On the pH and concentration response of an evanescent field absorption sensor using a coiled-shape plastic optical fiber

Evanescent field absorption coefficient has been measured in a coiled-shape plastic optical fiber sensor in presence of methylene-blue dye. This measurement shows that the evanescent field absorption coefficient is a nonlinear function of the dye concentration. The nonlinearity might be due to adsorption or accumulation of dye molecules on the core surface of the fiber. Dependence of the evanescent field absorption coefficient on pH level has also been investigated, which indicates non-linearity increases for higher pH levels.