Preparation of Metal-Coated FBG Sensors for Integration into Bioinspired Adaptive Structural Sensing Systems

Adaptive structures rely on integrated sensing systems to monitor and respond to changes in their environment. Optical fiber sensors (OFS), with their versatility, small size, electromagnetic passivity, and capability to operate in harsh conditions, including high and cryogenic temperatures, are promising candidates for embedding within adaptive systems. However, the brittle nature of bare optical fiber — fused-silica (SiO2) exposed by removing its polymer coating — presents challenges in durability and handling. This study focuses on enhancing the mechanical robustness and temperature sensitivity of optical fibers by depositing metallic coatings, specifically copper and nickel, onto the SiO2 surface through chemical deposition techniques. Two optical fibers with polyimide and two with acrylate coatings were processed through a series of steps: 1) removal and cleaning, 2) gold sputtering, 3) sensitization and activation, and 4) chemical deposition. Coating removal used acetone, followed by cleaning with a 15% NaOH solution. Since the SiO2 surface is non-conductive, an initial conductive layer was applied via gold sputtering. The sensitization and activation involved immersion in SnCl2/HCl and PdCl2/HCl solutions, respectively, to enable subsequent metal deposition. This activation step was required for the copper deposition to be uniform, while nickel deposition occurred without prior activation, producing a less homogeneous coating. The findings demonstrate the feasibility of electroless deposition for producing metal-coated optical fiber sensors. However, further optimization is required to improve adhesion, enhance coating uniformity, and evaluate the impact of layer thickness on sensor performance. This approach contributes to the development of resilient, embedded sensing elements crucial for adaptive structures.