Characterization of a quasi-distributed optical fiber chemical sensor

Techniques for distributed optical fiber 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 utilized to immobilize the indicator onto the optical fiber core. Thin (approximately 1 micrometer) porous glass films were deposited on the surface of the optical fiber core with indicator molecules entrapped in the matrix. An OTDR technique was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a N2 laser, produced blue light pulses at 440 nm which were launched into a 1 multiplied by 2 optical fiber coupler. A fiber with eight sensitive sections was splice 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 fiber and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1 by 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.