Polarization-preservation analysis for robust reflection-mode optical glucose sensing in turbid media

Non-invasive optical glucose sensing remains challenging in highly scattering media due to depolarization and absorption interference, and polarization scrambling under multiple scattering conditions. In reflection-mode geometries, polarization signals can contain mixed contributions from different optical mechanisms, which respond differently to glucose concentration and limit the effectiveness of conventional polarization metrics. In this work, polarization-resolved reflection measurements were investigated in skin-equivalent milk-based phantoms using visible illumination at 532 and 660 nm. Linear and circular polarization states were compared over glucose concentrations ranging from 50 to 400 mg dL^-1. To quantify polarization preservation under multiple scattering, a polarization-preservation ratio (PPR), derived from co- and cross-polarized circular components, was introduced and evaluated alongside the conventional degree-of-polarization (DOP) metric. Circular polarization combined with PPR exhibited stronger glucose-dependent correlations and improved measurement stability compared with linear polarization, particularly under reflection-mode detection. At 660 nm, the proposed approach maintained high correlation coefficients even under pigment interference, demonstrating improved robustness in turbid environments. These results indicate that polarization-preservation analysis provides a physically consistent and robust sensing strategy for reflection-mode optical glucose detection in scattering-dominant media.