Effects of fluorine on the properties of Yb/Ce co-doped aluminosilicate preforms prepared by MCVD with organic chelate precursor doping technique

Co-doped Yb/Ce aluminosilicate fiber preforms with less than 1 Wt% fluorine were prepared by MCVD with organic chelate precursor doping technique at 1400 °C. Fluorine content characterized by electron probe microanalysis decreased with increasing SiF4 flow and flow ratio of SiF4/SiCl4. Under maximum SiF4 flow the concentrations of Yb2O3 and CeO2 in the perform core were only as much as 17% and 25% under minimum SiF4 flow, respectively, but smaller refractive index change was only shown in fiber core. The reason was that the reaction of SiF4 with Al2O3,Yb2O3,CeO2 were intensified with increasing flow ratio of SiF4/SiCl4 and then resulted in producing more exhaust gas such as AlF3, YbF3 and CeF4. Raman spectra of core glasses with < 0.5 Wt% fluorine present different Raman shifts from F doped silica glass with 1–3.3 Wt% fluorine: 945 cm− 1 of SiF stretching bond was not observed, and with growth of fluorine content antisymmetric stretching vibrations of Si-O-Si linkages split from an envelope of 1188 cm− 1 into two peaks or three peaks shifted higher frequency. At the same time, Boson peak near 125 cm− 1 shifted to lower frequency. They were attributed to some minor changes of glass's network structure due to introducing lower fluorine into core glasses. On base of bonding energy measured by X-ray photoelectric spectroscopy, the plane graph describing the complex bonding distribution with F in core was supplied to in order to illustrate network structure change.