Advances and new applications using the acousto-optic effect in optical fibers

This work presents a short review of the current research on the acousto-optic mechanism applied to optical fibers. The role of the piezoelectric element and the acousto-optic modulator in the excitation of flexural and longitudinal acoustic modes in the frequency range up to 1.2 MHz is highlighted. A combination of the finite elements and the transfer matrix methods is used to simulate the interaction of the waves with Bragg and long period gratings. Results show a very good agreement with experimental data. Recent applications such as the writing of gratings under the acoustic excitation and a novel viscometer sensor based on the acousto-optic mechanism are discussed

All-fiber broadband spectral acousto-optic modulation of a tubular-lattice hollow-core optical fiber

We demonstrate a broadband acousto-optic notch filter based on a tubular-lattice hollow-core fiber for the first time. The guided optical modes are modulated by acoustically induced dynamic long-period gratings along the fiber. The device is fabricated employing a short interaction length (7.7 cm) and low drive voltages (10 V). Modulated spectral bands with 20 nm half-width and maximum depths greater than 60 % are achieved. The resonant notch wavelength is tuned from 743 to 1355 nm (612 nm span) by changing the frequency of the electrical signal. The results indicate a broader tuning range compared to previous studies using standard and hollow-core fibers. It further reveals unique properties for reconfigurable spectral filters and fiber lasers, pointing to the fast switching and highly efficient modulation of all-fiber photonic devices

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

Characterization and new applications of the acousto-optic effect in fiber gratings

In this work, the fundamentals of the acousto-optic modulation in diffraction gratings are presented. By means of a detailed analysis of the mechanical excitation modes applied to the modulator set, it was noticed that two predominant modes of acoustic excitation can be found in the optical fiber depending on the applied acoustic frequency. Through this characterization, it was possible to develop new applications, such as a fast acousto-optic add-drop multiplexing for optical channels, which produces one of the fastest switching device presented in the literature, a narrow tunable single channel dispersion compensator, a tunable optical filter based on the acousto-optic modulation of a phase-shifted fiber Bragg grating, a new method to control the spectrum of Bragg gratings during the writing process and a viscosity sensor, whose application can be considered the most promising.CAPES; CNPqNeste trabalho, os fundamentos da modulação acusto-óptica em redes de difração são apresentados. Através de uma análise detalhada dos modos de excitação mecânicos, mostrou-se que dois tipos predominantes de excitação acústica podem ser encontrados na fibra óptica dependendo da frequência acústica aplicada. Através da caracterização do modulador acusto-óptico, foi possível desenvolver novas aplicações, dentre as quais pode-se citar um dispositivo de inserção e retirada de canais em links ópticos cuja velocidade de chaveamento é uma das maiores apresentadas até agora na literatura, um compensador de dispersão sintonizável de banda estreita, um filtro óptico baseado na modulação acusto-óptica de uma rede de Bragg com uma diferença de fase, um novo método para controlar a escrita de redes de Bragg durante o processo de gravação utilizando-se da técnica da máscara de fase e um sensor de viscosidade, cuja aplicação pode ser considerada a mais promissora

Componentes em fibra ótica para comunicações óticas e sensores

Doutoramento em Engenharia FísicaNos últimos anos, a Optoelectrónica tem sido estabelecida como um campo de investigação capaz de conduzir a novas soluções tecnológicas. As conquistas abundantes no campo da óptica e lasers, bem como em comunicações ópticas têm sido de grande importância e desencadearam uma série de inovações. Entre o grande número de componentes ópticos existentes, os componentes baseados em fibra óptica são principalmente relevantes devido à sua simplicidade e à elevada de transporte de dados da fibra óptica. Neste trabalho foi focado um destes componentes ópticos: as redes de difracção em fibra óptica, as quais têm propriedades ópticas de processamento únicas. Esta classe de componentes ópticos é extremamente atraente para o desenvolvimento de dispositivos de comunicações ópticas e sensores. O trabalho começou com uma análise teórica aplicada a redes em fibra e foram focados os métodos de fabricação de redes em fibra mais utilizados. A inscrição de redes em fibra também foi abordado neste trabalho, onde um sistema de inscrição automatizada foi implementada para a fibra óptica de sílica, e os resultados experimentais mostraram uma boa aproximação ao estudo de simulação. Também foi desenvolvido um sistema de inscrição de redes de Bragg em fibra óptica de plástico. Foi apresentado um estudo detalhado da modulação acústico-óptica em redes em fibra óptica de sílica e de plástico. Por meio de uma análise detalhada dos modos de excitação mecânica aplicadas ao modulador acústico-óptico, destacou-se que dois modos predominantes de excitação acústica pode ser estabelecidos na fibra óptica, dependendo da frequência acústica aplicada. Através dessa caracterização, foi possível desenvolver novas aplicações para comunicações ópticas. Estudos e implementação de diferentes dispositivos baseados em redes em fibra foram realizados, usando o efeito acústico-óptico e o processo de regeneração em fibra óptica para várias aplicações tais como rápido multiplexador óptico add-drop, atraso de grupo sintonizável de redes de Bragg, redes de Bragg com descolamento de fase sintonizáveis, método para a inscrição de redes de Bragg com perfis complexos, filtro sintonizável para equalização de ganho e filtros ópticos notch ajustáveis.In the last years, the Optoelectronics has been established as a top field of research able to drive towards new technological solutions. The abundant achievements in the field of optics as well in optical communications have been of great importance and triggered a number of innovations. Among the large group of existing optical components, optical fiber components are mainly relevant due to three factors: their simplicity, their nature as an optical fiber based component and the capability of optical fiber to carry information. In this thesis the focus was on one of these optical components: optical fiber gratings, which have unique optical processing properties and are quite interesting for the development of devices for optical communications and sensing applications. The work started with a theoretical analysis applied to fiber gratings and continues with the most used fiber gratings production methods. The inscription of fiber gratings was also addressed in this work, where an automated inscription setup has been implemented for silica fiber, and experimental results showed a good approximation to the simulation study. Also, the inscription setup for gratings in polymer optical fiber was developed. A detailed analysis of the acousto-optic modulation in fiber gratings in silica and polymer optical fiber was presented. By means of a detailed analysis of the mechanical excitation modes applied to the modulator set, it was noticed that two predominant modes of acoustic excitation can be found in the optical fiber depending on the applied acoustic frequency. Through this characterization, it was possible to develop new applications. Studies and implementation of different devices based on fiber gratings were performed, using acousto-optic effect and regeneration process in optical fiber for photonic applications such as fast add-drop multiplexer, tunability of the fiber Bragg grating group delay, tunable phase-shift fiber Bragg grating, tunable mode coupler, method for inscription of complex fiber Bragg grating profiles, gain equalization filter and adjustable notch filters

Optical heterodyne micro-vibration detection based on all-fiber acousto-optic superlattice modulation

We propose a configuration of optical heterodyne micro-vibration detection based on an all-fiber acousto-optic superlattic modulation structure that acts as both frequency shifter and reflector, simultaneously. The vibration information within the frequency range between 1 Hz to 150 kHz of a piezoelectric mirror (PZM) has been experimentally measured by using this all-fiber optical heterodyne detection configuration. The minimal measurable vibration amplitude and the resolution are around 0.013 nm and 10 pm in the region of tens to hundreds of kilohertz, respectively. The configuration not only has advantages of compact size, easy alignment and non-contact measurement, but also gains high accuracy, which provides a promising alternative and could be applied in the compact and portable instruments based on optical heterodyne detection

Dynamic Feedback Pulse Shaping For High Power Chirped Pulse Amplification System

The topic of this proposal is the development of high peak power laser sources with a focus on linearly chirped pulse laser sources. In the past decade chirped optical pulses have found a plethora of applications such as photonic analog-to-digital conversion, optical coherence tomography, laser ranging, etc. This dissertation analyzes the aforementioned applications of linearly chirped pulses and their technical requirements, as well as the performance of previously demonstrated parabolic pulse shaping approaches. The experimental research addresses the topic of parabolic pulse generation in two distinct ways. First, pulse shaping technique involving a time domain approach is presented, that results in stretched pulses with parabolic profiles with temporal duration of 15 ns. After pulse is shaped into a parabolic intensity profile, the pulse is compressed with DCF fiber spool by 100 times to 80 ps duration at FWHM. A different approach of pulse shaping in frequency domain is performed, in which a spectral processor based on Liquid Crystal on Silicon technology is used. The pulse is stretched to 1.5 ns before intensity mask is applied, resulting in a parabolic intensity profile. Due to frequency to time mapping, its temporal profile is also parabolic. After pulse shaping, the pulse is compressed with a bulk compressor, and subsequently analyzed with a Frequency Resolved Optical Gating (FROG). The spectral content of the compressed pulse is feedback to the spectral processor and used to adjust the spectral phase mask applied on the pulse. The resultant pulse iv after pulse shaping with feedback mechanism is a Fourier transform, sub-picosecond ultrashort pulse with 5 times increase in peak power. The appendices in this dissertation provide additional material used for the realization of the main research focus of the dissertation. Specification and characterization of major components of equipments and devices used in the experiment are present. The description of Matlab algorithms that was used to calculate required signals for pulse shaping are shown. A brief description of the Labview code used to control the spectral processor will also be illustrated

Scaling symmetries and optimization of the refractive index profile in optical fibers

The optical fiber refractive index profile has a significant role in the optical devices fabrication like fiber Bragg gratings (FBG) and acousto-optic devices and the way guided light interacts in the presence of external factors like strain, stress or even bends. The analysis of the fiber profile gets to be feasible throughout understanding the properties of the optical guided modes such as the effective index, group index, and the chromatic dispersion. Taking a broader look at previous publications that use standard commercial optical fibers, it becomes noticeable that the ideal core-cladding step index profile gives incompatible results when it comes to the experimental versus the simulated dispersion curves. This difference is usually caused by various factors that changes the nominal geometrical and material parameters of the fiber. The stress resulted during the fabrication process, due to thermal and drawing effects is an example of many effects that causes the alteration of these parameters. In general, any irregularity in the fiber, geometrically speaking or material-wise, will produce a coupling of the energy of one mode to the others. Hence, studying coupled modes, whether they are induced by an inscribed FBG or an acousto-optic wave, provides useful information to test the effective refractive index profile required for an accurate theoretical simulation of fiber modes. This research analyzes the characterization of the fibers refractive index profile and the effect of scaling transformation on the dispersion curves. A fiber scaling through two degrees of freedom, geometrical scaling and refractive index difference scaling, gives a significant improvement on fitting the simulated dispersion curves with the experimental ones. However, in many cases, an additional cladding alteration is also needed and shows effectiveness, especially when we analyze a wide wavelength range. According to our final results, a scaling in the geometrical properties of the core and a perturbation of linearly decreasing refractive index in the cladding both describe the fiber profile correctly. As a consequence of our modified step index model, a perfect matching between the experimental and the theoretical dispersion curves is achieved. This process of fiber profile optimization, through the fiber scaling and cladding alteration, have also proven to be efficient, comprehensive and applicable for a wide range of commercial standard fibers