Abstract

We have shown that multiple-step upconversion pumping of Er:YLiF₄ at 1.50 μm into the metastable ⁴I_13/12 level excites blue (469.7 nm), yellow (560.6 nm), and green (551 nm) laser emission from the ²P_3/2,²H_9/2, and ⁴S_3/2 levels, respectively. This requires the energy of at least five erbium ions pumped into ⁴I_13/2 to populate the upper level for the blue transition, four for the yellow, and three for the green. We believe that the mechanism for upconversion consists of a sequence of pairwise energy transfer steps involving several metastable excited levels. Monolithic laser crystals, ~3 mm long and doped with 1% Er³⁺, were prepared with mirrors applied directly to the crystal surfaces. The thresholds for laser action were 25 mW, 50 mW, and 80 mW for the yellow, green, and blue lasers, respectively. This shows that high order excitation processes in upconversion lasers involving long lived (here ~10 ms) metastable levels can be rather efficient. Output powers of these lasers at 30 K reached 10 mW at 551 nm for 350 mW of pump light, 10 mW at 560.6 nm for 140 mW of pump light, and 0.7 mW at 469.7 nm for 350 mW of pump light. Given the quantum defect between the input and output photons, these are good overall efficiencies. Lasing was observed up to 80 K. This work extends an earlier study,¹ in which we obtained upconversion laser output at the three wavelengths by pumping in the near IR at 0.80 μm and 0.97 μm, to the observation of even higher order upconversion processes.

© 1992 Optical Society of America