Efficiency scaling of high power and high energy Tm-doped fiber lasers

Numerous applications in material's processing, medicine, metrology and sensing as well as several frequency conversion techniques such as high-harmonic generation, mid-IR or EUV generation would strongly benefit from stable, high-power laser sources emitting in the 2 µm wavelength region [1-5]. Thulium (Tm-) doped fiber lasers have already shown a significant scaling potential in terms of average power and high-energy extraction while preserving a high beam quality [6,7]. As the generation of inversion is not as straightforward as in Ytterbium-ions, obtaining a high efficiency is a significant challenge. Hence, slope efficiencies marginally exceeding 50% for pulsed systems, have been demonstrated employing matured ~790 nm diode-based cladding pumping. Consequently, there are high heat loads in the fiber, which cause thermal problems during high-power operation and, therefore, prevent scaling these systems beyond the kW-level so far [8]. The limitations imposed by the quantum defect can be largely overcome by pumping Tm gain media directly into the upper laser level, this technique is commonly known as in-band pumping. The most recent findings have identified the pump wavelength range of 1.6 µm to 1.7 µm as the optimal choice to achieve high efficiencies in combination with high energy extraction at highest average power levels from in-band pumped Thulium-doped fiber laser systems [9].