Evanescent wave analysis and experimental realization of refractive index sensor based on D-shaped plastic optical fiber

A simple D-shaped plastic multimode fiber optic sensor is proposed and demonstrated for refractive index testing. The D-shaped fiber is fabricated using the wheel side-polishing method. Experimental results exhibit a good linear relationship between the receiving optical power and the refractive index of the test medium. Different parameters, such as the depth, the length and the bending of the D-shaped region, are taken into account to analyze and optimize the sensitivity of the sensor. Normalized transmittance intensity decreases 13.4% with the refractive index increasing from 1.333 to 1.455 when the depth and length are 500 μm and 2 cm with excurvature radius of 5 cm and optical source wavelength of 652 nm. For different refractive indices of the test medium, we simulate the energy distribution of the D-type structure by using Finite Element Method, which achieves a good agreement between the experimental and theoretical results. Such a low-cost and easily fabricated structure will be competitive for remote and continuous measurement of refractive index in chemical, biological, and biochemical sensing areas.