Characteristic and sensing properties of near- and mid-Infrared optical fibres
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The work within this thesis investigates the characteristics and sensing properties of novel near- and mid-infrared tellurite and germanate glass fibres and their potential as sensing elements. An asymmetric splicing method for fusion-splicing tellurite and germanate glass fibres to standard silica fibre is demonstrated. The thermal and strain sensing properties of these glass fibres have been studied by analysing the properties of optical fibre Fabry-Perot cavities, which were formed when these high refractive index fibres were spliced to silica fibre, and fibre Bragg gratings. Using fibre F-P interferometer, the normalized thermal sensitivity of tellurite and germanate fibre was measured to be 10.76×10-6/°C and 15.56×10-6/°C respectively, and the normalized strain sensitivity of tellurite and germanate fibre was also measured with values of 0.676×10-6 /με and 0.817×10-6 /με respectively. These results show good agreement with measurements using fibre Bragg gratings in these fibres and are reasonably consistent with the values predicted using available published data for glasses of similar compositions. Tellurite and germanate glass fibres show potential as thermal sensing and load sensing elements compared with silica fibre. The design of an evanescent field gas sensor using tapered germanate fibre for methane gas species detection was investigated and modelled. This model shows the maximum gas cell length (sensing fibre length), detectable gas concentration range, and required gas cell length range for the expected minimum detectable gas concentration of a fibre evanescent field sensor, which gives guidance for the effective gas cell length choosen according to different minimum detectable gas concentration requirement in practise. The investigation of tellurite and germanate glass fibre characteristics and sensing properties offer guidance for their applications in sensing areas.