UP-CONVERSION MECHANISMS OF A PRASEODYMIUM-DOPED FLUORIDE FIBER AMPLIFIER

We develop a simple method leading to the determination of important p arameters for the praseodymium-doped fluoride fibre amplifier (PDFFA), such as the non-radiative energy transfer upconversion coefficient (c T(1)) or pump-excited state absorption cross section (sigma(ESAp)). We apply this method to five ZBLAN bulks of different concentration (fro m 10 000 down to 500 wt ppm) and to a ZBLAN:Pr3+ 500 wt ppm single-mod e fibre. The resulting non-radiative energy transfer upconversion coef ficients cT(1) are 5 s(-1) at 500 wt ppm and 9 s(-1) at 1000 wt ppm. W hen the pump wavelength is set at 1017 nm, sigma(ESAp) is very weak: l ess than 5 x 10(-26) cm(2). We measured that the P-3(0) population var ies according to the pump power raised to the power 2.7. We conclude t hat 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 pr eviously suggested and we propose a three-photon process. The applicat ion of our method requires the measurement of radiative emission branc hing ratios from levels (1)G(4), D-1(2) and P-3(0.1)-I-1(6) that we co mpare to the values calculated with the Judd-Ofelt analysis. We show t hat this analysis has to be used very cautiously with Pr3+ ions. Final ly, we discuss the distribution of Pr3+ ions in ZBLAN glass. The fitti ng of fluorescence decay curves from level D-1(2) with the Inokuti-Hir ayama model shows that the distribution of Pr3+ ions in ZBLAN is nearl y homogeneous and no clustering effects are detected. Nevertheless, th e non-exponential fluorescence decay curve of level (1)G(4) even at ve ry low concentrations is explained by the fact that the Pr3+ ions do n ot all have exactly the same environment in the glass.