Highly efficient, in-band pumped thulium-doped fibers for high-power ultrafast 2 µm wavelength laser systems

Ultrafast fiber laser sources emitting fs-pulses around 2 µm have many applications in medicine, metrology and sensing as well as in various frequency-conversion techniques. Thulium-doped fiber amplifiers are a promising platform for power scalable ultrafast amplification in this wavelength region. Usually, these ultrafast, high-power fiber laser systems were pumped at a wavelength around 790 nm and obtain slope efficiencies in the range of 50 % in the 100 W-class. Due to the high quantum defect obtained with this pump technique and the related high heat loads, considerable thermal challenges still must be overcome when scaling the power further. In this contribution we present a concept on highly efficient, high-power thulium-doped fiber amplifiers pumped at 1692 nm. This pump concept is suitable for high-power, high-energy, ultrafast Tm-doped fiber laser systems. In this proof of principle demonstration, we achieve a slope efficiency of 80% in a standard commercially available, thulium-doped photonic crystal fiber (PCF) with ~60 W of average power when pumping at 1692 nm compared to 47 % slope efficiency by pumping at 793 nm. In the simulation we investigated the heat load and core temperature evaluation along the fiber. These findings demonstrate an improvement in the amplification efficiency of large-mode area fiber amplifiers which are suitable for ultrafast operation on Yb-like efficiencies. The reduced heat load paves the way to even higher average powers from ultrafast Tm-doped fiber lasers with the potential to provide multi-mJ energy fs-pulses at kW-level average power from a single amplifier channel.