Micro-Raman investigation of X or c irradiated Ge doped ﬁbers A. Alessi a,⇑ , S. Girard b , C. Marcandella b , M. Cannas c , A. Boukenter a , Y. Ouerdane a a Laboratoire H. Curien, UMR CNRS 5516, Université Jean Monnet, 18 rue du Pr.Benoît Lauras 42000, Saint-Etienne, France b CEA, DAM, DIF, F-91297 Arpajon, France c Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, I-90123 Palermo, Italy article info Article history: Received 14 February 2011 Received in revised form 22 March 2011 Available online 26 March 2011 Keywords: Optical ﬁber Drawing condition Radiation effects Raman spectroscopy abstract Micro-Raman spectra have been recorded on Ge doped optical ﬁbers before and after 10 keV-X or c-ray irradiation up to doses of 1 MGy (X-ray) or 7.8 MGy (c-ray). Our data provide evidence that, at such dose levels, the glass matrix is not modiﬁed in a detectable way. We observed that varying the Ge doping lev- els from 0 to about 11 wt.%, X or c radiation sensitivity of the overall matrix remains unchanged. Such results are observed for ﬁbers obtained with drawing conditions within the usual range used for the fab- rication of specialty ﬁbers as radiation-tolerant waveguides. Our data support the potentiality of ﬁber- based sensors using glass properties, e.g. Raman scattering, for applications in harsh environments as those encountered in nuclear power plants. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Silica glass is widely employed in different technological ﬁelds [1,2] and one of the most important is the production of optical ﬁ- bers. Among the different classes of ﬁbers, the Ge doped ones are the most used types, mainly for telecommunications applications. Ge doping was ﬁrst used to increase the refractive index of the in- ner part (core) of the ﬁbers with respect to that of their external part (cladding) to ensure the light guiding [1]. The researcher’s attention on this class of ﬁber increased when the Ge doped ﬁbers revealed to be sensitive to UV exposures, allowing the writing of ﬁber Bragg gratings [1,3] and when the possibility to induce non linear effects was shown [1,4]. The ﬁber radiation responses, and particularly the one of Ge- doped waveguides, have been widely investigated [5–11]. Researcher’s objective was to improve the comprehension of radi- ation-induced processes for a better control on the ﬁber features needed by various applications. Two main mechanisms are proposed to explain the photosensi- tivity of the Ge-doped materials: the so called ‘‘color center’’ and that named ‘‘compaction effects’’. In the ﬁrst one, the photosensi- tivity is attributed to the generation or the conversion of point de- fects. These defects can explain, through their optical activities and the Kramers–Kroning relation, the measured refractive index changes in [6]. In the second model, the refractive index changes are attributed to a reorganization of the glass matrix as reported for example in [11]. The material density changes have been observed in UV irradi- ated Ge-doped ﬁbers [12,13]. Similarly, modiﬁcation of the matrix properties have been pointed out on germanosilicate samples irra- diated by 50 keV electron beam [14,15]. This information is not easily extrapolated to X and c irradiations, since the conversion to X or c doses is not well known. In parallel to the studies of radiation effects, the inﬂuence of the production parameters (deposition–fabrication temperature of the preform [16], tension, speed and temperature drawing of the ﬁber [17–19]) on Ge-doped glasses has been widely investigated too. In the present investigation, we report data regarding the Raman activity, before and after X and c irradiations, of various Ge-doped optical ﬁbers differing by their doping proﬁles and draw- ing parameters. We focused our study on high doses since optical ﬁbers are more and more considered for use in high dose radiation environments, mainly as part of strain or temperature sensors [20]. Many of these sensors and components are based on Raman or Brillouin scattering, so their performances will strongly depend on the glass matrix radiation response. 2. Experimental We have studied optical ﬁbers produced by iXFiber SAS. The samples have been obtained starting from two preforms realized using the modiﬁed chemical vapor deposition technique. The ﬁber named FGe is doped with different amounts of Ge according to a 4- steps radial distribution (maximum content of 11 wt.% at the core center, see Fig. 1). The ﬁber FGe has been produced using a drawing speed of 40 m/min, a tension of 70 g, and a furnace-tempera- ture of 1600 °C. At variance the ﬁbers, named FGeDi (i = 1,2,3), 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.03.021 ⇑ Corresponding author. E-mail address: antonino.alessi@univ-st-etienne.fr (A. Alessi). Nuclear Instruments and Methods in Physics Research B 269 (2011) 1346–1349 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb