0018-9499 (c) 2016 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2016.2644981, IEEE Transactions on Nuclear Science > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—We evaluate the potential of the cathodoluminescence (CL) spectroscopy to characterize the nature and the spatial distribution of point defects in the main classes of optical fibers (OFs): Telecom-grade, radiation-hardened, and radiation sensitive. Canonical samples, that are differently doped in their cores (Ge, N, P, Ce) or their claddings (F), have been investigated through CL technique using a 10 keV electron beam. Obtained results are compared with those obtained by photoluminescence spectroscopy. CL benefits and limits are discussed on the basis of the obtained experimental data. CL is shown to be efficient to investigate the kinetics of defect generation and bleaching under the electron exposure, being the unique technique allowing to determine in situ the spatial distribution of emitting defects in the fiber transverse cross sections. New insights are given for some of the defects related to the Ge, P, Ce and N dopants. Index terms– Optical fibers, cathodoluminescence, radiation, point defects, dosimetry, telecom. I. INTRODUCTION Silica-based Optical Fibers (OFs) can be divided into three different classes according to their employment in harsh environments: Telecom-grade, radiation-hardened, and radiation sensitive. The main effects that are observed when these optical fibers are exposed to ionizing or non-ionizing radiations are: (i) Radiation Induced Attenuation (RIA) that is responsible of the degradation of the transmitted signal caused by the radiation induced absorbing point defects; (ii) Radiation Induced Emission (RIE) resulting from the light emitted by some of the defects, being excited by the radiation; and (iii) the compaction that leads to the larger refractive index changes [1]. Telecommunication remains the main applicative field of the OFs, as their low losses, small size and multiplexing capacity made them far more attractive than copper cables [2]. In this typology of fibers, hereafter referred as telecom-grade, the OF I. Reghioua, S. Girard, A. Alessi, D. Di Francesca, A. Boukenter and Y. Ouerdane are with Univ-Lyon, Lab. Hubert Curien, F-42000 Saint-Etienne, France; (phone: (33)4 69663269; e-mail: imene.reghioua@univ-st-etienne.fr ; sylvain.girard@univ-st-etienne.fr; antonino.alessi@univ-st-etienne.fr ; diego.di.francesca@univ-st-etienne.fr ; aziz.boukenter@univ-st-etienne.fr; ouerdane@univ-st-etienne.fr). M. Raine and N. Richard are with CEA, DAM, DIF, F-91297 Arpajon Cedex, France (e-mail: melanie.raine@cea.fr; nicolas.richard@cea.fr ). M. Fanetti, L. Martin-Samos and M. Valant are with Materials Research Laboratory, University of Nova Gorica, Vipavska 11c 5270-Ajdovscina, Slovenia; (e-mail: mattia.fanetti@ung.si ; lmartinsamos@gmail.com; matjaz.valant@ung.si ). is doped with other elements, principally the germanium in its core, since the Ge dopant increases the refractive index permitting the light confinement in the fiber core [3]. Today, these OFs are exploited for a larger number of applications, some of them being associated with radiation environments. Their radiation tolerance is sufficient for operation at moderate dose levels (< 10 kGy), eg. for data transfer ([4], [5]) or sensing at infrared wavelengths. The increase of the silica refractive index is not the sole consequence of the Ge doping, its introduction in the silica matrix is also accompanied by formation of some Ge-related defects that can act as precursors for other types of radiation induced defects [6, 7]. Among the Ge related point defects we may cite: E'Ge, Ge(1), Ge(2) and Germanium Lone Pair Center (GLPC) [8]. For the aim of the present study, we focus our attention on the GLPC, which is one of the major luminescent defects present in germanosilicate glasses. Furthermore, as previously reported, these defects strongly affect the fiber radiation response [9]. GLPC consists in a twofold coordinated Ge, with two electrons forming a lone pair. This defect is characterized by two absorption bands at ~5.1 eV and ~3.8 eV, and two emission bands at ~4.2 eV and ~3.1 eV [10]. The development of radiation tolerant sensing devices for operation in severe harsh environments encouraged the development of a second category of OFs: the so-called radiation hardened [1]. Such OFs should resist to higher levels of dose (typically MGy dose levels) and to different types of radiation (from -rays to neutrons). Pure-silica-core (PSC) fibers with F-doped cladding have shown a higher tolerance to such radiations for a wide range of applications [11, 12]. For these fibers, the main point defects present before and/or after irradiation are: SiE', Non Bridging Oxygen Hole Center (NBOHC), Peroxy Radical (POR), Oxygen Deficient Centers (ODC(I), ODC(II)), Self-Trapped Hole (STH) and Self- Trapped Exciton (STE). Focusing our attention on the emitting centers in the UV/Visible spectral range, we remind that Si- ODC(II) is formed by a twofold coordinated Si with two nonbonding electrons forming a lone pair. This defect is responsible for an absorption band at ~5 eV and two emission bands at 4.4 eV and 2.7 eV [10, 13]. The NBOHC is constituted by a neutral oxygen dangling bond and is at the origin of an emission band at 1.9 eV. An absorption band at 2 eV [13, 14] and other bands in the range from 4 eV to 8 eV were assigned to NBOHCs [13]. N-doped core silica OFs can also be considered radiation hardened, because of their comparable radiation-induced losses to those of the PSC fibers in the near infrared (NIR) spectral range [15]. Moreover, under pulsed X-rays irradiation, these Cathodoluminescence Characterization of Point Defects in Optical Fibers I. Reghioua, Student Member IEEE, S. Girard, Senior Member IEEE, M. Raine, Member, IEEE, A. Alessi, D. Di Francesca, M. Fanetti, L. Martin-Samos, N. Richard, Member, IEEE, M. Valant, A. Boukenter, and Y. Ouerdane.