Fiber-based CPA system at 2050 nm with multi-µJ pulse energy at high repetition rate

High-power, ultrafast fiber lasers emitting in the wavelength region around 2 μm are in wide demand for applications such as high harmonic generation towards the soft-Xray, spectral conversion into the mid-infrared and semiconductor processing [1]. One of the main disadvantages of broadband laser sources in the range of 1.9 µm to 2.0 μm, however, is the overlap with atmospheric water-vapor absorption lines, which implies that high-power systems must be placed in a protective atmosphere or even vacuum [2]. Furthermore, crystal-based nonlinear frequency conversion towards the mid-IR would strongly benefit from a slightly longer wavelength as well. Thus, in this respect, it is advantageous to shift the central wavelength to a region above 2 µm where the impact of absorption is significantly reduced. Rare-earth elements offering gain above 2 µm wavelength include Thulium and Holmium, and both can be used as laser active ions in silica fibers [3]. Recent fiber-based demonstrations include cw-power levels of 100W and ns-pulses with energy of >1mJ at wavelengths between 2.0 and 2.1 µm [4,5]. Nevertheless, ultrafast fiber lasers appeared to be restricted to sub-µJ pulse energy, mainly due to the unavailability of low-nonlinearity Holmium- or Thulium/Holmium fibers.