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Optical spectroscopy investigation of fiber optic high temperature sensors.


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Bastola, Binod (2018). Optical spectroscopy investigation of fiber optic high temperature sensors. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 139 p.

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Optical fibers have become an integral part of our daily life in a variety of applications, most notably in telecommunications and non-invasive medical diagnostics. Derived products, including fiber Bragg gratings (FBGs) extend the field of applications to e.g. temperature and strain sensors that are already beyond the laboratory proof-of-concept level and that hold the promise to transform many areas of industries in an ever increasing number of applications in aerospace, structural integrity monitoring as well as in-site sensing in turbines and reactors. Especially the latter represent a harsh environment which makes the use of electronic sensors either completely impossible or which implies short service intervals. Often these industries employ optical techniques to conduct measurement in the optical fibers themselves, e.g. based on evanescent fields but only a few fiber-based techniques allow for distributed multiplexed sensing. The idea is to operate different sensors along the length of the fiber at different wavelengths, a concept that also capitalizes on the fact that most optical sensors are passive so that the fiber only carries the signal rather than the power supply. The aforementioned points translate into the many advantages of fiber based sensors: reliability, good sensitivity, multiplexing, and low maintenance costs so that in many industries, the technology is now already seen as a worthwhile alternative to established electronic sensors. Optical fibers are immune to electromagnetic field (EMI), compact and can be functionalized. This collaborative dissertation with an industrial partner specialized in developing practical, end–user–focused commercial sensor solutions, is based largely on fiber optic sensing. While the overall project dealt with glass fiber and Fiber Bragg Gratings (FBG) for temperature sensing at elevated temperature and in otherwise harsh environment, the focus of my work is on the physics and material science of FBGs at elevated temperatures. As typical application example, the atmospheric re–entry, of space vessels exposes materials to extreme thermal conditions under the influence of hypersonic velocity, which lead to critically high temperatures that might affect the structural integrity and thus the operational safety. These temperatures must therefore be monitored at a high repetition rate with sufficient precision to provide real time feedback to the crew. The reason of choosing optical fiber technology is not only because of fibers being themselves relatively inert but also because they are the ideal choice for long–term monitoring with embedding capability in composite structures. So, the thesis primarily addresses the capability of FBG to track temperatures, a property readily known and thoroughly investigated as such at low and moderate temperatures but which holds a couple of surprises and challenges for temperatures above 300 °C. Fiber Bragg gratings are periodic refractive index modulations in the core of a fiber for which the Bragg condition is met at a specific wavelength referred to as the Bragg wavelength. This wavelength is reflected and contains information about the grating. The length of this periodic structure is typically a few millimeters while the distance between two maxima of the refractive index modulation is in the range of 500 nm for an FBG matching the C-band of telecom. The permanent change in the physical characteristics with a spatial periodic modulation of the core index of refraction is most commonly created through transversely exposing the fiber core with a UV–beam and using a phase mask to generate an interference pattern of UV–optical field. The temperature sensitivity of the Bragg wavelength depends on the periodicity of the grating and on the effective refractive index of the fiber core. The dominant contribution to this stems from the temperature dependence of refractive index (thermo–optic effect) as compared to the thermal expansion of silica. At high temperatures, the FBG is however unstable which leads to a loss in reflectivity to the point that the signal becomes undetectable. For specific conditions during the post-fabrication process of the FBG, the reflectivity recovers after a certain time providing what is referred to as a regenerated FBG. These regenerated FBGs have a much better temperature stability but suffer from a substantially reduced reflectivity, typically in the range of 10 –15% of the initial value. While this regeneration process is now widely used, it still lacks fundamental understanding, in particular as the description of FBGs so far relies on a single grating that, once it has been erased, should not regenerate. The fundamental understanding of the regeneration process is furthermore complicated by the complexity of glass as a material system so that e.g., chemical compositions, stress relaxation phenomena, densification etc. are barely understood and models of the underlying mechanisms are still in their infancy. Therefore, in this thesis, I aim at correlating temperature dependent a) FBG parameters with b)in situ Raman spectroscopy and c) photoluminescence data to characterize the optical fibers and FBGs and the way fiber parameters (e.g., hydrogen loading, relaxation of the glass, fiber dopants and stresses) mitigate or impact the FBG regeneration process has been investigated. In the macroscopic study, an Erbium (Er) broadband source between 1530 –1565nm is coupled to an optical spectrum analyzer (OSA) through different fibers containing FBGs and their sensing capacities were investigated by monitoring wavelength shifts with changes in temperature. Annealing behavior of gratings in standard telecom grade fibers were investigated with a dedicated setup that allowed for temperature cycles below 1000 °C. The linear temperature response is influenced upon heating to 900 °C and subsequent cooling of FBGs back to ambient temperature and the observed temperature sensitivity is determined to be approximately 13 pm/K. The calibration curves are obtained to test the characteristics of RFBGs. This was done through a dedicated program in MATLAB to automatically extract all relevant experimental parameters from a time– and temperature–dependent sequence of spectra and led to a new methodology to enhance the recovered reflectivity of FBGs through regeneration between 700 –1000°C, exceeding the state of the art by over 400 %. The objectives of this work are thus to provide a better understanding of the regeneration mechanism from a phenomenological and microscopic perspective as well as suggestions on how to enhance the performance of regenerated FBGs. Regarding the methodology, the observation of the key parameters of FBGs was accomplished in collaboration with our industrial partner. I developed a dedicated Matlab code to improve the precision with which critical parameters could be extracted from the large amount of data which allowed me to systematically investigate the peak width that provides a direct handle on the core refractive index modulation. Still, the fundamental question remained about the mechanism of regeneration or in other words, what provides the memory to generate a new grating after the initial one was erased. I address this challenge through the introduction of a second grating that coexists from the very beginning and which is of opposite phase to the first while having a stronger thermal stability. All qualitative observations are covered through this simple model. With respect to the microscopic identification, Raman spectroscopy allowed me to identify fluorine as an undocumented component of the core in GF1B fibers. A comprehensive description of the glass composition, as it would be required to establish a complete microscopic model, is however still out of reach. In order to push for additional indicators at the microscopic level, I exploited the temperature dependence of a strong visible luminescence from the FBG under excitation of a blue cw laser and report on reversible and irreversible contributions of this luminescence. The two-grating model paves the way towards regeneration efficiencies above 100 %, an exciting perspective worth exploring further. As far as the spectroscopic investigation of the microstructure is concerned, two principle directions should be followed in the near future: for one, it is worthwhile investigating if any of the already observable properties (Raman signature and luminescence) correlates with the regeneration process of FBGs and for the other, it should be possible to extract thermal activation energies for these processes as a fingerprint of defects that can then be compared with independent studies in other fibers and by other experimental techniques.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ruediger, Andreas
Mots-clés libres: Énergie et matériaux
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 09 avr. 2019 21:15
Dernière modification: 09 avr. 2019 21:15
URI: https://espace.inrs.ca/id/eprint/8002

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