Measuring optical activity with the internal reflection in a glass prism

A novel internal reflection-based polarimeter and measurement method to characterize optically active substances is presented. The working principle relies on the phase difference acquired by s- and p-polarized light undergoing internal reflection: they interfere after passing through an analyzer and produce a minimum near the critical angle, whose position depends on the rotation angle imposed by the optically active sample. Since the location of the minimum does not depend on the laser power, the measurement is nearly insensitive to any power fluctuation. Furthermore, this low-cost device is rugged, very compact and stable, and practically immune to mechanical vibrations. It was used to measure concentrations of sucrose and fructose dissolved in distilled water and presented a resolution better than 0.04°.