The Interplay between Radiation-induced Attenuation, Photodarkening and Photobleaching at Pump Wavelength in Er- and Yb-doped Silica Optical Fibers
Résumé
Erbium-and ytterbium-doped silica-based fibers (EDF and YDF respectively) amplifiers are a reliable technology to achieve high-power fiber laser sources operating around 1 and 1.5 microns. These sources are of primary interest for a variety of industrial and medical applications. Their reduced weight, cost, size and power consumption also make EDF or YDF an attractive option to design compact and e±cient optical amplifiers for aerospace applications, in fiber optic gyroscopes (FOG) or in the high-power laser sources required for future optical inter-satellite links (OISL) or spacecraft-embedded remote sensing by LIDAR. Their implementation in harsh radiative environments, as space, is though complicated by the fact that silica fibers are damaged by ionizing radiations. An excess optical loss, referred to as "radiation-induced attenuation" (RIA), indeed builds up from UV to NIR spectral ranges. The RIA is due to intrinsic or dopant-related color centers that are formed by the trapping of free carriers released upon ionization of inner or valence electronic states. It can be probed by monitoring the continuous transmission decay, under irradiation, of a light launched in the fiber. But this probe light is likely to "bleach" the RIA (photobleaching, PB), even in the case of low-energy pump photons (at 980 nm typically). Besides PB, the pump may be responsible for an opposite "photodarkening" (PD) eAEect where the cooperative de-excitation of a few excited rare-earth ions leads to intra-gap charge transfers that again result in the formation of color centers. These PD-related color centers are of the same nature as part of those involved in the RIA. Therefore, the intrinsic RIA cannot be separated from PB and PD: the measured RIA at 980 nm results in fact from a complex interplay between RIA, PD and PB. A few empirical hardening routes were found that greatly improve the resistance of silica fibers against RIA and PD, based for instance on cerium co-doping or hydrogen loading. However, mechanisms of the RIA, PD and their PB remain unclear. There is still a crucial need in understanding the interplay between these eAEects to make decisive advances in its physical modelling. This is a strong prerequisite for allowing predictive simulations of the EDF or YDF operation in given radiative and pumping conditions. This presentation will specifically address these two steps: proper characterization and validated modelling. RIA is most often characterized using few meter-long fibers to comply with actual operation lengths. However, the pump and ASE powers distributions, contributing to PD or/and PB, are markedly inhomogeneous along such fibers. This precludes the assessment of their intrinsic impact on the measured length-integrated RIA, which then depends on extrinsic parameters as the amplifier length or pumping scheme. To approach the conditions of a local characterization, we present RIA measured at 980 nm on 1-2 cm-long fibers. Such short samples are free from amplification and ASE. The pump power is initially uniform along the fiber, equal to the well-defined power launched at the sample end. These conditions reveal new remarkable properties, both in EDF and YDF, as the existence of completely reversible local RIA equilibrium. Notably, these equilibrium levels are only determined by the couple [dose rate, pump power] given to the sample, and not by the total accumulated dose (they can decrease while the dose increases!). The eAEect of PD is negligible in EDF but very important in YDF. Pump-induced eAEects are shown to mitigate (mainly PB) or enhance (mainly PD) the radiation-induced degradation, depending whether the current RIA level is above or below the equilibrium PD level (reached when PD is balanced by PB in the absence of external irradiation). From these observations, the roles of intrinsic RIA, PD and PB can be separated. Local models, one for Er-and one for Yb-doped fibers, are introduced that apparently include the su±cient physics to ensure both qualitative and quantitative reproduction of experimental features. The way these models, based on non-linear coupled diAEerential equations, complement and modify the set of standard equations used to calculate the time-and lengthresolved pump, signal and ASE powers in usual fiber amplifier simulation tools is presented.