The actual challenge for space researchers is to increase the free space telecommunications data speed transfer. One of the most promising solutions is the optical communication systems. This technology can be used for the inter-satellite and/or satellite-ground links, reaching the TB/s speed for data transfer in the case of Dense Wavelengths Division Multiplexing (DWDM) based technologies. However, to achieve such systems, two main issues need to be overcome: the first one is to validate that no unexpected radiation effect appears when the optical amplifier working in the DWDM configuration and the second one is to estimate the degradation of the Erbium/Ytterbium co-doped boost (High Power - HP) amplifier performances during the space mission lifetime. In this last case, the used high powers will result in a complex response of the amplifier due to photobleaching, photodarkening and thermal effects. In this work, we estimate the radiation effects on an Er/Yb co-doped boost amplifier operating in a Dense WDM configuration. Both radiation hardened and a conventional versions of EYDFA have been considered. The obtained results allow estimating the performances of our fibers under exposure in such amplification setup and also to validate its potential for use in an actual space mission. We demonstrate the good radiation resistance of Er/Yb co-doped 12 μm core diameter fibers reaching 20 W of output power for telecommunication applications. This core diameter provides a fewmode optical output signal (with low dispersion) and with enough power to ensure the signal propagation trough the atmosphere. This study is fundamental as several phenomena such as Photo/Thermal bleaching, photo-darkening… are in competition due to the high-power light density in the fiber core and the system radiation response cannot yet be predicted by actual simulation tools.
In these ICSO proceedings, we review recent advances from our group concerning the radiation hardening of optical fiber and fiber-based sensors for space applications and compare their benefits to state-of-the-art results. We focus on the various approaches we developed to enhance the radiation tolerance of two classes of optical fibers doped with rare-earths: the erbium (Er)-doped ones and the ytterbium/erbium (Er/Yb)-doped ones. As a first approach, we work at the component level, optimizing the fiber structure and composition to reduce their intrinsically high radiation sensitivities. For the Erbium-doped fibers, this has been achieved using a new structure for the fiber that is called Hole-Assisted Carbon Coated (HACC) optical fibers whereas for the Er/Ybdoped optical fibers, their hardening was successfully achieved adding to the fiber, the Cerium element, that prevents the formation of the radiation-induced point defects responsible for the radiation induced attenuation in the infrared part of the spectrum. These fibers are used as part of more complex systems like amplifiers (Erbium-doped Fiber Amplifier, EDFA or Yb-EDFA) or source (Erbium-doped Fiber Source, EDFS or Yb- EDFS), we discuss the impact of using radiation-hardened fibers on the system radiation vulnerability and demonstrate the resistance of these systems to radiation constraints associated with today and future space missions. Finally, we will discuss another radiation hardening approach build in our group and based on a hardening-by-system strategy in which the amplifier is optimized during its elaboration for its future mission considering the radiation effects and not in-lab.
Rare-earth doped optical fibers (REDF, Er or Er/Yb-doped) are a key component in optical laser sources (REDFS) and amplifiers (REDFA). The high performances of these fiber-based systems made them as promising solution part of gyroscopes, telecommunication systems… However, REDFs are very sensitive to space radiations, so their degradation limits their integration in long term space missions. To overcome this issue, several studies were carried out and some innovations at the component level were proposed by our group such as the Cerium co-doping or the hydrogen loading of the REDF. More recently we initiated an original coupled simulation/experiment approach to improve the REDFA performances under irradiation by acting at the system level and not only at the component itself. This procedure optimizes the amplifier properties (gain, noise figure) under irradiation through simulation. The optimization of the system is ensured using a PSO (Particle Swarm optimization) algorithm. Using some experimental inputs, such as the Radiation Induced Attenuation (RIA) measurements and the spectroscopic features of the fiber, we demonstrate its efficiency to reproduce the amplifier degradation when exposed to radiations in various experimental configurations. This was done by comparing the obtained simulation results to those of dedicated experiments performed on various REDFA architectures. Our results reveal a good agreement between simulations and experimental data (with <2% error). Finally, exploiting the validated codes, we optimized the REDFA design in order to get the best performances during the space mission and not on-ground only.