A series of Yb-doped silica-based nanoparticles were fabricated in the MCVD process. Their compositions and doping
levels were well controlled from 0 to 0.53 wt% for Yb and from 0 to 2.1 wt% for Al. The nanoparticles are of about 21.3
± 4.6 nm in size, but their sizes and NIR absorption are influenced by the Al concentration. At a fixed Al doping level,
the NIR absorption depends on the Yb concentration. The nanoparticles exhibit a strong composition dependence of the
photodarkening induced by the X-ray irradiation.
Correlation between Yb<sup>3+</sup> optical properties and structural disorder of glass network has been investigated by using good
quality Yb-doped silica glasses fabricated by MCVD method. Absorption spectra strongly depend on the fictive
temperature, which is a good indicator of structural disorder in silica glass. Their dependences are affected by the
codopants (Al and F). Fictive temperature dependence of refractive index has been also elucidated.
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).