This communication describes the fabrication of Yb doped silica preforms by modified chemical vapor deposition
(MCVD) method using Yb(C<sub>11</sub>H<sub>19</sub>O<sub>2</sub>)<sub>3</sub> and AlCl<sub>3</sub> in vapor phase. In order to investigate the optical quality of the
preforms a systematic spectroscopic study was carefully carried out on various samples with different Yb and Al doping
levels. In this way, the influence of the preforms composition on the Yb spectroscopic properties was studied in details,
allowing the prediction of the composition favoring the laser emission with the highest efficiency. The predictions have
then been validated after pulling the preforms in large mode area fibers (LMA).