This paper proposes a new generic technique for fiber optic chemical sensing, discusses experiments which demonstrate the technique, and briefly treats possible extensions and applications. Our results indicate a more than 100-fold increase in the coupling of energy from a chemically sensitive coating to guided modes in the fiber core, when compared with other intrinsic sensor designs. Previous reports of distributed sensors making use of chemically sensitive dyes in optical fiber claddings describe sensors which depend on the direct coupling of light between the cladding and guided modes in the fiber core. This approach, particularly when used for distributed fluorescence sensors, is limited in sensitivity since coupling takes place through the evanescent field "tails" of the fiber's core-modes, which extend only a short distance into the fiber cladding and do not correspond well with the optical properties of light in the cladding. We describe an approach which can greatly increase the amount of energy coupled from fluorescent sensor molecules in a fiber coating to guided modes in the fiber core. In this approach, fluorescent molecules of a second type are present in the fiber core, absorb the fluorescence emission from the molecules in the sensor coating, and emit their own fluorescence in the core of the fiber. The design of the fiber maximizes the amount of this secondary fluorescence that is captured and subsequently guided by the fiber. The proportionately smaller amount of coating emission which is coupled into the fiber core by evanescent field effects is also converted to secondary fluorescence. Preliminary experiments have demonstrated this "two-stage fluorescence" approach using an oxygen-sensitive dye in a permeable silicone coating placed on the outside of a plastic fluorescent-core fiber.

Date Published: 19 January 1989
PDF: 7 pages
Proc. SPIE 0990, Chemical, Biochemical, and Environmental Applications of Fibers, (19 January 1989); doi: 10.1117/12.959980

Published in SPIE Proceedings Vol. 0990:
Chemical, Biochemical, and Environmental Applications of Fibers
Robert A. Lieberman; Marek T. Wlodarczyk, Editor(s)