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.
Résumé
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.
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