Abstract: A new method for measuring concentration of solutions using a fused tapering coupler of optical fiber is reported. According to the shape-curve function of a fused tapering fiber and the super-mode coupling theory,relation between output coupling visibility (CV) of the coupler and refractive index of the medium surrounding the coupler is investigated. A 2×2 fused tapering single-mode optical fiber coupler is immersed into a solution in the experiment. When light is injected into the coupler, the CV is changed with variation of solution-concentration (refractive-index) surrounding the fused tapering zone of the couple. The solution-concentration sensor is realized. The theoretical and experimental results are in good agreement.

Key words: coupling visibility, fused tapering coupler, refractive index, solution-concentration, optical fiber sensor