**5. Conclusion**

In conclusion, a room-temperature, diode-pumped, dual-crystal Yb:KGW laser operating as a Q-switched oscillator or regenerative amplifier has been developed where the gain bandwidth was extended by using Ng-cut + Np-cut or Ng-cut + Ng-cut configuration. It was shown that fine-tuning the mode sizes in the crystals in the resonator with high pump power is important to obtain the maximal output power since thermal effects change the operation point in the stability zone and mode matching conditions. It was demonstrated simply by shifting the position of the end mirror along optical axis in the resonator. Optimization of the laser resonator increased the output power from 18 to 24 W in case of Q-switched oscillator and from 17 to 21 W in case of regenerative amplifier. Such optimization of laser resonator improves not only output power but also stability of laser operation, especially for Ng-cut + Ng-cut crystal configuration, that is manifest in reduction of output power fluctuations.

The use of this regenerative amplifier enabled to create compact, high average power, high brightness, diode-pumped femtosecond Yb:KGW laser system. This laser, which utilized a CPA MOPA laser scheme, consisted of master oscillator, regenerative amplifier, and stretchercompressor module. It was capable of delivering 15 W of average output power with a pulse duration down to 182 fs high in a nearly diffraction limited output beam (*M*<sup>2</sup> ~1.2) at pulse repetition rates of 50–500 kHz.

This level of output power and quality of a laser beam are practically the same as the output power of Yb:KGW/Yb:KYW thin-disk lasers with medium level of output power [6, 9]. However laser heads based on volume laser media are considerably easier and cheaper compared with those based on thin-disk configuration. Subsequent development of multi‐ crystal laser approach will be able to increase output power on the level of high-power thindisk lasers.

The achieved level of radiation parameters allows to successfully use this femtosecond laser for ultrafast micromachining in various applications including sub-micron material process‐ ing, surface structuring, creating photonics devices, biomedical devices, microfluidics, displays, and solar applications.
