Novel special optical fibers nowadays can take advantage of several new preform production techniques. During the last years we have devoted our attention to the granulated silica method. It is one of the variants of the powder-in-tube technique and potentially offers a high degree of freedom regarding the usable dopants, the maximum possible dopant concentration, the homogeneity of the dopants, the geometry and minimal refractive index contrast. We developed and refined an approach for the production of doped granulated silica material based on the sol-gel process.Here, we present material analysis results of an ytterbium (Yb) doped, aluminum (Al) and phosphorous (P) co-doped glass on the basis of our sol-gel glass based granulated silica method as well as first measurements of two LMA fibers obtained from this material. For the material analysis we used advanced analysis techniques, such as HAADF-STEM and STEM-EDX spectroscopy to determine the composition of the material and the distribution of the dopants and the codopants. The chemical mapping of the STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale. Based on self-made LMA fibers, we measured the refractive index contrast of the sol-gelbased granulated silica derived core compared to the pure silica cladding. In addition we quantified optical characteristics such as the emission and absorption spectrum. The measured upper state lifetime of the optical active dopant ytterbium was 0.99ms, which in turn confirms the homogeneous distribution of the Yb atoms. The propagation losses were determined to be 0.2dB/m at 633nm and 0.02414dB/m at1550nm.

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The sol-gel based granulated silica preform fabrication method is presented as a versatile "rapid prototyping" platform for specialty optical fiber production, enabling arbitrary geometries, large flexibility of doping composition and concentration, and homogeneous dopant distributions.

Combining the sol-gel method for fiber material production with the granulated silica method for preform assembly results in a robust method that offers a high degree of freedom regarding both the composition and the geometry of the produced fiber. Using this method, two types of Yb-doped silica glass composition, that feature an excess in P concentration with respect to Al, have been prepared. The elemental distributions in a fiber core were analyzed by scanning transmission electron microscopy (STEM). The elemental mapping shows a similar localization of Al, P and Yb through the microstructure. In addition, the influence of the variation in the co-dopant concentration, with respect to Yb, on the fiber properties has been investigated. The results show an increase in the refractive index step and in the fiber’s transmission loss as the excess concentration of P increases. A significant contribution to the losses can be assigned to the existence of impurities such as iron, which was detected in our samples by mass spectrometer. Single exponential fluorescence decays with lifetimes of around 0.88 ms were measured for the two compositions. Finally, pumping at 976 nm a laser slope efficiency of 67% at 1031 nm was achieved for one of the fiber compositions.