## 2 Materials and Methods  
A two-step Ge-doped optical fiber produced by iXBlue Photonics Division [35] is studied in the present investigation. Such fiber has a total diameter of \(\sim\)125 \(\upmu\)m and a 62.5 \(\upmu\)m core diameter. The inner part of the core has a diameter of \(\sim\)40 \(\upmu\)m and a Ge content of \(\sim\)9 wt% (weight per cent), and such part is separated from the pure silica cladding by a ring made of 5 wt% Ge-doped silica, with a width of about 10 \(\upmu\)m. The chemical composition of the investigated sample has been previously reported [24]. The O\({}_{2}\) loading of the sample was performed by keeping the sample in a PARR-4651 high pressure vessel at \(\sim\)400 \({}^{\circ}\)C for \(\sim\)21 days in a 90-bar oxygen atmosphere, according to previous investigation [10]. We stress that the presence of the O\({}_{2}\) inside such a treated fiber was tested by monitoring, as a function of the position along fiber diameters, the Raman line of the molecular oxygen [10,36]. Such studies proved that the content of O\({}_{2}\) is uniform along the fiber cross section with micrometer precision [10,36]. Independent experimental evidence of the effectiveness of our O\({}_{2}\)-loaded treatment can also be inferred by the radiation-induced near-infrared luminescence of interstitial O\({}_{2}\), which is present in the loaded samples and absent in the untreated ones [37]. Cathodoluminescence and electron irradiation (at room temperature) were performed simultaneously using a field emission-scanning electronic microscope (SEM) JEOL JSM 7100f, equipped with a GATAN MONOCL4 detector (spectral range 300-750 nm, 1.7-4 eV). The electron beam is used both to excite the luminescence of point defects and simultaneously to irradiate the sample. For the present investigation, we acquired CL monochromatic images by monitoring the intensity of the incoming CL signal from the fiber at the wavelength of about 400 (\(\pm\)5) nm (i.e., 3.1 eV), then we extracted the luminescence profile as a function of the position from the center of the fiber (and of the number of scans, i.e., time). Based on previous studies, we are confident that the signal at 400 nm is due to the GLPC emission [24]. We employed the following parameters: electron energy 10 keV, a probe current of 4.2 nA, and an exposition time of about 20 s per scan. The investigated area was adapted to capture the core of the optical fiber, so the size of all the CL images is (\(\sim\)90 \(\times\) 70) \(\upmu\)m\({}^{2}\). We note that for acquiring CL spectra, the scanned area by the electron beam is much smaller to facilitate the response of only the selected zone. For the same experimental conditions (beam energy, probe current...), the dose rate of the CL spectra is much higher compared to the CL imaging, with a difference of about 1.7 \(\times\) 10\({}^{7}\) Gy/s. To avoid large irradiation effects, we used CL imaging to evaluate the GLPC luminescence kinetics. The electron penetration and the accumulated dose were evaluated by [PERSON] [38], and the results are displayed in Figure 1. Particularly, the interaction volume depends on the energy of the primary electrons. For an energy E = 10 keV, the electrons could penetrate up to \(\sim\)1.3 \(\upmu\)m. According to the Monte Carlo simulations, the estimated dose rate for the recorded CL images using the above conditions is about 3.36 \(\times\) 10\({}^{4}\) Gy/s. The first principle calculations presented in this work are based on Density Functional Theory (DFT) and were performed using the Quantum-ESPRESSO (QE) package [39]. We have adopted local-density approximation (LDA) for describing the exchange-correlation functional, together with norm-conserving pseudopotentials. Wave functions are expanded on a basis of plane waves up to a kinetic cutoff of 80 Ry. The expansion is carried out at the sole \(\Gamma\) point of the Brillouin zone, as justified by the size of the adopted supercell and the large band gap of our silica-based system [15].