A. Alessi

Evolution of Photo-induced defects in Ge-doped fiber/preform: influence of the drawing

We have studied the generation mechanisms of two different radiation-induced point defects, the Ge(1) and Ge(2) centers, in a germanosilicate fiber and in its original preform. The samples have been investigated before and after X-ray irradiation using the confocal microscopy luminescence and the electron paramagnetic resonance techniques. Our experimental results show the higher radiation sensitivity of the fiber as compared to the perform and suggest a relation between Ge(1) and Ge(2) generation. To explain our data we have used different models, finding that the destruction probability of the Ge(1) and Ge(2) defects is larger in fiber than in preform, whereas the generation one is similar. Finally we found that the higher radiation sensitivity of the fiber at low doses is essentially related to the presence of germanium lone pair center generated by the drawing.

Transient Radiation Responses of Optical Fibers: Influence of MCVD Process Parameters

A dedicated set of fibers elaborated via the Modified Chemical Vapor Deposition (MCVD) technique is used to study the influence of composition and drawing parameters on their responses to an X-ray pulse representative of the radiation environments associated with Megajoule class lasers. These canonical fibers were designed to highlight the impact of these parameters on the amplitude and kinetics of the transient pulsed X-ray Radiation Induced Attenuation (RIA) at room temperature. From preforms differing by their core composition, three optical fibers were elaborated by varying the tension and speed during the drawing process. No or only slight RIA change results from the tested variations in drawing process parameters of Ge-doped, F-doped, and pure-silica-core fibers. This study reveals that the drawing process is not the main parameter to be optimized in order to enhance the radiation tolerance of MCVD specialty optical fibers for the LMJ harsh environment. From the hardness assurance point of view, a specialty fiber sufficiently tolerant to this environment should be robust against changes in the drawing process. The origins of the RIA observed in the different fibers are discussed on the basis of spectral decomposition of their measured RIA spectra, using sets of defects from the literature and related to the different core dopants. This analysis highlights the limits of the well-known defect set to reproduce the RIA above 1 μm for Ge-doped fibers whereas self-trapped holes and chlorine-related species seem responsible for the transient responses of pure-silica-core and F-doped fibers.

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.

Twofold coordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO 2

We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to γ ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by γ radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern writing through ionizing radiation.

Influence of fluorine on the fiber resistance studied through the nonbridging oxygen hole center related luminescence

The distribution of Non-Bridging Oxygen Hole Centers (NBOHCs) in fluorine doped optical fibers was investigated by confocal microluminescence spectroscopy, monitoring their characteristic 1.9 eV luminescence band. The results show that these defects are generated by the fiber drawing and their concentration further increases after γ irradiation. The NBOHC concentration profile along the fiber provides evidence for an exponential decay with the fluorine content. This finding agrees with the role of fluorine in the fiber resistance and is discussed, from the microscopic point of view, by looking at the conversion mechanisms from strained bonds acting as precursors.

Influence of the manufacturing process on the radiation sensitivity of fluorine-doped silica-based optical fibers

In this work, we analyze the origins of the observed differences between the radiation sensitivities of fluorine-doped optical fibers made with different fabrication processes. We used several experimental techniques, coupling in situ radiation-induced absorption measurements with post mortem confocal microscopy luminescence measurements. Our data showed that the silica intrinsic defects are generated both from precursor sites and from strained regular Si-O-Si linkages. Our work also provides evidence for the preponderant role of the chlorine in determining the optical losses at about 3.5 eV. The results show that the manufacturing process of these fibers strongly affects their radiation response.

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.

Investigation by Raman Spectroscopy of the Decomposition Process of HKUST-1 upon Exposure to Air

We report an experimental investigation by Raman spectroscopy of the decomposition process of Metal-Organic Framework (MOF) HKUST-1 upon exposure to air moisture (  K, 70% relative humidity). The data collected here are compared with the indications obtained from a model of the process of decomposition of this material proposed in literature. In agreement with that model, the reported Raman measurements indicate that for exposure times longer than 20 days relevant irreversible processes take place, which are related to the occurrence of the hydrolysis of Cu-O bonds. These processes induce small but detectable variations of the peak positions and intensities of the main Raman bands of the material, which can be related to Cu-Cu, Cu-O, and O-C-O stretching modes. The critical analyses of these changes have permitted us to obtain a more detailed description of the process of decomposition taking place in HKUST-1 upon interaction with moisture. Furthermore, the reported Raman data give further strong support to the recently proposed model of decomposition of HKUST-1, contributing significantly to the development of a complete picture of the properties of this considerable deleterious effect.

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.

EPR on Radiation-Induced Defects in SiO2

Continuous-wave electron paramagnetic resonance (EPR) spectroscopy has been the technique of choice for the studies of radiation-induced defects in silica (SiO2) for 60 years, and has recently been expanded to include more sophisticated techniques such as high-frequency EPR, pulse electron nuclear double resonance (ENDOR), and pulse electron spin echo envelope modulation (ESEEM) spectroscopy. Structural models of radiation-induced defects obtained from single-crystal EPR analyses of crystalline SiO2 (α-quartz) are often applicable to their respective analogues in amorphous silica (a-SiO2), although significant differences are common.