Pump excited state absorption in holmium-doped fluoride glass

The primary excited state absorption processes relating to the I65 → I75 3 μm laser transition in singly Ho3+ -doped fluoride glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the I65 and I75 energy levels established the occurrence of two excited state absorption transitions from these energy levels that compete with previously described energy transfer upconversion processes. The I75 → I45 excited state absorption transition has peak cross sections at 1216 nm (esa =2.8× 10-21 cm2), 1174 nm (esa =1× 10-21 cm2), and 1134 nm (esa =7.4× 10-22 cm2) which have a strong overlap with the I85 → I65 ground state absorption. On the other hand, it was established that the excited state absorption transition I65 → S25 had a weak overlap with ground state absorption. Using numerical solution of the rate equations, we show that Ho3+ -doped fluoride fiber lasers employing pumping at 1100 nm rely on excited state absorption from the lowest excited state of Ho3+ to maintain a population inversion and that energy transfer upconversion processes compete detrimentally with the excited state absorption processes in concentrated Ho3+ -doped fluoride glass.