Experimental studies on the response of the fiber optic evanescent field absorption sensor

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 depend on the pH of the sample. As the pH decreases, the amount of nonlinearity and evanescent absorbance decrease. it has been shown that the nonlinearity and high evanescent absorbance at high pH value occur because of the adsorption of the positively charged dye molecules on the surface of the silica core of the plastic-clad silica (PCS) fiber, which is generally used for the sensor. A linear response has been obtained 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. The 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 first term is due to the simple Beer's law while the second term is the contribution of the pH dependent dye adsorption In addition, we have found that the increase in cope diameter increases the contribution of adsorption.