UV-pumped Tm:YLF lasers at 450 nm, 742 nm, and 1.9 µm

We investigated the feasibility of blue laser operation around 450 nm in Tm:YLF on the 1 D 2 → 3 F 4 transition by detailed spectroscopic characterization and modeling. Absorption cross sections as well as temperature- and concentration-dependent fluorescence lifetimes of the relevant multiplets were measured, enabling the determination of the cross-relaxation macroparameters. From visible fluorescence and excited-state absorption, we derived emission and gain cross sections for the 1 D 2 → 3 F 4 transition near 450 nm, and used them as the input for a five-level rate-equation model, which predicts that room-temperature lasing at 450 nm requires induced depopulation of the long-lived 3 F 4 level, e.g., via cascade lasing. Experiments under 360-nm pumping enabled efficient 1.9-µm laser operation with 44% slope efficiency, indicating a three-for-one pumping mechanism enabled by cross-relaxation processes. Cascade lasing 1 D 2 → 3 F 4 → 3 H 6 was not achieved at room temperature due to insufficient pump power, but cryogenic cascade laser experiments at 85 K enabled 39 mW blue output at 450 nm with simultaneous 1879-nm lasing. Furthermore, we demonstrate laser operation at 742 nm for the first time at temperatures below 100 K. These results motivate exploration of alternative hosts and UV-diode pumping for compact blue Tm 3+ lasers.