Enhanced Mid-Infrared Evanescent Wave Sensing via Synergistic Multi-Tapered Fiber Geometry and Optimized PDA Coating

This paper presents the development and optimization of a high-sensitivity mid-infrared optical fiber sensor based on evanescent wave spectroscopy. The sensor employs a GeAsSeTe (GAST) chalcogenide glass fiber, which is geometrically modified into a “taper-in-taper” microstructure to enhance the evanescent field. Light field simulations confirmed that reducing the fiber waist diameter increases the evanescent field energy ratio. Experimentally, fiber sensors with up to four secondary tapered regions (T4T) were fabricated, showing a significant increase in sensitivity to ethanol compared to a single-tapered fiber (T0), with the sensitivity rising from 0.03976 to 0.23569 a.u./vol%. Further sensitivity enhancement was achieved by functionalizing the optimal T4T fiber with a polydopamine (PDA) coating. The coating, applied via oxidative self-polymerization, improved surface hydrophilicity and provided adsorption sites for aromatic compounds. An optimal PDA precursor concentration of 2 mg·mL −1 was identified for benzaldehyde detection, balancing enhanced adsorption with evanescent field penetration. The finalized sensor (T4T-20DA) successfully detected low concentrations of model bio-analytes, sodium glutamate and creatinine, with linear responses and sensitivities of 0.0308 and 0.0344 a.u./(μmol·L −1 ), respectively. This study demonstrates that the synergistic combination of multi-scale tapering and controlled surface functionalization is a powerful strategy for creating highly sensitive and applicable mid-infrared fiber-optic sensors.