Optimal etching process and cladding dimension for improved coating of porous hemispherical ZnO nanostructure on FBG humidity sensor

Abstract

Abstract Fiber Bragg grating (FBG) based sensors require functional chemical coating for relative humidity (RH) sensing. However, the cladding of the fiber may impede the mechanical strain that is induced in the sensing region of the FBG. This study focuses on the humidity sensing performance of FBGs etched with several different diameters. Silica-based uniform FBG was chemically etched before being coated with a ZnO nanostructure reinforced with hexamethylenetetramine (HMT) additives. ZnO-HMT was produced using a low-temperature modified hydrothermal method and then coated on FBG etched with various diameters. Morphology observations with elemental analysis confirmed the formation and presence of ZnO-HMT coatings on the sensors. Results interpreted from an optical spectrum analyzer have shown wavelength shifts of 0.0004 nm/%RH with linearity of ∼98% for samples etched for 10 min. Both sensitivity and linearity deteriorated as the FBG was etched to thinner diameters. Sensor sensitivity improves by at least 20%–30% when compared to the uncoated version of FBG. It can be concluded that with zinc-based coatings, FBG sensing performance can be improved only to a certain extent through etching durations. Thinner cladding may not be suitable for use in combination with zinc-based nanostructures. The optimized FBG etching when used in conjunction with zinc oxide improves upon its non-etched counterparts and increases its potency in industrial applications.