## 5 Advanced Tools for High Average Power Laser Developments
### In Situ Thin Disk Deformation Measurements  
The high pulse energy generated by the thin disk lasers necessitates a large mode size on the disk in order to avoid optical damage. However, the large mode area is more sensitive to optical phase distortions (OPD) caused mainly by the thin disk deformations at high pump intensities. The OPD limits the achievable pulse energy and beam quality. Further, the spherical part of the OPD, related to defocusing, affects the stability zones of the resonator of the regenerative amplifier. Thus, the knowledge of the thin disk deformations is essential for the design of a stable resonator for single-mode operation (TEM00 transverse mode) within the range of thin disk radii of curvature (ROC) for the used range of pump intensities. The aspherical part of the OPD increases the mode mismatching and higher mode content. We have developed a compact setup for a precise measurement of the thin disk deformation based on a Hartmann-Shack wavefront sensor and a single-mode probe beam. It outperforms the interferometric methods in compactness, robustness, simplicity of alignment and resistance to mechanical vibrations. These properties allow one to use the setup _in situ_ and to observe the thin disk deformation under the laser operation. Last, but not least, the aberration evaluation is a standard part of the wavefront sensor software. The measurement precision is in the range of a few nanometers, which is sufficient to observe the thin disk deformation caused by the pressure of the cooling water [34]. In the setup shown in Figure 14, the single-mode fiber-pigtailed laser diode emitting at the wavelength of 852 nm is collimated and magnified by a telescope to cover the whole thin disk surface. The originally flat wavefront is modulated at the thin disk surface and imaged by the telescope to the wavefront sensor. The high quality reference flat or curved mirror is used to verify the wavefront flatness or to calibrate the thin disk radius of curvature measurement, respectively. The thermal camera is added to the system to study the relationship between the thin disk temperature and its deformations. We measured the ROC of a 220 \(\upmu\)m-thick Yb:YAG thin disk soldered on a CuW heat sink. The measurement was done in a non-lasing condition initially. The thin disk ROC increased from the  
Figure 14: _In situ_ thin disk deformation measurement setup. The setup is shown without the laser cavity, which can be added. LD, laser diodes; L, lens; \(\lambda\)/4, quarter-wave plate; BS, beam splitter; PBS, polarizing beam splitter; SPF, short pass filter; PCTRL, polarization controller [36]. initial 4 m without pumping by 120 mm for a 2.7-mm pump spot diameter and by 340 mm for a 5.8-mm pump spot size at the pump intensity of 1.5 kW/cm\({}^{2}\) in both cases [35]. The thin disk tends to be flatter when the pump is increased. Moreover, at the same pump intensity, the ROC change is more significant for larger pump spot sizes, because of the larger amount of heat induced in the thin disk. This is especially important for scaling up the output power by the pump spot size enlargement [34, 35, 36]. The _in situ_ wavefront deformation measurement setup was used to evaluate deformations under the conventional broadband pumping at 940 nm and the zero-phonon line pumping at 969 nm. The highest deformations are at 940-nm pumping in the non-lasing condition and are reduced in the lasing condition. We found a nonlinear temperature rise and a large aspheric deformation for the 940-nm pumping. The different temperatures for lasing and non-lasing conditions makes the resonator alignment more complicated, and if the lasing is lost, e.g., by resonator misalignment, the temperature can rise above the thermal thin disk damage threshold, which is about 200 \({}^{\circ}\)C. Further, the deformations are reduced for 969-nm pumping. In the case of 969-nm pumping, the temperature rise is linear, and the aspheric deformation is weak.