Modification of intrinsic refractive index optical fiber sensor via precise cladding removal and heterogeneous ZnO/Ag bi-layer coating

Abstract

Miniaturized refractive index sensor by combination of nanostructure thin films as a transformer sensitive layer and optical fiber as a signal carrier offers great potential for identifying the environmental features and understanding the novel sensor concepts. The partially unclad and bi-layer zinc oxide (ZnO) or silver (Ag) coated multimode glass and polymer optical fiber as a simple and reliable intrinsic fiber sensor was proposed in this work to detect the ambient refractive index changes (saline and crude oil having various concentrations) using two broadband sources of infrared radiation and ultraviolet-visible. The removing process to partially unclad the polymer and glass fiber was carried out precisely using our proposed dynamic monitoring process to prevent any interruption on propagating light by occurrence of damage on the core surface. The ZnO as an outer sensitive layer had three configuration which was spherical nanoparticle, horizontally and vertically oriented nanorods deposited on discontinuous Ag layer using mixture of electroless, dip coating and low temperature hydrothermal techniques because solo deposition technique was not possible. Ag nano-island shape deposition made the transition of evanescent wave to external media possibly through this semi-reflectance structure. ZnO coating avoided the formation of oxygen deficit defects, inhibited aging problem and trapping measurand molecules through mechanical interlock phenomena which altered its optical characteristics and improved sensitivity of the sensor. The x-ray diffraction spectra demonstrate that the level of crystallinity was higher for vertically oriented ZnO compared to others. Using field emission scanning electron microscope images, the width or length of vertically and horizontally oriented ZnO nanorods was measured. This shift is independent with the contact media. However, deep level emission depends strongly on the concentration of the contacted media. The shift observed for nanoparticle. The performance of fabricated probe to detect saline concentration changes for glass fiber coated with vertically oriented ZnO nanorods/Ag when IR light source employed, was supreme compared to the other samples and was reported to be 255.4 nm/RIU and 314.2 dB/RIU for wavelength and intensity sensing respectively. The durable polymer fiber coated with vertically oriented ZnO/Ag nanorods showed the intensity and wavelength sensitivity of 146.2 dB/RIU and 78.5nm/RIU respectively in identifying the variation of crude oil from 0 to 100%. The optimum length of glass and polymer fiber probe for maximum sensitivity was obtained for 3 cm and 2 cm respectively. The precise production techniques together with comprehensive analysis of the sensing mechanism lead to a deeper understanding of the liquid refractive index behavior applicable in quality control in water resource and oil reservoir.