JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 12, JUNE 15, 2014 2193 Mechanical Strength of Microstructured Optical Fibers C. Sonnenfeld, S. Sulejmani, T. Geernaert, S. Eve, M. Gomina, P. Mergo, M. Makara, K. Skorupski, H. Thienpont, and F. Berghmans Abstract—An experimental study of the mechanical reliability of microstructured optical ﬁbers (MOFs) is reported. Tensile tests were carried on ﬁve types of MOFs and two reference ﬁbers, and the tensile strengths were analyzed using Weibull statistics. Optical microscopy of the surfaces of rupture allowed identifying the criti- cal ﬂaws and determining the failure mechanisms. First, it appears that the MOFs have lower tensile strengths than standard optical ﬁbers. Second, the mechanical strength of MOFs was found to be related to the dimensions and morphology of the microstructure. Finally, fractographic examinations conﬁrmed that MOFs can fail from defects located in the vicinity of the air holes, in contrast to standard optical ﬁbers for which cracks always start propagating from defects located on the outer silica surface. Index Terms—Fractography analysis, microstructured optical ﬁbers (MOFs), optical ﬁber testing, statistical mechanics. I. INTRODUCTION M ICROSTRUCTURED optical ﬁbers (MOFs) have con- siderably evolved since their introduction in the mid 90s. In most cases, a MOF consists of a fused silica ﬁber with a regular pattern of air holes that run parallel to its axis and along its entire length. The waveguide properties of MOFs are determined by the speciﬁc arrangement of these air holes. The set of air holes is typically referred to as the “microstructure” of the MOF. The ability to tailor the number, size, shape, and position of these air holes provides exceptional design ﬂexibil- ity that allows obtaining ﬁbers with unique guiding properties [1], which are often not attainable with conventional step-index optical ﬁbers [2], [3]. Manuscript received July 25, 2013; revised March 28, 2014; accepted Febru- ary 27, 2014. Date of publication May 5, 2014; date of current version May 30, 2014. This work was supported by in parts the EU FP7 ICT Project PHOSFOS, the EU FP7 Marie Curie IAPP Project SMARTSOCKET, the Institute for the Promotion of Innovation through Science and Technology Flanders (IWT), the COST TD1001 action, and the IWT-SBO Project with Contract 120024 “Self Sensing Composites – SSC,” in particular, the Methusalem and Hercules Foun- dations Flanders. The work of C. Sonnenfeld and T. Geernaert was supported by the Research Foundation – Flanders (FWO). C. Sonnenfeld, S. Sulejmani, T. Geernaert, H. Thienpont, and F. Berghmans are with the Brussels Photonics Team (B-PHOT), Vrije Universiteit Brussel, 1050 Brussel, Belgium (e-mail: csonnenf@b-phot.org; ssulejma@b-phot.org; tgeernae@b-phot.org; hthienpo@b-phot.org; fberghma@vub.ac.be). S. Eve and M. Gomina are with the University of Caen-Basse- Normandie, CRISMAT UMR 6508/ENSICAEN/CNRS, 14050 Caen, France (e-mail:sophie.eve@ensicaen.fr; moussa.gomina@ensicaen.fr). P. Mergo, M. Makara, and K. Skorupski are with the Laboratory of Op- tical Fibres Technology, Maria Curie-Sklodowska University, 20-031 Lublin, Poland (e-mail:pawel.mergo@poczta.umcs.lublin.pl; mariusz.makara@poczta. umcs.lublin.pl; kskorups@hermes.umcs.lublin.pl). Color version of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JLT.2014.2322201 One of the application areas of MOFs is in the ﬁeld of optical ﬁber sensors. For example, the combination of MOFs and ﬁber Bragg grating sensors (FBGs) for strain sensing applications has been reported in a number of papers [4]–[7]. The integra- tion of MOFs equipped with FBGs into ﬁber-reinforced polymer composites and the capacity of monitoring in situ the deforma- tion of a material has also already been reported [8]–[10]. The use of MOFs in applications, where the ﬁbers experience ther- momechanical loads calls for a thorough assessment of their mechanical reliability, and hence, for extended studies that aim to determine their mechanical failure characteristics. This is the subject of our paper. The mechanical reliability of standard optical glass ﬁbers used in telecommunication applications has already been extensively studied [11]. Fused silica is a brittle material that experiences stress corrosion: in a non-inert environment, cracks that initi- ate from surface defects grow and propagate under any applied stress [12]. The time to failure of optical glass ﬁbers, thus, de- pends on their stress history. For ﬁbers experiencing low strain levels, as in most telecommunication applications, failure de- pends on the presence of structural defects. Defects in optical ﬁbers have been studied for many years [13] with the aim to better understand their development and to predict failure. A ﬁrst type of defects results from production irregularities lead- ing, for example, to impurities at the glass-coating interface. They are typically very occasional. The second type of defects stems from particles contained within the ﬁber preform or em- bedded at the ﬁber surface. Such contamination originates from the draw environment conditions or from surface debris during splicing events. The occurrence of this second type of defects has decreased over the years along with the improvement of manufacturing processes. To ensure a minimum strength, weak spots are eliminated by proof-testing the ﬁbers up to stress levels of typically 0.7 GPa [13]. This procedure removes the weakest defects and yields ﬁber lengths of a known minimum strength. Crack growth models have been developed that start from the assumption that the strength is controlled by the initial defect geometry. For glass ﬁber, ﬂaws are usually located on their cladding surface (corresponding to the silica ﬁber exterior lateral surface). It has been shown that the strength distribution of a pristine ﬁber is usually narrow and unimodal and that its mechanical behavior can be predicted with satisfactory accuracy [13]. Compared to standard optical ﬁbers, MOF cross-sections feature a smaller glass surface area, while the structure along the axis obviously involves a larger glass surface area. The inner hole surface, deﬁned as the lateral surfaces of the air holes of the microstructure region, is not coated and is directly exposed to 0733-8724 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.