Fabrication and characterisation of Bragg grating based polymeric optical devices

Abstract

The work presented in this thesis is mostly located in the field of polymer optical fibre (POF) based sensors. Polymeric materials are flexible, biocompatible, they have higher failure strain, and they have lower Young modulus compared with silica materials. The latter characteristic renders POF Bragg grating (POFBG) sensors more sensitive to fibre stress and pressure compared with the conventional silica based sensors. However, the POF technology is not mature enough and it has some drawbacks, including the high optical attenuation, poor fibre handling and connectorisation methods, and time-consuming POFBG inscription procedures. In this thesis, these issues are addressed. First, the attenuation effects on the performance of POFBG and Bragg grating based Fabry-Pérot (FP) interferometric sensors were comprehensively investigated and discussed. Then, POFBG inscriptions with the common He-Cd and the KrF laser systems were experimentally demonstrated. In the latter case, a POFBG inscription with only one laser pulse is presented for the first time, which is the fastest Bragg grating inscription time reported until now. A demountable POF connectorisation method with low connection loss and a simple, well-controlled thermal annealing process applied on POFBGs were also demonstrated in this thesis. The thermal annealing was initially used for multiplexing purposes due to the ability of fibre to shrink in length under specific thermal exposure. The fibre shrinkage can blue-shift the Bragg wavelength of the POFBG permanently. In this thesis, it is shown for the first time that shifting the Bragg wavelength to longer wavelengths permanently is also feasible. The fibre annealing was also used to improve the lifetime and the performance of POFBG devices. The sensors fabricated in this work were used to develop novel applications, such as ultrasound detection, liquid level monitoring, and direct erythrocyte concentration measurement.