Progress in the fabrication of optical fibers by the sol-gel-based granulated silica method

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.