Design and Implementation of Optical Fiber Sensor Based on Surface Plasmon Resonance for Detection of Water Contamination

Abstract

To identify and treat the increasing levels of pollution in the water, new technology had to be found. The Optical Fiber sensor-based on the Surface Plasmon Resonance (SPR) phenomena mechanism has been a successful performance sensing and presents high refractive index (RI) sensitivity. This thesis investigates how to improve the performance of current designs of OFs based on SPR. The thesis includes two parts; the first part includes a theoretical design to Single-mode fibers (SMF’s) that had been designed using the Finite Element Method (FEM) through the simulation software COMSOL Multiphysics. The influence of OF diameter and doping concentration of cladding on the optical properties of SMF was studied. And the practical part includes the design and implementation of the OFs based on SPR to detect pollution in water. The simulation results had been in good agreement with experimental results. In this thesis, a simulation was performed, to develop the performance of the OF pollution sensor by testing the change of the SMF structure parameters, such as the diameter of the cladding with different thicknesses, adding impurities such as germanium oxide to the base material of the fiber, which is silica in the cladding area with different concentrations, and studying the best thickness and best concentration of impurities to obtain the best performance of the optical fiber sensor in Water pollution sensor with sodium chloride salt (for example). From the obtained results it had been found that the sensors with less diameter of micrometer range were more sensitive to refractive index change of applied analyte. The higher sensitivity was obtained with a fiber diameter of 6.5 µm. Also, it had been found that as GeO2 concentration increase (with fixed cladding thickness) the sensor was more sensitive to the change of analyte refractive index, the best results were obtained with a concentration of 0.19 wt%. The experimental part of the thesis was based on Mach-Zehender interferometry technology, in which a SMF was designed as a water pollution sensor.Sodium chloride salt and sugar were used as examples of the pollutants with different concentrations (0-0.45 mol/liter). The sensor performance was studied for different sensor arm diameters, which were (63.5, 51.58, 39.68, and 20 µm). A portion of the SMF was chemically etched by immersion in a hydrofluoric acid solution with a dilution ratio (1 mm3 acid: 1 mm3 distilled water), which resulted in a tapering effect, reducing the thickness of the cladding and keeping the core intact. The variation of the refractive index of salty and sugary solutions that were surrounding the sensor lead to a change in the optical properties of the transmitted signal. in this work, a wavelength shifting had been obtained due to this variation. The influence of sensor diameter on the sensitivity of the system had been studied. From the results, we found that the sensitivity was higher for sensors with a diameter equal to 20 µm and they were equal to 1130 pm/mol.l-1 and 1205 pm/mol.l-1 for the salty and sugary solution respectively. Finally, the important and basic part of the thesis was conducted, which is the manufacture of a sensor based on surface plasmon resonance. The SPR sensors were prepared by coating a golden metallic film which thickness (42 nm) on a chemically etched single-mode fiber with a thickness of 20 micrometers, which achieved the best results of sensitivity to water pollution according to the results of the previous process stage, The results of the high sensitivity of the optical fiber sensor were obtained based on the surface plasmon resonance phenomenon. The experimental results showed high sensitivity, reaching 1315 pm/mol.l-1 for a salty solution with distilled water, 1705 pm/mol.l-1 for the salty solution with tap water, as well as 2222 pm/mol.l-1 for sugar solution with distilled water, and 1925 pm/mol.l-1 for sugar solution with tap water. This means that these sensors which are based on SPR could be very useful in the field of water pollution detection.