Sensors and Actuators A 250 (2016) 29–34 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna Optical fiber sensor system for remote and multi-point refractive index measurement Kivilcim Yüksel Electronics Engineering Department, Izmir Institute of Technology, IZTECH, Gülbahce Kampüsü, 35430 Izmir, Turkey a r t i c l e i n f o Article history: Received 7 March 2016 Received in revised form 25 August 2016 Accepted 2 September 2016 Available online 8 September 2016 Keywords: Fiber optics sensors Fresnel reflection OTDR Refractive index sensing a b s t r a c t A Fresnel-reflection-based RI sensor using SMF fiber tips as sensing points interrogated by multi- wavelength OTDR from a distant location (up to several tens of kilometers) has been reported. The advantage of the system compared to previous work is that the distance between sensor points is not limited by the spatial resolution of OTDR. Experimental work demonstrated that the proposed sensor is capable of measuring refractive index of liquid chemicals with a precision of 1.7 × 10 −4 . This sensor prototype have strong conveniences (simple installation requirements, fast response and reliability in harsh environment) compared to previous Fresnel-based RI sensors which makes it a very good option for environmental monitoring systems. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Refractive index (RI) is an inherent characteristic of a substance that is closely related to the physical and chemical properties mak- ing the refractometry necessary in many fields. Examples include quality control of foods during processing and packaging, adul- teration detection (edible oils, gasoline or automotive lubricants) [1], monitoring of environmental pollution [2], biomedical applica- tions, and process monitoring of composite materials [3]. Most of these areas require in-situ, self-calibrated, maintenance-free and easy-to-use refractive index measurement capabilities. Commer- cially available refractometers based on Abbe configuration present some challenges in addressing these requirements largely due to the manner they are in contact with the specimen, their size and their power requirements. In parallel to the above-mentioned issues in refractive index measurement, another major concern for many industries is to have a compact system which provides multiple, minimally inva- sive sensor points interrogated from a remote location. This concern is of particular interest in harsh environments such as chemical and nuclear sites, waste water processing units and river pollution assessment locations. Optical fiber sensors have a great potential for such industrial monitoring tasks thanks particularly to their durability against harsh environments in addition to their small E-mail address: kivilcimyuksel@iyte.edu.tr dimensions, fast response, and immunity to electromagnetic inter- ference. Fiber optic sensors which have been widely investigated during the last decades are mainly based on fiber gratings sen- sors (long period fiber gratings [4,5], tilted fiber Bragg gratings [6]), fiber interferometers [7], and surface plasmon resonance (SPR) [8]. Recently, Fresnel reflection-based sensors gained a renewed interest as they provide simple and low-cost solution [9–14]. The operation principle is basically based on the measurement of Fres- nel reflection coefficient at the interface (sensor tip) between the optical fiber and the specimen. Fresnel-reflection-based sensors proposed in the literature differ from each other essentially by their method in interrogating the sensor tip. Among these approaches, there is no universal solution that is standardized for the practi- cal implementations (the related state-of-the-art is summarized in Table 1). The contribution of this article is twofold: first, the performance parameters of a RI sensor based on a commercial OTDR (single wavelength) has been evaluated which will provide a detailed com- plementary information to the missing parts of the previous (OTDR- based) work. Second, a multi-wavelength OTDR interrogation scheme is proposed which permits to take measurements on sev- eral sensor points from a remote terminal (up to several tens of kilo- meters). The main advantage of the proposed system compared to the previous work is that it can be used on several sensor tips with- out imposing limitations on the distances between sensor points. Therefore the system can be easily tailored for different application requirements (i.e. number of sensor tips and the distance between http://dx.doi.org/10.1016/j.sna.2016.09.003 0924-4247/© 2016 Elsevier B.V. All rights reserved.