  The results of measurements of the emission and absorption cross section for
the transition 2F5/2 -> 2F7/2 of Yb3+ ions in Al2O3 and P2O5 doped silica
fibers are presented. Different techniques based on spectroscopic data, and
more direct technique involving optical saturation of the transition are
employed. The data obtained indicate an essential difference in cross section
spectra of Yb3+ ions in alumosilicate and phosphosilicate fibers.
