BNNT coating of FBGs written in tapered optical fiber for hydrophilic gas sensing

Fiber Bragg gratings (FBGs) were fabricated in tapered fibers using the plane-by-plane technique and a fs-IR laser. A thin layer of boron nitride nanotubes (BNNTs) was deposited onto the FBGs through a dip-coating process from a BNNT water solution. The reflection spectra of FBGs were then recorded when they were placed above the water solution of ammonia (NH₃), hydrogen chloride (HCl) or liquid bromine (Br₂) in an open cylinder container. It was demonstrated experimentally that, due to the thin BNNT coating, the returned losses of FBGs were increased when they were surrounded with such hydrophilic gas vapors, such as ammonia, bromine and hydrogen chloride. However, when the FBGs were in other atmospheres such as air, methanol and acetone vapors, no change of the FBG reflection spectra was observed. The results of this work indicates that BNNT coating on FBGs plays an important role for hydrophilic gas detection with high sensitivity and reusability due to their quick release of adsorbed gases. The proposed sensor has the capacity of multiparameter sensing and can be used in harsh environments.