City Research Online

Abstract

As a rapid progressing technical field, fibre-optic sensors based on different schemes can work properly over a wide temperature range and sense the change of many important parameters. In this thesis, two major types of fibre optic sensors are comprehensively investigated, which are the fibre thermometer based on the fluorescence lifetime detection scheme of some specific fluorescent materials and fibre optic sensors based on the wavelength interrogation scheme of some specific fibre Bragg gratings.

As well known, these two types of fibre optic sensors are normally most suitable for comparatively low or medium temperature range. Difficulties still exist for these two types of sensors to work properly at an extended higher temperature region. To enable these types of sensors to operate efficiently and stably at higher temperature regions, some key problems remain to be solved. The aim of the work in this thesis is to extend their working temperature range to above 800 °C by exploring some novel fluorescent materials and novel photosensitive optical fibres.

In the aspect of fluorescence based fibre thermometer, several types of fluorescent materials were tested to evaluate their fluorescence performance, and the Tm doped YAG crystal was finally chosen to act as the fluorescent sensing material in a new developed fibre thermometer after extensive test of its fluorescence characteristics. The experiments carried out in the laboratory confirmed that the crystal has a monotonic and smooth fluorescence lifetime decay characteristics from room temperature to 1200 °C and an observable strong fluorescence intensity over such a temperature range. A novel fibre thermometer has been developed by using the Tm doped YAG as the sensing material and a phase locked detection (PLD) circuit as the signal processing scheme. The results obtained after comprehensive tests performed in laboratory and industrial fields clearly indicate that the fibre thermometer has very good long term performance stability, fast dynamic response and high temperature measurement resolution.

In the aspect of FBG-based sensors, the focus has been on the development of novel photosensitive fibres, which can be used to fabricate strong FBGs with superior high temperature sustainability. By co-doping Sb or In into germanium doped silica fibres, several new photosensitive fibres have been developed by means of modified chemical vapour deposition (MCVD) method. Strong gratings have been fabricated by exposing pieces of these fibres to the laser emission of an UV excimer laser working at 248 nm. Annealing tests on these gratings confirmed that the gratings were able to survive an annealing temperature of 800 V or even 900 °C for 24 hours or more and still retain an observable reflectivity in the reflective spectrum. The results obtained from these tests fully show the potential of using these gratings in industrial applications.

Based on the extensive experimental tests on the photosensitivity of the fibres and on the high temperature sustainability of the gratings, the mechanisms concerned with the decay of the gratings and the photosensitivity of the fibres were further investigated. A novel theoretical model, named cation hopping, was presented to account for the experimental results obtained. According to this model, the vacancies existing in germanosilicate fibre are important in acquiring a high degree of photosensitivity, and the cations hopping away from the original localized positions during UV emission exposure are responsible for the changes in the refractive index.

As an important application of fibre optic sensors, heat flux measurement -was performed by using both the fluorescent lifetime-based fibre thermometer and FBG-based fibre optic sensors. Results thus obtained show the great prospect of using these fibre sensors in actual industrial applications.

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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