## 1 Introduction  
High average power lasers are needed to increase the processing speed in industrial applications and to increase the signal-to-noise ratio in many scientific applications. Typically, the output of a weak oscillator is increased in a single or multiple amplifiers, which is denoted as the master oscillator power amplifier (MOPA). In short-pulse laser systems, the chirped pulse amplification (CPA) is utilized, where the MOPA procedure involves stretching the pulses prior to the amplification and a subsequent compression of amplified pulses [1]. In recent years, the number of diode-pumped solid state lasers has dramatically increased thanks to their high efficiency, reliability, flexibility and compactness [2]. Increasing the average power of lasers needs active laser media with a low quantum defect, efficient heat removal and good overlap between their absorption line and pump radiation. In short-pulse high average power lasers, the popular Nd:YAG (neodymium-doped yttrium aluminium garnet) active medium has been replaced by Yb:YAG (ytterbium-doped yttrium aluminium garnet). Compared to the Nd:YAG, the Yb:YAG has a lower quantum defect and broader emission bandwidth, allowing for the generation of picosecond or even femtosecond pulses. The other Yb-doped materials used in lasers are, e.g., Yb:LuAG (lutetium aluminum garnet), Yb:YLF (yttrium lithium fluoride), Yb:CaF\({}_{2}\) (calcium fluoride), Yb:KYW (potassium yttrium tungstate), Yb:KGW (potassium gadolinium tungstate), Yb:CALGO (Yb:CaAlGdO\({}_{4}\)) and Yb:glass, especially in the form of optical fibers [3; 4; 5; 6; 7].  
The efficient heat removal is achieved by the specific gain material geometry enabling large heat flow from the laser material to a heat sink. Among the most popular geometries are fiber [8], thin-disk [9] and Innoslab concepts [10; 11]. These geometries are mainly used for moderate (kHz) and high (MHz) pulse repetition rates of laser amplifiers. In fibers the radial heat flow removes the heat. The maximum pulse energy is limited by the small fiber aperture. A thin disk works as an active mirror with cooling from the back side. Small disk thickness in order of hundreds of micrometers enables very efficient heat removal by axial heat flow, but results in a small gain per bounce. The Innoslab concept uses a thin slab with side cooling. A folded beam path is used, and the beam size is increased at each pass. Higher pulse energies (>1 J) at lower repetition rates (10 Hz, 100 Hz) have been achieved by using conventional designs [12], thick disks [13; 14; 15; 16], total reflection active mirrors (TRAM) [17] or multi-slabs [18; 19]. Some of them employ operation at cryogenic temperatures using the advantages of higher thermal conductivity and higher gain at low temperatures [20; 21].  
In this paper, we present a review of the current status of high average power diode-pumped lasers being developed at the HiLASE Centre. First, the laser facility and its aims are introduced. The description of the latest results of the thin disk-based regenerative amplifiers follows. The development is supported by several diagnostic and optimization methods, developed at the HiLASE, that are mentioned in the last section.