ConclusionsIn this paper, a highly sensitive liquid concentration sen-sor based on a single-mode–multimode–single-mode (SMS) fiberstructure cascaded with a FBG was exploited and experimentallydemonstrated. The characteristic that the difference of measure-ment sensitivity for different length of MMF is negligible wasdemonstrated by simulation and experiment. The sucrose solu-tions with different concentration are compounded as tested liquid.When the concentration is ranged from 0 to 26%, the measurementsensitivity is about 0.52 nm/%, which means the measurement res-olution about 0.002% could be implemented if the wavelength shiftmeasurement resolution of the optical spectrum analyzer is 1 pm.Utilizing this structure, which is provided with the advantagesas simple structure, easy to fabricate and low cost, can achievethe measurement of liquid concentration with high sensitivity. Itshould be noted that, when measuring the concentration of differ-ent solutions it needs to use the calibration relationship betweenthe RI and the solution concentration for conversion