Energy transfer processes in Er3+-doped and Er3+,Pr3+-codoped ZBLAN glasses

We present a detailed characterization of energy transfer processes in Er3+-doped and Er3+,Pr3+-codoped ZBLAN bulk glasses. For several Er3+ (0.25-8.75 mol%) and Pr3+ (0.25-1.55 mol%) concentrations, we investigate energy transfer upconversion (ETU) and cross relaxation in Er3+ as well as energy transfer (ET) from Er3+ to the Pr3+ codopant. The measured parameters of ETU from the Er3+ I-4(13/2) and I-4(11/2) levels are comparable to those of LiYF4:Er3+. ETU from I-4(13/2), in particular, possesses a factor of 3 larger probability than ETU from I-4(11/2). The parameters of ET from the Er3+ I-4(13/2) and I-4(11/2) levels to the Pr3+ codopant are larger than the corresponding ETU parameters. ET effectively quenches the I-4(13/2) intrinsic lifetime of 9 ms down to 20 mu s for the highest Er3+ and Pr3+ concentrations investigated, and is more efficient than ET from I-4(11/2), because the corresponding absorption transition in Pr3+ has a large oscillator strength and back transfer is inhibited by fast multiphonon relaxation from the corresponding Pr3+ level. In both cases, the ET parameters depend on Er3+ concentration in a similar way as the ETU parameters but depend only weakly on Pr3+ concentration. This shows that energy migration within the Er3+ I-4(13/2) and I-4(11/2) levels is fast. The presented results are important for the choice of the appropriate operational regime of the erbium 3-mu m fiber laser.