable change in the accommodation after the age of 35-40 years. The work done during the thesis led to articles in journals and several contributions to congresses, as well as to a patent being comercially exploited. / La acomodación, cambio dióptrico del ojo, permite una visión nítida a diferentes distancias. El interés que despierta su estudio es grande debido a diferentes factores: la progresiva pérdida de acomodación con la edad, la restauración de la acomodación en pacientes pseudfáquicos, su posible relación con el desarrollo de la miopía y las patologías asociadas a la acomodación. Aunque las técnicas más habituales para la medida de la acomodación son las técnicas subjetivas, presentan el problema de sobrevalorar la amplitud de acomodación debido a la pseudoacomodación, rango en el que sin cambio dióptrico del cristalino se ve nítidamente gracias a la profundidad de foco. Entre las objetivas se han propuesto diversas técnicas como la retinoscopía, autorrefractómetros o aberrómetros. En los últimos años se ha sugerido que una correcta medida de la acomodación no sólo debería tener en cuenta el desenfoque si no el global de las aberraciones que afectan a la calidad de la imagen retiniana. El objetivo de esta tesis ha sido desarrollar un nuevo método para la medida de la respuesta acomodativa basado en la técnica de doble paso y su aplicación al estudio de la acomodación con la edad. La técnica de doble paso consiste en proyectar una fuente puntual de luz en la retina y registrar la imagen formada tras su reflexión en la retina y doble paso por los medios oculares. Esta técnica da información acerca de la calidad óptica del ojo y tiene en cuenta los dos factores que afectan básicamente a la calidad de la imagen retiniana: aberraciones y difusión intraocular, lo que la convierte en una candidata ideal para su aplicación en la medida de la respuesta acomodativa. En la primera parte de la tesis se ha desarrollado un método y un montaje para la medida de la respuesta acomodativa con un sistema de doble paso y se ha validado mediante comparación con medidas realizadas con un aberrómetro de Hartmann-Shack. El montaje experimental desarrollado combina ambas técnicas, doble paso y Hartmann-Shack, y además está optimizado para la medida de la respuesta acomodativa con un test de fijación de campo abierto que permite una estimulación de la acomodación más natural que los test internos. El método desarrollado se basa en hallar la vergencia de la luz que maximiza la calidad de la imagen de doble paso. Para ello se registran diversas imágenes de doble paso correspondientes a diferentes vergencias de la luz y la de mayor calidad se relaciona con la respuesta acomodativa. Las medidas realizadas con el sistema de doble paso se han comparado con medidas simultáneas realizadas con un aberrómetro de Hartmann-Shack, obteniendo resultados similares con ambas técnicas. Una vez validado el método para la medida de la respuesta acomodativa con el sistema de doble paso, se ha aplicado al estudio de los cambios en la acomodación con la edad realizando medidas en una población extensa con un rango amplio de edad. La respuesta acomodativa medida en los pacientes jóvenes es mayor que en otros trabajos, básicamente debido a que la técnica empleada está basada en la calidad de la imagen retiniana y no en el desenfoque como sucede en los otros estudios. Mientras que en pacientes mayores, los resultados concuerdan con los de otros estudios. La curva de estímulo-respuesta acomodativa y su pendiente muestran un empeoramiento con la edad, observándose que la edad que marca los cambios más notables está en torno a los 35-40 años. El trabajo realizado durante el desarrollo de la tesis ha dado lugar a publicaciones en revistas y diversas contribuciones a congresos, así como una patente actualmente explotada comercialmente.

Page generated in 0.0591 seconds