Liquid concentration measurement based on SMS fiber sensor with temperature compensation using an FBG

A highly sensitive liquid concentration sensor based on a single-mode–multimode–single-mode (SMS) fiber structure cascaded with a FBG is utilized and experimentally demonstrated. The two kinds of dips that caused by the modal interference and FBG transmission are observed in the measured spectrum. Dip of SMS would shift while the liquid concentration is changed, but the dip of FBG is kept still. When the ambient temperature changed, the two kinds of dips would shift simultaneously with different sensitivities. The experimental results indicate that it is an effective method to compensate for the temperature perturbation via introducing a FBG. The approximate linear relationship between liquid concentration and spectrum dip wavelength shifts is obtained experimentally. Measurement sensitivity up to 0.52 nm/% is showed with the concentration range of sucrose solution from 0 to 26%, which means the measurement resolution about 0.002% could be implemented if the wavelength shift measurement resolution of the optical spectrum analyzer is 1 pm.