Two Fibers Enhance Monitoring

Michael D. Wheeler

Fiber optic sensors containing probes that react to a triggering event such as a chemical spill can guard against widespread pollution from industrial plants. However, the fluorophores must be separated to accurately monitor their response to an excitation pulse, limiting spatial resolution. Researchers at the University of Wisconsin solved this problem with a monitoring system that uses a second fiber to transmit the fluorescent response.

A two-fiber setup avoids the spatial resolution limits of traditional optical fiber sensors. The fibers are evanescently coupled, enabling the fluorophore sensor regions to be spaced more closely than dictated by their fluorescence lifetimes.

"The fibers are effectively coupled 'evanescently,' with the coupling being 'mediated' by the fluorophore," explained lead researcher Peter Geissinger. One fiber contains sensor regions that can be fractions of a millimeter apart, and the second contacts the fiber at these sensor regions, enabling evanescent excitation of the fluorophores. Once fluorescence occurs, the emitted light can be detected through this second fiber by the evanescent fields in the fiber cladding.

In this setup, the spacing of the sensor regions is no longer dictated by the fluorescence lifetimes on one fiber. With the second fiber periodically contacting the first, the distance between the adjacent sensor regions along the second fiber is large enough to delay the light pulses.

Monitoring and testing

Geissinger's group constructed four setups aimed at testing different aspects of the evanescent fiber-to-fiber coupling scheme. All hold promise in improving the resolution of distributed fluorescent sensors for environmental monitoring.

Another prospective application is in pharmaceutical testing. Vast numbers of compounds created in discrete regions on optical fibers could be combined, synthesized and read out optically to determine their potential effectiveness as drugs.

Details of the work appear in the March 1 issue of Analytical Chemistry.