## 2 Simulation and Experimental Procedure
### RIA Measurements  
In order to evaluate the performances of EDFA under the ionizing radiation, the simulation code needs to know how the transmissions at the pump and signal wavelengths are degraded under radiation, so RIA measurements were also carried out. To evaluate the evolution of RIA as a function of the absorbed dose, the EDF, introduced in Section 2.1, was irradiated in the MOPERIX facility of the Laboratoire Hubert Curien (Saint-Etienne, France). This is characterized by a Tungsten tube operating with a 100 kV voltage, that gives a mean energy fluence of 40 keV.  
In the previous work of our research group, [PERSON] et al. [28] showed that adding periods of high pump power during the fiber testing reduces the measured RIA at the pump and signal wavelengths. They used an experimental setup for the RIA measurements that allowed them to inject, considering time slots of 15 min, the pump signal into the FUT during the 80% of the time, while during the remaining 20%, only a low-power light source was injected to allow the RIA spectral acquisition. The percentage of time in which the pump signal is not injected into the fiber can cause an overestimation of the RIA obtained during the operation of an EDFA. Indeed, during the fiber testing, there is a lower contribution of the PB to the RIA reduction. This RIA overestimation was potentially identified as one of the causes of the slight observed disagreement between measured and simulated gain degradation kinetics on a similar EDFA, observed at high TIDs in a previous work [28]. For this reason, in this work, we developed a new experimental setup for RIA measurements, that was able to reduce the percentage of time during which the pump is not injected into the fiber.  
Figure 2 reports the experimental setup used for the RIA measurement. The signals of two sources, the 1999 CHP pump diode at 980 nm by 3S Photonics (Nozay, France) and a halogen white light source from Ocean Optics (Orlando, FL, USA), were injected through an optical switch (belonging to the Thorlabs PRO8000-4 rack) into the EDF. The white light signal was then collected, thanks to another switch synchronized to the first one, with an IR spectrometer (the NIRQuest spectrometer by Ocean Optics), while the pump was recorded using the YOKOGAWA Optical Spectrum Analyzer (OSA) (Tokyo, Japan). The considered EDF sample had a length of 19 cm.  
The two-switch system allows one signal to pass through the EDF at a time. Using this type of system, we can adjust the percentage of time the pump signal is injected into the EDF through a LabVIEW 2015 program. Obviously, the pump signal cannot be injected 100% of the time into the EDF, as we need the white light signal to determine the impact of ionizing radiation on the fiber transmission. However, we can obtain very high percentages of time in which the pump signal is injected into the EDF, such as 99%.  
Figure 2: Experimental setup of the RIA measurement used in this work.  
In this work, two different dose rates were investigated: 0.28 Gy/s (SiO\({}_{2}\)) and 0.093 Gy/s (SiO\({}_{2}\)) up to a TID of 3 kGy. Moreover, at each dose rate, two irradiation runs were carried out, employing different percentages of time in which the pump was injected into the EDF, as highlighted in Table 4. The pump power used in these measurements was 50 mW, the same used for gain degradation measurements. This kind of measurement was carried out at a temperature of 25 \({}^{\circ}\)C.