A U-shaped optical fiber sensing system designed to measure the refractive index of liquid had been
proposed. The sensing mechanism of U-shaped optical fiber was discussed. A general single-mode
fiber was bent into U-shaped and partially cladding of U-shaped fiber was corroded by HF acid buffer
solution. Powers of different diameters of U-shaped fibers had been measured by many experiments.
The results showed that the diameter of U-shaped fiber cladding 40 μm and the diameter of U-shaped
was 1 cm were suitable to measure liquid refractive index. Then, this U-shaped optical fiber was
immersed in liquid, such as pure water, ethanol, acetone and isopropanol, respectively. The evanescent
field of the U-shaped fiber should be modulated by the liquid. The optical signal in the U-shaped fiber
was measured with the optical spectrum analyzers(OSA). Finally, the experimental results were
analyzed, and the spectra in the air was selected as a reference. The relative intensity was obtained for
the different liquid. 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.