Measurements of glycerine concentration in water solution by using long-period grating refractometer

We measured the glycerine in water solution with different concentration by using a long-period grating. As a refractometer, it is desirable that the surrounding refractive index is the linear relationship with concentration of the solution. The results of measurements show that the linear range of glycerine concentration is from 0 to 40% with the transmission minimum shift, whereas it is from 0 to 20% with the wavelength minimum shift.