Silica optical fibers doped with Yb3+ and Er3+

Two novel modifications of the MCVD process extended with the 'solution doping' technique are presented and used for reproducible preparation of preforms with a high content of P2O5 (up to 20 mol.%) and rare-earth ions (up to 36000 ppm) in the Al2O3-P2O5-SiO2 core. Erbium-doped and ytterbium-sensitized silica optical fibers have been fabricated by this method and used for the investigation of fiber lasers emitting at 1550 nm when pumped at 1064 nm. The relation between the core composition, conditions during the stages of the preform fabrication and the final fiber properties are discussed resulting in a conclusion that lasing of the fibers is influenced by central perturbations of the refractive- index profile. Fibers which can be characterized by a slope efficiency of up to 20% and laser threshold of 400 - 500 mW measured under Fresnel reflection from the fiber end have been fabricated.