Ge- and Al-related point defects generated by gamma irradiation in nanostructured erbium-doped optical fiber preforms

Erbium-doped amplifiers (EDFAs) are of special interest for space applications. In this environment, the ionizing radiations decrease the gain of these optical amplifiers, due to the ionization of defects precursors, mainly linked to dopants as Germanium (Ge), Aluminum (Al), or Phosphorus (P). The aim of this work is to study the influence of the Ge and Al relative concentration on the radiation resistance of different nanostructured fiber preforms, manufactured by Modified Chemical Vapor Deposition (MCVD), in which various types of nanoparticles (Er@SiO₂-NP, Al₂O₃-NP, and Er@Al₂O₃-NP) have been introduced in the silica matrix. The radiation resistance of these fibers has been compared with that of standard MCVD Er-doped preforms. All of them have been characterized by optical absorption and Electronic Paramagnetic Resonance (EPR) spectroscopies before and after irradiation with a total gamma dose of 5.9 kGy. EPR results show that Al-related defects are not observed in fiber preforms with Ge concentrations higher than 4.4 wt%. We also demonstrated that NP technology can limit the formation of Aluminum-Oxygen Hole Centers (AlOHCs), reducing the Radiation-Induced Attenuation at the energy of interest for EDFAs.