## 1 Introduction  
During the past decades, high-harmonic generation (HHG) has been extensively studied to generate extreme ultraviolet (XUV) and soft-X-ray frequencies supporting the generation of pulses down in the attosecond range [1]. Additionally, the spectral, spatial and temporal properties of the HHG radiation make it a versatile tool in atomic and molecular spectroscopy [2]. To realize a matched source for the envisaged spectroscopic applications, much work has been done in controlling the shape of the HHG spectrum by either modifying the driving laser field [3,4] or using multicolor fields [5]. Using shaped driving laser fields is also a promising way to increase the HHG conversion efficiency. The high average power of optimized HHG sources allow now coherent diffraction imaging [6] which paves the way for using HHG sources for biological and medical applications [7], especially bearing in mind the ongoing effort to push HHG into the water window [8,9]. Two major scaling laws render HHG difficult for practical applications, namely:  
(i) The cutoff law:  
\[\hbar\omega_{\mathrm{max}}=I_{p}+3.17\cdot I\cdot\lambda^{2} \tag{1}\]  
It defines \(\hbar\omega_{\mathrm{max}}\) the maximum photon energy of the XUV radiation which depends on the ionization potential of the atom or molecule \(I_{p}\), the laser peak intensity \(I\) and \(\lambda\) the wavelength of the driving laser. And (ii) the wavelength dependent conversion efficiency:  
\[\eta(\lambda)\propto\lambda^{-6.5} \tag{2}\]  
Taking Equations (1) and (2) into account for designing an HHG source it can be seen that, for a given laser intensity, the wavelength should be as long as possible to obtain the highest photon energy, which on the other hand results in a poor conversion efficiency. These two equations describing only single atom response already suggest the necessity to make a compromise. Optimization becomes even more important if collective effects are also considered. A higher laser intensity will not only give rise to higher photon energies, but also the conversion efficiency is reduced because almost all neutral atoms will be ionized before the intensity maximum is reached. Additionally, the freed electrons form a plasma which defocusses the laser and reduces the intensity further. The most important measure to avoid excessive ionization before the peak of the laser pulse is using driving pulses as short as possible [3]. The wavelength dependent conversion efficiency is also governed by phase-matching between XUV and driving laser pulse. The main sources of dephasing are the different index of refraction of the neutral gas, but also a major contribution stems from the dispersion of the generated plasma, the parameters of which strongly depend on shape, intensity and wavelength of the driving laser pulse [3]. So for almost all HHG optimization strategies it is beneficial to start with ultrashort laser pulses with controlled parameters. One of the key techniques for controlling the shape and intensity of the driving laser pulses is spectral broadening in a noble gas-filled hollow-core fiber (HCF) and subsequent compression [10] and/or shaping [3]. For implementation of this scheme several optimizations of the setup can be done (Figure 1). Besides reduction of the inner diameter of the HCF to achieve the necessary intensities for self-phase modulation for laser systems with relatively low output peak powers, the design of the HCFcan be improved, e.g., hollow-core fiber photonic crystal fibers can lead to soliton self-compression [11]. To meet the desires for few-cycle pulse generation differential pumping schemes have been implemented to avoid ionization of the gas and accompanied coupling losses due to defocusing as well as higher order spectral phases which cannot be compensated appropriately by chirped mirrors [12]. A third way is to change the nonlinear medium, e.g., to a Raman-active molecular gas where the Raman activity allows spectral broadening beyond the scope of self-phase modulation.