Thermo-optic discrimination of aqueous solution composition using a multimodal interference fiber optic sensor

The precise optical identification of aqueous solutions with different compositions is challenging for samples that have a similar refractive index at a certain temperature. However, their thermo-optic response, which describes how the refractive index varies with temperature, provides simple means for discrimination. We report on the use of a conventional fiber optic sensor based on multimodal interference to distinguish aqueous solutions of Tris and fructose based on thermo-optic effects. At room temperature, aqueous solutions of these components have indistinguishable properties. In fact, our sensor exhibited a linear dependence on concentration in both cases, with similar sensitivities of 0.2179 nm/% for Tris and 0.2264 nm/% for fructose, indicating that their discrimination is hindered based on concentration. On the other hand, by changing the temperature controllably, from 25°C to 45°C in increments of 2.5°C, we were able to change the refractive index enough to clearly discriminate between the samples at the same concentration. The thermal sensitivity measured was 0.14433 nm/°C for Tris and 0.1852 nm/°C for fructose, allowing for evident differentiation among samples in the range of35°C to 45°C. Our findings demonstrate that the proposed thermo-optic discrimination approach is effective and offers a promising practical solution; the optical fibers do not need to be prepared in a special way and the temperature control can be carried out with general-purpose laboratory equipment, like a hot plate, over a temperature range that is easily attainable. Overall, the results presented are relevant to industrial and research applications involving advanced monitoring and fine processing control.