Bd. Gupta et Sk. Khijwania, EXPERIMENTAL STUDIES ON THE RESPONSE OF THE FIBER OPTIC EVANESCENT FIELD ABSORPTION SENSOR, Fiber and integrated optics, 17(1), 1998, pp. 63-73
In the present study the response of the fiber optic evanescent field
absorption sensor has been analyzed experimentally. The dependence of
evanescent absorbance on sample (dye) concentration has been found to
be nonlinear, in contradiction to theoretical predictions. The amount
of nonlinearity as well as evanescent absorbance has been found to dep
end on the pH of the sample. As the pH decreases, the amount of nonlin
earity and evanescent absorbance decrease. it has been shown that the
nonlinearity and high evanescent absorbance at high pH value occur bec
ause of the adsorption of the positively charged dye molecules on the
surface of the silica core of the plastic-clad silica (PCS) fiber, whi
ch is generally used for the sensor. A linear response has been obtain
ed at pH close to 2.0 at which the presence of a tremendous amount of
H+ ions in the sample restricts the adsorption of the dye molecules. T
he following empirical relation has been found to fit the experimental
data: gamma = 1.1C + 1.6(pH)C-3/2(1/3) where gamma is the evanescent
absorption coefficient and C is the concentration of the dye. The firs
t term is due to the simple Beer's law while the second term is the co
ntribution of the pH dependent dye adsorption In addition, we have fou
nd that the increase in cope diameter increases the contribution of ad
sorption.