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User operation at the European X-ray Free-Electron Laser Facility started at the SASE1 undulator beamline in fall 2017. The majority of the experiments utilize optical lasers (mostly ultrafast) for pump-probe-type measurements in combination with X-ray pulses. This manuscript describes the purposedeveloped pump-probe laser system as installed at SASE1, implemented features and plans for further upgrades. research papers J. Synchrotron Rad. (2019). 26, 328-332 Guido Palmer et al. Pump-probe laser system at FXE and SPB/SFX 329

We present, what is to our knowledge, the first detailed lasing investigation of cryogenic Yb:YLF gain media in the E//a-axis. Compared to the usually employed E//c-axis, the a-axis of Yb:YLF provides a much broader and smooth gain profile, but this comes at the expense of reduced gain product. We have shown that, despite the lower gain, which (i) increases susceptibility to cavity losses, (ii) raises lasing threshold, and (iii) inflates thermal load, efficient and high-power lasing could be achieved in the E//a axis as well. A record continuous-wave (cw) powers above 300 W, cw slope efficiencies of 73%, and a tuning range covering the 995-1020.5 nm region were demonstrated. In quasi-cw lasing experiments, via minimization of thermal effects, slope efficiencies can be scaled up to 85%. In gain-switched operation, sub-50-µs long pulses with a peak power exceeding 2.5 kW at multi-kHz repetition rate were attained. We measured a beam quality factor below 1.5 for laser average powers up to 100 W and below 3 for laser average powers up to 300 W. Power scaling limits due to thermal effects, laser dynamics in pulsed pumping, and multicolor lasing operation potential were also investigated. The detailed results presented in this manuscript will pave the way towards development of high-power and high-energy Yb:YLF oscillators and amplifiers with sub-500-fs pulse duration.

We report, what is to our knowledge, the highest average power obtained directly from a Yb:YLF regenerative amplifier to date. A fiber front-end provided seed pulses with an energy of 10 nJ and stretched pulsewidth of around 1 ns. The bow-tie type Yb:YLF ring amplifier was pulse pumped by a kW power 960 nm fiber coupled diode-module. By employing a pump spot diameter of 2.1 mm, we could generate 20-mJ pulses at repetition rates between 1 Hz and 3.5 kHz, 10 mJ pulses at 5 kHz, 6.5 mJ pulses at 7.5 kHz and 5 mJ pulses at 10 kHz. The highest average power (70 W) was obtained at 3.5 kHz operation, at an absorbed pump power level of 460 W, corresponding to a conversion efficiency of 15.2%. Despite operating in the unsaturated regime, usage of a very stable seed source limited the power fluctuations below 2% rms in a 5 minute time interval. The output pulses were centered around 1018.6 nm with a FWHM bandwidth of 2.1 nm, and could be compressed to below 1-ps pulse duration. The output beam maintained a TEM 00 beam profile at all power levels, and possesses a beam quality factor better than 1.05 in both axis. The relatively narrow bandwidth of the current seed source and the moderate gain available from the single Yb:YLF crystal was the main limiting factor in this initial study. a gain bandwidth of 10 nm for the E//a axis at cryogenic temperatures [1]. Moreover, unlike Yb:YAG, the emission profile is rather smooth, which minimizes gain-narrowing effects, that could potentially enable amplification of sub-250 fs long pulses. At the same time, parameters of Yb:YLF such as thermal conductivity, thermal expansion coefficient, thermo-optic coefficient (dn/dT) are better than for RT Yb:YAG. On the other hand, the emission cross section of Yb:YLF at 80 K is only around 0.7 × 10 −20 cm 2 for the E//a axis, which is around 3 times lower than RT-Yb:YAG and 14 times lower when compared to cryogenic Yb:YAG. However, the longer fluorescence lifetime, τ=1990 µs [17], partly balances for the reduced gain, resulting in a σ em τ-product of 1.4 x10 −23 cm 2 s in comparison with 2 × 10 −23 cm 2 s and 9.4 × 10 −23 cm 2 s of RT and cryogenic Yb:YAG, respectively. As a result of the low emission cross section, one also suffers from a rather high saturation fluence (14 J/cm 2 ) in Yb:YLF, which creates challenges in optimizing extraction from amplifiers. As an example, for a stretched pulsewidth of 1 ns, the cavity optics has an estimated laser induced damage threshold (LIDT) of around 6.3 J/cm 2 , and long-term damage free operation usually requires operating the amplifier at a lower/safer value of ∼3 J/cm 2 or below. Operating the amplifier at a fluence value much lower than the saturation fluence: (i) reduces the extraction efficiency of the circulating amplified pulse, (ii) increases output energy fluctuations upon undesired perturbations, and (iii) results in circulation of a Gaussian beam profile in the amplifier (which is known to have 2 times higher fluence compared to flattop beams). Note that, despite these disadvantages, if the am...