Ultrafast Tm-Doped Fiber Amplifier with 1 kW Average Output Power

Summary form only given. Ultrafast thulium-doped fiber lasers emitting in the 2 μm wavelength region have recently attracted a significant amount of attention regarding applications in material processing, life sciences, as well as in fundamental science, e.g. high harmonic generation, particle acceleration, mid-IR generation [1,2]. In addition to the application-relevant emission wavelength, Tm-doped silica fibers possess many interesting characteristics, such as a broad gain bandwidth spanning from 1700 nm to 2100 nm and more than 70% slope efficiency by exploiting cross-relaxation processes [3]. Furthermore, the longer operation wavelength reduces the impact of detrimental nonlinear effects in fiber lasers, and allows for a larger mode field area with diffraction-limited beam quality as compared to the well-established ytterbium-doped fiber laser systems [4]. In this contribution, we report on a new average power record from a thulium-doped fiber amplifier. We use state-of-the-art laser diodes to reach a combined pump power of more than 2 kW at 793 nm. The use of a polarization maintaining thulium-doped photonic crystal fiber allows for an optical-to-optical efficiency of more than 60% and a pump-limited average output power of 1150 W. After compression to 265 fs pulse duration, 1060 W of average power and 50 MW of peak power at 80 MHz pulse repetition rate are achieved, which represents a performance level that is currently not achievable with a single ultrafast ytterbium-doped fiber amplifier. Additionally, we confirmed a close to diffraction-limited beam quality (M 2 <;1.1) up to the highest power level. From the simulation of the amplification process, the average heat-load of the fiber reaches 95 W/m at full power level, without the onset of transverse mode instability [5]. This average heat-load is significantly higher than the critical value of about 30 W/m, which was found to be the threshold for transverse mode instability in ytterbium-doped fiber lasers.