Optical EngineeringNonlinearity of absorbance with sample concentration and path length in evanescent wave spectroscopy using optical fiber sensors
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The dependence of absorbance on sample concentration and sample length in evanescent wave spectroscopy, carried out by means of weakly guiding step index optical fiber sensors, is investigated theoretically. The absorbance is found to depend on the product of the sample bulk attenuation coefficient (α), the length of the fiber in contact with the sample (L), and the inverse of the normalized frequency (V) of the sensing section of the waveguide. When the mode volume of the fiber sensor is filled, the absorbance is predicted to vary nonlinearly with concentration (or sample length) at low values of αL/V and to increase as the cube root of αL/V at high values. However, if the mode volume of the sensing section of the fiber is only partially filled, a linear dependence of absorbance on αL/V is indicated. The results of the model are compared with experimental data already published by various groups.