We develop a simple method leading to the determination of important parameters for the praseodymium-doped fluoride fibre amplifier (PDFFA), such as the non-radiative energy transfer upconversion coefficient or pump-excited state absorption cross section . We apply this method to five ZBLAN bulks of different concentration (from 10 000 down to 500 wt ppm) and to a ZBLAN: 500 wt ppm single-mode fibre. The resulting non-radiative energy transfer upconversion coefficients are at 500 wt ppm and at 1000 wt ppm. When the pump wavelength is set at 1017 nm, is very weak: less than . We measured that the population varies according to the pump power raised to the power 2.7. We conclude that the upconversion mechanism leading to the excitation of states in the PDFFA is different from all that which has been previously suggested and we propose a three-photon process. The application of our method requires the measurement of radiative emission branching ratios from levels and that we compare to the values calculated with the Judd - Ofelt analysis. We show that this analysis has to be used very cautiously with ions. Finally, we discuss the distribution of ions in ZBLAN glass. The fitting of fluorescence decay curves from level with the Inokuti-Hirayama model shows that the distribution of ions in ZBLAN is nearly homogeneous and no clustering effects are detected. Nevertheless, the non-exponential fluorescence decay curve of level even at very low concentrations is explained by the fact that the ions do not all have exactly the same environment in the glass.

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