Direct upper-state pumping of the 2.8 mum, Er(3+):YLF laser

Direct resonant pumping of the (4) < e1 > I < /e1 > (11/2 ) upper state of the 2.8-mum Er(3 ):YLF laser at 0.97 mum is shown to be more efficient than other pumping schemes which have been used for this transition. Upper-state pumping avoids nonradiative losses arising from phonon and self-quenching decays inherent in other pumping schemes. Efficient performance of the 2.8-mum laser is maintained on condition that extremely shallow pump absorption is avoided. Reduced efficiency for shallow pump absorption is attributed to upconversion loss arising from high pumping rates