## 4 Discussion  
In order to have a better understanding of the basic mechanisms at the origin of the fiber degradations shown in Section 3, a fitting routine of all the RIA spectra was performed. To this end, we choose to use a set of Gaussian curves already proposed in the literature to describe the absorption bands associated with a set of color centers known to be present in the irradiated optical fibers (OFs) [14], and described as follows:  
\[G(E)=\frac{A}{\sqrt{2\pi\sigma^{2}}}e^{-\frac{(E-E_{\rm C})^{2}}{2\sigma^{2}}} \tag{6}\]  
where A is the area of the band, \(E_{\rm C}\) is the center of the optical absorption band, while the standard deviation \(\sigma\) is related to the Full Width at Half Maximum (FWHM) as \(\mathrm{FWHM}=2\sqrt{2\ln 2}\sigma\). While to date the decomposition of RIA in the UV-visible range of silica-based materials is rather well known and understood, still a number of issues need to be clarified with regard to certain color centers typically identified in OFs, such as strain-assisted self-trapped holes (s-a STH) [34; 35; 36]. To the fit purpose, the values used for the \(E_{\rm C}\) Gaussians centers and for their FWHMs remain fixed parameters, whereas the amplitude A is left as a free parameter eVolving during the irradiation. Being all samples PSC, in the fitting routine we have considered only intrinsic defects and eVentually impurities, such as Chlorine related centers [24; 37]. Since the only source of extrinsic processes in the fibers under test is attributed to the OH content [38], we started our analysis from the _low_-OH content sample, being this fiber associated with the lowest  
Figure 4: Radiation-induced attenuation (RIA) spectral growth during the irradiation (left) up to 300 kGy(SiO\({}_{2}\)) and spectral decrease during the recovery (right) in the _medium_-OH+H\({}_{2}\)+TT sample. Both insets show the respective RIA kinetics at wavelengths of 450 nm and 650 nm. probability for the process expressed in Equation (3). The result of the fitting routine obtained on the RIA at 300 kGy is shown in Figure 5a. This analysis highlights the large contribution of the color center centered at 3.26 eV (380 nm) attributed to Cl\({}^{0}\), as well as the contributions of the STHs with two absorption bands centered at 2.61 eV (475 nm) and 2.16 eV (574 nm), corresponding to the STH\({}_{1}\) and STH\({}_{2}\) point defects respectively. Furthermore, it was deliberately chosen not to introduce in the fitting routine the absorption band related to the Cl\({}_{2}\) given the significant error that would be committed when assessing its amplitude, considering that it is centered at 3.78 eV (328 nm) and then at quite far energies from those for which our experimental data are available. Table 2 reports the best-fit parameters of the Gaussian absorption bands of each point defect used in the decomposition of the three fibers responses. To compare the different spectral profiles, we have reported in Figure 5b the normalized spectra, with respect to the RIA values measured at 2.8 eV (450 nm). This normalization was done in order to qualitatively highlight the disparity between the three samples. Indeed, the appearance of a new spectral feature in the range 1.7-2.7 eV (730-460 nm) is eVident with analogous spectral characteristic compared to the one linked to NBOHC centered at -2.0 eV, strongly supporting the NBOHC generation through Equation (3), in accordance with the OH amount registered in the pristine optical fiber. To isolate the NBOHC contribution, we have then decided to perform the fitting routine on the differences between the normalized RIA (\(\Delta\)RIA) of the _high_-OH and _medium_-OH samples with respect to the _low_-OH sample. In addition, considering the different models of NBOHC contribution around 2 eV we have decided to adopt the model reported by [PERSON] et al. in [39] rather than the more standard symmetrical Gaussian peaked at -2 eV reported in [14]. It is important to remark that the adopted model reconstructs the