ESKİŞEHİR TECHNICAL UNIVERSITY JOURNAL OF SCIENCE AND TECHNOLOGY A- APPLIED SCIENCES AND ENGINEERING 2018, 19(4), pp. 815 - 830, DOI: 10.18038/aubtda.413304 *Corresponding Author: kivilcimyuksel@iyte.edu.tr Received: 09.04.2018 Accepted: 15.10.2018 IMPLEMENTATION OF LONG-PERIOD FIBER GRATINGS FOR REFRACTIVE INDEX MEASUREMENT OF LIQUIDS Cansu İDE 1 , Kıvılcım Yüksel ALDOĞAN 1, * 1 Electrical - Electronics Engineering Department, İzmir Institute of Technology, 35430, İzmir, Turkey ABSTRACT Refractive index (RI) is one of the optical parameters that is of paramount interest in characterizing liquid solutions and sol- vents. Increased public awareness and legal requirements for quality standards in different sectors have created a need for reliable, fast, automated, remote and/or portable refractive index sensors with multi-measurement capability. The fiber-based RI sensors are very good candidates in providing attractive solutions in such application areas and have been attracting a growing interest by the scientific community. In this work, a fiber optic refractive index sensor has been investigated. It uses a long-period fiber grating (LPFG) and an Optical Spectrum Analyzer (OSA), as sensor head and interrogator unit, respectively. New measurements of refractive indices of common solvents and solutions at the low-loss telecom wavelength band have been successfully demonstrated by using the proposed sensor. The reported measurement results are in excellent agreement with our simulation results and reference measurements realized by conventional measurement techniques. Keywords: Fiber optic sensor, Long-period fiber grating, Refractive index measurement 1. INTRODUCTION Refractive index (RI) is considered as one of the physical properties of material such as boiling point, melting point and density. The measurement of RI is frequently used in a variety of fields, such as chemical industry, food quality and safety analysis, bio-medical applications, environmental imaging systems, petroleum and oil industry and material analysis. There is an increasing interest in recent years in determining the exact value of the refractive index of the materials as it is a critical parameter in all aforementioned areas. During the last thirty years, several research groups have published different types of fiber-based refractive index sensors. These sensors include fiber Bragg gratings (FBG), long period gratings (LPGs) in a simple or tapered fiber structures, photonic crystal fibers (PCF), micro-interferom- eters and sensors using surface Plasmon resonance (SPR) effects [1–3]. In this study, we focused on the long-period fiber grating (LPFG) based-refractometers inscribed into standard single mode fiber. They are compact and do not require the use of specialty fibers, side polish- ing fibers and/or post processing. They can be used directly to measure the change in the refractive index of the surrounding medium due to the cladding modes based coupling mechanism. And they may be as sensitive as SPR based refractive index sensors with some sensitivity enhancement methods [4–7]. In the framework of this work, we first of all, studied the operation principles of LPFG including the modelling aspects of this sensing element. For this purpose, transmission spectra of LPFGs to the exter- nal refractive index changes have been simulated by employing the two-layer geometry. Then, experi- mental work has been realized on different concentrations of glucose-water, glycerol-water solutions, and various chemicals. Our experimental results show an excellent agreement with the theory which demonstrated the capability of measuring RI of liquids in the telecommunication wavelength range.