RARE-EARTH DOPED MG-SILICATE NANOPARTICLES IN SILICA FIBER: MOLECULAR DYNAMICS SIMULATIONS FROM THE PREFORM TO THE FIBER

An enhancement of the spectroscopic performance of rare-earth-doped silica optical fibers is still required for new photonics applications. An interesting route to tailor their optical behavior consists in embedding rare-earth ions within dielectric nanoparticles in the core of optical fibers. Experimentally, such nanoparticles can be produced in situ through spontaneous phase separation phenomenon within a MgO-SiO2 binary melt, during melt/quench sequences of MCVD fabrication process of the preform 1,2. Then, fibers are obtained by drawing at high temperature a preform containing nanoparticles. First report on the drawing process reveals an elongation of the nanoparticles in the drawing direction as well as a breakup of the larger ones 3. In this Molecular dynamics study, we use a new simple transferable model 4 to show that phase separation occurring in the MgO-SiO2 binary melt leads to the separation of liquid phases with mixed composition: Si-rich Mg-poor phases on one hand, Mg-rich Si-poor phases on the other hand. These latter phases, the so-called nanoparticles, are amorphous, non-spherical and exhibit a wide range of sizes. Mg-O coordination and MgO content increase with the nanoparticle size. With rare-earth doping, the larger nanoparticles are over-concentrated in luminescent ions. We show that the rare-earth clustering effect is prevented, compared with a pure silica matrix. Finally, at high temperature, we apply a uniaxial elongation to the nanostructured preform to mimic the experimental drawing step leading to the fiber. We report here on the effects of this drawing process on the nanoparticles characteristics.