Green up-conversion in Yb3+–Tb3+ and Yb3+–Tm3+–Tb3+ doped fluoro-germanate bulk glass and fibre

Yb3+–Tb3+ and Yb3+–Tm3+–Tb3+ doped fluoro-germanate glass samples were fabricated in bulk and fibre form. Bright up-conversion has been obtained by exciting the samples with a 975 nm laser source, and the power dependence of the up-conversion intensities with respect to the excitation power has been characterised. Two mechanisms must be taken into account to explain the population of the 5D4 level of the terbium ions in these experiments; namely multi excited state absorption (MESA) and cooperative up-conversion (CU). While in the Yb3+–Tm3+–Tb3+ samples both MESA and CU contribute simultaneously to the 5D4:Tb3+ population, in the Yb3+–Tb3+ case a pure CU mechanism is responsible for the 5D4:Tb3+ population. In the fibre geometry, the re-absorption effect has been observed. The detrimental role of the re-absorption has been explained by characterising the up-conversion spectrum at different fibre lengths.