Upconversion-pumped ultrafast 2.3 µm laser

Upconversion pumping in rare-earth-doped materials offers access to low-gain and unconventional transitions, unlocking new spectral regions for laser operation. We introduce a scheme for femtosecond pulse generation at 2.3 µm by exploiting photon-avalanche-driven upconversion pumping of thulium-doped crystals at 1.45 µm, combined with a bandgap-engineered GaSb-based semiconductor saturable absorber mirror for self-starting, high-power mode-locking. The upconversion-pumped Tm:LiYF 4 laser operating on the 3 H 4 → 3 H 5 transition delivers Watt-level continuous-wave output at 2.3 µm, achieving a record optical efficiency relative to absorbed pump power of over 30%, the highest reported for any Tm laser in this spectral range. Stable ultrashort pulse generation that fully exploits the emission profile of the gain medium is further demonstrated, producing 282 fs soliton pulses at a high average power of 478 mW, representing a substantial performance leap over prior demonstrations. These results highlight the strong synergy of optimized upconversion excitation and tailored nonlinear cavity dynamics, establishing a robust platform for next-generation ultrafast coherent light sources for molecular spectroscopy and frequency down-conversion.