Fiber Bragg gratings in microstructured optical fibers for stress monitoring Thomas Geernaert *a , Geert Luyckx b , Eli Voet b , Tomasz Nasilowski a , Karima Chah a , Martin Becker c , Hartmut Bartelt c , Waclaw Urbanczyk d , Jan Wojcik e , Wim De Waele b , Joris Degrieck b , Francis Berghmans a , Hugo Thienpont a a Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium b Department of Materials Science and Engineering, Ghent University, 41 Sint-Pietersnieuwstraat, Gent 9000, Belgium c Institute of Photonic Technology, Albert-Einstein-Strasse 9, D-07745 Jena, Germany d Wroclaw University of Technology, 27 Wybrzeze Wyspianskiego, 50-370 Wroclaw, Poland e Maria Curie-Sklodowska University, Pl. Marii Curie-Sklodowskiej 5, 20-031 Lublin, Poland ABSTRACT Combining the functionalities of fiber Bragg gratings (FBGs) and microstructured optical fibers (MOFs) offers promising technological perspectives in the field of optical fiber sensors. Indeed, MOFs could overcome some of the limitations of FBGs in conventional fibers for sensor applications. The added value of MOFs stems from the ability to design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to directional strain but not to temperature. For this purpose we use a MOF with a phase modal birefringence on the order of 8×10 -3 . A FBG in this MOF yields two Bragg peak wavelengths, with a wavelength separation that depends on the phase modal birefringence of the MOF. We characterize these FBGs for transversal loads on a bare fiber and compare the results with simulated sensitivities. Then, we embed the sensor in a composite coupon and measure the response of the Bragg peak wavelengths as a function of the applied transversal pressure on the composite material. This allows drawing conclusions on the advantages of FBGs in MOFs for sensing applications. Keywords: fiber Bragg gratings, microstructured optical fibers, photonic crystal fibers, optical fiber sensing, structural health monitoring 1. INTRODUCTION Fiber Bragg gratings (FBG) are widely recognized as one of today’s most valuable optical fiber components in optical telecommunication, fiber laser technology and optical fiber sensing systems [1]. Measuring axial strain and structural damage are among the most important applications of FBG-based sensor technology. FBGs are indeed commonly used to measure axial strain. However to rigorously monitor the structural health or assess damage inside composite materials, it is also necessary to map the transversal stresses inside that material, because they can cause catastrophic damage such as delamination. Because the composite material is usually only a few millimeters thick, it is not straightforward to assess the strain in this transversal direction. For this purpose and as already reported, optical fibres with FBGs can be embedded in composite materials without compromising the structural integrity of the host material. FBGs fabricated in microstructured optical fibers (MOF) have been reported in [2]-[7]. MOFs are a relatively new type of optical fiber that can be optimized for a large range of applications by tailoring the number, the size, the position and the geometry of the air holes that form the confining microstructure around a (sometimes doped) fiber core region [8]. Owing to this design flexibility, FBG-based sensors in MOFs promise optimized and selective sensitivities via dedicated air hole geometries. *thomas.geernaert@vub.ac.be; phone 0032 (0)2 629 1381; fax 0032 (0)2 629 3450; tona.vub.ac.be Photonic Crystal Fibers III, edited by Kyriacos Kalli, Proc. of SPIE Vol. 7357 73570I · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.820441 Proc. of SPIE Vol. 7357 73570I-1