Randomized trial evaluates refractive index dispersion for identifying alcoholic beverages, suggesting a new quality control method.
A fiber optic refractometer designed to measure the refractive index of a liquid sample over a broad wavelength range is proposed and implemented. The sensor head structure is simple consisting on a section of coreless fiber inserted into a multimode transmission line and partially covered with a gold thin film. The multimode interference (MMI) and surface plasmon resonance (SPR), generated in the multimode-coreless-multimode (MCM) structure, produce a transmission spectrum with three distinctive loss bands, one in the visible and two in the near-infrared range. When the SPR-MMI device is immersed in a liquid sample, the SPR and MMI conditions are modified, resulting in a redshift of the loss bands. By tracking their spectral positions, the refractive index dispersion of the liquid can be constructed using the Cauchy´s equation. The sensor response was first calibrated using different Cargille oils with known refractive indices, and subsequent measurements with liquid samples showed good agreement between the experimental refractive index dispersion and those calculated using Sellmeier models. The proposed device was also used to obtain the optical fingerprint of six alcoholic beverages, enabling their discrimination, and demonstrating that the differences in the dispersion curves are associated with both the ingredients and the manufacturing process. Overall, the SPR-MMI device and the analysis methodology presented here constitute a feasible and simple technique for quality control, discrimination, and, in certain cases, estimation of alcohol content in alcoholic beverages.
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Galván-Navarro et al. (2026) studied this question.