Doped sol-gel materials for the production of optical fibers

The sol-gel technology offers highly interesting prospects for fiber fabrication. The main advantage is the possibility to incorporate any dopants and co-dopants soluble in a suitable solvent. Generally, these dopants are homogeneously dissolved in the matrix. The objective of this work is to produce fiber cores with the sol-gel technique, to fabricate ytterbium doped sol-gel fiber lasers, and to investigate their properties. The sol-gel route was adapted to successfully build ytterbium doped sol-gel fiber lasers. This wet chemistry process involves water and alcohol, but the hydroxyl groups of these liquids highly attenuate guided light. The fiber core deposition was therefore extended with thermal annealing at 1800 °C for vitrification and to minimize the hydroxyl group content in the fiber core. In case of a mono-mode fiber laser the core was built with three 2.7 µm layers. These layers were deposited with this intermediate vitrification applied between subsequent layers. In the case of multi-mode fibers six layers were applied similarly. After collapsing, the preforms were drawn to fibers. The produced fibers were investigated in view of structural and optical properties. The sol-gel fibers show cores with a radial structure in their cross sections. This structure is also known from the modified chemical vapor deposition (MCVD) technique. The stress distribution within sol-gel fibers is in the same range as in MCVD fibers. The fibers show losses of 31 dB/km at 1100 nm. Laser action has been demonstrated. With thresholds between 32 mW and 90 mW a slope efficiency of up to 73 % has been reached. The maximum output of 44 mW was limited by the available pump power.