Butterfly-Shaped Fiber Structure: A Novel Dual-Parameter Fiber Sensor for NaCl and Sucrose Detection in Aqueous Solutions via RI Modulation

The development of sensitive and selective optical fiber sensing devices is essential for detecting chemical compounds in the environment, biomedical, and food quality applications. In this paper, a dual-point optical fiber sensor is demonstrated, utilizing a novel butterfly-shaped fiber structure to detect sodium chloride (NaCl) and sucrose in aqueous solutions through refractive index (RI) modulation. The butterfly-shaped structure was fabricated by wrapping a section of single-mode fiber (SMF) to form two loops cascaded on either side of a plastic capillary tube. Each fiber loop represents the butterfly's wings, the upper wing (UW) and the lower wing (LW). The UW and LW serve as separate Mach-Zehnder interferometers (MZIs), which represent separate sensing regions, namely, sensing points 1 and 2, respectively. Each sensing point can respond separately to RI variations induced by NaCl and sucrose concentrations. Experimental results show that the proposed dual-parameter sensor exhibits high sensitivities of 273.212 nm/RIU and 341.366 nm/RIU, with excellent linearity of 99.4% and 99.6%, respectively, to RI changes over a range of NaCl and sucrose concentrations from 0 to 25%. The proposed sensor configuration provides a compact, low-cost, and efficient solution for real-time multi-analyte detection in food safety, health diagnostics, and environmental analysis applications.