  The role of some glass network modifiers on the quantum efficiency of the
near-infrared fluorescence from the 3H4 level of Tm3+ ion in silica-based doped
fibres is studied. Modifications of the core composition affect the
spectroscopic properties of Tm3+ ion. Adding 17.4 mol% of AlO3/2 to the core
glass caused an increase of the 3H4 level lifetime up to 50 $\mu$s, 3.6 times
higher than in pure silica glass. The quantum efficiency was increased from 2%
to approximately 8%. On the opposite, 8 mol% of PO5/2 in the core glass made
the lifetime decrease downto 9 $\mu$s. These changes of Tm3+ optical properties
are assigned to the change of the local phonon energy to which they are
submitted by modifiers located in the vicinity of the doping sites. Some
qualitative predictions of the maximum achievable quantum efficiency are
possible using a simple microscopic model to calculate the non-radiative
de-excitation rates.
