Dissertations / Theses on the topic 'Fiber optic distributed temperature sensing'

Distributed temperature and strain sensing is demanded for a wide range of applications including real-time monitoring of industrial processes, health monitoring of civil infrastructures, etc. Optical fiber distributed sensors have attracted tremendous research interests in the past decade to meet the requirements of such applications. This research presents a multiplexed sensor array for distributed temperature and strain sensing that can multiplex a large number of UV-induced sensors along a single fiber. The objective of this research is to develop a quasi-distributed sensing technology that will greatly increase the multiplexing capacity of a sensor network and can measure temperature and strain with a high accuracy and high resolution. UV-induced intrinsic Fabry-Perot interferometric (IFPI) optical fiber sensors, which have low reflectance and low power loss, are good candidates for multiplexed sensors networks. Partial reflectors are constructed by irradiating photosensitive fiber with a UV laser beam. A pair of reflectors will form a Fabry-Perot interferometer that can be used for temperature and strain sensing. A sensor fabrication system based on a pulsed excimer laser and a shadow mask is developed. A spectrum-based measurement system is presented to measure the interference fringes of IFPI sensors. A swept coherent light source is used as the light source. The spectral responses of the IFPI sensors at different wavelengths are measured. A frequency division multiplexing (FDM) scheme is proposed. Multiple sensors with different optical path differences (OPD) have different sub-carrier frequencies in the measured spectrum of the IFPI sensors. The multiplexing capacity of the sensor system and the crosstalk between sensors are analyzed. Frequency estimation based digital signal processing algorithms are developed to determine the absolute OPDs of the IFPI sensors. Digital filters are used to select individual frequency components and filter out the noise. The frequency and phase of the filtered signal are estimated by means of peak finding and phase linear regression methods. The performance of the signal processing algorithms is analyzed. Experimental results for temperature and strain measurement are demonstrated. The discrimination of the temperature and strain cross sensitivity is investigated. Experimental results show that UV-induced IFPI sensors in a FDM scheme have good measurement accuracy for temperature and strain sensing and potentially have a large multiplexing capacity.<br>Ph. D.

Detailed characterization of coastal aquifers is essential for proper management of coastal groundwater resources, and coastal areas in general. Still, there is a general lack of systematic monitoring and detailed characterization of the seawater intrusion (SWI). Part of this lack is related to the limitations of traditional tools and methodologies. In this thesis we aim to provide new approaches and methodologies for the characterization of coastal aquifers, and in particular for the SWI dynamics. At a first stage, three relatively new monitoring systems were tested to characterise the SWI: Cross-Hole Electric Resistivity Tomography (CHERT), Time Laps Induction Logging (TLIL) and Fiber Optics Distribute Temperature Sensing (FO-DTS). We did that by installing the fiber optic cable and CHERT electrodes outside the borehole casing, and allowing to deploy additional tools through the borehole. This multipurpose approach allowed to deploy multiple tools in parallel, decreasing installation costs and improving characterization by combining complementary information. In fact the combination of the three methods allowed to discern different levels of SWI and multiple dynamics. From these techniques we were interested in further exploring the use of FO-DTS. However, before that we had to study the use of temperature as a tracer of the SWI. Although the fundamentals of heat transport in groundwater are well known, the use of temperature as a natural tracer for SWI is limited. Numerical modelling was used to explore the potential of temperature for studying SWI dynamics at field scale. The results were used to propose a framework for the interpretation of thermal data at the SWI. The framework gives guidelines to identify preferential paths of the SWI, flow direction and even roughly estimate groundwater flux rates, in multilayered coastal aquifers. We concluded that temperature has a potential for SWI motorisation largely unexplored. Based on the promising results from the modelling exercise, we tested the FO-DTS for monitoring of the SWI continuously during one year and a half. This is commonly called a passive application of the FO-DTS. We were able to identify different levels of the SWI, flow directions and short and long term dynamics. The interpretation confirmed the framework proposed previously. FO-DTS can also be used actively, thus by heating the fiber optic cable and measuring the evolution of the heating and cooling of the cable. An active FO-DTS application was designed to measure groundwater fluxes and thermal properties of the media. The obtained flux rates are in agreement with independent estimates. Further research is needed to adapt the analytical approach used for the interpretation of the heating curves to account for the effect of the heat storage in the cable material and surrounding elements. Still, the method shows great potential for its application in quantifying discharge fluxes towards the sea, and in general to quantify groundwater fluxes in porous media. Additionally, traditional methods for the interpretation of pumping testing data were revised. A new approach to extract drawdowns from measured heads during pumping in coastal aquifers is proposed. This approach ease and reinforce the use of this traditional hydraulic characterization technique in coastal aquifers. Many questions remain unanswered, and many others arise from the exploration of these new and traditional techniques. The wide range of information that can be obtained with FO-DTS well deserves more research, in order to facilitate its general use. The revision of traditional methods, such as the classic pumping test, or well-known theories, like the use of temperature as a natural tracer, may be necessary for their application in coastal aquifers. Still, the set of methods proposed in this document expands the tools and the resolution available to characterise the SWI, contributing to expand the available knowledge about coastal aquifers.<br>Una caracterización detallada de los acuíferos costeros es esencial para la gestión de los recursos hídricos costeros, y en general de las zonas costeras. A pesar de su importancia, en general no existe un monitoreo sistemático de la zona de intrusión salina (SWI). Parte de esta carencia se debe a limitaciones en las herramientas y metodologías tradicionalmente empleadas. El objetivo de esta tesis es proporcionar nuevos enfoques y herramientas para la caracterización de los acuíferos costeros, y en particular de la SWI. Inicialmente, tres sistemas de monitoreo fueron testados en campo para caracterizar la SWI: Cross-Hole Electric Resistivity Tomography (CHERT), Time Laps Induction Logging (TLIL) and Fiber Optics Distribute Temperature Sensing (FO-DTS). Para poder operar las tres herramientas en paralelo, el cable de fibra óptica y los electrodos de la CHERT se instalaron entre el entubado y el sedimento. De este modo redujimos costes de instalación y mejoramos el proceso de caracterización con la combinación de información complementarias. De hecho, la combinación de las tres permitió´ detectar diferentes niveles y dinámicas de la SWI. De estas tres técnicas, nos hemos querido concentrar en explorar el uso de la FO-DTS para el monitoreo de acuíferos costeros. Sin embargo, antes de todo tuvimos que estudiar el uso de la temperatura como trazador de la SWI. A pesar de que los fundamentos del transporte de calor en agua subterránea están establecidos, el uso de la temperatura como trazador natural de la SWI es limitado. Para estudiar la SWI se han hecho dos modelos numéricos. De los resultados obtenidos surge un posible marco teórico para la interpretación de datos térmicos de la SWI. Usando la distribución vertical de temperaturas podríamos diferenciar niveles de SWI, la dirección de flujo, e incluso aproximar la velocidad del mismo. Con estos resultados llegamos a la conclusión de que el uso de la temperatura para el seguimiento de la SWI tiene todavía un gran potencial sin explorar. Basándonos en estos resultados testamos la FO-DTS para el monitoreo de la SWI en nuestro emplazamiento experimental durante un año y medio. Los datos obtenidos con la FO_DTS permitieron identificar la respuesta de la SWI a eventos a distintas escala temporales. De este modo, los datos confirmaron las conclusiones obtenidas con los modelos numéricos. La FO-DTS puede ser usada también de forma activa, es decir, calentando el cable de fibra óptica y midiendo el calentamiento y posterior enfriamiento. En la tesis proponemos una metodología activa de la FO-DTS para medir la velocidad del flujo de agua subterránea y estimar las propiedades térmicas del medio teniendo en cuenta el efecto del almacenamiento de calor en el cable. Los valores obtenidos concuerdan con estimaciones independientes. El método puede ser aplicado tanto para la caracterización del flujo de descarga al mar en acuíferos costeros, como en cualquier medio poroso saturado. Finalmente, se revisa la interpretación de los tradicionales ensayos de bombeo. De esta revisión surge proponer una alternativa para separar los descensos de los niveles medidos durante el bombeo, y filtrar el ruido. Esta metodología facilita y refuerza el uso de este tipo de ensayos en acuíferos costeros. La FO-DTS proporciona diferentes tipos de información sobre la SWI, lo que bien justifica profundizar en su estudio para poder generalizar su uso. Por otro lado, la revisión de métodos tradicionales, como los ensayos de bombeo, o de metodologías establecidas, como el uso de la temperatura como trazador, podrían beneficiarse de cierta revisión para adaptarlas a las condiciones especificas de los acuíferos costeros. En cualquier caso, el conjunto de métodos presentados en este documento expande las herramientas y resolución disponibles para la caracterización de la SWI en acuíferos costeros.

[EN] Fiber grating sensors and devices have demonstrated outstanding capabilities in both telecommunications and sensing areas, due to their well-known advantageous characteristics. Therefore, one of the most important motivations lies in the potential of customized fiber gratings to be suitably employed for improving the interrogation process of optical fiber sensors and systems. This Ph.D. dissertation is focused on the study, design, fabrication and performance evaluation of customized fiber Bragg gratings (FBGs) and long period gratings (LPGs) with the double aim to present novel sensing technologies and to enhance the response of existing sensing systems. In this context, a technique based on time-frequency domain analysis has been studied and applied to interrogate different kind of FBGs-based sensors. The distribution of the central wavelength along the sensing structures has been demonstrated, based on a combination of frequency scanning of the interrogating optical pulse and optical time-domain reflectometry (OTDR), allowing the detection of spot events with good performance in terms of measurand resolution. Moreover, different customized FBGs have been interrogated using a technology inspired on the operation principle of microwave photonics (MWP) filters, enabling the detection of spot events using radio-frequency (RF) devices with modest bandwidth. The sensing capability of these technological platforms has been fruitfully employed for implementing a large scale quasi-distributed sensor, based on an array of cascaded FBGs. The potentiality of LPGs as fiber optic sensors has also been investigated in a new fashion, exploiting the potentials of MWP filtering techniques. Besides, a novel approach for simultaneous measurements based on a half-coated LPG has been proposed and demonstrated. Finally, the feasibility of FBGs as selective wavelength filters has been exploited in sensing applications; an alternative approach to improve the response and performance of Brillouin distributed fiber sensors has been studied and validated via experiments. The performance of the reported sensing platforms have been analyzed and evaluated so as to characterize their impact on the fiber sensing field and to ultimately identify the use of the most suitable technology depending on the processing task to be carried out and on the final goal to reach.<br>[ES] Los sensores y dispositivos en fibra basados en redes de difracción han mostrado excepcionales capacidades en el ámbito de las telecomunicaciones y del sensado, gracias a sus excelentes propiedades. Entre las motivaciones más estimulantes destaca la posibilidad de fabricar redes de difracción ad-hoc para implementar y/o mejorar las prestaciones de los sensores fotónicos. Esta tesis doctoral se ha enfocado en el estudio, diseño, fabricación y evaluación de las prestaciones de redes de difracción de Bragg (FBGs) y de redes de difracción de periodo largo (LPGs) personalizadas con el fin de desarrollar nuevas plataformas de detección y a la vez mejorar la respuesta y las prestaciones de los sensores fotónicos ya existentes. En este contexto, una técnica basada en el análisis tiempofrecuencia se ha estudiado e implementado para la interrogación de sensores en fibra basados en varios tipos y modelos de FBGs. Se ha analizado la distribución de la longitud de onda central a lo largo de la estructura de sensado, gracias a una metodología que conlleva el escaneo en frecuencia del pulso óptico incidente y la técnica conocida como reflectometria óptica en el dominio del tiempo (OTDR). De esta manera se ha llevado a cabo la detección de eventos puntuales, alcanzando muy buenas prestaciones en términos de resolución de la magnitud a medir. Además, se han interrogado varias FBGs a través de una técnica basada en el principio de operación de los filtros de fotónica de microondas (MWP), logrando así la detección de eventos puntuales usando dispositivos de radio-frecuencia (RF) caracterizados por un moderado ancho de banda. La capacidad de sensado de estas plataformas tecnológicas ha sido aprovechada para la realización de un sensor quasi-distribuido de gran alcance, formado por una estructura en cascada de muchas FBGs. Por otro lado, se han puesto a prueba las capacidades de las LPGs como sensores ópticos según un enfoque novedoso; para ello se han aprovechados las potencialidades de los filtros de MWP. Asimismo, se ha estudiado y demostrado un nuevo método para medidas simultáneas de dos parámetros, basado en una LPG parcialmente recubierta por una película polimérica. Finalmente, se ha explotado la viabilidad de las FBGs en cuanto al filtrado selectivo en longitud de onda para aplicaciones de sensado; para ello se ha propuesto un sistema alternativo para la mejora de la respuesta y de las prestaciones de sensores ópticos distribuidos basados en el scattering de Brillouin. En conclusión, se han analizado y evaluado las prestaciones de las plataformas de sensado propuestas para caracterizar su impacto en el ámbito de los sistemas de detección por fibra y además identificar el uso de la tecnología más adecuada dependiendo de la tarea a desarrollar y del objetivo a alcanzar.<br>[CAT] Els sensors i dispositius en fibra basats en xarxes de difracció han mostrat excepcionals capacitats en l'àmbit de les telecomunicacions i del sensat, gràcies a les seus excel¿lents propietats. Entre les motivacions més estimulants destaca la possibilitat de fabricar xarxes de difracció ad-hoc per a implementar i/o millorar les prestacions de sensors fotònics. Esta tesi doctoral s'ha enfocat en l'estudi, disseny, fabricació i avaluació de les prestacions de xarxes de difracció de Bragg (FBGs) i de xarxes de difracció de període llarg (LPGs) personalitzades per tal de desenvolupar noves plataformes de detecció i al mateix temps millorar la resposta i les prestacions dels sensors fotònics ja existents. En este context, una tècnica basada en l'anàlisi temps-freqüència s'ha estudiat i implementat per a la interrogació de sensors en fibra basats en diversos tipus i models de FBGs. S'ha analitzat la distribució de la longitud d'ona central al llarg de l'estructura de sensat, gràcies a una metodologia que comporta l'escaneig en freqüència del pols òptic incident i la tècnica coneguda com reflectometria òptica en el domini del temps (OTDR). D'esta manera s'ha dut a terme la detecció d'esdeveniments puntuals, aconseguint molt bones prestacions en termes de resolució de la magnitud a mesurar. A més, s'han interrogat diverses FBGs a través d'una tècnica basada en el principi d'operació dels filtres de fotònica de microones (MWP), aconseguint així la detecció d'esdeveniments puntuals utilitzant dispositius de ràdio-freqüència (RF) caracteritzats per un moderat ample de banda. La capacitat de sensat d'aquestes plataformes tecnològiques ha sigut aprofitada per a la realització d'un sensor quasi-distribuït a llarga escala, format per una estructura en cascada de moltes FBGs. D'altra banda, s'han posat a prova les capacitats de les LPGs com a sensors òptics segons un enfocament nou; per a això s'han aprofitat les potencialitats dels filtres de MWP. Així mateix, s'ha estudiat i demostrat un nou mètode per a mesures simultànies de dos paràmetres, basat en una LPG parcialment recoberta per una pel¿lícula polimèrica. Finalment, s'ha explotat la viabilitat de les FBGs pel que fa al filtrat selectiu en longitud d'ona per a aplicacions de sensat; per això s'ha proposat un sistema alternatiu per a la millora de la resposta i de les prestacions de sensors òptics distribuïts basats en el scattering de Brillouin. S'han analitzat i avaluat les prestacions de les plataformes de sensat propostes per a caracteritzar el seu impacte en l'àmbit dels sistemes de detecció per fibra i a més identificar l'ús de la tecnologia més adequada depenent de la tasca a desenvolupar i de l'objectiu a assolir.<br>Ricchiuti, AL. (2016). Design and fabrication of customized fiber gratings to improve the interrogation of optical fiber sensors [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/66343<br>TESIS<br>Premiado

The need to consider groundwater and surface water as a single resource has fostered the interest of the scientific community on the interactions between surface water and groundwater. The region below and alongside rivers where surface hydrology and subsurface hydrology concur is the hyporheic zone. This is the region where water exchange determines many biogeochemical and ecological processes of great impact on the functioning of rivers. However, the complex processes taking place in the hyporheic zone require a multidisciplinary approach. The combination of innovative point and distributed techniques originally developed in separated disciplines is of great advantage for the indirect identification of water exchange in the hyporheic zone. Distributed techniques using temperature as a tracer such as fiber-optic distributed temperature sensing can identify the different components of groundwater-surface water interactions based on their spatial and temporal thermal patterns at the sediment-water interface. In particular, groundwater, interflow discharge and local hyporheic exchange flows can be differentiated based on the distinct size, duration and sign of the temperature anomalies. The scale range and resolution of fiber-optic distributed temperature sensing are well complemented by geophysics providing subsurface structures with a similar resolution and scale. Thus, the use of fiber-optic distributed temperature sensing to trace flux patterns supported by the exploration of subsurface structures with geophysics enables spatial and temporal investigation of groundwater-surface water interactions with an unprecedented level of accuracy and resolution. In contrast to the aforementioned methods that can be used for pattern identification at the interface, other methods such as point techniques are required to quantify hyporheic exchange fluxes. In the present PhD thesis, point methods based on hydraulic gradients and thermal profiles are used to quantify hyporheic exchange flows. However, both methods are one-dimensional methods and assume that only vertical flow occurs while the reality is much more complex. The study evaluates the accuracy of the available methods and the factors that impact their reliability. The applied methods allow not only to quantify hyporheic exchange flows but they are also the basis for an interpretation of the sediment layering in the hyporheic zone. For upscaling of the previous results three-dimensional modelling of flow and heat transport in the hyporheic zone combines pattern identification and quantification of fluxes into a single framework. Modelling can evaluate the influence of factors governing groundwater-surface water interactions as well as assess the impact of multiple aspects of model design and calibration of high impact on the reliability of the simulations. But more importantly, this modelling approach enables accurate estimation of water exchange at any location of the domain with unparalleled resolution. Despite the challenges in 3D modelling of the hyporheic zone and in the integration of point and distributed data in models, the benefits should encourage the hyporheic community to adopt an integrative approach comprising from the measurement to the upscaling of hyporheic processes.

Thesis (Ph. D.) -- University of Maryland, College Park, 2003.<br>Thesis research directed by: Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Optic fiber sensors are employed in a variety of applications for the remote measurement of various parameters such as strain, pressure, or temperature. These sensors offer an array of benefits as well including light weight, compactness, and high resolution. In particular, Fabry-Perot interferometers (FPIs) maintain these benefits and can also be made to withstand extremely high temperatures. This advantage of the FPI allows it to be used in harsh environments where many other tools for parameter measurement could not survive. An FPI strain sensor is constructed and tested which has the capabilities to be used at high temperatures of over 1000°C for applications in gas turbine engine testing. This paper discusses the need for high temperature strain sensors in engine testing and this sensor’s capabilities in this application.

Temperature sensing is one of the most common and needed sensing technique, especially in harsh environment like a coal gasifier or an airplane engine. Single crystal sapphire has been studied in the last two decades as a candidate for harsh environment sensing task, due to its excellent mechanical and optical properties under extreme high temperature (over 1000°C). In this research, a sapphire wafer based Fabry-Perot (FP) interferometer sensor has been proposed, whose functional temperature measurement can go beyond 1600°C. The size of the sensors can be limited to a 2cm-length tube, with 2mm outer diameter, which is suitable for a wide range of harsh environment applications. The sensors have shown linear sensing response during 20~1200°C temperature calibration, with high sensitivity and resolution, and strong robustness, which are ready for the field test in real-world harsh environment.<br>Master of Science

<p>Much of the analytical and computational work necessary for the development of distributed fiber-optic strain sensors using photo induced Bragg gratings is presented. The one dimensional wave equation is solved for a slowly varying sinusoidal modulation of the index of refraction. The solution is found to take the form of a fourier transform for low reflectivity «15%) gratings. As a result, the process can be inverted, and if the phase and amplitude of the reflected light can be measured over frequency, the phase and amplitude of the bragg grating as a function of length can be computed using the inverse fourier transform. These results are computationally verified, and then further analysis of critical engineering parameters is carried out. A measurement system and procedure are described. A method of writing long, low-reflectivity bragg grating is proposed.</p><br>Master of Science

Distributed sensing is highly desirable in a wide range of civil, industrial and military applications. The current technologies for distributed sensing are mainly based on the detection of optical signals resulted from different elastic or non-elastic light-matter interactions including Rayleigh, Raman and Brillouin scattering. However, they can measure temperature or strain only to date. Therefore, there is a need for technologies that can further expand measurement parameters even to chemical and biological stimuli to fulfill different application needs. <br />This dissertation presents a fully-distributed fiber-optic sensing technique based on a traveling long-period grating (T-LPG) in a single-mode fiber. The T-LPG is generated by pulsed acoustic waves that propagate along the fiber. When there are changes in the fiber surrounding medium or in the fiber surface coating, induced by various physical, chemical or biological stimuli, the optical transmission spectrum of the T-LPG may shift. Therefore, by measuring the T-LPG resonance wavelength at different locations along the fiber, distributed measurement can be realized for a number of parameters beyond temperature and strain. <br />Based on this platform, fully-distributed temperature measurement in a 2.5m fiber was demonstrated. Then by coating the fiber with functional coatings, fully-distributed biological and chemical sensing was also demonstrated. In the biological sensing experiment, immunoglobulin G (IgG) was immobilized onto the fiber surface, and the experimental results show that only specific antigen-antibody binding can introduce a measurable shift in the transmission optical spectrum of the T-LPG when it passes through the pretreated fiber segment. In the hydrogen sensing experiment, the fiber was coated with a platinum (Pt) catalyst layer, which is heated by the thermal energy released from Pt-assisted combustion of H2 and O2, and the resulted temperature change gives rise to a measurable T-LPG wavelength shift when the T-LPG passes through. Hydrogen concentration from 1% to 3.8% was detected in the experiment. This technique may also permit measurement of other quantities by changing the functional coating on the fiber; therefore it is expected to be capable of other fully-distributed sensing applications.   <br /><br>Ph. D.

Internal erosion is the cause of a significant percentage of damages involving water-retaining structures all over the world. In most cases, however, early detection of the phenomenon allows avoiding dramatic accidents. Developing monitoring techniques capable of detecting ongoing internal erosion with sufficient advance represents therefore a significant step towards a higher level of flood safety. Temperature measurements performed in embankment dams have been proven to be effective in identifying leakages consequent to internal erosion. This work investigates the effectiveness of distributed fibre-optic temperature sensing for the early detection of internal erosion in river and sea dikes. The attention is focused on dikes prone to backward erosion piping, a mechanism that affects the foundation layer of structures resting on sandy soils. The measurements performed in a large-scale piping test are presented and discussed. The test enabled the recognition of the main factors that influence the onset of thermal anomalies in regions affected by piping. Subsequently, the effect of the above-mentioned factors was quantitatively studied with the aid of finite element modelling and dimensional analysis. It was discovered that early detection of piping cannot completely rely on the propagation of a thermal front (hot or cold depending on the season) that moves from the waterside to the landside with different speed in intact and eroded regions. Indeed, depending on the permeability and thickness of the sandy layer and on the size of the embankment, the duration of the flood event might not be sufficient for the front to reach a pipe not yet fully developed. However, another mechanism is exploitable for detection: the effect of the flow velocity on the natural vertical temperature gradient. Since this gradient decreases with depth, the effectiveness of this mechanism depends on the depth at which piping occurs. The thesis also contains some tips for an effective numerical modelling of the advection-diffusion equation in permeable soils, both intact and affected by piping. Finally, the thesis describes a small-scale setup that was developed to investigate the temperature distribution in the surroundings of a pipe. The model incorporates a distributed fibre-optic sensor with a sub-centimetre spatial resolution. Such resolution was achieved adopting a novel sensing technology, in which the spectral shift of the Rayleigh backscatter is measured using optical frequency-domain reflectometry. Preliminary results show how the cross-sensitivity of the sensor to temperature and strain influenced the measurements and how this was partially overcome.<br>L'erosione interna è causa di una percentuale significativa di danni riguardanti le opere di ritenuta idraulica. Nella maggior parte dei casi tuttavia l'identificazione precoce del fenomeno permette di evitare incidenti drammatici. Sviluppare tecniche di monitoraggio in grado di individuare l'erosione interna in atto con sufficiente anticipo rappresenta pertanto un passo significativo verso una maggiore sicurezza idraulica. Questo lavoro indaga l'efficacia della misure distribuite di temperatura per l'individuazione precoce dell'erosione interna negli argini fluviali e costieri. La ricerca si concentra sugli argini suscettibili a sifonamento, intesa come l'erosione retrogressiva che interessa lo strato di fondazione di strutture poggianti su terreni sabbiosi. Vengono presentate e discusse le misure realizzate in un esperimento di sifonamento a grande scala. Il test ha permesso l'identificazione dei principali fattori che influenzano l'insorgenza delle anomalie termiche nelle regioni affette da sifonamento. Successivamente, l'effetto dei suddetti fattori è stato studiato quantitativamente con l'ausilio della modellazione agli elementi finiti e dell'analisi dimensionale. E’ emerso che l'individuazione precoce del sifonamento si non può basare esclusivamente sulla propagazione di un fronte termico (caldo o freddo a seconda della stagione) che si muove dal fiume verso campagna con velocità differente nelle zone integre ed erose. Infatti, in funzione della permeabilità e spessore dello strato sabbioso e della dimensione del rilevato, la durata della piena può non essere sufficiente affinché il fronte raggiunga un sifone non ancora pienamente sviluppato. Tuttavia si può sfruttare un meccanismo alternativo: l'effetto della velocità del flusso idraulico sul gradiente verticale naturale. Poiché tale gradiente diminuisce con la temperatura, l'efficacia di questo meccanismo dipende dalla profondità a cui il sifonamento si manifesta. La tesi contiene anche alcuni consigli per una modellazione numerica efficiente del problema di avvezione-diffusione in terreni permeabili, sia integri che affetti da sifonamento. Infine la tesi descrive un apparato sperimentale sviluppato per studiare in piccola scala la distribuzione di temperatura nei dintorni di un sifone. Il modello incorpora un sensore distribuito in fibra ottica con risoluzione spaziale sub-centimetrica. Tale risoluzione è stata ottenuta adottando una tecnica di misura innovativa basata sull'analisi della componente di Rayleigh nel dominio delle frequenze. I risultati preliminari mostrano l'influenza sulle misure della contemporanea sensibilità del sensore a temperatura e deformazione e come questo problema è stato parzialmente risolto.

This thesis explores and evaluates the temperature measuring capabilities of a phase-sensitive optical time-domain reflectometer (φ-OTDR), which exploits Rayleigh backscattering in normal single mode optical fibers. The device is constructed and its setup explained, and a protocol for making temperature measurements with it is developed. Performance tests are made and the device is shown to achieve fully distributed temperature measurements on fibers hundreds of meters in length with a spatial resolution of 1 m and a temperature resolution of 0.1 K. In addition, the capabilities of the device to measure normal strain in the measurement fiber are tested using the same approach, albeit with less success. The device is capable of very precise measurements, making it very sensitive to the environmental conditions around the measuring fiber but also susceptible to disturbances. Some discussion is had on how to avoid or deal with these disturbances. Furthermore, the technique is shown to be able to run in conjunction with other φ-OTDR measurement techniques from the same device simultaneously.<br>Det här examensarbetet utforskar och utvärderar förmågorna att mäta temperatur hos en fas-känslig optisk tidsdomän-reflektometer (φ-OTDR), som utnyttjar bakåtriktad Rayleigh-spridning i vanliga optiska singelmodfibrer. Anordningen konstrueras och dess komponentstruktur förklaras, och ett protokoll tas fram för att utföra mätningar med den. Prestandatester utförs och anordningen visas kapabel att göra fullt distribuerade temperaturmätningar längs hundratals meter långa fibrer, med en rymdsupplösning på 1 m och en temperaturupplösning på 0.1 K. Dessutom testas förmågan att mäta normaltöjning hos testfibern med samma metod, dock med mindre framgång. Anordningen är väldigt känslig för förhållandena i omgivningen runt mätningsfibern, vilket gör den kapabel till mätningar med mycket hög precision, men också mottaglig för störningar. Lite diskussion hålls kring hur dessa störningar kan undvikas eller hanteras. Vidare visas att mätningstekniken kan köras samtidigt som andra φ-OTDR-baserade tekniker från samma anordning.

Fiber optic temperature sensing has become a well-defined field in the past few decades [1] through the use of Fiber Bragg Gratings, Fabry-Perot interferometry, and pyrometry, to list several techniques in use today.  The use of fiber optics offers significant advantages over electronic sensing in terms of size and insensitivity to harsh conditions such as extreme temperatures and corrosive environments.  The availability of optical sapphire materials, including fibers, has allowed the creation of fiber optic sensing elements able to continuously operate at temperatures of 1600"C [2] or more, thus outstripping the abilities of many commonly used thermocouples (excluding platinum types R, S, and B) [3] which will also exhibit a sensitivity to electromagnetic fields.  <br />In addition to the aforementioned benefits, fiber optic sensing techniques provide a great deal of accuracy in temperature measurement over the entire working range of the sensor.<br /><br />The work documented in this thesis consists of efforts to minimize the overall footprint of a sapphire based extrinsic Fabry-Perot interferometry (EFPI) temperature sensing element, as well as strides made in multiplexing the same element and reducing the error potential from cross sensitivity of the thermometer with applied strain.  This work has been variously funded by Pratt & Whitney and the Department of Energy.<br>Master of Science

ITC/USA 2006 Conference Proceedings / The Forty-Second Annual International Telemetering Conference and Technical Exhibition / October 23-26, 2006 / Town and Country Resort & Convention Center, San Diego, California<br>In this paper we will present a new fiber sensor integrated monitor (FSIM) to be used in an embedded instrumentation system (EIS). The proposed system consists of a super luminescent diode (SLD) as a broadband source, a novel high speed tunable MEMS filter with built in photodetector, and an integrated microprocessor for data aggregation, processing, and transmission. As an example, the system has been calibrated with an array of surface relief fiber Bragg gratings (SR-FBG) for high speed, high temperature monitoring. The entire system was built on a single breadboard less than 50 cm² in area.

Due to their compact size, sensitivity, and ability to be multiplexed, intrinsic Fabry-Perot interferometers (IFPIs) are excellent candidates for almost any multi-point temperature or strain application, and it is well-known that using a single-mode lead-in fiber, a multi-mode fiber section as the Fabry-Perot cavity, and an additional single-mode fiber as the tail results in a structure that generates strong interference fringes while remaining robust. Though the basic principles behind these sensors are understood, to the best of the authorâ s knowledge there hasnâ t been a thorough investigation into the design and fabrication of a chain of multiplexed IFPI sensors for industrial use in an environment where serious issues associated with the size of the test coupon, sensor placement, and mechanical reinforcement of the fiber could arise. This thesis details the preparation and results of this investigation. It turns out that fabricating a sensor chain with appropriate sensor spacing and excellent temperature response characteristics proved a significant challenge, and issues addressed include inter-sensor interference, high-temperature mechanical reinforcement for bare fiber sections, and high bending losses. After overcoming these problems, a final sensor chain was fabricated and characterized. This chain was then subjected to a battery of tests at the National Energy Technology Laboratory (NETL), where four multiplexed sensors were installed on a 2â x2â coupon in a simulated gas turbine environment. Final results are presented and analyzed. The work that went into developing this chain lays the foundation for future efforts in developing quasi-distributed temperature sensors by identifying potential obstacles and fundamental limitations for certain approaches.<br>Master of Science

Distributed fibre optic sensing (DFOS) is essential for structural health monitoring (SHM) of strain changes induced during the lifetime of a structure. Among different DFOS systems, the Brillouin Optical Time Domain Reflectometry (BOTDR) takes the advantages of obtaining full frequency spectrum to provide strain and temperature information along the optic fibre. The key parameters of distributed fibre optic sensors, spatial and frequency resolution, are strongly linked with the time-frequency (T-F) localisation in the system in three parts: pulse, hardware design and optical fibre. T-F localization is fundamentally important for the communication system, whereas in this study the importance of the T-F localisation to the spatial and frequency resolution, repeatability and the measurement speed are introduced in BOTDR. In this dissertation, the development of DFOS is first introduced, including both traditional methods and new developed designs. The literature review shows the signal to noise ratio (SNR) of BOTDR can be improved by investigating its T-F localisation. In the hardware design, in order to improve the T-F localisation in hardware architecture, a Short-Time Fourier Transform-Brillouin Optical Time-Domain Reflectometry (STFT-BOTDR), which implements STFT over the full frequency spectrum to measure the distributed temperature and strain along the optic fibre, is applied so that the conventional frequency sweeping method can be replaced for high resolution and fast speed measurement, providing new research advances in dynamic distributed sensing. The STFT based BOTDR has better T-F localisation, which in turn provides an opportunity for off-line post signal processing that is more adaptable for fast speed measurements. The spatial and frequency resolution of dynamic BOTDR sensing is limited by the Signal to Noise Ratio (SNR) and the T-F localization of the input pulse shape. The T-F localized input pulse shape can enhance the SNR and the spatial and frequency resolution in STFT-BOTDR. In this study, simulation and experiments of T-F localized different pulses shapes are conducted to examine the limitation of the system resolution. The result indicates that a rectangular pulse should be selected to optimize the spatial resolution and a Lorentzian pulse could be chosen to optimize the frequency resolution, while a Gaussian shape pulse can be used in general applications for its balanced performance in both spatial and frequency resolution. Meanwhile, T-F localization is used for pulse T-F localisation optimisation. A set of Kaiser-Bessel functions is used to simulate different pulse shapes and to compare their parameters in terms of T-F localisation and their Brillouin scattering spectrum. A method using an iterative filtering algorithm to achieve the optimised pulse in terms of T-F localisation is introduced to converge the Effective-pulse Width (TEW) in the time-domain and Effective-pulse Linewidth (FEL) in the frequency domain to identify the fundamental limitations. The optimised pulse can be fitted with a 7th order Gaussian (super-Gaussian) shape and it offers the best experimental performance compared to a Rectangular pulse. The sensitivity of a sensor to strain or temperature variations due to distributed Brillouin scattering is closely related to the power distribution on the Brillouin scattering spectrum which is related to the property of the optic fibre. The performance of a highly nonlinear fibre that can generate a higher Brillouin scattering signal is compared to that of a standard single mode fibre. The results show that much higher SNR of the Brillouin scattering spectrum and smaller frequency uncertainties in the sensing measurement can be achieved by using a highly nonlinear fibre for comparable launched powers. With a measurement speed of 4 Hz, the frequency uncertainty can be 0.43 MHz, corresponding to 10 με in strain or 0.43°C in temperature uncertainty for the tested highly nonlinear fibre. In contrast, for a standard single mode fibre, the value would increase to about 1.02 MHz (25 με or 1.02°C), demonstrating the advantage of the tested highly nonlinear fibre for distributed strain/temperature sensing. Results show that, by using a small effective area highly nonlinear fibre, the strain or temperature resolution can be improved because it generates stronger Brillouin scattering signal with high SNR and high Q factor spectrum, both of which determine the optimal averaging time in a single measurement. In general, the STFT-BOTDR can achieve 1 m spatial resolution, 10 με frequency resolution on a 10 km fibre with measurement speed at about 2.5 kHz.

The health monitoring of smart structures in civil engineering is becoming more and more important as in-situ structural monitoring would greatly reduce structure life-cycle costs and improve reliability. The distributed strain and temperature sensing is highly desired in large structures where strain and temperature at over thousand points need to be measured simultaneously. It is difficult to carry out this task using conventional electrical strain sensors. Fiber optic sensors provide an excellent opportunity to fulfill this need due to their capability to multiplex many sensors along a single fiber cable. Numerous research studies have been conducted in past decades to increase the number of sensors to be multiplexed in a distributed sensor network. This dissertation presents detailed research work on the analysis, design, fabrication, testing, and evaluation of an intrinsic Fabry-Perot fiber optic sensor for quasi-distributed strain and temperature measurements. The sensor is based on two ultra-short and broadband reflection fiber Bragg gratings. One distinct feature of this sensor is its ultra low optical insertion loss, which allows a significant increase in the sensor multiplexing capability. Using a simple integrated sensor interrogation unit and an optical spectrum based signal processing algorithm, many sensors can be interrogated along a single optical fiber with high accuracy, high resolution and large dynamic range. Based on the experimental results and theoretical analysis, it is expected that more than 500 sensors can be multiplexed with little crosstalk using a frequency-division multiplexing technology. With this research, it is possible to build an easy fabrication, robust, high sensitivity and quasi-distributed fiber optic sensor network that can be operated reliably even in harsh environments or extended structures. This research was supported in part by U.S. National Science Foundation under grant CMS-0427951.<br>Ph. D.

A fatigue test has been performed on 7075-T651 aluminum specimens which were bonded with polyimide coated optical fibers with discrete Bragg gratings. These fibers were bonded with AE-10 strain gage adhesive. The results indicate that lower strain amplitudes do not produce cause for concern, but that larger strain amplitudes (on the order of 3500 μ) may cause some sensors to become unreliable. The strain response of acrylate coated optical fiber strain sensors bonded to aluminum specimens with AE-10 and M-Bond 200 strain gage adhesives was investigated with both axial and cantilever beam tests. These results were compared to both the strain response of conventional strain gages and to model predictions. The results indicate that only about 82.6% of the strain in the specimen was transferred through the glue line and fiber coating into the fiber. Thus, multiplying by a strain transfer factor of approximately 1.21 was sufficient to correct the optical fiber strain output. This effect was found to be independent of the adhesive used and independent of the three-dimensional profile of the glue line used to attach the fiber. Finally, this effect did not depend on whether the fiber had a polyimide or an acrylate coating. Further investigation was conducted on the feasibility of using optical fiber strain sensors for monitoring subcritical damage (such as matrix cracks) in fiber reinforced composite materials. These results indicate that an array of optical fibers which monitor the strain profile on both sides of a composite panel may be sufficient for these purposes<br>Master of Science

La Thèse a été réalisée en cotutelle entre l’Université Jean Monnet de Saint Etienne (France) et l’université de Palerme (Italie) et en collaboration avec l’Agence nationale pour la gestion des déchets radioactifs (ANDRA). Le travail de recherche concerne l'étude des capteurs répartis à fibres optiques (utilisant la lumière rétrodiffusée Raman et Brillouin) destinés à l’observation et à la surveillance des ouvrages dédiés au stockage géologique de déchets radioactifs de haute activité et moyenne activité à vie longue (projet CIGEO : centre industriel de stockage géologique). Dans ce contexte, les fibres optiques sont envisagées en tant qu’élément sensible de capteurs pour la mesure de paramètres environnementaux tels que la température ou la déformation. Les conditions environnementales extrêmes d’exploitation entrainent la dégradation des propriétés de la fibre optique et des performances du capteur. La réponse des capteurs Brillouin et Raman est influencée par la présence de rayonnement y et d’hydrogène dans l’atmosphère Cigéo. Dans les deux cas, une diminution de la transmission linéique de la fibre optique est observée. Ce phénomène est dû à l’atténuation induite par radiation (RIA) ou par hydrogène (HIA) réduisant ainsi la portée de la mesure. En outre le rayonnement y et l’hydrogène provoquent des erreurs sur la mesure de température ou de déformation pour les deux types de capteurs. L’étude a permis d’établir les lignes directrices pour la sélection et le développement des capteurs repartis à fibre optique appropriés pour travailler dans l’environnement Cigéo<br>This PhD thesis is conducted in the joint supervision of both the University Jean Monnet of Saint Etienne (France) and the University of Palermo (Italy) in collaboration with the French national agency for the management of radioactive wastes (ANDRA). The aim of the Thesis is to evaluate the performances of distributed optical fiber sensors (based on Raman and Brillouin scattering technologies) that will be employed for monitoring industrial site for deep geological disposal for high and long-lived intermediate level activity radioactive waste (HL-W and LL/IL-W, respectively), called Cigéo. In this context, the distributed optical fiber sensors will provide a time and spatial cartography of the strain and temperature inside the disposal cell. The severe environment of Cigéo requires the sensor evaluation taking into account the resulting degradation of the sensing optical fiber. The sensor response is affected by y-rays and hydrogen presences inside the storage cells. In both cases a decrease of the optical fiber transmission, due to the radiation or hydrogen induced attenuation (RIA or HIA) is observed and limits the sensing distance range of the sensor. Moreover, the two different environment constraints lead to errors in the temperature or strain evaluation for both sensor technologies. This Thesis work establishes the guidelines to select and design distributed optical fiber sensors suitable to operate in radiation environment such as Cigéo one

The financial requirements between fiber optic sensors and conventional sensors are gradually declining, which, despite many advatages and wide range of applicationas, has slowed down the demand for these sensors. With the demand for fiber optic sensors also grow the requirements for the parameters of these sensors. This thesis deals with distributed temperature fiber optic sensors. Non--linear phenomen in optical fibers, such as Raman scattering is used for measuring with these sensors. This scatterin was used to obtain information about temperature, which effected the optical cable under the test.

Nowadays, composite materials have become the main construction materials for aeronautical structures, replacing traditional materials commonly used in this industry. However, composite structures are still characterized by conservative designs due to the lack of understanding from a physics-based approach their damage propagation and failure mechanisms. Since repairs of composite structures are a crucial part of the long-term use of composites in aerospace, the stakeholders require an in depth understanding of the physics of disbonding in composites. The aim of this research is a numerical evaluation of mode II disbonding on Center Cut Plies (CCPs) specimens with a preliminary study on material characterization performed through a Distributed Sensing System (DSS). In order to fulfill the objectives of this research, CCPs specimens manufactured from unidirectional fiber prepregs will be considered. The specimens will be produced from unidirectional prepregs in order to minimize the effects of residual stress fields introduced into the specimen during the curing process. That residual stress field will be evaluated and monitored making use of a Distributed Sensing System (DSS) optical fiber mounted and embedded within the laminas of the specimen. In addition, an analytical and numerical approach through FEM analysis will be adopted and validated to verify the experimental results obtained from the DSS.

Předložená disertační práce zkoumá možnosti použití nového typu polovodičového MGY- Laseru elektricky laditelného v širokém spektrálním rozsahu a zabývá se možnostmi jeho nasazení v optovláknové senzorové síti založené na metodě FBG (Fiber Bragg Grating). Výzkum byl započat komplexními dlouhodobými testy reálného měřícího scénáře z oblasti stavebnictví, sestaveného pro účely ověření limitujících aspektů současných technik. Inženýrské aplikace nabízejí velké množství vzájemně se vylučujících požadavků pro návrh strukturálních senzorových systémů. Tyto požadavky jsou sdíleny mnoha dalšími technologickými oblastmi, což přispívá k vysokému stupni univerzálnosti použití dosažených výsledků. Na základě posouzení stavu současné techniky a aplikačních požadavků byly v práci nejprve identifikovány aspekty, které mají být výzkumem zlepšeny. V dalším kroku byl detailně charakterizován MG-Y laser Syntune/Finisar S7500. Na základě dat získaných měřením byla zkoumána nová metoda spojitého řízená vlnové délky záření laseru. Provedené experimenty vedly nejen k návrhu nového způsobu spojité regulace vlnové délky ale také k vytvoření prostředků pro vlastní kalibraci systému na základě jeho vnitřních vlastností (podélných módů rezonátoru).

Cette thèse porte sur l’étude des systèmes de mesure répartie de la rétrodiffusion Brillouin dans une fibre optique, que l’on nomme B-OTDR et qui sont sensibles à la température et à la déformation de la fibre. Les solutions d'interrogateurs actuelles permettent l'instrumentation de grandes infrastructures. Cependant, il existe des limites, inhérentes au phénomène physique utilisé, qui ne permettent pas d'adresser certaines applications spécifiques. Ces limites portent sur la distance de mesure maximale accessible par ces interrogateurs mais également sur la décorrélation de la mesure des paramètres de température et de déformation. Nous avons donc cherché, au cours de cette thèse, à adresser des solutions à ces problématiques. Un nouveau système de ré-amplification optique, basé sur les technologies EDFA, a été mis au point. Associé à un système B-OTDR, cette solution nous a permis de réaliser une mesure distribuée de température sur 150 km de fibre avec une répétabilité de 1,5 °C. Cette avancée propose, à notre connaissance, les meilleurs résultats obtenus avec un tel système et nous permet d’envisager son déploiement pour la surveillance d’infrastructures du transport de l’énergie sur de longues distances. Nous avons dans un second temps conçu un nouvel interrogateur, utilisant la rétrodiffusion Brillouin, et permettant la décorrélation des mesures de température et de déformation sur une seule et unique fibre optique. Ce nouvel interrogateur, caractérisé et breveté durant cette thèse, a permis de réaliser une mesure indépendante de température et de déformation sur un câble spécifique inséré dans un puits de forage. Les résultats de ces mesures ont montré à la fois des variations de température et de déformation sur le câble, donnant des informations précieuses à l’opérateur du puits. Le nouvel interrogateur mis au point, permet une séparation de ces deux paramètres avec une répétabilité inférieure à 1 °C et 20 μm/m pour une distance de l’ordre du kilomètre. Pour des distances de l’ordre de la dizaine de kilomètres, la répétabilité de mesure est de 3 °C et 75 μm/m. Ce résultat fait l'état de l'art dans la séparation température/déformation par B-OTDR. Enfin, les différents travaux réalisés pour répondre à ces deux problématiques ont abouti au développement d’un prototype d’interrogateur qui laisse envisager une mesure simultanée de la température, de la déformation, des vibrations acoustiques et de la pression hydrostatique. Ce prototype conduit à des perspectives intéressantes pour une solution complète de surveillance d’infrastructures<br>This thesis deals with distributed optical fiber sensors, especially Brillouin sensors called B-OTDR that are sensitive both to temperature and strain. Some actual limitations due to the scattering phenomenon avoid deployment of this technology for some specific applications. These limitations are about the maximum length measured by the sensor but also the double sensitivity to strain and temperature. From our current B-OTDR sensor, we propose solutions to address these issues.A new re-amplification module using EDFA has been developed and characterized associated to a B-OTDR system. A 150 km distributed temperature measurement with 1,5 °C repeatability has been reached. This record performance let us consider a deployment for very long infrastructure monitoring for the energy transport. In another hand, we developed a new sensing device using Brillouin scattering and able to separate temperature and strain. This patented device has been characterized and used on a specific cable inserted in a well drilling. Results have shown a combination of strain and temperature variations on the cable that give very useful information to the well operator. We have reached a temperature repeatability of 1 °C and a strain repeatability of 20 μm/m for 1 km fiber. At 10 km, temperature repeatability has been evaluated to be 3 °C and strain to 75 μm/m. These results are to the state of art for temperature/strain separation using B-OTDR.These works have led to a new distributed measurement prototype that could address simultaneously temperature, strain, acoustic vibrations, and pressure sensing. Some interesting perspectives could result from these works to address a complete infrastructure monitoring using fiber optic sensing

M.Ing.<br>This thesis demonstrates the measurement of temperature distribution across a metal plate and the non-contact measurement of torsion in a rotating shaft. After a thorough theoretical study of the origin of fibre Bragg gratings, methods of fabrication and erasing of gratings, methods of demodulating the signals from gratings, and methods of discriminating between strain and temperature variations in gratings, simulations and experiments were done to prove the feasibility of both temperature distribution measurement and non-contact torsion measurement. For the measurement of temperature distribution, a 450 x 450 x 1.6 mm type 304 stainless steel plate was used. Two 4,7Ω 25W resistors were used as heat sources, and five fibre Bragg gratings measured the temperature at strategic points on the plate. The measured values were interpolated appropriately and a three-dimensional representation of the temperature distribution was shown on a personal computer monitor. The measurement of torsion was first done on a non-rotating 25 mm diameter solid shaft with torque applied. Both resistive strain gauges and fibre Bragg gratings were attached to the shaft and the measurement results compared well. Up to 90 Nm of torque was applied to the shaft. The non-contact measurement of torsion on a rotating shaft was done with fibre Bragg gratings and involved the use of graded index (GRIN) lenses for transmitting optical information between the light source, the gratings, and back to the spectrum analyser through free space. A special Böhler steel with high yield strength was used, and the set-up was mounted on a lathe. The one end of the shaft was clamped in the lathe chuck, and the other was fixed to an automobile drum brake system, allowing the application of torque on the rotating shaft. Up to 97 Nm of torque was applied to the shaft rotating at 190 rpm, and the results are within 10% of the expected values. As far as we are aware, this is the first time that torsion is measured by fibre Bragg gratings without the need to have physical contact with the shaft, i.e. slip rings, to transmit the information to and from the sensors. The experimental results of this thesis confirm that fibre Bragg grating sensors can be applied usefully in temperature distribution measurement, as well as in the measurement of torsional stress in rotating shafts.

Water temperature in rivers and streams is an important factor for aquatic ecosystem health. Measurement of stream temperature has traditionally been accomplished by point temperature measurements, continuous point temperature loggers, and more recently, airborne remote sensing techniques such as Forward-Looking Infrared Radar (FLIR) or Thermal Infrared Radiometry. While each of these measurement techniques has certain advantages, none allows for the combined spatial and temporal information provided by Distributed Temperature Sensing (DTS). DTS employs fiber optic signals to measure temperature and is a relatively new temperature measurement technology for hydrologic sensing applications. Nine DTS stream temperature datasets were collected in the Middle Fork John Day River (MFJDR) as part of a basin-wide stream monitoring effort. The datasets encompassed five 1-3 kilometer long reaches, some monitored over three summers (2009-2011). In contrast to existing stream temperature measurement technologies, DTS can provide stream temperature data in both the spatial and temporal domains. Techniques and challenges of interpreting DTS stream temperature data were documented, and three applications of the technology to stream temperature monitoring were explored. Cold water patches, potentially used by fish as thermal refugia during stream temperature maximums, were located using DTS. No identified cold patch exceeded 2.31°C cooler than ambient stream temperature. Tributary inflows provided some of the most temperature-differentiated cold patches. These findings provide a reference for the degree of thermal heterogeneity in the MFJDR system and beg the question of whether fish respond to small (<3°C) spatial temperature variations. Theoretical predictions of stream mixing potential (Richardson number and cavity flow mixing predictions) suggested that increasing stream thermal heterogeneity would require channel modification to decrease stream flow velocity in select areas. The combined spatial and temporal coverage of a DTS stream temperature dataset on the Oxbow Conservation Area allowed diagnosis of a 2°C longitudinal stream temperature decrease observed in multiple Thermal Infrared Radiometry (TIR) and Forward-Looking Infrared Radiometry (FLIR) datasets collected on that reach. Advection velocity and channel depth, rather than groundwater or tributary inflows, were the main cause of the decrease, and the magnitude of the decrease peaked in the early afternoon, disappearing completely by evening. This finding suggests caution for interpretation of FLIR and TIR stream temperature datasets, which represent "snapshot" temperature measurements. For these datasets, knowledge of flow conditions (velocity and depth) may help avoid misinterpretation of temporally-transient temperature anomalies. Diurnal slope periodicity was observed in linear-like spatial trends in four DTS datasets, and an analysis was made to examine this subtle spatially and temporally varying phenomenon. The phase of the diurnal slope variation differed between river reaches, suggesting that propagation of larger-scale thermal waves might be one driving mechanism. Temporally-constant offsets between slope magnitudes within reaches suggested some intra-reach differences in heat fluxes.<br>Graduation date: 2012

In recent years, wireline temperature profiling methods have evolved to offer new insight into fractured rock hydrogeology. An important advance in temperature logging makes use of boreholes temporarily sealed with flexible impervious fabric liners so that the water column is static and effects of cross-connection are eliminated. For this project a characterization technique was developed based on combining fibre optic distributed temperatures sensing (DTS) with active heating within boreholes sealed with underground liners. DTS systems provide a temperature profiling method that offers improved temporal resolution when compared with wireline trolling based techniques. The ability to collect temperature profiles rapidly in time can improve understanding of transient processes. In this study the advantage of a sealed borehole environment for temperature investigations is demonstrated. Evidence for identifying active groundwater flow under natural gradient conditions using DTS heat pulse testing is presented through a comparison with high resolution geologic logging and hydraulic datasets.

The Middle Fork John Day Basin in Northeastern Oregon is prime habitat for spring Chinook salmon and Steelhead trout. In 2008, a major tributary supporting rearing habitat, Big Boulder Creek, was restored to its historic mid-valley channel along a 1 km stretch of stream 800 m upstream of the mouth. Reduction of peak summer stream temperatures was among the goals of the restoration. Using Distributed Temperature Sensing (DTS) Fiber Optic Technology, stream temperature was monitored prior to restoration in June 2008, and after restoration in September 2008, July 2009, and August 2009. Data gathered was used to determine locations of groundwater and hyporheic inflow and to form a stream temperature model of the system. The model was used both to develop an evaluation method to interpret components of model performance, and to better understand the physical processes important to the study reach. A very clear decreasing trend in surface temperature was seen throughout each of the DTS stream temperature datasets in the downstream 500 m of the study reach. Observed reduction in temperature was 0.5°C (±0.10) in June 2008, 0.3°C (±0.37) in September 2008, 0.6°C (±0.25) in July 2009, and 0.2°C (±0.08) in August 2009. Groundwater inflow was calculated to be 3% of the streamflow for July 2009 and 1% during the August 2009 installation. Statistically significant locations of groundwater and hyporheic inflow were also determined. July 2009 data was used to model stream temperature of the 1 km (RMSE 0.28°C). The developed model performance evaluation method measures timelag, offset, and amplitude at a downstream observed or simulated point compared with the boundary condition, rather than evaluating the model based on error. These measures are particularly relevant to small scale models in which error may not be a true reflection of the ability of a model to correctly predict temperature. Breaking down model performance into these three predictive measures was a simple and graphic method to show the model's predictive capability without sorting through large amounts of data. To better understand the model and the stream system, a sensitivity analysis was conducted showing high sensitivity to streamflow, air temperature, groundwater inflow, and relative humidity. Somewhat surprisingly, solar radiation was among the lowest sensitivity. Furthermore, three model scenarios were run: a 25% reduction in water velocity, a 5°C increase in air temperature, and no groundwater inflow. Simulations of removal of groundwater inflows resulted in a 0.5°C increase in average temperature over the modeled time period at the downstream end, further illustrating the importance of groundwater in this stream system to reduce temperatures.<br>Graduation date: 2012

Bohrlochintegrität ist unerlässlich für die erfolgreiche und nachhaltige Produktion und Injektion von Fluiden aus Reservoirgesteinen, wie beispielsweise bei der Nutzung von Kohlenwasserstoffen, Geothermie oder Standorten für geologische Speicherung. Um die Integrität einer Bohrung über seine Lebenszeit zu gewährleisten, ist vor allem eine erfolgreiche primäre Komplettierung nötig. Besonders die Zementation der Rohre stellt dabei ein großes Risiko dar, weil durch die natürlichen Gegebenheiten im Bohrloch viele Faktoren Einfluss auf die Zusammensetzung und Verteilung der Zementsuspension haben. Diese Studie untersucht das Monitoring-potenzial von faseroptischer ortsverteilter Dehnungsmessung DSS (distributed strain sensing). Ergänzend zu faseroptischer ortsverteilten Temperaturmessung DTS (distributed temperature sensing), welche seit mehr als zwei Jahrzehnten in der Industrie Anwendung findet, kann jeder Ort einer Glasfaser zusätzlich Informationen über den mechanischen Spannungszustand geben. Experimentelle und analytische Arbeiten wurden durchgeführt, um die Auswirkung von Laständerungen auf einer Faser zu quantifizieren. Desweiteren wurde der Einfluss komplexer mehrschichtiger Bohrlochkabel auf Dehnungsmessergebnisse untersucht. Ein faseroptisches Messkabel wurde im Zuge dieser Arbeit im Ringraum entlang der Produktionsrohrtour einer Bohrung installiert. Die gemessenen Geländedaten zeigen Ergebnisse aus zwei Arbeitsschritten der Fertigstellung der Bohrung - der Filterverkiesung und der Zementation. Aufgrund der Dichtedifferenz von Kies und Bohrspülung wurde am Kabel ein Dehnungseffekt gemessen. Die Teufe, in welcher der Dehnungseffekt auftritt, korreliert mit Wireline Gamma-Gamma-Dichtedaten, welche im gleichen Zeitfenster gemessen wurden. Die anschliessende Kompaktion des Kieskopfes wurde durch das Glasfaserkabel in Form einer zunehmenden mechanischen Belastung erfasst. Während der anschliessenden Zementation der Rohrtour wurde ein Dehnungseffekt in der Mischzone von Flüssigkeiten mit unterschiedlichen rheologischen Eigenschaften gemessen. Anhand eines Experiments konnte bestätigt werden, dass fluidrheologische Parameter (wie die Fluidviskosität) mit einem faseroptischen Messkabel quantifiziert werden können. Hierfür werden Fluidscherspannungen gemessen, welche durch das Fliessen von Fluiden an der Kabeloberfläche hervorgerufen werden (amtliches Zeichen zur Patentanmeldung: EP 19171265.2). DSS-Messungen erweiten das Verständnis von Fluidverdrängungsvorgängen in Bohrlöchern und ermöglichen eine Beurteilung von Komplettierungsvorgängen in Echtzeit.<br>Borehole integrity is fundamental for the successful and sustainable utilization of hydrocarbons, geothermal energy and sites for geological storage. The success of the primary well completion is necessary to ensure the integrity of a well over its lifetime. In particular, the casing cementation represents a great risk because many factors have an influence on the composition and distribution of the cement suspension due to the natural conditions in the borehole. This study investigates the monitoring potential of fiber-optic distributed strain sensing (DSS) using a measurement cable which is installed in the annulus of a well. Similar to distributed temperature sensing (DTS), which is used for temperature monitoring in industry applications for more than two decades, fibers additionally convey information about their mechanical stress state. Laboratory as well as analytical work was performed to quantify the effect of load changes on a fiber. In addition, the influence of complex multilayered downhole cable on the strain response is examined. The presented field data shows results from two stages of the well completion - the gravel packing and the cementation. Due to the difference in density of gravel and drilling fluid, a deformation is measured on the cable. The depth at which the stretching effect occurs correlates with wire-line gamma-gamma density data measured in the same time window. The subsequent compaction of the gravel head, which was not revealed by the logging measurement, was detected by the fiber optic cable in the form of an increasing mechanical load on the cable. During cement pumping, fluid shear stresses create a measurable load on the cable, especially in the mixing zone of liquids with dfferent rheological properties. Based on this observation, an experiment was designed and conducted which aims at measuring fluid rheological parameters such as fluid viscosity. For this purpose, the fluid shear stresses acting on the fiber optic sensing cable in the flow path are measured (patent application number: EP 19171265.2). DSS measurements extend the understanding of fluid displacements in wellbores and allow an assessment of well completion process in real time.

Polymer electrolyte membrane (PEM) fuel cells are being studied for use as high efficiency power plants in alternative fuel vehicles. To maintain high efficiency the operating temperatures of the membranes in these fuel cells must be closely monitored and controlled. However, the environment inside of the fuel cell is not favorable for traditional temperature sensing, so a new optical-fiber-based, distributed temperature sensor was proposed to address this need. This thesis investigates the properties of neodymium-doped optical fiber for use as a distributed temperature sensor for PEM fuel cells. The optical absorption spectrum was measured to identify the energy band structure and determine upconversion excitation schemes. The temperature coefficient of the Nd³⁺-doped silica fiber fluorescence decay time was measured for several bands of emission. Finally, two-photon upconversion was attempted from the lower excited states of Nd:YAG and Nd:silica.<br>Graduation date: 2012

The development and transition of optical fiber sensors from experimental stage to practical applications largely depends on manufacturing cost and simplicity. To date, in-fiber grating sensors are largely manufactured by ultraviolet lasers despite higher fabrication cost and complexity. Besides, ultraviolet radiation can only write gratings in doped fibers. Therefore, reaping the benefits of existing fibers such as pure silica fiber, photonics crystal fibers etc. cannot be achieved using this technique. In contrast, uses of ultra-fast lasers have the potential to eliminate or minimize those drawbacks. However, extensive fabrication and packaging research is required for ultrafast laser technology to mature and offer grating based sensors fabrication in industrial scale. This dissertation presents design and fabrication of fiber optic sensors using femtosecond laser for measurement of ambient refractive index and temperature. The femtosecond laser operating at 780 nm with pulse duration of 172 fs and pulse repetition rate of 1 kHz is used to study bulk index modification and fabricate fiber long period and short period gratings. Effective and reliable fabrication of in-fiber gratings requires spatial control of refractive index written in optical fiber. With an aim to better control spatial index modulation in direct ultrafast writing, primary focus of this work is given to write single-shot submicron periodic voids in bulk glass. Femtosecond pulse filamentation in glass is studied to understand the morphology of bulk index change written by ultrashort pulses. Laser writing parameters (such as beam diameter, pulse energy, scanning speed, depth of focus, etc.) are then further tuned to write pulse filamentation induced refractive index change in optical fibers suitable for fiber grating fabrication. In order to design and tailor grating’s spectrum, measurement of in-fiber index is introduced in this work. We propose fiber Bragg grating based Fabry-Perot cavity structure (cavity length, L= 10 mm) to characterize femtosecond pulse filamentation induced refractive index change in the core of standard SMF. In addition, Mach-Zehnder interferometer (MZI) is proposed as an alternative yet effective and low cost tool to measure in-fiber index change. Comsol simulation is used to validate the quantification of index change. Measured index change is used in Optiwave simulation to design fiber long period gratings in standard telecommunication and pure silica core fibers. To increase fabrication reliability, we introduce inscription of helical long period gratings using a custom made rotary stage. Tapered photonic crystal and microfiber based Mach-Zehnder interferometer is also investigated for ambient refractive index measurement. Miniature fiber Bragg grating written in microfiber Mach-Zehnder interferometer is used in this work for multi-parameter sensing as well as temperature compensated refractive index sensing. Microfiber Bragg gratings buried in materials of higher thermal expansion coefficient is also proposed to significantly enhance temperature sensitivity.<br>Graduate<br>0548, 0794, 0775<br>fariduvic@gmail.com

[EN] Structural Health Monitoring (SHM) is a discipline that quantitatively assesses the integrity and performance of infrastructures, relying on sensors, and support the development of efficient Maintenance and Rehabilitation (M&R) plans. Optical Multicore Fiber (MCF) Shape Sensors offer an innovative alternative to traditional methods and enable the reconstruction of the deformed shape of structures directly and in real-time, with no need of computation models or visual contact and exploiting all the advantages of Optical Fiber Sensors (OFS) technology. Despite the intense research efforts centered on this topic by research groups worldwide, a comprehensive investigation on the parameters that influence the performance of these sensors has not been conducted yet. The first part of the thesis presents a numerical study that examines the effects of strain measurement accuracy and core position errors on the performance of optical multicore fiber shape sensors in sensing three-dimensional curvature, which is at the basis of shape reconstruction. The analysis reproduces the strain measurement process using Monte Carlo Method (MCM) and identifies several parameters which play a key role in the phenomenon, including core spacing (distance between outer cores and sensor axis), number of cores and curvature measured. Finally, a set of predictive models were calibrated, by fitting the results of the simulations, to predict the sensors performance. Afterward, an experimental study is proposed to evaluate the performance of optical multicore fiber in sensing shape, with particular focus on the influence of strain sensors length. Two shape sensors were fabricated, by inscribing long (8.0 mm) and short (1.5 mm) Fiber Bragg Gratings (FBG) into the cores of a multicore seven-core fiber. Thus, the performance of the two sensors was assessed and compared, at all the necessary phases for shape reconstruction: strain sensing, curvature calculation and shape reconstruction. To conclude, an innovative approach, based on the Saint-Venant's Torsion Theory, is presented to determine the twisting of multicore fiber and to compensate the errors due to twisting during shape reconstruction. The efficiency of the theoretical approach was then corroborated performing a series of twisting tests on a shape sensor, fabricated by inscribing FBGs sensors into an optical spun multicore seven-core fiber. The investigation of the mechanical behavior of multicore optical shape sensors has synergically involved diverse disciplines: Solid Mechanics, Photonics, Statistics and Data Analysis. Such multidisciplinary research has arisen from the prolific cooperation between the Institutes of the Institute of Science and Technology of Concrete (ICITECH) and the Institute of Telecommunications and Multimedia Applications (iTEAM) - Photonics Research Labs (PRL) - of Universitat Politècnica de València (UPV), in addition to valuable collaboration with other members of the European ITN-FINESSE project, to which this work belongs. This research work aims to enhance the performance optical multicore fiber shape sensors and support the development of new sensor geometries, with great potential for structural health monitoring applications.<br>[ES] La Monitorización de la Salud Estructural (MSE) evalúa cuantitativamente la integridad y el comportamiento de las infraestructuras y permite desarrollar planes eficaces de Mantenimiento y Rehabilitación (M&R), utilizando los datos de los sensores. Sensores de forma basados en fibra óptica multinúcleo ofrecen una alternativa a los métodos tradicionales y permiten la reconstrucción de la deformada de estructuras de forma directa y en tiempo real, sin necesidad de modelos de cálculo o contacto visual y con todas las ventajas de la tecnología de los Sensores de Fibra Óptica (SFO). A pesar de los grandes esfuerzos en la investigación centrada en este tema por parte de los grupos de investigación de todo el mundo, todavía no se ha realizado una investigación exhaustiva que estudie los parámetros que influyen en el comportamiento de estos sensores. En la primera parte de la tesis se presenta un estudio numérico en el que se examinan los efectos de la precisión de la medición de la tensión y los errores de posición del núcleo en el comportamiento de los sensores de forma basados en fibra óptica multinúcleo para definir la curvatura tridimensional, que es la base de la reconstrucción de la forma. El análisis reproduce el proceso de medición de la tensión utilizando el método de Monte Carlo (MC) e identifica una serie de parámetros que desempeñan un papel en el proceso, entre ellos la separación del núcleo (distancia entre los núcleos exteriores y el eje del sensor), el número de núcleos y la curvatura medida. Por último, se calibró un conjunto de modelos de predicción ajustando los resultados de las simulaciones para predecir el comportamiento de los sensores. A continuación, se propone un estudio experimental para evaluar el comportamiento de los sensores de forma basado en fibra óptica multinúcleo, con especial atención en la influencia de la longitud de los sensores de deformación. Se fabricaron dos sensores de forma, inscribiendo Fiber Bragg Gratings (FBG) con longitudes de 8,0 mm y 1,5 mm en los núcleos de una fibra multinúcleo de siete núcleos. Así, se evaluó y comparó el comportamiento de los dos sensores en todas las fases necesarias para la reconstrucción de la forma, incluyendo la medición de la tensión, el cálculo de la curvatura y la reconstrucción de la forma. Para concluir, se presenta un enfoque innovador, basado en la Teoría de la Torsión de Saint-Venant, para determinar la torsión de la fibra multinúcleo y compensar los errores debidos a la torsión durante la reconstrucción de la forma. La eficiencia del enfoque teórico fue verificada realizando una serie de pruebas de torsión en un sensor de forma, fabricado inscribiendo los sensores de FBGs en una fibra óptica multinúcleo torcida y siete núcleos. La investigación del comportamiento mecánico de los sensores ópticos de forma multinúcleo ha involucrado sinérgicamente diversas disciplinas: Mecánica del sólido, Fotónica, Estadística y Análisis de datos. Esta investigación multidisciplinaria ha surgido de la prolífica cooperación entre el Instituto de Ciencia y Tecnología del Hormigón (ICITECH) y el Instituto de Telecomunicaciones y Aplicaciones Multimedia (iTEAM) - Laboratorio de Investigación Fotónica (LIF) - de la Universidad Politécnica de Valencia (UPV), además de la valiosa colaboración con otros miembros del proyecto europeo ITN-FINESSE, al que pertenece este trabajo. Este trabajo de investigación puede permitir mejorar el comportamiento de los sensores de forma basados en fibra óptica multinúcleo y apoyar el desarrollo de nuevas geometrías de sensores, con un gran potencial para aplicaciones de control de la salud estructural.<br>[CA] Structural Health Monitoring (SHM) avalua quantitativament la integritat i el comportament de les infraestructures i permet desenrotllar plans eficaços de Maintenance and Rehabilitation (M&R), utilitzant les dades dels sensors. Optical Multicore Fiber (MCF) Shape Sensors oferixen una alternativa als mètodes tradicionals i permeten la reconstrucció de la forma de la deformació de les estructures de forma directa i en temps real, sense necessitat de models de càlcul o contacte visual i amb tots els avantatges de l'Optical Fiber Sensors (OFS) Technology. A pesar dels grans esforços en la investigació centrada en aquest tema per part dels grups d'investigació de tot el món, encara no s'ha realitzat una investigació exhaustiva que estudie els paràmetres que influïxen en el comportament d'aquestos sensors. En la primera part de la tesi es presenta un estudi numèric en què s'examinen els efectes de la precisió del mesurament de la tensió i els errors de posició del nucli en el comportament dels sensors de forma basats en fibra òptica multinucli per a definir la curvatura tridimensional, que és la base de la reconstrucció de la forma. L'anàlisi reproduïx el procés de mesurament de la tensió utilitzant el mètode de Monte Carlo (MC) i identifica una sèrie de paràmetres que exercixen un paper en el procés, entre ells la separació del nucli (distància entre els nuclis exteriors i l'eix del sensor), el nombre de nuclis i la mesura de la curvatura. Finalment, es va calibrar un conjunt de models de predicció ajustant els resultats de les simulacions per a predir el comportament dels sensors. A continuació, es proposa un estudi experimental per a avaluar el comportament dels sensors de forma basat en fibra òptica multinucli, amb especial atenció en la influència de la longitud dels sensors de deformació. Es van fabricar dos sensors de forma, inscrivint Fiber Bragg Gratings (FBG) amb longituds de 8,0 mm i 1,5 mm en els nuclis d'una fibra multinucli de set nuclis. Així, es va avaluar i es va comparar el comportament dels dos sensors en totes les fases necessàries per a la reconstrucció de la forma, incloent el mesurament de la tensió, el càlcul de la curvatura i la reconstrucció de la forma. Per a concloure, es presenta un enfocament innovador, basat en la Teoria de la Torsió de Saint-Venant, per a determinar la torsió de la fibra multinucli i compensar els errors deguts a la torsió durant la reconstrucció de la forma. L'eficiència de l'enfocament teòric va ser verificada realitzant una sèrie de proves de torsió en un sensor de forma, fabricat inscrivint els sensors de FBGs en una fibra òptica de set nuclis de filat múltiple. La investigació del comportament mecànic dels sensors òptics de forma multinucli ha involucrat sinèrgicament diverses disciplines: Mecànica del sòlid, Fotónica, Estadística i Anàlisi de dades. Aquesta investigació multidisciplinària ha sorgit de la prolífica cooperació entre l'Institut de Ciència i Tecnologia del Formigó (ICITECH) i l'Institut de Telecomunicacions i Aplicacions Multimèdia (iTEAM) - Laboratori de investigación fotònica (LIF) - de la Universitat Politècnica de València (UPV), a més de la valuosa col·laboració amb altres membres del projecte europeu ITN- FINESSE, al qual pertany aquest treball. Aquest treball d'investigació pot permetre millorar el comportament dels sensors de forma basats en fibra òptica multinucli i ajudar al desenrotllament de noves geometries de sensors, amb un gran potencial per a aplicacions de control de la salut estructural.<br>Floris, I. (2020). Optical Multicore Fiber Shape Sensors. A numerical and experimental performance assessment [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/148715<br>TESIS