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Abstract

Starting with a brief, yet comprehensive review of optical fibre Bragg gratings (FBGs) and their various sensing applications, this thesis covers the characterisation and high-temperature sensing potential of fibre Bragg gratings written in specialised optical fibres. Following work done to optimise the grating-writing system set-up, different types of FBGs (type I and type IIA) were written in various photosensitive fibres (germanium and boron-germanium doped fibres) including in several specially fabricated photosensitive fibres (tin-erbium-germanium, erbium- high germanium and antimony-erbium-germanium doped fibres). The gratings were fabricated through a phase-mask technique by using 248 nm ultra-violet (UV) light from a krypton-fluoride (KrF) excimer laser. The photosensitivity of the fibres and the on-line growth of the gratings during the fabrication process were studied and compared. The thermal sustainability of the gratings written in various photosensitive fibres, including a chemical composition grating (CCG), were tested and thus the operational ranges of temperature measurement, using these FBGs, were determined and compared.

The thermal stability (reflected in the decay) of the gratings was studied and analysed both in terms of the refractive-index modulation (correlated to the reflectivity of the grating) and the effective refractive index (depending on the blue-shift in the Bragg wavelength of the gratings). Type I uniform gratings written in one of the most UV-photosensitive fibres (boron-germanium doped fibre) and one of the most promising fibres for high temperature sensing applications (tin- erbium-germanium doped fibre) were used in this thermal stability analysis, by adopting the power-law and the master aging curve approach. The operational lifetimes of the gratings at relatively lower temperatures were predicted by the accelerated-aging test.

To extend the work further, the thermal response of the pre-annealed gratings was studied over a wide range of temperatures. Linear and non-linear regressions were fitted to the experimentally measured data and the deviation between the regression and the actual data was compared for each grating. Non-linear fittings were found to reflect the optimised regression for the grating calibration for temperature measurement.

Finally, following a brief introduction to the simultaneous measurement of strain and temperature, a new development is applied for the measurement of both parameters over wider ranges (temperature: 20-600°C, strain: 0-2000|ie). The scheme is based on a combined fluorescence and grating based technique and thus, gratings were written in specially fabricated tin-erbium-germanium and antimony-erbium-germanium doped fibres to demonstrate the technique. A scheme for a strain-independent temperature measurement is also demonstrated by writing type I and type IIA gratings in the same fibre (erbium-high germanium doped fibre).

The significance of the work is considered and the suggestions for future work are made.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QA Mathematics T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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