Optical properties of Yb-doped laser fibers in dependence on codopants and preparation conditions

In the last years, the performance of ytterbium doped high power silica fiber lasers has been remarkably improved. Increasing attention has been paid to the role of material composition, impurities and atomic defects, because the extreme power load and the complicated fiber structure make great demands on material properties and preparation technology. Recently, with the further growth of output power, the problem of photodarkening has been identified as a critical issue for fiber laser devices. Here we report on developments aimed at increasing efficiency and power stability of ytterbium doped laser fibers. The optical properties of preforms and fibers made by MCVD and codoped with high phosphorus contents were investigated for a wide range of the ytterbium concentration. Moreover, the influence of the collapsing atmosphere on the properties was studied. The observed spectral effects, photodarkening and laser efficiency were correlated with changes in fiber composition and process conditions. It could be shown that phosphorus codoping leads to remarkably different and advantageous fiber properties compared with the usual aluminium codoping.