## 3 Results
### RIA Measurements Results  
The main parameter that affects the gain under radiation is the RIA. But, because of the PB effect induced by the high pump power, the RIA must be measured in conditions as close as possible to the ones of an EDFA. The focal point of this work is to measure the RIA while injecting the pump with the highest percentage possible, in order not to overestimate the RIA level, and then to compare the measured behavior of EDFA under irradiation with the simulated one. The RIA was monitored up to a maximum dose of 3 kGy and during the post-irradiation recovery phase, which lasts 2 h for the dose rate of 0.28 Gy/s and 6 h for 0.093 Gy/s.  
Figure 6 shows the spectral RIA obtained at the maximum dose for the four different irradiation conditions, whose specifications are listed in Table 4. The obtained RIAs agree with those reported in [25; 28] and are mainly due to the tail of the Al-OHC point defects, whose absorption band is centered in the visible range around 540 nm, and to a not-yet-well-known defect absorbing in the IR [25]. Thus, whereas at 980 nm, the RIA is associated with the presence of the Al-OHCs, at 1550 nm, on the other hand, the RIA is linked to the unknown center. For both dose rates, it can be noted that the RIA is lower as the percentage of time the pump signal was injected into the EDF increases: the higher the percentage of time the pump signal was injected into the fiber, the greater the impact of the PB and the lower the RIA.  
Furthermore, it can be observed that for the same pump percentage for both dose rates (97%), the RIA is lower for the lower dose rate, as already observed for this EDF but also for other fibers. This is due to the fact that with a lower dose rate the defects have more time to recombine during the irradiation run [41; 42].  
Figure 7a,b show the RIA as a function of the absorbed dose, at the pump and signal wavelengths, respectively.  
The RIA values at 980 nm and 1550 nm, reported in Figure 7a,b, were averaged over a 4 nm wide range around the wavelength of interest, obtaining a relative error of the RIA less than 3%.  
Figure 6: Spectral RIA obtained at 3 kGy TID for the four different case studies.  
At both wavelengths, the RIA increases almost linearly with the dose, as expected for aluminosilicate matrix. As already observed in Figure 6, by increasing the pump percentage, and getting closer and closer to the EDFA real application case, the RIA amplitude is reduced. Moreover, a lower dose rate corresponds to a lower RIA. It is interesting to note that, in Figure 7a, during the recovery phase, the RIA at 980 nm decreases slightly, with a recovery rate of 10%, whereas the RIA at 1550 nm (Figure 7b) shows a slight recovery only at the highest investigated dose rate and with the lower percentage of time using the pump. For the other irradiation runs (#2, #3 and #4), no recovery is observed at 1550 nm. This may be explained as follows: the not-well-known defect that characterizes the RIA at 1550 nm and that is generated under these irradiation conditions, is created at lower concentration but it appears also to be more stable.  
It is worth noticing that he RIA values employed in the calculation code were measured on a 19 cm-long sample, while the optimal active fiber length for the EDFA is 4.3 m. The impact of PB varies as a function of the sample length, which means that it should not be uniform. However, the RIA measurement cannot be carried out on a sample of the same length as the fiber used for the gain degradation measurements, which is 4.3 m, as the losses of the pristine fiber result in being too high [28]. We can only make the hypothesis that the measured RIA is a good approximation of the real situation.