We report a diode-pumped Yb:KGW laser that is capable of operating as a Q-switched oscillator or as a regenerative amplifier with average power of more than 20 W. The laser is based on a dual-crystal configuration where the pump thermal load is distributed over relatively long two crystals. It permits a sufficiently large number of passes with low passive losses and maximizes the energy extraction efficiency. The amplification bandwidth was extended by spectral combining of two Yb:KGW crystals with spectrally shifted gain maxima, that allows to mitigate spectral gain narrowing and provides pulse length down to 200 fs after compression in a stretcher-compressor module. The output power saturated with increasing pump power and output beam quality was defined by aberration of thermal lenses. Optimization of laser cavity allows us to compensate thermal lens partially and provide output beams with quality M2<1.2. Efficient frequency doubling and tripling of high-power femtosecond Yb:KGW laser is demonstrated in a nonlinear BBO crystal. Second or third harmonic generation with respective conversion efficiency of 55% or 24% was achieved in a single-pass configuration.
Ultrashort pulse laser systems are widely used in many areas such as microprocessing of various materials, the
generation of terahertz radiation, nonlinear optics, medical tomography, chemistry, and biology due to the high peak
power and large spectral width. For a practical usage of the femtosecond lasers, they must be fairly compact and stable.
These conditions are most fully met when laser media are used that allow direct pumping with the radiation from
semiconductor injection lasers, which are more compact, reliable, and inexpensive than pumping with solid-state lasers.
Since Ytterbium-doped crystals have a broad luminescence band for generating femtosecond pulses less than 500 fs wide,
they are attractive as materials for lasers with direct diode pumping. Moreover, the position of the central luminescence
wavelength of Yb:KGW and Yb:KYW crystals makes them promising priming sources of femtosecond pulses for
amplifiers that operate at wavelengths close to 1 μm (Yb:KGW, Yb-glass, Nd-glass, Yb:YAG, etc.)
We developed a femtosecond generator based on the Yb:KYW crystal with direct pumping by the radiation of a laser
diode with fiber output of the pump radiation. The use of such pumping, as well as of chirped mirrors to compensate
intracavity dispersion, made it possible to generate a continuous sequence of optical pulses 90 fs wide at a frequency of
87.8 MHz with a mean radiation power of more than 1 W. The product of the pulse width by the spectral width is close
to the theoretical limit, and this indicates that there is no frequency modulation.