Upconversion mechanisms of a praseodymium-doped fluoride fibre amplifier

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 (cT(1)) or pump-excited state absorption cross section (sigma(ESAp)). We apply this method to five ZBLAN bulks of different concentration (from 10 000 down to 500 wt ppm) and to a ZBLAN:Pr3+ 500 wt ppm single-mode fibre. The resulting non-radiative energy transfer upconversion coefficients cT(1) are 5 s(-1) at 500 wt ppm and 9 s(-1) at 1000 wt ppm. When the pump wavelength is set at 1017 nm, sigma(ESAp) is very weak: less than 5 x 10(-26) cm(2). We measured that the P-3(0) population varies according to the pump power raised to the power 2.7. We conclude that the upconversion mechanism leading to the excitation of P-3(0-1)-I-1(6) 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 (1)G(4), D-1(2) and P-3(0.1)-I-1(6) that we compare to the values calculated with the Judd-Ofelt analysis. We show that this analysis has to be used very cautiously with Pr3+ ions. Finally, we discuss the distribution of Pr3+ ions in ZBLAN glass. The fitting of fluorescence decay curves from level D-1(2) with the Inokuti-Hirayama model shows that the distribution of Pr3+ ions in ZBLAN is nearly homogeneous and no clustering effects are detected. Nevertheless, the non-exponential fluorescence decay curve of level (1)G(4) even at very low concentrations is explained by the fact that the Pr3+ ions do not all have exactly the same environment in the glass.