The use of multimode optical fibre as an intrinsic
chemical sensor, with application in on-line analysis in
the process industry, is described. The technique of
attenuated total reflection spectroscopy is applied to the
unclad section of the fibre which is in contact with the
chemical being detected. A model based on selective mode
propagation is developed to relate the evanescent
absorption coefficient of the fibre probe to the bulk
absorption coefficient of the absorbing species.
An experimental system was constructed to verify the
theoretical model. Evanescent wave absorption in an aqueous
dye solution was performed using multimode fused silica
fibre which was unclad at the sensing region. In order to
produce modes close to cutoff in the sensing region,
tunnelling modes were launched into the clad lead-in fibre.
The measured evanescent absorbance of the dye solution was
found to vary linearly with the exposed core length and to
exhibit a square root dependence on concentration. The
former effect is predicted from theory while the latter is
attributed to adsorption on the core surface which obeys a
Debye-Huckel-type concentration dependence. In adddition, a
concentration enhancement of two orders of magnitude was
observed due to this adsorption.
Because the adsorption process is of an irreversible
nature, the potential for evanescent wave spectroscopy on
fused silica fibre is severly limited for ionic solutions.
The effect may be exploited, however, in disposable probes
to give increased sensitivity due to the enhanced surface
concentrations.