Experimental study of liquid refractive index sensing based on a U-shaped micro-fiber

A U-shaped micro-fiber sensing system designed to measure the refractive index of liquid had been proposed. The sensing mechanism of U-shaped micro-fiber was discussed. A general single-mode fiber was bent into U-shaped and the cladding of U-shaped fiber was corroded by HF acid buffer solution. Powers of U-shaped micro-fiber with different diameters had been measured by many experiments and the appropriate designed parameters were obtained. Then, this U-shaped micro-fiber was immersed in liquids, such as pure water, ethanol, acetone and isopropanol. The evanescent field of the U-shaped fiber should be modulated by the liquid and the optical signal in the U-shaped micro-fiber was measured by the optical spectrum analyzers (OSA). Experimental results were analyzed. The spectra in the air were selected as a reference and the relative intensity of different liquids was obtained. These results showed that the relative intensity of the liquid had a good linear relationship. This sensing device could accurately demarcate refractive index of liquid. It is simple, low cost, and it can also be applied in measuring the level of liquid.