Optimization of the Radiation Response of Backup Optical Fiber Amplifiers for Space Missions

We investigated the X-ray radiation impact on the performances of “backup” Erbium Doped Fiber Amplifiers (EDFAs) and Erbium-Ytterbium Doped Fiber Amplifier (EYDFA). These devices are exposed to the space radiation constraints, unpowered in the “OFF” mode, up to their hypothetic activation during the mission. This atypical profile of use must be considered as it could lead to a higher radiation vulnerability. To carry this study, active optical fibers differing in terms of radiation hardness, have been exposed to steady-state X-rays under different profiles of amplifier use: the gain degradation is measured by varying the pumping conditions, that drive the photobleaching phenomenon, during the radiation exposure: “ON” (100% ON), “OFF” (0% ON), and “OFF-ON-OFF” with powering 2% or 10% of the time. Our results show that the impact of the pump and its related photobleaching efficiency depend on the selected active fiber to build the amplifier. An important result is that the “backup” EDFA designed with radiation-hardened Ce-doped fiber does not suffer from an extra-degradation due to the “OFF” mode, contrary to the one designed with a non-rad-hard fiber. For this latter one, it is indeed possible to reduce the extra-gain degradation by powering the amplifier for short time periods during the “OFF” state, otherwise the EDFA recovers this extra-gain degradation in a few hours after turning “ON” in our test conditions. For the tested radiation-tolerant EYDFA, an enhanced vulnerability is observed for the “OFF” amplifier, that can be reduced too by briefly powering the amplifier. The origins of these effects are discussed basing on the properties of point defects related to the aluminosilicate (EDFA) and phosphosilicate (EYDFA) glasses of the fiber.