Detection of saline-based refractive index changes via bilayer ZnO/Ag-coated glass optical fiber sensor

Abstract

The combination of sensitive nanostructure thin films and optical fiber offers the great prospective for understanding the novel sensor concepts. The partially unclad and bilayer zinc oxide (ZnO)/silver (Ag)-coated multimode glass fiber as a simple and reliable probe is proposed in this work to detect the ambient refractive index changes using two broadband sources of IR and UV–Vis. The wavelength and intensity of propagating light both are modulated when the saline concentration is varied. The appropriate etching time for partially removing the cladding suitable for both IR and UV–Vis sources is 53 min. The fabricated ZnO/Ag/fiber sensor exhibits excellent repeatability and high sensitivity to saline with different concentrations from 0 to 30% at room temperature. Among the sensors probe, higher sensitivity is observed for ZnO/Ag/fiber sample when IR is used as a light source. In this sensor by changing the refractive index of the media from ~ 1 to ~ 1.38, the normalized intensity drop to 0.6 of its maximum value and corresponding wavelength shifted from ~ 1559 to ~ 1585 nm. The high sensitivity of fabricated probe is attributed to two phenomena related to bilayer structure of sensing probe: first, the uncontinuous Ag coating which makes the optical tunneling to the outer layer be possible and, second, altering the optical properties of ZnO by oxygen absorbance through interaction of saline by ZnO nanostructure and changing the refractive index of the deposited layer. The wavelength and intensity are found to be less sensitive for both partially unclad and ZnO/Ag/fiber once UV–Vis is used as a light source, which is due to slighter penetration of evanescent wave in the cladding part compared to IR source.