Pump excited state absorption in holmium-doped fluoride glass

摘要

The primary excited state absorption processes relating to the ⁵I₆ → ⁵I₇ 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 ⁵I₆ and ⁵I₇ 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 ⁵I₇ → ⁵I₄ excited state absorption transition has peak cross sections at 1216 nm (σesa =2.8x10⁻²¹ cm²), 1174 nm (σesa =1x10⁻²¹ cm²), and 1134 nm (σesa =7.4x10⁻²² cm²) which have a strong overlap with the ⁵I₈ → ⁵I₆ ground state absorption. On the other hand, it was established that the excited state absorption transition ⁵I₆ → ⁵S₂ 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.