|dc.description.abstract||The work in this thesis is concerned with the development of optical sensors for the
structural health monitoring of systems with inaccessible components in the presence of
ionising radiation. Two primary approaches to determine structural health are
investigated, these being the implementation of distance measurement sensors (to assess,
for example, the occurrence of creep/cracking) and gas sensors (to assess the occurrence
of outgassing which can be indicative of chemical ageing).
Distance measurement sensors are developed using fibre optic Fabry-Pérot interferometry
(FPI) measured in reflection. A fast-Fourier transform (FFT) of the reflected interference
spectrum is incorporated to demodulate the signal and extract cavity length information.
To mitigate noise and enhance measurement accuracy and sensitivity, spectrum
reconstruction in the form of a function-fitting algorithm (FFA) is developed, the input
of which is fed by the FFT output. The FFA demonstrates measurement improvements
of approximately one-and-a-half orders of magnitude.
To situate the fibre FPI sensors within spatially confined, closely positioned components,
turning mirrors are fabricated on two cores of multi-core fibre (MCF) which redirect the
fibre-guided light perpendicular to the fibre axis in opposite directions. This allows for
the absolute distance between component parts to be determined as Fabry-Pérot cavities
are formed on both sides of the MCF. Three different turning-mirror fabrication
processes are developed, the results from each are compared and discussed.
The distance measurement capabilities are expanded to that of a two-point measurement
system to allow for tilt measurement competencies. This facilitates further
comprehensiveness with regards to structural health monitoring. Further, two optical gas
sensors are investigated, both based on the principle of absorption spectroscopy. One
consists of evanescent wave generation in a tapered fibre and the other of incoherent
broadband cavity-enhancement. Modelled and experimental results are presented and
limitations are discussed.||en