The Influence of Geometric Shape on the Performance of Refractive Index Sensors Based on Plastic Optical Fibers: Simulations and Experimental Assessment

In this work, we present a comparative study on the influence of the geometric shape of plastic optical fiber (POF) sensors in their response to refractive index (RI) of external media. Three different geometric shapes were simulated using the finite-element method (FEM), one being a simple U-shaped POF sensor (FU) and the other two based on the same U shape but with one and two additional small curves in the fiber, FO1 and FO2, respectively, allowing for an increased exposure of the evanescent field. The results of simulations showed that the shapes of sensors FO1 and FO2 would allow for higher sensitivities compared to that of the simple U-shaped sensor FU and that FO2 would show an even higher sensitivity due to an increased evanescent field exposure enabled by the additional curves on its shape. A set of POF sensors were fabricated according to the designs simulated and then tested in several sucrose solutions corresponding to RI from 1.3330 to 1.4242. The POF sensors manufactured showed sensitivities (optical power loss) of 36%/RIU, 102%/RIU, and 217%/RIU for FU, FO1, and FO2, respectively, from air to pure water (RI 1.000 to 1.333), and 330%/RIU, 656%/RIU, and 904%/RIU for FU, FO1, and FO2, respectively, from pure water to sucrose solution (RI 1.333 to 1.381). The correlation between the results from simulations and experiments shows that POF sensors based on RI sensing, such as chemical sensors, gas sensors, and biosensors, can be significantly improved with small modifications in their shape design.