The effect of thermal and mechanical processing on the fluorescence lifetime of Yb-doped silica preforms and fibers for use in nanostructured-core fiber lasers

Abstract

In this paper, we investigate the influence of various nanostructured-core fiber fabrication processes, such as preform elongation or fiber drawing, on the fluorescence lifetime of Yb³⁺ ions. The optical fiber preform was prepared using Modified Chemical Vapor Deposition (MCVD) method combined with Al₂O₃ nanoparticle doping. The optical fiber preform was subjected to various processing treatments involving heat and mechanical stresses, i.e. preform elongation and fiber drawing, and the fluorescence lifetime was measured in all stages of fiber fabrication, i.e. original preform, elongated preform (cane), fiber and overcladded fiber. It was found that the time-resolved photoluminescence properties of Yb³⁺ ions in silica glass are strongly dependent on the processing of the material. The fluorescence lifetime of the 2F5/2 level of Yb³⁺ ions decreased with the heat and mechanical treatment, which was explained by the break-up of Al₂O₃ nanoparticles, diffusion of dopants and changes in the Yb³⁺ phonon environment as well as clustering of the Yb³⁺ ions. The fiber drawing exhibited a stronger effect compared to preform elongation which was ascribed to the high rate of cooling and mechanical stresses during the drawing process. In general, the heat and mechanical processing of Yb-doped optical fiber preforms leads to a deterioration of time-resolved photoluminescence properties.