Droplet bent multimode fiber sensor for temperature and refractive index measurement

Abstract

This thesis presents an original research work on bent multimode fiber sensor for refractive index (RI) and temperature sensing. The fiber sensor structure is formed by a successive splicing between single mode-multimode-single mode (SMS) fibers. A droplet bent shape is introduced in the multimode fiber section for excitation of modes into the acrylate coating. The excitation of higher-order modes into the acrylate coating is particularly interesting due to high thermo-optic coefficient (TOC) of acrylate which could improve temperature sensitivity. While evanescent field interaction of modes at the acrylate surface with surrounding material could be used for RI and temperature sensing. These modes experienced phase changes due to surrounding temperature and refractive index changes, consequently shifting the spectra of the sensor. This study covers numerical simulation using BeamPROP, sensor fabrication and experimental work. The sensor structure was simulated using BeamPROP software to understand light distribution and mode distribution inside the bent multimode fiber from the field pattern such that the bent structure is suitable for sensing. Different bending radiuses of 3.5 mm, 5 mm, and 7.5 mm were fabricated and tested. The highest refractive index sensitivity was obtained from 3.5 mm bent sensor with sensitivity of 42.41 nm/refractive index unit (RIU) tested between 1.30-1.395 RIU. Meanwhile, highest temperature sensitivity of 1.317 nm/°C was attained using 5 mm bent sensor between 25 °C to 35 °C. The low cost and simple sensor fiber structure is desirable in many applications including liquid food industry, and water quality monitoring system.