Electrooptic electric field sensor for dc and extra-low-frequency measurement

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The thesis reports the results of the research carried out towards the development of an electrooptic sensor for DC and extra low frequency electric field measurement. Available cubic electrooptic crystals were compared from the sensor sensitivity point of view. A new figure of merit was used taking into account the attenuation of the electric field in the dielectric crystal and its shape. The effect of optical activity in 23 cubic crystals was analyzed using the concept of Poincare sphere. The cubic crystals were further characterised for the charge relaxation time constant to estimate their performance in DC field measurements. Crystals of Bismuth Germanate and Lithium Niobate were identified as suitable materials for the DC field sensor. The selected crystals were found suitable at extra-low-frequencies. DC field measurements, without the rotation of the crystal, were possible only with Lithium Niobate. However, its performance was influenced to a great extent by the effect of stimulated conductivity. The quarter-wave plate and the crystal of Lithium Niobate were identified as the main sources of temperature instability. A new method of temperature compensation of the quarter-wave plate is proposed. Due to the temperature instability of Lithium Niobate, mainly attributed to the pyroelectric effect and natural birefringence, it is difficult to use the sensor in practical applications. The performance of the sensor is significantly affected by the presence of an external space charge. The proposed method of its elimination using an artificial extension of the sensing element did not reduce the space charge effect adequately. The response of the sensor in a space charge environment was found to be linear and independent of the space charge density. This enabled measurements of static fields in a unipolar environment. The direct field measurements in bipolar environment suffered from a drift which is intolerable in practical measurements. The minimum detectable electric field of this sensor in the frequency range from 1 to 200Hz was 1V/m, with a signal to noise ratio equal to 0dB and a resolution of 1V/m. The static field measurements were limited to measurements of pulses with a duration of 200s, due to a long term drift of photodetectors. The minimum detectable level of DC electric field was 2.4kV/m.EPSR

An investigation for the development of an integrated optical data preprocessor

The successful fabrication and demonstration of an integrated optical circuit designed to perform a parallel processing operation by utilizing holographic subtraction to simultaneously compare N analog signal voltages with N predetermined reference voltages is summarized. The device alleviates transmission, storage and processing loads of satellite data systems by performing, at the sensor site, some preprocessing of data taken by remote sensors. Major accomplishments in the fabrication of integrated optics components include: (1) fabrication of the first LiNbO3 waveguide geodesic lens; (2) development of techniques for polishing TIR mirrors on LiNbO3 waveguides; (3) fabrication of high efficiency metal-over-photoresist gratings for waveguide beam splitters; (4) demonstration of high S/N holographic subtraction using waveguide holograms; and (5) development of alignment techniques for fabrication of integrated optics circuits. Important developments made in integrated optics are the discovery and suggested use of holographic self-subtraction in LiNbO3, development of a mathematical description of the operating modes of the preprocessor, and the development of theories for diffraction efficiency and beam quality of two dimensional beam defined gratings

Tunable superlattice p-i-n photodetectors: characteristics, theory, and application

Extended measurements and theory on the recently developed monolithic wavelength demultiplexer consisting of voltage-tunable superlattice p-i-n photodetectors in a waveguide confirmation are discussed. It is shown that the device is able to demultiplex and detect two optical signals with a wavelength separation of 20 nm directly into different electrical channels at a data rate of 1 Gb/s and with a crosstalk attenuation varying between 20 and 28 dB, depending on the polarization. The minimum acceptable crosstalk attenuation at a data rate of 100 Mb/s is determined to be 10 dB. The feasibility of using the device as a polarization angle sensor for linearly polarized light is also demonstrated. A theory for the emission of photogenerated carriers out of the quantum wells is included, since this is potentially a speed limiting mechanism in these detectors. It is shown that a theory of thermally assisted tunneling by polar optical phonon interaction is able to predict emission times consistent with the observed temporal response

Thin-film Lithium Niobate Photonics for Electro-optics, Nonlinear Optics, and Quantum Optics on Silicon

Ion-sliced thin-film lithium niobate (LN) compact waveguide technology has facilitated the resurgence of integrated photonics based on lithium niobate. These thin-film LN waveguides offer over an order of magnitude improvement in optical confinement, and about two orders of magnitude reduction in waveguide bending radius, compared to conventional LN waveguides. Harnessing the improved confinement, a variety of miniaturized and efficient photonic devices are demonstrated in this work. First, two types of compact electrooptic modulators are presented – microring modulators, and Mach-Zehnder modulators. Next, two distinct approaches to nonlinear optical frequency converters are implemented – periodically poled lithium niobate, and mode shape modulation (grating assisted quasi-phase matching). Following this, stochastic variations are added to the mode shape modulation approach to demonstrate random quasi-phase matching. Afterward, broadband photon-pair generation is demonstrated in the miniaturized periodically poled lithium niobate, and spectral correlations of the biphoton spectrum are reported. Finally, extensions of the aforementioned results suitable for future work are discussed

Multiheterodyne tunable sources for the interrogation of fiber optic sensors applied to acoustic emissions and ultrasound

Mención Internacional en el título de doctorLight is a very useful tool for measuring high frequency and low amplitude mechanical vibrations. Thanks to the interference process and under certain circumstances we can obtain a specific sort of optical sources called multimode multiheterodyne sources, that are very useful to read several optical wavelengths at the same time on a single photodetector and distinguishing them from each other. This characteristic makes them suitable for interrogating fiber optic sensors. In this thesis, I analyze several fiber optic sensor readout methods that mix multiheterodyne techniques, multimode techniques, and interferometry techniques to measure ultrasound and acoustic mechanical waves. These mechanical waves occur when periodic forces are applied to mechanical structures. This disturbs the layout of atoms and may lead to cracks or the complete collapse of the structure. Therefore, the characterization and measurement of such vibrations are of great importance when performing structure health monitoring (SHM) and non-destructive evaluation (NDE). This thesis aims to solve this problem by implementing several systems that employ light-based technology to measure and characterize mechanical vibrations up to 1 MHz of frequency and sub-nano-strain (lower than 10-3 ppm) level of resolution. The proposed systems involve new features and parameters more settable compared to more conventional approaches of optical sensor reading processes and therefore they offer wider possibilities. A total of three systems have been implemented and tested: First, an electro-optic dual optical frequency comb source to read fiber Bragg gratings for dynamic measurements. This set up reaches 120 kHz of mechanical frequency detection. The second system is based on a self-heterodyne acousto-optic comb that reads a random fiber grating sensor. In this case, the system can detect up to 1 MHz of mechanical vibrations. Finally, the third is based on a compact electro-optic dual optical frequency comb that is used to read low reflectivity fiber Bragg gratings with a dispersion interferometer. This system can detect a maximum of 135 kHz of mechanical frequencies. The results of this thesis improve previous systems achievements to satisfy the specifications required to date in this application, both in mechanical bandwidth and in strain amplitude. They also show the potential of these multimode sources for high-precision optical sensing.La luz es una herramienta muy útil para medir vibraciones mecánicas de alta frecuencia y baja amplitud. Gracias al proceso de interferencia y bajo determinadas circunstancias podemos obtener un tipo específico de fuentes ópticas, denominadas fuentes multimodo multiheterodinas, que son muy útiles para leer varias longitudes de onda ópticas al mismo tiempo en un solo fotodetector y distinguirlas entre sí. Esta característica hace que estas fuentes ópticas sean adecuadas para la lectura de sensores de fibra óptica. En esta tesis, analizo varios métodos de lectura de sensores de fibra óptica que mezclan técnicas multiheterodinas, técnicas multimodo y técnicas de interferometría para medir ultrasonidos y ondas mecánicas acústicas. Estas ondas mecánicas se producen cuando se aplican fuerzas periódicas a las estructuras mecánicas. Esto perturba la disposición de los átomos y puede provocar grietas o el colapso completo de la estructura. Por lo tanto, la caracterización y medida de dichas vibraciones son de gran importancia a la hora de monitorizar el estado de las estructuras y de realizar una evaluación no destructiva. Esta tesis tiene como objetivo resolver este problema mediante la implementación de varios sistemas que emplean tecnología basada en la luz para medir y caracterizar vibraciones mecánicas hasta frecuencias de 1 MHz y nivel de resolución sub-nano-deformación (menor que 10-3 ppm). Los sistemas propuestos implican nuevas características y parámetros más configurables en comparación con los enfoques más convencionales de procesos de lectura de sensores ópticos y, por lo tanto, ofrecen posibilidades más amplias. A lo largo de la tesis se presentan tres sistemas de medida: El primero está basado en un doble peine de frecuencias ópticas (dual comb) electroóptico que es capaz de leer sensores de fibra óptica basados en redes de Bragg (FBG) en régimen dinámico. Este sistema ha sido probado con una frecuencia máxima de detección de 120 kHz. En segundo lugar, se presenta un sistema basado en un selfheterodyne comb acustoóptico para leer sensores de fibra con distribución aleatoria de la rejilla en el núcleo (random grating). Este sistema es capaz de detectar señales de vibración de hasta 1 MHz. El tercer sistema presentado se basa en un doble peine de frecuencias ópticas (dual comb) electroóptico compacto que se utiliza para leer sensores FBG de baja reflectividad con un interferómetro de dispersión. Este sistema puede detectar hasta 135 kHz de vibraciones mecánicas. Los resultados de esta tesis mejoran los obtenidos en sistemas anteriores a fin de satisfacer las especificaciones requeridas hasta la fecha en esta aplicación, tanto en el ancho de banda mecánico como en la amplitud de la deformación. También muestran el potencial de estas fuentes multimodo para la detección óptica de alta precisión.Quiero agradecer la financiación de este trabajo dada por el Ministerio de Educación, Cultura y Deporte para la Formación de Profesorado Universitario FPU2016 (Beca FPU16/03695) y a través del proyecto PARAQUA (TEC2017-86271-R), así como por el Ministerio de Ciencia, Innovación y Universidades a través de las ayudas de movilidad EST18/00617.Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: Óscar Esteban Martínez.- Secretario: Marta Ruiz Llata.- Vocal: Pedro Alberto da Silva Jorg

Phase-reversal travelling-wave optical modulators for broadband and bandpass applications

This thesis is concerned with the analysis, fabrication, and characterisation of phase-reversal optical modulators for both broadband and bandpass applications. The operation of modulation is described together with the parameters by which optical modulators are characterised. Direct and indirect modulation are discussed. Particular attention is given to optical modulators constructed using titanium-indiffused lithium niobate technology. For these devices, the most important characteristics determining the frequency response and efficiency are studied. The technique of phase-reversal is examined as a means of artificial phase-matching. Systematic procedures for the design of phase-reversal electrode patterns are proposed. A novel class of equalising modulator is presented. A simulation of a digital lightwave system based on an equalising phase-reversal device is constructed. Transmission lines on anisotropic substrates are examined. The limitations of a quasi-static analysis are highlighted. A full-wave treatment utilising the method of lines is presented for the study of electrooptic modulators. Full-wave analysis is employed to study conventional coplanar waveguide electrooptic modulators. A new modulator structure, based on a fin line, is presented for high frequency operation. A comprehensive analysis of this structure is undertaken, including a modal analysis of a dielectric discontinuity. Designs are developed toward the demonstration of a device operating above 30GHz. A novel phase-matching technique, particularly suited to the fin line configuration, is discussed. The major processes involved in the fabrication of titanium- indiffused lithium niobate devices are briefly described. The techniques by which high speed modulators are measured are discussed, including a novel method by optical down-conversion. Experimental measurements are presented for devices operating in the frequency range 0 to 40GHz. The thesis is concluded, with suggestions for future avenues of research

Optical sampling using wideband electro-optic modulators

A simple system for optical and microwave signals analysis based on the optical sampling technique is presented. It is novel in requiring only low-frequency detection and electronic components. This is made possible by the use of a commercially available LiNbO3 intensity modulator for processing the light wave. The ultimate performance of the system is discussed, and comparisons with standard detection systems are also mad