A liquids refractive index spectrometer

Abstract

In this paper, we report about the use of an optical fiber sensor for measurements of the refractive index (η) of liquids as a function of wave-length (λ). The method uses only one optical fiber. Different laser diodes are used to carry out measurements at several wavelengths. The principle of measurement is based on the detection of the intensity of reflected light. Periodical intensity modulated light of a given λ is transmitted using an optical liquid fiber through the sample contained in a quartz glass cuvette, being reflected by a mirror located at the rear side of the cuvette back to the same fiber, through which it arrives to a photodetector after passing through a beam splitter that blocks the original beam. The signal amplitude is measured using a Lock-in amplifier synchronized at the modulation frequency as a function of the distance between the fiber tip and the cuvette for different samples of well-known η. A linear calibration curve is obtained by plotting the refractive index versus the amplitude maximum value, which can be used to measure the value of η for an unknown sample, as demonstrated by measurement of the refractive index of sodium chlorine and glucose in water solutions (in the range between 0 and 25 weight%) for different wave-lengths. Good agreement has been obtained with values reported and/or measured with an Abbe refractometer values. A dispersion relation, η (λ), is obtained for these samples. The described refractometer has a sensitivity of 0.56V/RIU, its linearity is of more than 99% in the investigated range between 1 and 1.5 RIU. The limit of detection was estimated as 1.4×10−3RIU (RIU=Refractive Index Unit) at the used wave-lengths.