Improvement of the Tm:H4 level lifetime in silica optical fibers by lowering the local phonon energy

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 fibers is studied. Modifications of the core composition affect the spectroscopic properties of Tm³⁺ ion. Adding 17.4 mol% of AlO_3/2 to the core glass caused an increase of the ³H₄ level lifetime up to 50 μs, 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 PO_5/2 in the core glass made the lifetime decrease down to 9 μs. 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.