Enhancement of evanescent fluorescence from fiber-optic sensors by thin-film sol-gel coatings

Abstract

A theoretical analysis and an experimental demonstration are presented to show the increase in collected evanescent fluorescence for a fiber-optic sensor having a high refractive-index (nr), titanium sol-gel, thin-film coating. Simulations indicated that the maximum collected fluorescence should increase and shift to smaller film thicknesses as nr increases and also predicted an interference color-filtering effect. Experimentally, collected fluorescence increased by as much as 6× over that from a bare fused-silica fiber having a numerical aperture of 0.60. Simulations and experimental data were consistent with a decrease in the effective nr as film thickness increases. Electron micrographs of the sol-gel structure supported this observation and showed that the structure differs significantly from that of films formed on a planar glass substrate. High nr, sol-gel thin films are a potentially inexpensive approach to significantly increasing the signal from fiber sensors.