Speciality polymer optical fibres for sensing applications

Abstract

The ultimate goal of our research is to develop an all-polymer fibre optic system for biomedical sensing application. This thesis introduces a paradigm shift in polymer optical fibre sensors that the light source and sensing elements are combined in a single laser-dye-doped polymer optical fibre without a coupling device. The light source is created by pumping the fibre with low-cost diode-pumped solid-state (DPSS) laser at any point along the fibre. Moreover, the sensing elements are allowed to be created anywhere along the POF as well, providing a low-cost technology platform for disposable biomedical sensors. Our POFs are drawn by the preform drawing method. A reliable POF draw tower has been designed and built, which can handle prefroms with diameter ranging from 6mm to 40mm and POF with diameter less than 3% tolerance. A specific pre-polymerization procedure is necessary to prevent bubble formation during the early stage of the polymerization of monomers. The light source is created when the POF is doped with organic laser dye and pumped by a low-cost DPSS laser below 50mW by side-illumination fluorescence (SIF) technique. The Rh640 doped POF with cladding structure is fabricated and it is also calculated the optical conversion efficiency of the fibre is 1.509% with the laser dye concentration of 10ppm. The SIF signal can be increased by tapering or bending method which is further studied experimentally, such that the fluorescence power in the fibre core is at least 4 times greater than that was achieved when side-illuminating without bends. The fluorescence power was further improved to greater than 10 times for multiple bends. The modelling of bending mechanism is done as well which supports and explains the experimental results. There are two novel pumping schemes, based on a Switch-pumping (SwP) technique and a Scan-pumping (ScP) technique with single or multiple laser diodes, are invented to overcome the power degradation problems arising from the photobleaching of the dyes. The total output power deterioration is less than 10% when the dye doped POF has been continuously pumped by the laser diode(s) to give 1μW optical output for 90min.