Long-term stability tests of intrinsic Fabry-Perot optical fibre sensors at high temperatures
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The main aim of this thesis is to develop and gain a better understanding of intrinsic Fabry-Pérot optical fibre sensors in high temperature environments. This thesis will describe the characteristics, manufacturing process, and applications of intrinsic Fabry-Pérot optical fibre sensors and their suitability for temperature sensing in high temperature environments. Intrinsic Fabry-Pérot optical fibre sensors with different types of sensing elements were manufactured and investigated throughout this project. The types of sensors differ at the dopant of their Fabry-Pérot cavity (sensing element), such as Gedoped core, F-doped depressed cladding and pure SiO2. Their long-term phase stability response at temperatures up to 1150oC over up to 4 months continuously monitoring is presented. Most promising results were given by pure SiO2 sensors up to 1000oC with a minimum temperature drift of less than 1oC. Above that temperature, all types of sensors showed temperature drifts from 20oC up to 100oC due to permanent changes of the core refractive index. At elevated temperatures, permanent changes of the core refractive index arise due to dopant diffusion in the optical fibre sensor, from core to cladding and vice versa, as well as tapering phenomena leading to phase response drifting and modulated behaviours. The presence of dopant in the Fabry-Pérot cavity proved to affect the phase stability of the sensors, especially for temperatures above 1000°C. An investigation of how dopant diffusion affects the core radius by making it larger is presented in this thesis as well. Using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray (EDX) Spectroscopy, optical fibre sensors tested at temperatures up to 1150°C over long periods are investigated for dopant diffusion. After 85 days at temperatures above 900°C, germanium concentration in the core of the sensor has been dropped down to the 60% of its initial dopant concentration. Through calculations of FWHM and germanium wt. % concentration, the 40% of germanium dopant that have been diffused towards the cladding expanded the core radius by 0.9μm. This can lead to expanding tapering phenomena along the optical fibre and transformation of the single mode core to a multimode core, for specific wavelengths. Also, a modelled theoretical analysis of a 2nd order mode cavity interference in the fundamental cavity has been conducted proving its correlation with the modulated phase response at high temperatures.