Highly Efficient, In-Band Pumped, Thulium-Doped Fibers in High-Power Amplifier and mJ Q-Switched Regime

High-power, ultrafast, thulium-doped (Tm) fiber-laser systems operating at a center wavelength around 2 μm are very beneficial for various applications in metrology and sensing, as well as for frequency conversion techniques such as high- harmonic generation [1]. With longer pulses, in the ns-region, Tm-fiber lasers could be an ideal platform for laser-plasma sources due to their high beam quality [2]. With the state-of-the-art, ~790 nm cladding pumping of Tm-doped, fused silica fiber amplifiers, the achieved slope efficiencies are barely above 50 % for ultrafast pulse amplification [3–5]. Furthermore, in the ns-region, targeting energy extraction at mJ-levels, the efficiencies do not exceed more than 15 % [6]. In addition, due to the low quantum-efficiency, there is a high heat load generated that could cause thermal problems during high-power operation and, therefore, prevents scaling these systems beyond the kW-level [7]. The quantum-defect related limitations can be overcome by pumping Tm gain media directly in the upper laser level. For high-power CW operation, the most common approach is to pump Tm gain media around 1.55 μm or 1.9 μm, which has already demonstrated remarkable slope efficiencies [8], [9]. However, due to the high emission cross-section, the maximum storable energy at these pump wavelengths is quite low [10]. Therefore, a more suitable pump wavelength around 1.7 μm should be considered for highly energetic laser operation.