We investigated the codoping of different high silica glasses (GeO2-SiO2-B2O3-SiO2) with flouring with respect to the use of such glasses in actual optical fibres devices (fibre Bragg gratings, fibre lasers and amplifiers, optical filters, etc). It could be shown that there is a strong interaction between fluorine and the other dopants germanium and boron during gas phase deposition of the glassy layers. The doping concentration in dependence on the experimental conditions (gas concentrations, flow velocity, temperature) can be understood and quantitatively described on the basis of the chemical thermodynamics (equilibrium chemistry) of the systems. This makes possible the defined preparation of complex fiber structures in terms of refractive index profiles and multi element distributions.
