Eye-safe, diode-pumped all-solid-state lasers working in the 2 μm region are favourable for many applications including remote sensing, medicine, and gas sensing. Tm-doped crystals such as Tm:YAG and Tm:LuAG are among the commonly employed laser gain media for this purpose. These systems are usually pumped by laser diodes operating around 780 nm [1]. The corresponding stokes limit for laser slope efficiency is then about 39%. However, for highly-doped samples, the two-for-one cross-relaxation process enables laser efficiencies exceeding this limit, and a slope efficiency of 59% has been reported from a 785 nm Ti:Sapphire pumped Tm:YAG laser, indicating a cross-relaxation efficiency of around 1.5 (one 780 nm pump photon generating 1.5 laser photons around 2 μm) [2]. As an alternative pumping approach, Stoneman et al. pointed out the possibility to pump Tm-based lasers around 681 nm, due to the much stronger absorption line at this wavelength [3]. Compared to 785 nm pumping, using the stronger 681 nm line warrants utilization of shorter Tm-crystals, which minimizes self-absorption losses [3]. Moreover, 681 nm pumping could also facilitate usage of thinner crystals in thin-disk geometry and/or could decrease the required number of bounces on the thin-disk for sufficient absorption of the pump light [4]. Moreover, pumping at a wavelength with stronger absorption coefficient creates a more densely populated gain medium, which could increase the effectiveness of cross-relaxation process. As a drawback, the stokes limited slope efficiency reduces to around 34% for 681 nm pumping, and decreased absorption lengths might also induce stronger thermal effects. In their early work, Stoneman et al. used a Ti:Sapphire laser operating at 681 nm to pump a Tm:YAG crystal, but the obtained slope efficiency was only around 35%, indicating a weak cross-relaxation process [3].

© 2019 IEEE

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