Comparison of Er- and Er, Pr-doped YLF 2.8-µm lasers

Development of the 3µm laser based on the4I11/2–4I13/2 transition in Er-doped crystals is proceeding slowly because the up-conversion process used to empty the long-lived terminal laser level causes complicated population dynamics that make it difficult to optimize the performance of the laser. We have found that by co-doping these high-concentration Er crystals with small amounts of Pr, the 4I13/2 lifetime can be drastically reduced through energy transfer to the Pr 3F3,4 levels, which allows this transition to be operated as a quasi-four-level laser. The 4I13/3 lifetime in Er:YLF is approximately 14 ms; with the addition of 0.1% Pr, it is reduced to 750 µs with only a slight decrease in the lifetime of the 4I11/2 level. Experiments are now under way to compare the performance of the up-conversion laser with the co-doped laser, and preliminary measurements using an Ar+ laser pump source indicate that both 10% Er: YLF and 20% Er, 0.1% Pr:YLF can be operated cw, with the co-doped sample having the higher threshold. Both lasers should operate most efficiently by directly pumping the Er 4I11/2 level, which will soon be practical with laser diodes operating at 980 nm. We will present results of pumping both the 4I9/2and 4I11/2 levels with a Ti:Al2O3 source using various concentrations of Er and Er, Pr in YLF.