Diego Di Francesca

Coupled Theoretical and Experimental Studies for the Radiation Hardening of Silica-Based Optical Fibers

We applied theoretical and experimental spectroscopy tools to ad hoc silica-based “canonical” samples to characterize the influence of several dopants and of some drawing process parameters on their radiation sensitivities. We present in this paper, the recent advances and results occurring from our coupled approach. On the experimental side, we studied the doping influence on the response of optical fibers and showed that changing the drawing parameters has a negligible influence on the fiber response in the case of specialty fibers. We focus mainly on the SiE defect that is observed through Electronic Paramagnetic Resonance (EPR) measurements in all canonical samples. On the theoretical side, we exhibit the improvements obtained in the calculations of electronic and optical properties of defects by using Many Body Perturbation Theory through the use of the GW approximation and the resolution of the Bethe-Salpeter equation instead of the Density Functional Theory (Local Density Approximation). To continue to strengthen the link between experiment and simulation, we have performed first-principles calculations of EPR parameters of some silica-based defects. The first results allowing for an attribution of EPR E signals to structural models are presented. In particular, we confirm that the EY center is originated by an unpaired electron in a sp3 state at a three-fold coordinated silicon atom.

X-ray irradiation effects on fluorine-doped germanosilicate optical fibers

We report an experimental investigation on the effects of fluorine codoping on the radiation response of Ge-doped Optical Fibers (OFs) obtained by three different drawing conditions. The OFs were irradiated with 10 keV X-rays up to 300 Mrad and studied by online Radiation-Induced-Attenuation (RIA) measurements. Confocal Micro- Luminescence (CML) and Electron Paramagnetic Resonance (EPR) were also employed to investigate the permanent radiation-induced-defects. The variation of the Germanium-Lone-Pair-Center (GLPC) and Non-Bridging- Oxygen-Hole-Centers (NBOHC) concentration with the radiation dose is investigated by CML, whereas the ones of the induced Ge(1), Ge(2) and Eʹ centers by EPR. No relevant differences are found in the RIA of the three fibers, as well as in the induced concentrations of Ge(1) and Ge(2) and in the decrease of the GLPC, showing minor relevance of changing the drawing conditions. We found that fluorine codoping does not affect the RIA and that, unexpectedly, the fluorine co-doped zones of the OFs show an enhanced photoluminescence of the radiation induced NBOHC enabling to suggest the presence of both Si and Ge variants. Moreover, an overall increase of the radiation induced Eʹ(Ge) centers is registered in relation to the presence of fluorine showing that this codopant has relevant effects.

Gamma and x-ray irradiation effects on different Ge and Ge/F doped optical fibers

We performed electron paramagnetic resonance (EPR) measurements on γ and X ray irradiated Ge doped and Ge/F co-doped optical fibers. We considered three different drawing conditions (speed and tension), and for each type of drawing, we studied Ge and Ge/F doped samples having Ge doping level above 4% by weight. The EPR data recorded for the γ ray irradiated fibers confirm that all the samples exhibit a very close radiation response regardless of the drawing conditions corresponding to values used for the production of specialty fibers. Furthermore, as for the X irradiated materials, in the γ ray irradiated F co-doped fibers, we observed that the Ge(1) and the Ge(2) defects generation is unchanged, whereas it was enhanced for the EGe. In the various fibers, the comparison of the γ and X-ray induced concentrations of these kinds of Ge related defects indicates that the two irradiations induce similar effects regardless of the different employed dose rates and sources. Confocal microscopy luminescence results show that the starting content of the Germanium Lone Pair Center (GLPC) is neither strongly affected by the Ge content nor by the drawing conditions, and we consider the similarity of the GLPC content as key factor in determining many of the above reported similarities.

Radiation Response of Ce-Codoped Germanosilicate and Phosphosilicate Optical Fibers

We report an experimental investigation on the effects of Ce-codoping in determining the radiation response of germanosilicate and phosphosilicate Optical Fibers (OFs) in the UV-Visible domain and up to doses of 1 MGy(SiO2). We show that the addition of Ce strongly impacts the Radiation Induced Attenuation (RIA) of both types of fibers. In the first case the radiation induced losses increase, whereas in the second one decrease. By combining the online RIA measurements with the Electron Paramagnetic Resonance (EPR) ones, we are able to infer the basic microscopic mechanisms taking place under irradiation, which involve the cerium codopant and some of the known Ge-related or P-related defects. More precisely, we found that part of the Ce atoms are incorporated in the glass matrix as Ce3+ ions by the production process and act as electron donor centers under irradiation. Consequently, the concentrations of radiation induced hole centers of Ge and P are drastically reduced. The reported results give an insight into possible ways of exploiting Ce codoping to control the radiation sensitivity of the OFs. Moreover, the OFs doped with cerium and phosphorous show a strongly reduced saturation effect at high radiation doses that make them a potential candidate for RIA-based dosimetry applications in a wide range of radiation doses.

Proton Irradiation Response of Hole-Assisted Carbon Coated Erbium-Doped Fiber Amplifiers

We investigated the behavior of a new class of erbium-doped fiber amplifier (EDFA) when exposed to 63 MeV protons. The EDFA is designed with a radiation hardened hole-assisted carbon coated (HACC) Er3 + -doped optical fiber. The particular structure of this HACC fiber allows to permanently incorporate an optimal amount of D2 or H2 gases into its core, reducing its radiation sensitivity without degrading the EDFA performances. Irradiations up to a fluence of 7.5 ×1011 p/cm2 confirm the excellent tolerance of this HACC-EDFA component. It exhibits a limited decrease of ~ 0.6 dB of its ~ 27 dB gain for this fluence corresponding to an ionization dose of 100 krad(Si). Such a device can then survive to the radiative environments associated with both todays space missions and future more challenging applications.

Near infrared radio-luminescence of O2 loaded radiation hardened silica optical fibers: A candidate dosimeter for harsh environments

We report on an experimental investigation of the infrared Radio-Luminescence (iRL) emission of interstitial O2 molecules loaded in radiation hardened pure-silica-core and fluorine-doped silica-based optical fibers (OFs). The O2 loading treatment successfully dissolved high concentrations of oxygen molecules into the silica matrix. A sharp luminescence at 1272u2009nm was detected when 2.5u2009cm of the treated OFs were irradiated with 10u2009keV X-rays. This emission originates from the radiative decay of the first excited singlet state of the embedded O2 molecules. The dose, dose-rate, and temperature dependencies of the infrared emission are studied through in situ optical measurements. The results show that the iRL is quite stable in doses of up to 1 MGy(SiO2) and is linearly dependent on the dose-rate up to the maximum investigated dose-rate of ∼200 kGy(SiO2)/h. The temperature dependency of the iRL shows a decrease in efficiency above 200u2009°C, which is attributed to the non-radiative decay of the excited O2 molec...

O 2 -Loading Treatment of Ge-Doped Silica Fibers: A Radiation Hardening Process

The effects of a high-pressure O2-loading treatment on the radiation response of Ge-doped optical fibers (OFs) were investigated. We found that the incorporation of high concentration of interstitial molecular oxygen remarkably enhances the resistance to ionizing radiation of Ge-doped OFs in the UV-Visible domain and, at the same time, improves the transmission of UV light in the unirradiated OF sample. By comparison with previously reported results, the O2-loading treatment turned out to increase the radiation resistance of Ge-doped OFs more efficiently than F or Ce codoping. The understanding of such amelioration relies in basic radiation-induced mechanisms that were characterized with three complementary experimental techniques: Confocal microluminescence (CML), online radiation-induced attenuation (RIA), and electron paramagnetic resonance (EPR). We have shown that the almost intrinsic oxygen-deficient character of germanosilicate fibers can be overturned by forcing O2 diffusion in the glass matrix. The Germanium lone pair centers, which are precursor defects invariantly present in the as-drawn Ge-doped OFs, are converted to some other yet-undetermined species. Consequently, the usual chain of radiation-activated processes leading to the creation of Ge(1) and Ge(2) is substantially suppressed. The experiments have also highlighted an increased production of oxygen-excess related defects under irradiation. Although in terms of RIA, the tradeoff between the oxygen-excess and oxygen-deficient defects is already a positive one, it is conceivable that the radiation resistance of Ge-doped OFs can be further improved by optimizing the O2-loading treatment.

Irradiation Temperature Influence on the In Situ Measured Radiation Induced Attenuation of Ge-Doped Fibers

We report an experimental investigation on the radiation-induced attenuation (RIA) in the ultraviolet-visible domain for Ge-doped optical fibers, during X-ray (10 keV) exposure at different temperatures. The objective is to characterize the impact of the irradiation temperature on the RIA levels and kinetics. Our data highlight that for dose exceeding 1 kGy(SiO2) the RIA spectrum changes with the irradiation temperature. In particular, for wavelengths below 470 nm the RIA depends both on the dose and on the irradiation temperature, whereas at higher wavelengths the RIA depends only on the dose. From the microscopic point of view the origin of this behavior is explained by a larger impact of the irradiation temperature on the Ge(1) defects generation mechanism with respect to the one of GeX defects, which appears as poorly temperature sensitive in the tested range. This finding prevents us from easily establishing a conclusive relation between the generation mechanisms of these two types of defects. The lower content of radiation induced Ge(1), in fiber irradiated at higher temperature, is supported by the electron paramagnetic resonance (EPR) results acquired after the irradiation. In situ RIA and postmortem EPR data show a significant correspondence of the Ge(1) growth as a function of the dose. Confocal microscopy luminescence experiments indicate that the non-bridging oxygen hole center concentration is higher at 473 K in comparison with those observed at 300 and 373 K.

Cerium Codoping Effect on the Radiation Response of Germanosilicate and Phosphosilicate Multimode Optical Fibers

We investigate the effects of cerium codoping on the radiation response of germanosilicate and phosphosilicate optical fibers in the UV-Visible spectral range. The samples were irradiated by X-rays up to 1 MGy(SiO2) dose level.

On-site Regeneration Technique for Hole-Assisted Optical Fibers Used In Nuclear Facilities

In this paper, we demonstrate and highlight a proof of concept for the feasibility of an innovative technique to regenerate on-site irradiated optical fiber links in nuclear facilities. Using Hole-Assisted optical fibers (HAOF), a longitudinal gas-loading is easy to perform thanks to the fibers dedicated holes located in the outer part of the cladding. All along the fiber length, gas ( H2 or D2) diffuses from the holes into the silica matrix, interacts with radiation induced point defects and passivates them, reducing the Radiation Induced Attenuation (RIA) levels. The validity of our approach is demonstrated considering the changes occurring at infrared wavelengths during the H2 treatment of a MGy irradiated single mode Ge-doped HAOF. Within just a few hours, a reduction of about 50% is observed for the RIA at 1550 nm of the 10 MGy irradiated HAOF, acting only from one of its two ends. An additional study is done on a set of fibers with various core dopants (F, Ge, P) and without holes to give an overview of the pertinence of developing HAOF fibers with these dopants for various applications. Using HAOF and this recovery technique appears very promising for samples based on pure-silica, Ge or F-doped cores and operating in the ultraviolet-visible spectral domains such as plasma diagnostics. This approach exhibits another interesting feature which may be extension to higher dose ranges and lifetime of P-doped distributed dosimeters used in high energy physics facilities or nuclear power plants.