Nd:Yb-codoped silica fibers for high power fiber lasers: fluorescence and laser properties

We have doped the core of Ytterbium (Yb) laser fibers additionally with Neodymium (Nd) to exploit wavelength-multiplexed high-power pump systems, which are commercially available. By pumping such a Nd:Yb-codoped fiber with the 808/940/978 nm-diode system, we could demonstrate CW output powers of more than 1 kW with high laser slope efficiency. In order to get a better understanding of this laser medium, we studied the fluorescence and the laser behavior of Nd:Yb fibers with different rare earth concentrations in comparison to a fiber doped solely with Nd. A theoretical model for the calculation of the fluorescence decay curve and spectrum as well as the laser characteristic and wavelength was developed, that takes the energy transfer process from Nd to Yb ions into account. Comparing the experimental and theoretical results, the behavior of the Nd:Yb high power fiber laser is understood as a collective emission of both ion types within the same wavelength region. These investigations contribute to the optimization of high power fiber lasers under the viewpoint of thermal load.