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 [1,2]. Thulium (Tm)-doped fibers represent a very attractive platform for Q-switched systems emitting ns pulses in the 2µm 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 [3]. However, in those fiber systems targeting mJ pulse energy, the slope efficiency is barely above 35 % [4]. Due to the large quantum defect of ~ 60 % such systems face considerable thermal challenges [5,6]. 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.