## 3 Results  
### In Situ CL Investigation of GLPC Kinetics in the \(O_{2}\)-Loaded Sample  
In this section, we present experimental results of the CL in situ measurements. Figure 2 illustrates the normalized CL spectra of the Ge-doped optical fiber in the O\({}_{2}\)-loaded and unloaded samples. We can easily observe that the luminescence at 400 nm is the main emission band in the core of both samples, and it is attributed (according to the literature) to the GLPC defects [24]. The normalized spectra are very similar and there is no change in the shape of the GLPC emission at 400 nm in the CL spectra, which indicates, at least from the spectroscopic point of view, that we are studying the same kind of centers. We then followed on-line the kinetics of the emitting point defects using CL imaging. Many CL monochromatic images at 400 nm (\(\pm\)5 nm) were recorded as a function of the irradiation time up to \(\sim\)30 min, then the luminescence profiles were extracted from these images following the method used previously [24]. In  
Figure 1: Geant4 simulation of the penetration depth and deposited energy distribution of 106 electrons (10 keV) in SiO\({}_{2}\). (**a**) (Y,Z) 2D cartography, (**b**) (X,Y) 2D cartography. Figure 2: Normalized CL intensity taken in the center of the core of the Ge-doped optical fiber in the O\({}_{2}\)-loaded and untreated samples. first (Figure 3a) and the last (Figure 3b) recorded monochromatic images of the sample. The different Ge-doped zones are distinguished by blue circles, where the central part is the one with \(\sim\)9 wt% of Ge and the surrounding part is doped with \(\sim\)5 wt% of Ge, as mentioned above. Comparing the two images, we can observe an increase of the luminescence signal after 30 min of irradiation. Moreover, the outer part seems to be brighter than the inner one. This indicates that the electron irradiation leads to an increase in the luminescence of the GLPC at 400 nm. In the first image, the CL signal is low since GLPC centers were bleached by the O\({}_{2}\)-loading treatment as previously reported [10]. Such behavior differs from what is observed for nonloaded fibers [24], where the starting signal is high and is reduced by electron irradiation until it reaches a stable value [24]. With the aim of creating a better visualization of the above reported images, radial profiles were extracted following the method previously explained [24]. Figure 4a demonstrates the radial profiles of the first and the last CL monochromatic images, whereas Figure 4b presents the in situ kinetics of the GLPC luminescence as a function of the electron irradiation, which in this case is represented by the number of scans in the different Ge-doped regions. Basing on the results reported in Figure 4, we can notice that in the radial profile of the first scan, the intensity of the GLPC signal is slightly lower in the higher Ge-doped zone (i.e., for a distance from the center of the core \(<\) 20 \(\upmu\)m). After 30 min of irradiation, the CL signal at 400 nm indicates an increase, which is still smaller in the higher Ge-doped region  
Figure 4: (**a**) Radial profiles of the first and the last recorded monochromatic CL images at 400 nm. (**b**) In situ kinetics of the GLPC CL at 400 nm as a function of the electron irradiation in the different Ge-doped zones. The estimated dose is 670 kGy per scan. In (**a**) vertical dashed lines are used to indicate the zoning of the fiber in terms of the core (Ge content of 9 and 5 wt%) and the cladding regions. Figure 3: CL monochromatic images acquired at 400 nm of the O\({}_{2}\) loaded fiber of (**a**) the first scanned image, (**b**) after 30 min of electron irradiation. than in the outer 5 wt% Ge-doped region. Following on line the kinetics in both zones (Figure 4b), we can observe that the increase in the GLPC signal is more efficient in the zone doped with 5 wt% as compared to the 9 wt% Ge-doped zone. Indeed, in the lower-doped zone, the signal increased by a factor of \(\sim\)2.