Influence of aluminum-phosphorus codoping on optical properties of ytterbium-doped laser fibers

Ytterbium-doped high-power fiber lasers with high beam quality are promising devices for a variety of applications. Extreme power load and complicated fiber structures make great demands on material properties and preparation technology. Recently, with the further increase of output power, the problem of photodarkening has been identified as a critical issue for fiber laser devices. Detailed knowledge of optical properties of materials and fibers are needed for the successful development of laser fibers, aimed at increasing efficiency and power. It is well known that the properties of silica based rare earth doped fibers can be influenced and remarkably improved by the incorporation of further dopants. Here, the influence of combined aluminium-phosphorus codoping on the optical properties of Yb doped laser fibers was investigated. Preforms and fibers were prepared by MCVD and solution doping both with phosphorus and aluminium excess. The samples were characterized concerning the radial distribution of refractive index and dopant concentrations and the absorption and emission properties in the UV/VIS/NIR region. The observed spectral effects and active properties (laser efficiency, photodarkening) were correlated with changes in the fiber composition. It could be shown, that the combined doping leads to effects which deviate from a simple additivity and which can be beneficially utilized for the improvement of the laser fiber performance.