Simple system for measuring optical rotation of glucose solution using liquid-crystal grating

We demonstrate an optical system for measuring the concentrations of optically active media using liquid-crystal polarization gratings (LCPGs). The optical rotation angle is determined by measuring the intensities of two diffracted light beams from an LCPG combined with a quarter-wave plate (QWP). The intensity ratio is used to evaluate the optical rotation angle, minimizing the dependence on changes in light source intensity and wavelength and reducing the influence of temperature-drift-induced LC birefringence shifts. We demonstrate the system by measuring the concentration of a glucose-water solution. The measurement error caused by the slight wavelength dependence of the QWP's retardation is assessed numerically. (C) 2015 The Japan Society of Applied Physics