The main topic of this thesis work is photosensitivity and photo-structuring of optical fibers and bulk glass. Although research in the field of photosensitivity in glass and optical fibers has been ongoing for more than three decades, the underlying mechanisms are still not well understood. The objective was to gain a better understanding of the photo-response by studying photosensitivity from a thermodynamic perspective, as opposed to established research focusing on point defects and structural changes, and strain and stress in optical fibers. Optical fibers was mainly used for experimental studies for two reasons; first, photosensitivity in fibers is more pronounced and more elusive compared to its bulk counterpart, and secondly, fibers provide a simplified structure to study as they experimentally can be seen as one-dimensional.Initially, ablation experiments on bulk glass were performed using picosecond infrared pulses. With a design cross section of 40x40 μm, straight channels were fabricated on the top (facing incident light) and bottom side of the sample and the resulting geometries were analyzed. The results show a higher sensitivity to experimental parameters for bottom side ablation which was ascribed to material incubation effects. Moreover, on the top side, the resulting geometry has a V-shape, independent of experimental parameters, related to the numerical aperture of the focusing lens, which was ascribed to shadowing effects.After this work, the focus shifted towards optical fibers, UV-induced fiber Bragg gratings (FBGs) and thermal processing with conventional oven and with a CO2 laser as a source of radiant heat.First, a system for CO2 laser heating of optical fibers was constructed. For measuring the temperature of the processed fibers, a special type of FBG with high temperature stability, referred to as "Chemical Composition Grating" (CCG) was used. A thorough characterization and temperature calibration was performed and the results show the temperature dynamics with a temporal resolution of less than one millisecond. The temperature profile of the fiber and the laser beam intensity profile could be measured with a spatial resolution limited by the grating length and diameter of the fiber. Temperatures as high as ~ 1750 °C could be measured with corresponding heating and cooling rates of 10.500 K/s and 6.500 K/s.Subsequently, a thorough investigation of annealing and thermal regeneration of FBGs in standard telecommunication fibers was performed. The results show that thermal grating regeneration involves several mechanisms. For strong regeneration, an optimum annealing temperature near 900 C was found. Two different activation energies could be extracted from an Arrhenius of index modulation and Braggv iwavelength, having a crossing point also around 900 °C, indication a balance of two opposing mechanisms.Finally, the thermal dynamics and spectral evolution during formation of long period fiber gratings (LPGs) were investigated. The gratings were fabricated using the CO2 laser system by periodically grooving the fibers by thermal ablation. Transmission losses were reduced by carefully selecting the proper processing conditions. These parameters were identified by mapping groove depth and transmission loss to laser intensity and exposure time. / Huvudtemana i denna avhandling är fotokänslighet och fotostrukturering av optiska fibrer och bulk glas. Trots att forskning inom fotokänslighet i glas och optiska fibrer har pågått under mer än tre decennier är de bakomliggande mekanismerna ännu inte klarlagda. Syftet var att få en bättre förståelse för fotoresponsen genom att studera fotokäsligheten ur ett termodynamiskt perspektiv, i motsats till etablerad forskning med fokus på punktdefekter och strukturförändringar, samt mekaniska spännings effekter i optiska fibrer. Optiska fibrer användes för flertalet av de experimentella studierna av två skäl; för det första är fotokänsligheten i fibrer större och dessutom vet man mindre om bakomliggande mekanismer jämfört med motsvarande bulk glas, och för det andra kan fibrer vara enklare att studera eftersom de experimentellt kan ses som en endimensionell struktur.Inledningsvis utfördes ablaherings experiment på bulk glas med en infraröd laser med pikosekund pulser. Raka kanaler med ett designtvärsnitt på 40x40 μm tillverkades på ovansidan (mot infallande ljus) och bottensidan av provet och de resulterande geometrierna analyserades. Resultaten visar en högre känslighet för variationer i experimentella parametrar vid ablahering på undersidan vilket kan förklaras av inkubations effekter i materialet. Dessutom är den resulterande geometrin på ovansidan V-formad, oavsett experimentella parametrar, vilket kunde relateras till den numeriska aperturen hos den fokuserande linsen, vilket förklaras av skuggningseffekter.Efter detta arbete flyttades fokus mot optiska fibrer, UV inducerade fiber Bragg gitter (FBG), och termisk bearbetning med konventionell ugn samt även med en CO2-laser som källa för strålningsvärme.Först konstruerades ett system för CO2-laservärmning av fibrer. För mätning av temperaturen hos bearbetade fibrer användes en speciell sorts FBG med hög temperaturstabilitet, kallade ”Chemical Composition Gratings” (CCG). En grundlig karaktärisering och temperaturkalibrering utfördes och temperaturdynamiken mättes med en tidsupplösning på under en millisekund. Temperaturprofilen i fibern, och laserns strålprofil, kunde mätas med en spatiell upplösning begränsad av gitterlängden och fiberns diameter. Temperaturer upp till ~1750 °C, vilket är högre än mjukpunktstemperaturen, kunde mätas med korresponderande uppvärmnings- och avsvalningshastighet på 10.500 K/s och 6.500 K/s.Därefter gjordes en omfattande undersökning av värmebearbetning och termisk regenerering av FBG:er i telekomfiber. Resultaten visar att termisk gitter-regenerering aktiveras av flera olika mekanismer. Värmebearbetning vid en temperatur omkring 900 °C resulterade i starka gitter efter en regenerering vid en temperatur på 1100 °C. Två olika aktiveringsenergier kunde extraheras från en Arrhenius plot avseende brytningsindexmodulation och Braggvåglängd, med en skärningspunkt tillika runt 900 °C, vilket indikerar en avvägning mellan två motverkande mekanismer vid denna temperatur.Slutligen undersöktes temperaturdynamiken och de spektrala egenskaperna under tillverkning av långperiodiga fibergitter (LPG). Gittren tillverkades med CO2-vi iilasersystemet genom att skapa en periodisk urgröpning medelst termisk ablahering. Transmissionsförluster kunde reduceras med noggrant valda processparametrar. Dessa parametrar identifierades genom mätningar av ablaherat djup och transmissionsförlust som funktion av laserintensitet och exponeringstid. / <p>QC 20150924</p>

Ao longo dos últimos anos, redes de Bragg em fibras ópticas (FBG, do inglês Fiber Bragg Gratings) vêm sendo frequentemente utilizadas como sensores de deformação e de temperatura. O problema da indistinguibilidade entre esses dois parâmetros físicos, presente durante medidas realizadas por esse tipo de sensor, tem sido bem resolvido com o uso de duas FBGs com comprimentos de onda distintos. Muito embora esse artifício tenha apresentado bons resultados, ele também oferece algumas desvantagens, sendo uma delas a necessidade de duas fontes de luz para diferentes comprimentos de onda. Em virtude disto, este trabalho apresenta um sistema capaz de realizar medidas de temperatura e deformação, simultaneamente, utilizando apenas uma fonte de luz. O método baseia-se na inscrição de duas redes de Bragg com comprimentos de onda próximos (no caso, 810 e 860 nm) na mesma posição da fibra óptica. Apesar de a separação entre os comprimentos de onda das FBGs ser aparentemente pequena (cerca de 50 nm), o sistema respondeu precisamente a variações de deformação e temperatura. Dessa forma, a utilização de apenas uma fonte de luz no sistema é corretamente justificada, uma vez que, com essa alteração, o custo do sistema é substancialmente reduzido. Ademais, o uso de comprimentos de onda em torno de 800 nm também barateia o sistema, pois os CCDs usados neste intervalo espectral são menos onerosos do que aqueles tradicionalmente usados em comprimentos de onda de comunicações ópticas (1,55 &mu;m). / In recent years, fiber Bragg gratings (FBGs) have been frequently used as strain and temperature sensors. Several studies have tackled the problem of distinguishing between these two physical parameters using a dual-wavelength sensor. Although these sensors have shown good results, they have a few drawbacks, one of them being the need for two light sources with different wavelengths. We present an approach for simultaneous strain and temperature sensing which uses only one light source. The method relies on writing FBGs with nearby wavelengths (for instance, at 810 and 860 nm) at the same section of the fiber. Even though the Bragg wavelengths are separated by just a few nanometers (about 50 nm), it is possible to accurately measure variations in strain and temperature. One of the major advantages of this approach is the use of a unique light source, what reduces substantially the system cost. Another advantage is the lower cost of array detectors at 800 nm when compared to those of telecom wavelengths (1,55 &mu;m).

A novel detection technique to estimate the amount of chirp in fiber Bragg gratings (FBGs) is proposed. This method is based on the fact that reflectivity at central wavelength of FBG reflection changes with strain/temperature gradient (linear chirp) applied to the same. Transfer matrix approach was used to vary different grating parameters (length, strength and apodization) to optimize variation of reflectivity with linear chirp. Analysis is done for different sets of ‘FBG length-refractive index strength’ combinations for which reflectivity vary linearly with linear chirp over a decent measurement range. This work acts as a guideline to choose appropriate grating parameters in designing sensing apparatus based on change in reflectivity at central wavelength of FBG reflection. A novel high sensitive FBG strain sensing technique using lasers locked to relative frequency reference is proposed and analyzed theoretically. Static strain on FBG independent of temperature can be measured by locking frequency of diode laser to the mid reflection frequency of matched reference FBG, which responds to temperature similar to that of the sensor FBG, but is immune to strain applied to the same. Difference between light intensities reflected from the sensor and reference FBGs (proportional to the difference between respective pass band gains at the diode laser frequency) is not only proportional to the relative strain between the sensor and reference FBGs but also independent of servo residual frequency errors. Usage of relative frequency reference avoids all complexities involved in the usage of absolute frequency reference, hence, making the system simple and economical. Theoretical limit for dynamic and static strain sensitivities considering all major noise contributions are respectively of the order of 25 pε/ Hz and 1.2nε /

In this thesis, a method to improve survivability is developed for fiber Bragg gratings under high velocity impact in dynamic body armor shape sensing applications by encasing the fiber in silicone. Utilizing the slipping of the fiber within the silicone channel, a proportionality relationship between the strain of the fiber to the acceleration of the impacting projectile is found and is used to obtain the rate of the back-face deformation. A hybrid model is developed to handle errors caused by the stick-slip of the fiber by fitting an inverse exponential to stuck sections found in a captured strain profile and double integrated to transform the stuck section to its equivalent slipping. Displacement errors below 10% was achieved using the hybrid model. A graphical user interface with a step-by-step walkthrough and a fiber Bragg grating interrogation system was designed for test engineers to utilize this technology. Test engineers from the Army Test Center in Aberdeen, MD were trained on this technology and successfully captured and processed shots using this technology. A method for cutting Silicon through wire-EDM machining is developed by utilizing the photoconductive properties of Silicon. Cut rates for unilluminated and illuminated Silicon was compared and a 3x faster cut was achieved on the illuminated cuts.

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No description available.

The higher-order modes (HOMs) of an optical fiber has been demonstrated as a new dimension to transmitting signals with the development of mode-division multiplexing (MDM) technique. This dissertation aims to explore the HOMs as an extra degree of freedom for device innovation. In particular, with femtosecond (FS) laser point-by-point (PbP) inscription technique which opens up a unique possibility to explore the HOMs for device innovation, we design, fabricate, and characterize novel-structured fiber Bragg gratings (FBGs) written in the step-index two-mode fibers. We also develop a numerical model for the PbP gratings which has the potential for inverse design problem. Chapter 2 begins with a general framework of MDM with adaptive wavefront shaping in few-mode fibers (FMFs) and multimode fibers (MMFs), followed by two examples in slightly more detail. The fabrication setup and an short overview of the FS laser system will also be covered. In Chapter 3, we show the design, fabrication, and characterization of off-axis Bragg gratings in a step-index two-mode fiber (TMF). Through measuring the transmission and reflection spectra along with the associated reflected mode intensity profiles under different input polarization, we experimentally investigate the off-axis TM-FBGs (FBGs in a TMF) with multiple characteristics reported for the first time to our best knowledge. To highlight, we report the laser-induced birefringence exhibits strong offset dependence, the reflectivity heavily depends on the offset and polarization, and particularly the mode pattern can be controlled solely through polarization. The design and characterization of cross-axis TM-FBGs are presented in Chapter 4. Specifically, these gratings show six primary reflection peaks, which are identified through mode-decomposition based on the intensity profiles through nonlinear optimization problem. We also show in this chapter the development of a numerical model for the general PbP gratings, implementation of this model into standard coupled-wave analysis shows reasonable agreement to the experimental findings. In Chapter 5, discussions and suggestions for future studies are given. / Doctor of Philosophy / The higher-order modes (HOMs) of an optical fiber has been demonstrated as a new space for signal transmission, in the ``mode space'' one can use the modes as distinct multiplexing channel and therefore increase the data capacity of a single fiber. This work aims to explore if the the higher-order modes can also add some extra degree of freedom for device innovation. In particular, we use femtosecond (FS) laser point-by-point (PbP) technique for device fabrication, since the structural change induced by this fabrication methods is highly localized, typically ranging from a few hundred nanometers to a few micrometers. Hence this particular fabrication technique offers a unique possibility of exploiting the HOMs for device innovation. In this work, we fabricate, and characterize fiber Bragg gratings (FBGs) with novel structural designs written within the step-index two-mode fibers, with multiple characteristics reported for the first time as far as we know. We also develop a numerical model for the PbP gratings which has the potential for inverse design problem.

Structural health monitoring (SHM) has become a strong national interest because of the need of reliable and accurate damage detection methods for aerospace, civil and mechanical engineering infrastructure. Health monitoring of these structures usually requires the sensors to have such features as large area coverage, maintenance free or minimum maintenance, ultra-low cost per measurement point, and capability of operation in harsh environments. Fiber Bragg grating (FBG) has attracted considerable interest for this application because of its compactness, electromagnetic immunity, and excellent multiplexing capability. Several FBG multiplexing techniques have been developed to increase the multiplexing number and further reduce the unit cost. To the author's best knowledge, the current demonstrated maximum multiplexing number are 800 FBG sensors in a single array using optical frequency domain reflectometry (OFDR), whose maximum fiber span is limited by the coherence length of light source. In this work, we proposed and demonstrated a wavelength-scanning time-division multiplexing (WSTDM) of 1000 ultra-weak FBGs for distributed temperature sensing. In comparison with the OFDR method, the WSTDM method distinguishes the sensors by different time delays, and its maximum operation distance, which is limited by the transmission loss of the fiber, can be as high as tens of kilometers. The strong multiplexing capability and low crosstalk of the ultra-weak FBG sensors was investigated through both theoretical analysis and experiment. An automated FBG fabrication system was developed for fast FBG fabrication. With this WSTDM method, we multiplexed 1000 ultra-weak FBGs for distributed temperature sensing. Besides the demonstrated temperature measurement, the reported method can also be applied to measure other parameters, such as strain, pressure. / Ph. D.

Essa dissertação de mestrado é dedicada ao estudo, desenvolvimento e caracterização da resposta dinâmica de transdutores de deformação mecânica e de temperatura baseados em grades de Bragg inscritas em fibras ópticas (FBG). Como não se encontram referências diretas na literatura da área sobre a caracterização da resposta dinâmica de sensores de temperatura e deformação a FBG, percebeu-se a importância de realizar um trabalho com ênfase nesse aspecto. O desenvolvimento de transdutores FBG de deformação e de temperatura com resposta dinâmica rápida é de grande importância, já que as grades de Bragg são afetadas, de maneira intrínseca, simultaneamente pelas duas grandezas e que existe um amplo leque de aplicações em que uma delas, ou ambas, variam rapidamente. Nesses casos, diferenças em termos de tempos de resposta entre os sensores utilizados para medição de deformação e os usados para compensação de temperatura podem acarretar em erros significativos nas medidas, o que pode até inviabilizar o uso dos sensores a grades de Bragg em tais aplicações. Este trabalho apresenta um compêndio da teoria da tecnologia das FBG para aplicação em sensores ópticos de deformação e de temperatura. Buscando contribuir para o domínio das técnicas de fabricação de sensores a FBG no Brasil, também são apresentados os resultados experimentais da utilização de grades de Bragg inscritas em fibras ópticas fabricadas no laboratório da Subdivisão de Sensores (EFO-S), da Divisão de Fotônica do Instituto de Estudos Avançados (IEAv) do Comando-Geral de Tecnologia Aeroespacial (CTA), como elementos sensores em ensaios de deformação e de temperatura. Os resultados de caracterizações, tanto de FBG produzidas no IEAv quanto de sensores de deformação e de temperatura comerciais das empresas Micron Optics e Fiber Sensing, foram obtidos por meio de três interrogadores diferentes e são comparados e apresentados neste trabalho. Em uma análise realizada dos resultados apresentados de sensores a FBG, utilizados para medição de temperatura e de deformação mecânica em alguns arranjos experimentais, foram observados comportamentos dinâmicos não lineares que apresentam-se como um obstáculos a serem vencidos para a viabilização da aplicação desta técnica à medição desses parâmetros quando os mesmos são rapidamente variáveis. / This dissertation is devoted to the study, development and characterization of the dynamic response of mechanical strain and temperature transducers based on Bragg gratings inscribed in optical fibers (FBG). Since there are no direct references in the literature on the characterization of the dynamic response of FBG based temperature and strain sensors, it was realized the importance of conducting a study with emphasis on this aspect. The development of FBG strain and temperature transducers with fast dynamic response is of great importance, since the Bragg gratings are intrinsically and simultaneously affected by the two quantities and there is a wide range of applications in which one or both quantities vary rapidly. In such cases, differences in response times between the sensors used for measurement of strain and used for temperature compensation can result in significant errors, which may even preclude the use of FBG sensors in such applications. This work presents a compendium of the theory related to FBG technology for application in optical strain and temperature sensors. Seeking to master the techniques of manufacture of the FBG sensors in Brazil, experimental results from the use of FBG manufactured in the laboratory of Sensors Subdivision (EFO-S), of Photonics Division of Institute of Advanced Studies (IEAv) of General Command for Aerospace Technology (CTA) as sensor elements in strain and temperature trials are also presented. The results of both characterizations, of FBG produced in IEAv and strain and temperature commercial sensors, produced by Micron Optics and Fiber Sensing manufacturers, were obtained from three different interrogators and are compared and presented in this paper. In an analysis of the results of FBG sensors used for measuring temperature and mechanical deformation in some experimental arrangements, nonlinear dynamic behaviors were observed. Such behaviors present themselves as an obstacle to be overcome to achieve the feasibility of applying this technique to measurement of these parameters when they are rapidly changing.

Essa dissertação de mestrado é dedicada ao estudo, desenvolvimento e caracterização da resposta dinâmica de transdutores de deformação mecânica e de temperatura baseados em grades de Bragg inscritas em fibras ópticas (FBG). Como não se encontram referências diretas na literatura da área sobre a caracterização da resposta dinâmica de sensores de temperatura e deformação a FBG, percebeu-se a importância de realizar um trabalho com ênfase nesse aspecto. O desenvolvimento de transdutores FBG de deformação e de temperatura com resposta dinâmica rápida é de grande importância, já que as grades de Bragg são afetadas, de maneira intrínseca, simultaneamente pelas duas grandezas e que existe um amplo leque de aplicações em que uma delas, ou ambas, variam rapidamente. Nesses casos, diferenças em termos de tempos de resposta entre os sensores utilizados para medição de deformação e os usados para compensação de temperatura podem acarretar em erros significativos nas medidas, o que pode até inviabilizar o uso dos sensores a grades de Bragg em tais aplicações. Este trabalho apresenta um compêndio da teoria da tecnologia das FBG para aplicação em sensores ópticos de deformação e de temperatura. Buscando contribuir para o domínio das técnicas de fabricação de sensores a FBG no Brasil, também são apresentados os resultados experimentais da utilização de grades de Bragg inscritas em fibras ópticas fabricadas no laboratório da Subdivisão de Sensores (EFO-S), da Divisão de Fotônica do Instituto de Estudos Avançados (IEAv) do Comando-Geral de Tecnologia Aeroespacial (CTA), como elementos sensores em ensaios de deformação e de temperatura. Os resultados de caracterizações, tanto de FBG produzidas no IEAv quanto de sensores de deformação e de temperatura comerciais das empresas Micron Optics e Fiber Sensing, foram obtidos por meio de três interrogadores diferentes e são comparados e apresentados neste trabalho. Em uma análise realizada dos resultados apresentados de sensores a FBG, utilizados para medição de temperatura e de deformação mecânica em alguns arranjos experimentais, foram observados comportamentos dinâmicos não lineares que apresentam-se como um obstáculos a serem vencidos para a viabilização da aplicação desta técnica à medição desses parâmetros quando os mesmos são rapidamente variáveis. / This dissertation is devoted to the study, development and characterization of the dynamic response of mechanical strain and temperature transducers based on Bragg gratings inscribed in optical fibers (FBG). Since there are no direct references in the literature on the characterization of the dynamic response of FBG based temperature and strain sensors, it was realized the importance of conducting a study with emphasis on this aspect. The development of FBG strain and temperature transducers with fast dynamic response is of great importance, since the Bragg gratings are intrinsically and simultaneously affected by the two quantities and there is a wide range of applications in which one or both quantities vary rapidly. In such cases, differences in response times between the sensors used for measurement of strain and used for temperature compensation can result in significant errors, which may even preclude the use of FBG sensors in such applications. This work presents a compendium of the theory related to FBG technology for application in optical strain and temperature sensors. Seeking to master the techniques of manufacture of the FBG sensors in Brazil, experimental results from the use of FBG manufactured in the laboratory of Sensors Subdivision (EFO-S), of Photonics Division of Institute of Advanced Studies (IEAv) of General Command for Aerospace Technology (CTA) as sensor elements in strain and temperature trials are also presented. The results of both characterizations, of FBG produced in IEAv and strain and temperature commercial sensors, produced by Micron Optics and Fiber Sensing manufacturers, were obtained from three different interrogators and are compared and presented in this paper. In an analysis of the results of FBG sensors used for measuring temperature and mechanical deformation in some experimental arrangements, nonlinear dynamic behaviors were observed. Such behaviors present themselves as an obstacle to be overcome to achieve the feasibility of applying this technique to measurement of these parameters when they are rapidly changing.