Spatial distribution effects and laser efficiency in Er/Yb-doped fibers

Abstract

It is well known that the efficiency of silica based Er/Yb fiber lasers and amplifiers can be greatly enhanced by a high level of phosphorus co-doping. The interaction of phosphorus with the rare earths and with the further co-dopant aluminium during the preparation process leads to peculiar effects concerning chemistry of dopant incorporation, diffusion behaviour and microscopic glass structure. Until now however, it has been disregarded that these interactions give rise to strong radial and even axial concentration profiles in preform and fiber (for the codopants phosphorus and aluminium, but also for the rare earth components erbium and ytterbium) with remarkable consequences for the laser efficiency. Here, we have studied the incorporation mechanism and the diffusion behaviour (diffusion coefficients in dependence on temperature and concentrations) by X-ray microprobe analysis concerning the dopant interaction effects. Moreover, we have investigated both fluorescence intensities and lifetimes, radially and axially resolved in the fiber preforms. The results were correlated with the properties of double clad fiber lasers made from these preforms. First attempts have been undertaken to interpret and control the incorporation mechanism on the basis of model calculations regarding phosphorus diffusion and chemistry.