**1. Introduction**

Energetic (dozens of μJ) optical pulses with femtosecond (fs) pulse lengths and hence ultra‐ high focused intensities in the range of 1012–1016 Wcm−2 are capable of ablating a wide range of materials including metals, semiconductors, ceramics, polymers, biological tissue, and dielectrics [1–3]. Femtosecond laser ablation has been demonstrated to be a powerful tool for various technologies [2]. Due to rapid energy delivery, the laser-plasma interaction is avoided and heat-affected zones in the irradiated targets are strongly localized with minimal residual damage. This allows generation of well-defined microstructures with high quality and reproducibility [1–3].

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Femtosecond laser sources based on Yb-doped laser materials became promising tool for various technological and industrial applications including femtosecond lasers. Among Ybmedia ytterbium tungstates (Yb:KYW/Yb:KGW), crystals Yb:KY(WO4)2 (Yb:KYW) or Yb:KGd(WO4)2 (Yb:KGW) exhibits an attractive set of parameters that makes it as one of the best choices for high-power fs lasers operating around 1 μm [4]. Bandwidth of Yb:KGW/ Yb:KYW is sufficient for amplification of sub-200 fs pulses, but typical pulse length on the output of Yb:KYW amplifier system is limited on the level 300–400 fs [5, 6] primarily due to gain narrowing [7, 8]. A promising method to reduce the effect of gain narrowing and to increase the effective gain bandwidth is to combine laser media with separated gain maxima and to overlap broadband gain. Using Yb:KYW crystals with different orientation of crystal‐ lographic axes, e.g. Ng- and Np-cut orientation, this approach has been realized in [9]. Another way for increasing gain bandwidth is using special spectral filters introducing controlled losses at maximum gain spectrum [7].

In this review chapter we present combination of those approaches to a double-slab regener‐ ative amplifier (RA). Each slab is pumped separately, which enables additional possibility to control gain. Furthermore as a seed source, we used high-power master oscillator (MO) based on Np-cut Yb:KYW crystal with output pulse length ~100 fs and central wavelength agreed well with spectral gain profile of regenerative amplifier. A highly efficient stretcher and compressor based on single transmitted diffraction grating are used for stretching and recompressing initial pulses after master oscillator.
