Characterisation of a quasi distributed optical fibre chemical sensor

Techniques for distributed optical fibre chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete, distributed signals were obtained. Fluorescein sodium was chosen as a pH indicator for this work because of its well known properties and high fluorescent intensity. A low temperature sol-gel glass manufacturing process was utilised to immobilise the indicator onto the optical fibre core. Thin (-1jim) porous glass films were deposited on the surface of the optical fibre core with indicator molecules entrapped in the matrix. An OTDR technique was employed to obtain the signal at specific positions along the fibre. A dye laser, pumped by a N2 laser, produced blue light pulses at 440 nm which were launched into a 1×2 optical fibre coupler. A fibre with eight sensitive sections was spliced to a 50:50 coupler. The indicator molecules were excited by the blue light via the evanescent wave. Part of the fluorescent light from the indicator molecules was coupled back into the fibre and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1×2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have potential for applications in environmental and safety monitoring.