Using a new liquid-crystal polarization modulator for a polarimetric glucose sensor

This paper presents a new liquid-crystal polarization modulator comprising a nematic liquid-crystal variable retarder (LCVR) and two quarter-wave plates for the measurement of glucose concentrations. Rather than using a conventional Faraday modulator, this study firstly adopts a LCVR driven by sinusoidal signals to modulate the polarization angle of the light as a sinusoidal function. It is found that the variation frequency of the polarization angle doubles that of the driving signal, and the modulation depth is not proportional to the driving amplitude. These features are quite different from a Faraday modulator. Therefore, a signal demodulation algorithm which developed to adapt the particular features of the LC modulator enables the polarization rotation angle corresponding to the glucose concentration to be derived. The proposed measurement method has a minimum resolution of 0.05 degrees corresponding to a glucose concentration of 0.2 g/dl. Compared to its conventional counterparts, the developed polarimeter has a simpler structure, fewer optical elements, and a cheaper modulator component.