Crystal-length optimization in 681-nm pumped Tm:YAG and Tm:LuAG lasers: from low threshold to SESAM Q-switching

Abstract

We systematically quantify how crystal length governs efficiency, tuning bandwidth, and passive Q-switching in 681 nm tapered-diode-laser (TDL) pumped Tm:YAG (1.5 at.%) and Tm:LuAG (2 at.%) oscillators. By scanning four lengths (3, 5, 7, and 9 mm), we identify a three-way trade-off among pump absorption, quasi-three-level reabsorption, and length-dependent passive loss. In continuous-wave (CW) operation, maximum extraction occurs at 7 mm in both hosts, while the minimum absorbed lasing thresholds are obtained with the 3 mm crystals (35–45 mW). In the experiments, instantaneous slope efficiencies reach 60% (Tm:YAG) and 52% (Tm:LuAG), corresponding to photon-quantum efficiencies of 1.78 and 1.54. Using an intracavity quartz birefringent filter, we achieve >220 nm of continuous tuning and reproducible two-color operation; in Tm:LuAG, the longest crystals impose a short-wavelength cutoff (below ∼1.92 μm), so shorter lengths are required for full spectral access. Passive Q-switching with a SESAM is strongest for 7 mm Tm:LuAG, producing 1.36 μs pulses with 1.2 W peak power. These results establish crystal-length selection rules for compact, red-pumped 2 μm sources balancing efficiency, tunability, and passive Q-switching.