Fabrication and Interrogation of Refractive Index Biosensors Based on Etched Fiber Bragg Grating (EFBG).

We discuss the fabrication and interrogation of a fiber-optic refractive index sensors based on wet-etched fiber Bragg grating (EFBG). The fabrication is based on chemical etching of an FBG through an HF-free etching solution (ammonium fluoride and sulfuric acid), which progressively depletes the fiber cladding exposing the core to the outer medium. Microscope inspection of the fiber and real-time detection of the Bragg wavelength allow controlling the sensitivity. The proposed interrogation method is based on a spline interpolation, that measures the change of Bragg wavelength when the FBG is exposed to variations of the refractive index in the surrounding medium. An experimental validation has been carried out, for small refractive index variations (up to $1.85\times 10^{-3}$ RIU), in order to verify the progressive change of sensitivity through fiber etching. The proposed EFBG sensing unit is a building block for functionalized fiber optic biosensors.