Highly efficient, in-band pumped, thulium-doped, Q-switched fiber laser

High power laser sources in the 2 μm wavelength region and emitting ns-pulses have many applications in material processing, biology as well as for laser plasma sources targeting the extreme ultraviolet region. Thulium (Tm)-doped fibers represent a very attractive platform for Q-switched systems emitting ns pulses in the 2 μm wavelength range. Usually, high-power, Tm-doped fiber amplifiers are pumped at 790 nm. By exploiting Cross-Relaxations (CR) the slope efficiency can be significantly increased beyond the Stokes limit. However, in those fiber systems targeting mJ pulse energy, the slope efficiency is barely above 35 %. Due to the large quantum defect of approximately 60 % such systems face considerable thermal challenges. In this contribution an in-band pumped Q-switched oscillator is demonstrated. This source is used to seed an in-band pumped, Tm-doped, rod-type amplifier which delivers up to 6.1 mJ pulse energy and up to 128 W average power with 77% slope efficiency.