Encapsulated fiber Bragg grating sensor for strain and temperature measurements Leandro de Marchi Pintos, Ivo de Lourenço Junior, Jean Carlos Cardozo da Silva Electrical Engineering Graduate Program – PPGEE Federal University of Technology – Paraná Pato Branco, Brazil jeanccs@utfpr.edu.br Hypolito José Kalinowski Graduate Program on Electrical Engineering and Applied Computer Science – CPGEI Federal University of Technology – Paraná Curitiba, Brazil hjkalin@utfpr.edu.br Abstract — This paper presents an effective method of encapsulation of a fiber Bragg grating (FBG) for measurements of temperature and strain. The packaging technique consists in embedding two fibers Bragg gratings sensors inside a polymeric material with different geometric characteristics. The characterization of the optical transducer was performed through temperature and strain changes. A matrix equation was used from measurements of the two wavelength shifts. The preliminary results obtained demonstrate the feasibility of the encapsulation technique allowing measurements of temperature and strain in smart structures and harsh environments. The experimental procedure provided robustness to the sensor and the matrix equation approach has the potential for strain and temperature independent measurements. Keywords-component: Optical fiber sensors, fiber Bragg gratings, optical fiber sensors packaging. I. INTRODUCTION Optical sensors based on fiber Bragg gratings (FBG) have emerged as important sensing elements for measurements of temperature, strain, pressure, chemical and biological agents, among other parameters [1]. These sensors offers attractive characteristics that make them very suitable such as small size and lightweight, immunity to electromagnetic interference (EMI), electrically passive operation, high sensitivity and low losses [2], [3]. However, optical fiber sensors are fragile in their bare form and are subjected to breakage when being handled improperly especially during packaging or subjected to installation to harsh working conditions [1]. Fiber Bragg Gratings sensors can be used as a single sensing element or multiple sensors in the same fiber optic network. The multiplexing ability of these sensors, due to their narrowband wavelength reflection, has uses in a number of important areas and it is the ideal device for structural monitoring applications [4], [5]. These sensors also can be wavelength encoded, this characteristics eliminates the problems of amplitude or intensity variations. One significant limitation of FBG sensors is their cross- sensitivity (simultaneous sensitivity to temperature and strain) [6]. This property creates a problem for discrimination of strain and temperature responses. In order to circumvent this issue, two sensor elements which have very different responses to strain and temperature are located at the same point on the structure [7]. Based on these problems it is necessary to provide some approaches to distinguish the cross- sensitivity that ensure the reliability of the monitoring system. Several studies have been reported for producing encapsulated fiber Bragg grating for simultaneous measurement of temperature and strain in order to prevent the occurrence of cross measures and to provide greater robustness to optical sensors. Ferreira da Silva et al. [8] developed a package structure to embed optical sensors on a flexible polyvinyl chloride (PVC) skin foil. These researchers using a standard industrial fabrication processes developed a smart skin foil to package the optical sensor. Therefore, the sensor can be used in harsh environments such as severe weather conditions, high electronic noise or dangerous chemical compounds. Wu et al. [9] measured the responses of a packaged FBG using an epoxy resin to encapsulate the optical sensor. Tian et al. [10] has been developed a packaging technique which the temperature effect is eliminated through the use of two FBG attached on the upper and lower surfaces of a uniform strength cantilever that can be used for strain, displacement or acceleration measurements. Mokhtar et al. [11] presented a novel approach to better reducing the dimensions of sensor packages for simultaneous measurement of temperature and strain also using FBG sensor to be applicable to the monitoring of large scale concrete structures. In this work a similar approach to [11] is being applied and therefore provides the advantage as the sensors are placed inside the packaging. The sensors are embedded in the polymeric structure without the use of cyanoacrylate glue as adhesive. The use of adhesives is a complicating factor in the most of fiber optic sensors applications due to the complexity