Deep-ocean pressure measurements are a necessary component for ocean characterization and oceanographic monitoring. Some principle applications such as tsunami detection and ocean floor subsidence are reliant on deep-ocean pressure measurement data. The deep ocean is a challenging environment especially for pressure measurements; discerning pressure changes that are a small fraction of the ambient pressure calls for intelligent engineering solutions. An ocean-deployable concept model of a pressure sensor is developed. The design is based on a diaphragm transducer intended for measuring hydrostatic pressure changes on the order of 1 centimeter of water (cmH2O) while exposed to ambient pressures several orders of magnitude greater for up to 2500 meters of water (mH2O). Two laboratory-scale pressure sensors are fabricated to test the fundamental principle of the proposed concept at lab-safe pressures. One is a single-sided sensor exposed to atmospheric pressure. The second sensor is a two-sided design that operates at a defined target depth pressure and measures the differential pressure across both faces of the diaphragm. The sensor design built for atmospheric pressure testing observed a mean experimental sensitivity of 6.05 pm/cmH2O in contrast to 6 pm/cmH2O determined theoretically. The percent error between the experimental and theoretical values is 0.83%. The second design was tested at target depth pressures of 10, 20, 40, and 60 psi (7, 14, 28, and 42 mH2O) and performance was within 5.8%, 2.8%, 0.7%, 4.0% respectively when considering percent error of the mean experimental and theoretical. The repeatability was sufficient for a given sample and pressure response within the range proposed in theory when a pressure preload was present to the diaphragm. Future work will aim at developing a design concept that incorporates a piston and is tested at a higher hydrostatic pressure system, and within ocean waters. A deployment plan and consideration of challenges associated with ocean testing will be accounted for. / Graduate

Esta tesis investiga la aplicación de conceptos de codificación al diseño de sensores ópticos basados en redes de difracción de Bragg de Bragg (FBG). Específicamente, se presenta el diseño, la caracterización y la validación experimental de dispositivos de detección codificados personalizados que se pueden diseñar y fabricar como dispositivos FBG súper estructurados (SSFBG). El objetivo de esta tesis es mejorar la capacidad y el rendimiento general de los sistemas de detección óptica basados en sensores FBG convencionales. Para ello, se han propuesto tres metodologías de codificación de dispositivos de detección SSFBG, con el objetivo de dotar a cada sensor con información adicional útil para la identificación de cada sensor incluso en condiciones de superposición. Un sensor codificado basado en FBGs es una estructura FBG cuya forma se ha adaptado a una palabra-código ortogonal, de tal manera que su longitud de onda central se puede distinguir inequívocamente de otras señales en el espectro. El diseño de los sensores SSFBG codificados se realiza modificando el espectro de reflexión de dispositivos FBG multibanda, esto se logra traduciendo las palabras-código ortogonales en los términos de amplitud y fase de los sensores FBG. La codificación en amplitud de los sensores SSFBG consiste en traducir las palabras-código \textit{"Optical Orthogonal Codewords"} (OOC), desarrolladas para sistemas de comunicaciones de acceso múltiple por división de código óptico (OCDMA), en el patrón de reflexión de los dispositivos. La codificación en amplitud y fase se ha propuesto mediante dos enfoques diferentes: en el primero, palabras-código de amplitud y fase personalizadas ($ a_ {k} $, $ f_ {k} $) fueron diseñadas específicamente para exhibir un comportamiento ortogonal obtenido por la combinación de dos palabras-código. La técnica de interrogación basada en una fuente dual sintonizable fue específicamente diseñada para recuperar la medición diferencial de los sensores y decodificar efectivamente su información. El segundo enfoque utiliza las secuencias \textit{"Discrete Prolate Spheroidal Sequences"} (DPSS), que son secuencias mutuamente ortogonales desarrolladas para sistemas de comunicaciones. Se demostró el uso de estas estructuras como elementos de detección ortogonales con patrones específicos de fase y amplitud. La fabricación y validación experimental de los dispositivos SSFBG propuestos se realizaron para demostrar el rendimiento de los sensores inclusive en condiciones de superposición espectral. La longitud de onda central de los sensores se recupera con éxito en las tres metodologías, además, el error del sistema de detección se caracterizó en términos de los parámetros de diseño. / Esta tesi investiga l'aplicació de conceptes de codificació al disseny de sensors òptics basats en xarxes de difracció de Bragg de Bragg (FBG) . Específicament, es presenta el disseny, la caracterització i la validació experimental de dispositius de detecció codificats personalitzats que es poden dissenyar i fabricar com a dispositius FBG súper estructurats (SSFBG) . L'objectiu d'esta tesi és millorar la capacitat i el rendiment general dels sistemes de detecció òptica basats en sensors FBG convencionals. Per a això, s'han proposat tres metodologies de codificació de dispositius de detecció SSFBG, amb l'objectiu de dotar a cada sensor amb informació addicional útil per a la identificació de cada sensor inclús en condicions de superposició. Un sensor codificat basat en FBGs és una estructura FBG la forma de la qual s'ha adaptat a una paraula-codi ortogonal, de tal manera que la seua longitud d'ona central es pot distingir inequívocament d'altres senyals en l'espectre. El disseny dels sensors SSFBG codificats es realitza modificant l'espectre de reflexió de dispositius FBG multibanda, açò s'aconseguix traduint les paraules-codi ortogonals en els termes d'amplitud i fase dels sensors FBG. La codificació en amplitud dels sensors SSFBG consistix a traduir les paraules-codi extit \textit{"Optical Orthogonal Codewords"} (OOC) , desenrotllades per a sistemes de comunicacions d'accés múltiple per divisió de codi òptic (OCDMA) , en el patró de reflexió dels dispositius. La codificació en amplitud i phase s'ha proposat per mitjà de dos enfocaments diferents: en el primer, paraules-codi d'amplitud i fase personalitzades ($ a_ {k} $, $ f_ {k} $) van ser dissenyades específicament per a exhibir un comportament ortogonal obtingut per la combinació de dos paraules-codi. La tècnica d'interrogació basada en una font dual sintonizable va ser específicament dissenyada per a recuperar el mesurament diferencial dels sensors i descodificar efectivament la seua informació. El segon enfocament utilitza les seqüències \textit{"Discrete Prolate Spheroidal Sequences"} (DPSS), que són seqüències mútuament ortogonals desenrotllades per a sistemes de comunicacions. Es va demostrar l'ús d'estes estructures com a elements de detecció ortogonals amb patrons específics de fase i amplitud. La fabricació i la validació experimental dels dispositius SSFBG proposats es van realitzar per a demostrar el rendiment dels sensors inclusivament en condicions de superposició espectral. La longitud d'ona central dels sensors es recupera amb èxit en les tres metodologies, a més, l'error del sistema de detecció es va caracteritzar en termes dels paràmetres de disseny. / This thesis investigates the application of encoding concepts to the design of optical sensors based on fiber Bragg grating (FBG) devices. Specifically, we present the design, characterization and experimental validation of custom encoded sensing devices that can be designed and manufactured as super-structured FBG (SSFBG) devices. The aim of this thesis is to enhance the capacity and the overall performance of the optical sensing systems based on conventional FBG sensors. To do so, three encoding methodologies of SSFBG sensing devices have been proposed, aiming to endow each sensor with additional information useful to identify each sensor even under overlapping conditions. An encoded FBG-based sensor is a FBG structure whose shape has been tailored after an orthogonal codeword in such a way that their central wavelength can be distinguished unequivocally from other signals in the spectrum. The design of encoded SSFBG sensors is performed by modifying the reflection spectrum of multi-band FBG devices, this is achieved by translating orthogonal codewords into the amplitude and phase terms of the FBG sensors. Amplitude encoding of SSFBG sensors consists in translating the binary optical orthogonal codewords (OOCs), developed for optical-code division multiple-access (OCDMA) communications systems, into the reflection pattern of the devices. Amplitude $\&$ phase encoding has been proposed in two different approaches: in the first one, custom amplitude and phase codewords ($a_{k}$, $f_{k}$) were specifically devised to exhibit orthogonal behavior by combining the two codewords. The dual-wavelength tunable interrogation technique was also specifically designed to retrieve the differential measurement of the sensors and effectively decode their information. The second approach uses the discrete prolate spheroidal sequences (DPSS), which are mutually orthogonal sequences developed for communications systems. We demonstrated the use of this structures as orthogonal sensing elements with definite phase and amplitude patterns. The manufacturing and experimental validation of the proposed SSFBG devices were carried out to prove the overlap-proof performance of the devices. The central wavelength of the sensors is successfully retrieved in the three methodologies, additionally, the error of the sensing system was characterized in terms of the design parameters. / Triana Infante, CA. (2018). Enoding optial FBG sensors to enhance the capacity of optial sensing systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/114824 / TESIS

[ES] Con el continuo desarrollo de materiales y tecnologías de fabricación durante las últimas tres décadas, la atenuación de la transmisión de las fibras ópticas de polímero (POF) se ha reducido considerablemente. Las POF son ventajosas para las redes domésticas, así como para las interconexiones de almacenamiento, y tienen ventajas significativas para muchas aplicaciones de detección, que incluye el límite alto de tensión elástica, alta resistencia a la fractura, alta flexibilidad en la flexión, alta sensibilidad a la tensión y coeficientes termoópticos negativos. Esta tesis consigue mejorar la tecnología de irradiación con láser para POF e investiga dispositivos especiales basados en redes de difracción en POF para comunicaciones ópticas, microondas, fotónica y detección. En particular, desarrollamos la tecnología de fabricación rápida de FBGs en POF con un estudio detallado y la optimización de los parámetros de fabricación de redes de difracción de Bragg en fibra (FBG). Los resultados más destacados incluyen un FBG uniforme de 8 dB con un solo pulso láser Nd: YAG (26 nsm) (8 ns) basado en fibra dopada con BDK, que es el tiempo más corto presentado hasta ahora para la fabricación de FBGs en POF. La irradiación de fibras ópticas de polímero utilizando diferentes materiales basado en el láser KrF a 248 nm permitió demostrar un mejor rendimiento en comparación con el sistema que emplea el láser He-Cd a 325 nm. Además, se fabricaron FBGs uniformes en POFs de índice escalón dopadas con TS en menos de 1 segundo mediante la repetición de pulsos con baja energía. Finalmente, el estudio de la irradiación UV con pulsos de baja energía para la fabricación de redes de difracción estables permitió ahorrar energía en el proceso de fabricación de FBGs en POF, como uno de los principales requisitos para la producción en masa. Basándonos en la tecnología de fabricación mejorada, nos centramos en la fabricación de redes de difracción con diferentes estructuras: se fabricó un FBG con desplazamiento de fase utilizando dos pulsos de 15 ns a 248 nm KrF superpuestos por el método de Moiré; el primer FBG con chirp sintonizable se logró utilizando un solo pulso corto del láser, que abrió nuevas perspectivas a las aplicaciones basadas en redes de difracción con chirp en POF; también se propuso un nuevo método basado en gradientes térmicos para obtener FBG con chirp en POF basadas en FBG uniformes, y se demostró como la forma más conveniente publicada hasta la fecha para lograr este tipo de FBGs no uniformes en POF; y, finalmente, también se han fabricado redes de difracción de largo período utilizando un proceso de fabricación de corto tiempo, especialmente en comparación con investigaciones anteriores. En la última parte de la tesis, y en base a los dispositivos basados en redes de difracción obtenidos a lo largo de este trabajo, se han propuesto varias aplicaciones . De manera similar a los FBG en fibra de sílice, los FBGs con chirp en POF tienen muchas aplicaciones futuras en las áreas de comunicaciones ópticas y de los sensores. Este documento describe la aplicación de detección de tensión basada en una FBG con chirp sintonizable en POF, su aplicación para detección térmica en sistemas biomédicos; e ilustra el potencial de los dispositivos de dispersión sintonizables en el campo de las comunicaciones ópticas, bien como compensación de dispersión o en fotónica de microondas. / [CAT] Amb el continu desenvolupament de materials i tecnologies de fabricació durant les últimes tres dècades, l'atenuació de la transmissió de les fibres òptiques de polímer (POF) s'ha reduït considerablement. Els POF són avantatjosos per a les xarxes domèstiques, així com per a les interconnexions d'emmagatzematge, i tenen avantatges significatius per a moltes aplicacions de detecció, inclosos els límits de tensió elàstica alta, alta resistència a la fractura, alta flexibilitat en la flexió, alta sensibilitat a la tensió i potencials coeficients termoópticos negatius. Aquesta tesi va millorar la tecnologia d'irradiació amb làser per a POF i va investigar dispositius basats en xarxes difracció de Bragg (FBG) especials en POF per a comunicació òptica, microones, fotònica i detecció. En particular, desenvolupem la tecnologia de fabricació ràpida de FBG en POF amb un estudi detallat i l'optimització dels paràmetres per a la seua fabricació. Els punts destacats dels resultats inclouen un FBG uniforme de 8 dB amb un sol pols del làser Nd: YAG (266 nm) (8 ns) basat en fibra dopada amb BDK, que és el temps més curt reportat per a la fabricació de POF FBG. La irradiació de fibres òptiques de polímer utilitzant diferents materials sota el sistema de làser KrF a 248 nm va permetre demostrar un millor rendiment en comparació amb el sistema de làser Kimmon de 325 nm. A més, els FBG uniformes en el POF dopat amb TS d'índex escalonat es van aconseguir amb menys d'1 segon mitjançant la repetició de polsos de control i l'energia baixa de pols. Finalment, l'estudi de la irradiació d'energia de pols per a la fabricació de FBGs estables va permetre estalviar energia en el procés de fabricació de FBGs en POF, com un dels principals objectius de la producció en massa. Basant-nos en la tecnologia de fabricació millorada, ens centrem en la fabricació de diferents estructures de xarxes de difracció: es va fabricar un FBG amb desplaçament de fase utilitzant dos polsos de 15 ns a 248 nm KrF superposats pel mètode de Moiré; el primer FBG amb chirp sintonitzable es va aconseguir utilitzant un sol pols curt de làser, que va obrir les aplicacions basades en FBG amb chirp en POF; també es va proposar un nou mètode amb gradients tèrmics per a obtindre FBG en POF basat en FBG uniformes, i es va demostrar com la forma més convenient publicada fins hui per a aconseguir FBG POF estimulada; i, finalment, també s'han aconseguit xarxes de llarg període utilitzant un procés de fabricació de curt temps, especialment en comparació amb investigacions anteriors. Finalment, sobre la base dels dispositius de xarxes de difracció obtinguts al llarg d'aquest treball, s'han proposat diverses aplicacions potencials en aquesta tesi. De manera similar que per als FBG amb silici, el FBG amb chirp en POF té moltes aplicacions potencials en comunicacions òptiques i a l¿àrea de sensors. Aquest document descriu l'aplicació de detecció de tensió basada en FBG amb chirp sintonitzable en POF; a més, l'aplicació de detecció tèrmica en sistemes biomèdics; i el potencial dels dispositius de dispersió sintonitzables en les comunicacions òptiques, com per eixample a la compensació de dispersió o a la fotònica de microones. / [EN] With the continuing development of material and fabrication technologies over the last three decades, the transmission attenuation of polymer optical fibers (POF) has been greatly decreased. POFs are advantageous for home networks as well as storage interconnections and have significant advantages for many sensing applications, including high elastic strain limits, high fracture toughness, high flexibility in bending, high sensitivity to strain and potential negative thermo-optic coefficients. This thesis improved the laser irradiation technology for POF and investigated special grating devices in POF for optical communication, microware photonics and sensing. In particular, we developed fast POF grating fabrication technology with a detailed study and optimization of the polymer optical fiber Bragg grating (POFBG) fabrication parameters. Highlights of the results include an 8 dB uniform POFBG with one single Nd:YAG (266nm) laser pulse (8 ns) based on BDK doped fiber, which is the shortest time ever reported for POFBG fabrication. The irradiation of polymer optical fibers using different materials under 248 nm KrF laser system allowed to demonstrate a better performance compared with 325 nm Kimmon laser system. Furthermore, uniform FBGs in step-index TS doped POF were achieved with less than 1 second by means of controlling pulse repetition and low pulse energy. Finally, the study of low UV pulse power irradiation for fabricating stable gratings allowed to save energy in the POF grating fabrication process, as one of the main goals for mass production. Based on the improved fabrication technology, we focused on the fabrication of different grating structures: a phase-shifted FBG was fabricated by using two 15 ns 248 nm KrF pulses overlapped by Moiré method; the first tunable chirped FBG was achieved by using a single laser short pulse, which opened the applications based on chirped POF BGs; also a novel thermal annealing method was proposed to obtain chirped POFBGs based on uniform FBGs, and proved as the ever published most convenient way to achieve chirped POFBG; and finally, long period gratings have been also achieved by using a short time fabrication process, specially when compared with previous research. Finally, based on the grating devices obtained throughout this work, several potential applications have been proposed in this thesis. Similarly, to silica chirped FBG, chirped FBG in POF have many potential applications in optical communications and sensing area. This document described the potential strain sensing application based on tunable chirped POFBG; also, the thermal detection application in bio-medical systems; and the potential of tunable dispersion devices in optical communications, i.e., dispersion compensation or microwave photonics. / Min, R. (2019). Polymer optical fiber gratings for microwave photonics and communications application [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/125473 / TESIS

Volvo Cars is in a change of producing only electric and hybrid cars by 2025.Subcomponent testing is a crucial part to ensure the quality of the individual buildingblocks in an electric machine. Any way of making these tests more reliable and less timeconsuming is of great interest at Volvo. Force and temperature on bearings are especiallyhard to measure accurately, because of their placement and dynamic behavior. Accurateand reliable measurements is also a vital part in creating realistic Computer-AidedEngineering (CAE) models for simulation purposes. Simulations on bearings could lead tobetter bearing choices and accelerate the design process. This could increase bearing lifeand increase the Electrical Vehicle (EV) range due to minimized friction losses. FiberBragg Grating (FBG) sensors is a technology that has some key advantages overconventional sensors. They are immune to EMI, smaller in size, can have multiple sensorsin one fiber and can measure multiple physical quantities at the same time. Volvo Cars isinterested in investigating whether this sensor technology could be a candidate forreplacing some of the current measurement setup configurations.The project was divided into three parts, validating sensor equipment, find method forinstallation and measurement on a bearing and development of a CAE model for bearinglosses and heat transfer. To validate the sensor equipment a Measurement SystemAnalysis (MSA) was performed on two FBG fibers, one FBG isolated from strain fortemperature measurement and one FBG array with multiple sensing points. From theMSA it could be seen that the FBG temperature sensor had a total uncertainty of 3.4 °CThe FBG array had a strain uncertainty of 1.04 μ𝜀 and a temperature uncertainty of 0.4 °C.The uncertainty of both the FBG array and the FBG temperature sensor is highlydependent on the calibration of the sensitivity constant. The force measurement on thebearing was done with a concept based on the wavelength difference, produced by strain,between two FBG sensors. The concept was tested in a dynamic component rig where anaxial force could be applied, and the wavelength difference measured. The temperatureon the outer ring of the bearing was measured using an FBG isolated from strain. The testresults were promising, but since the FBG is sensitive to temperature and strain theincreased temperature difference between the two fibers affected the results. Thecalibration method needs to be compensated for the increased temperature differencebetween the fibers which is generated when the rotational speed is increased, and thiscould not be done with a single temperature measurement. The two developed CAEmodels was both constructed in MATLAB and showed similar behavior with experimentaltests done by others. To validate the models, physical test for heat transfer and bearinglosses should be performed.

Récemment, de nouvelles techniques ont été développées pour la fabrication des réseaux de Bragg à profil complexe. Ces composants photoniques sont utilisés dans plusieurs applications émergentes telles que la compensation de la dispersion pour les systèmes de communication de longue portée, les lasers à fibre, multiplexeurs et détecteurs optiques. Le diagnostic après inscription devrait fournir les informations nécessaires pour l’amélioration de la fabrication des réseaux de Bragg. Nous savons que les propriétés spectrales du réseau de Bragg sont liées au profil d’indice Δn. Les techniques de mesure directes, telles que la diffraction latérale de Krug, permettent de retrouver l’amplitude de modulation d’indice le long du réseau. Cependant, ces techniques sont insensibles aux fluctuations de phase. Une méthode alternative de caractérisation indirecte fondée sur l’algorithme de Layer-Peeling (LP) a été proposée. Toutefois elle ne peut pas être appliquée à la caractérisation des réseaux longs en raison de la propagation du bruit de calcul. Dans cette thèse nous avons présenté une nouvelle technique pour la mesure directe de l’amplitude et de la phase du profil d’indice le long du réseau de Bragg fondée sur la luminescence bleue (LB) induite par l’irradiation UV. Nos résultats expérimentaux de la mesure du profil de modulation d’indice sont en bonne correspondance avec la méthode de Krug. La méthode que nous proposons peut être appliquée à la caractérisation des réseaux longs. Elle permet de retrouver simultanément l’amplitude de modulation d’indice Δnac(z), la fonction du chirp et détecter le changement de l’indice moyen Δndc(z). / N the last decade new techniques were developed for fabrication of sophisticated Fiber Bragg Gratings (FGBs). This has been motivated by the emergence of many applications such as dispersion compensation for long-haul communication systems, DFB fiber lasers, optical add/drop multiplexers, and optical sensors. Post-fabrication diagnostics should provide relevant information to enhance the FBG fabrication process. It is well known that the FBG spectral properties are related to the index profile Δn. Direct measurement techniques, such as the side diffraction method reported by P. Krug, allow determining the index modulation amplitude along the FBG. Nevertheless, these techniques provide no information about phase fluctuations. An alternative method of indirect characterization, based on the Layer-Peeling (LP) algorithm, consists in Bragg grating profile reconstruction from its complex reflectivity. However, the LP method is unstable when applied to characterize long FBGs (>1mm) due to the error propagation effect. In this thesis we have shown the principle of a novel technique for the direct measurement of amplitude and phase variations of the index modulation along an FBG based on the blue luminescence (BL). Our experimental results are in a good agreement with the according Krug characterization. The proposed method of FBG characterization in amplitude and phase using the UV induced BL can be applied to long gratings (up to tens of centimeters) having complex index modulation profiles. It allows retrieving simultaneously the index profile modulation Δnac(z) and the chirp function, localizing phase shifts, and also detecting the mean index change Δndc(z).

No description available.

The understanding of how structural components endure loads, in particular variable loads, is that these components gradually, over some period of time depending on the nature of the loading and the material, develop a microcrack. After some additional time and loading, the microcrack grows to a size that might be detected. Beyond that point, the microcrack propagates in a manner that can be reliably predicted by computer analysis codes. Consequently, one can define different stages for the life of a structural component. These are: 1) the period prior to the formation of a microcrack, 2) the period of microcrack growth, and finally 3) the period of crack growth. To date, structural health monitoring approaches that seek to detect cracks offer no insight into the extent of deterioration occurring in the initial stage that is a precursor to the formation of the microcrack or its growth. However, an approach that would facilitate monitoring the extent of the deterioration that takes place during this stage promises to improve life prediction capabilities of structural components. The challenge, thus, is to develop quantitative assessment of damage accumulation from the earliest stages of the fatigue process and to provide a structure's signature that is dependent of the damage stage. One such signature is the structure's response to forced excitation. The realization of such a goal would help in advancing structural health monitoring procedures using interrogative system identification techniques and determine sensitivities of physical parameters to damage. Additionally, vibration-based spectral quantities are related to physical properties of the structure under test. In this thesis, nonlinear response to parametric excitation is exploited for nonlinear system identification of metallic and composite beam-mass systems before damage initiation through intermediate states of damage progression to failure. Parametric identification procedure combines linear and higher order spectral analysis of vibration measurements and perturbation techniques for the derivation of the approximate solution of the system nonlinear governing differential equation. The possibility of using optical Fiber Bragg Grating sensors technology for damage localization is also assessed. Spectral moments and quantities obtained from fiber optic strain measurements are evaluated near and away from cracks to assess the relation between these moments and cracks. Variations in parameters representing natural frequency, damping and effective nonlinearities for different levels of progressive damage in a beam-mass system have been determined. Their percentage variations have been quantified to establish their sensitivities to damage initiation. The results show that damping and effective nonlinearity parameters are more sensitive to damage conditions than the natural frequency of the first mode. Crack localization is assessed by means of optical fiber technology for a composite beam-mass system. The results show that noise levels in fiber optic signals are high in comparison to strain gage signals. Of particular interest, however, is the observation that the nonlinear response is more pronounced near the cracks than away from them. / Ph. D.

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. / Ph. D.

The use of optical fiber sensors is increasing widely in civil, industrial, and military applications mainly due to their, (a) miniature size, (b) high sensitivity, (c) immunity from electro-magnetic interference, (d) resistance to harsh environments, (e) remote signal processing ability, and, (f) multiplexing capabilities. Because of these advantages a variety of optical fiber sensing techniques have evolved over the years having potentials for myriad of applications. One very challenging job, for any of these optical fiber sensing techniques, is to implement a stand alone system with the design and development of all the signal processing models along with the necessary hardware, firmware, and software satisfying the real-time signal processing requirements. In this work we first develop the equations for the system model of the wavelength modulated extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor, and then design and build all the hardware and software necessary to implement a stand-a / Ph. D.

Optical resonators such as silica micro-spheres and micro-toroids can support whispering gallery modes (WGMs), where light circulates near the resonator surface and is confined by the total internal reflection at the dielectric boundary. Such resonators can exhibit very high quality (Q) factors, since the resonator surface can maintain atomic level smoothness. The combination of high Q factors and small resonator volumes has led to a wide range of applications in sensing, optomechanics, nonlinear optics, and quantum optics. In this dissertation, we introduce a new type of whispering gallery resonators (WGRs) based on micro-droplets in an immiscible liquid-liquid system. Within such an all-liquid platform, it is possible to achieve highly nonlinear coupling between light and liquid that can potentially lead to single-photon level optical nonlinearity. Specifically, we experimentally characterize a droplet (D~500um) of index matching fluid submerged in the water as a high-Q optical resonator, where we use an optical fiber taper to couple light into the droplet through non-contact evanescent coupling. The highest Q-factor observed in the experiment is 2x10^7 which closely matches the upper limit of intrinsic Q-factor set by the material absorption. Given with such a high Q factor, the WGM can exert strong radiation pressure on the droplet interface, push it outward, increase the length of optical path, and produce a red-shift in WGM resonance. Our experimental results have found that the ratio of those resonance shifts and the optical power coupled into the resonator is approximately 60 fm/μW. The result closely matches to our steady-state estimation based on solving the coupled Maxwell-Navier-Stokes equation. To investigate the dynamic interplay of light and liquid, we develop a harmonic oscillator (HO) model to describe the time-domain behaviors of the coupled optofluidic system. We find a good agreement between theoretical predictions and our experimental data. The shift of WGM resonance can potentially be induced by thermal effects. To estimate the magnitude of thermal effects, we also investigate the thermally induced nonlinear behaviors of WGMs in a cylindrical fiber resonator (D~125um), where we change the mechanism of heat dissipation by changing the cladding material (e.g. air and water). For direct temperature measurements, we use a fiber optical resonator with a fiber Bragg grating (FBG) inscribed in the fiber core to observe temperature shifts induced by the high-Q WGMs. Our result shows that the temperature increase in the fiber resonator in the water is 0.13 C, whereas the fiber resonator in air shows ~4.5 C increase in temperature. Our results suggest that the relatively high thermal conductivity of water suppresses thermal nonlinearity by ~50 times, and that the red-shifts of WGMs can largely be attributed to radiation pressure effect. / Ph. D.