On the pH response of fiber optic evanescent field absorption sensor having a U-shaped probe: An experimental analysis

An investigation is made of the response of fiber optic evanescent field absorption sensors in respect of the concentration of dye as well as its pH value. Patent blue VF dye is chosen as the measured sample, and it is found that the evanescent absorbance of the dye is a nonlinear function of dye concentration, which is in contradiction with the theory of Beer-Lambert law of absorption, which states that the absorbance of the sample is linearly related to the sample concentration. Further, it is also observed that the degree of nonlinearity is a function of the pH value of the measured sample; it (the nonlinearity) decreases with the decrease in sample pH. The reason is attributed to the accumulation of positively charged chromophores on the bare silica surface of the fiber in the case of higher pH values. Justification of it is also made on the basis of the results obtained through free-beam method. A U-shaped sensor probe is used for the investigation because the sensitivity of the system improves due to the higher penetration depth achieved in this case; this is experimentally shown too.