Efficient Tm:YAG and Tm:LuAG Lasers Pumped by Red Tapered Diodes

Summary form only given. 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. 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). 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. Compared to 785 nm pumping, using the stronger 681 nm line warrants utilization of shorter Tm-crystals, which minimizes self-absorption losses. 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. 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. In this work, we report highly efficient Tm:YAG and Tm:LuAG lasers pumped with high brightness tapered diodes operating around 681 nm [5]. The lasers crystals had a Tm-doping of 6%, a length of 7 mm, and absorbed around 98% of the incident TM polarized pump light. In the experiments, we have achieved slope efficiencies up to 52% and 51% in cw operation from Tm:YAG and Tm:LuAG lasers, respectively (Fig. 1). This efficiency indicates a cross -relaxation efficiency of around 1.55, which is similar to what was shown with the regular 785 nm diode pumping [2]. To the best of our knowledge, this is the fi rst report of diode pumping of Tm-doped solid-state lasers at 681 nm, as well as the fi rst usage of tapered diodes as pump sources in Tm-systems.