Characterization of a quasi-distributed optical fiber chemical sensor

A distributed optical fiber pH sensor based on time resolved fluorescence is investigated. 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 N ₂ laser, produced 1 ns light pulses at 490 nm which were launched into a 1 by 2 optical fiber coupler. A 100 meter fiber with eight sensor sites was spliced to the coupler. The indicator molecules were excited via the evanescent wave. Part of the fluorescent emission 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 in the range 6 - 12. The system appears to have potential for applications in environmental and safety monitoring.