## 3 Results
### Ab Initio Calculations of Bulk Dixacgerminare Defects  
To identify the relevant bleaching/creation mechanisms of GLPC point defects in O\({}_{2}\)-loaded germanosilicate optical fibers, we have carried out a few dedicated first principle calculations. Since DIOG has been proposed as one of the possible defects involved in the oxygen-induced bleaching mechanisms of GLPC previously [10], we have obtained by first principle relaxation several DIOG point defects in a Ge-doped silica supercell model [15]. The DIOG structure involves an isosceles quasi-equilateral triangle OGeO, in which the two oxygen atoms of the O\({}_{2}\) molecule are bonded to the Ge of the GLPC, as schematically illustrated in Figure 6a. Figure 5: Normalized radial profiles, to the luminescence intensity detected at a distance of 15 \(\upmu\)m from the core center, of the last recorded monochromatic images at 400 nm in the O\({}_{2}\)-loaded sample (squares) and in the unloaded one (diamonds). The molecular oxygen atoms form Ge-O bonds with an average Ge-O distance of 1.82 (\(\pm\)0.03) A to be compared with the typical Ge-O bond distance (1.73-1.78 A) in GeO\({}_{2}\)[42]. Furthermore, the O-O distance of 1.61 (\(\pm\)0.01) A increases by about 30% with respect to the one of the free O\({}_{2}\) molecules (1.21 A), resulting in a narrow O-Ge-O angle (Figure 6a) with mean value of 52.3\({}^{\circ}\) (\(\pm\)1.0\({}^{\circ}\)), in agreement with the geometric parameters reported previously [31]. Such a small angle involves considerably strained \(p\)-orbitals. Despite the likely stress on orbitals and consequent destabilization of the bonds, our calculations indicate that the DIOG configuration is a very robust bulk configuration, and 70% of our DIOGs could be properly relaxed with force convergence thresholds as small as those typically used (\(\sim\)0.0025 eV A\({}^{-1}\)) for vibrational spectra calculations [39; 42]. The presence of a DIOG defect will cause the appearance of defect levels in the band gap of silica. In particular, the highest occupied molecular orbital (HOMO) level of a DIOG configuration is located \(\sim\)2 eV above the top of the silica valence band, and it corresponds to \(p\)-orbitals of the O\({}_{2}\) oxygen atoms which are bonded to the GLPC (Figure 6b). The formation energies of DIOG and GLPC centers have been calculated following the procedure outlined previously [15]. The calculated formation energy of the DIOG provides a mean value of \(\sim\)4.5 eV, while the formation energy for a GLPC defect is \(\sim\)0.6 eV larger. The latter discrepancy indicates that the absorption of an oxygen molecule at a GLPC site is a favorable process. This is understandable since the DIOG represents a step towards a healing path for the Ge-doped silica network, since it restores the fourfold coordination of the Ge atom. Overall, these results indicate that the dioxagermirane defects are good candidates for explaining the interaction of the excess oxygen with Ge lone pair defects (i.e., GLPC) in bulk silica. In fact, the latter interaction has been proven to annihilate a large portion of GLPC/ODC-II defects [10]. To better characterize the excess-oxygen absorption process at a GLPC, NEB simulations have been performed on Ge-doped silica to further evaluate if the absorption of an O\({}_{2}\) molecule at a GLPC site is a favorable process. Thus, silica configurations containing the DIOG defect and conversely the GLPC + O\({}_{2}\) configurations have been considered to be initial and final reaction configurations (Figure 7a,d). The calculated activation energies for the reaction of oxygen release from a DIOG defect span from 1.0 to 1.6 eV, with an average value of approximately 1.3 eV. Such an activation energy value falls in the upper bound of the range of activation energy values [\(\sim\)0.9 to 1.3 eV] reported in the literature for the migration of oxygen in silica [33; 43; 44].