Measuring liquid-refractive-index based on a fused fiber-optic coupler

In this paper, a new method for measuring the liquid-refractive-index using a fused tapering coupler of optical fiber is presented. Firstly, according to the shape-curve function of a fused tapering fiber and super-mode coupling theory, the relationship between the output coupling visibility (CR) of the coupler and the refractive index of the medium surrounding the coupler is investigated. Then, a 2x2 fused tapering single-mode optical fiber coupler is immersed into a liquid in experiment. When the light is injected into the coupler, the coupling visibility (CR) can be changed with the variation of the refractive-index of liquid surrounding the fused tapering zone of the couple. The measurement of the liquid-refractive-index can be realized. Finally, the theoretical and experimental results are in good agreement.