Studies of measurement techniques for indirect chemical sensing based on fluorescence spectroscopy and applications for fibre-optic sensors
This thesis describes experimental and theoretical studies of interrogation systems for determining fluorescent decays of order a few microseconds. The studies have enabled optimised design of interrogators for sensing oxygen using a fluorescent polymerencapsulated ruthenium complex. Two basic interrogation methods were explored, using blue LED excitation. The Rapid Lifetime Detection (RLD) scheme, a fluorescence interrogation method based on direct interrogation of the decay curve following pulsed excitation was generalised, and a novel method for optimising measurement precision derived. The effect of background light on the optimum was quantified. Dissolved (aqueous) oxygen concentration was measured to a precision of 1 part per billion using a 1 second response time (the peak fluorescence power was only 12.5±0.5pW). A second interrogation method, where the phase delay between an intensity modulated excitation source and the resultant fluorescence is processed to make measurements, was for the first time, fully analysed for measurement of exponential decays. When measuring fluorescence lifetimes in the range 2.9-3.3μs, a precision of 2.3 x 10-10 s Hz-0.5 was achieved. (The peak fluorescence power was 500±25pW). A novel combination of ruby optical temperature sensor insert and oxygen sensing layer was demonstrated as a simultaneous temperature and oxygen sensor. A new fluorescence calibration standard consisting of thermally stabilised titanium-dopedsapphire sample was constructed to calibrate and test the indicators. This work was sponsored by a BRITE EuRam European project, which helped determine the priorities of the research.