## 3 Kilowatt-Class Picosecond Thin-Disk Lasers  
Applications, such as EUV generation, would benefit from picosecond lasers of high beam quality and average power from 100 W to more than 1 kW at a 1-100-kHz repetition rate [26]. Another application interesting for the industrial market that would profit from lasers of these parameters is processing of carbon fiber-reinforced plastics [27]. The thin-disk laser concept can meet all of these demanding requirements with high efficiency and is suitable for diode-pumped solid state lasers. Figure 3 shows the scheme of the thin-disk laser concept [9,28]. The laser gain media has the shape of a thin disk. The front surface of the thin disk is anti-reflection coated, and the back surface is high-reflection coated, both coatings functional for the laser and pump wavelengths. Thus, the thin disk can be used as an active mirror. The back surface is bonded to a heat sink, which is most often water cooled. The heat sink is made from material with high thermal conductivity, such as a copper-tungsten (CuW) alloy or diamond. The heat is removed from the thin disk mainly by axial heat flow thanks to its large surface-to-volume ratio. Thermal lensing, mechanical stresses and nonlinear effects experienced by a laser pulse in the active material are lower compared to the bulk arrangements. Further, the low  
Figure 2: Application potential of the laser systems soon available at HiLASE R & D Laser Centre (prospective applications for future upgrades in blue) [25]. EUV, extreme ultraviolet; BEUV, beyond EUV; FEL, free-electron laser; LIDT, laser-induced damage testing. transverse temperature gradient minimizes transversal thin disk deformations, and a high quality beam, close to the fundamental mode, can be produced. The fundamental mode is characterized by the ability to be focused to the smallest spot with a certain lens and initial beam diameter. A very small spot size diameter, in order of 10 \(\upmu\)m, is crucial for some applications. On the other hand, very small thickness causes small single-pass gain and small pump light absorption. Therefore, materials with higher doping concentrations of active ions are used because higher doping increases gain and pump light absorption. The typical gain per reflection is between 1.1 and 1.4 [28]. Thus, if the thin disk is used for pulse amplification, a regenerative amplifier or a multi-pass amplifier needs to be used to provide high total gain. For the amplification of ultrashort (ps, fs) pulses, the CPA technique is incorporated. The optical pumping by laser diodes of the wavelength matched to the absorption line of the active medium increases the overall efficiency and reduces the thermal load. The pump light is often fiber coupled for easy handling and homogenization of the beam of the laser diodes. The pump beam is collimated and imaged by a parabola mirror to the thin disk (see the Figure 3). Prisms and the parabola mirror retro-reflect the pump to the thin disk about 20-30 times, so over 90% of the pump power is absorbed. High pump power densities in the homogenously-pumped area are thus achieved. The output laser power can be increased in a simple way by increasing the diameter of the laser beam and pump spot size if the pump power density stays constant [9,28]. The typical materials used for thin disks are ytterbium-doped ones. Yb:YAG is the most common among them. Its quasi-three-level operation (at room temperatures and above) needs high pumping power density, which is obtained by diode pumping. The central laser wavelength is 1030 nm. Bands around 940 nm or 969 nm are used for pumping. The 940-nm pumping is less efficient due to the higher quantum defect, but at the same time less expensive pump diodes with an unstabilized wavelength can be employed. On the contrary, the 969-nm pumping offers a 1.5-times lower quantum defect and, so, reduces the heat generated in the laser medium. Thanks to direct pumping to the upper laser level, this scheme is called zero-phonon line pumping (ZPL).