Methods for resonator internal frequency doubling in efficiency maximized adaptive disk lasers in variable operation

在基于激光的制造中，许多应用受益于发射可见辐射的激光器。与常用的波长为1.03 µm的近红外辐射相比，可见激光辐射可提高吸收率，从而提高工艺稳定性。目前，没有已知的激光活性材料允许直接产生具有kW范围内的高平均功率的可见辐射。然而，在连续波操作中，这样的功率可以在绿色光谱范围内通过腔内倍频薄盘激光器以1.03 μm的波长发射来实现。在腔内倍频的情况下，输出耦合度、腔内功率以及晶体内部的红外和绿色波之间的相位匹配彼此非线性耦合。这使得这种激光器非常容易受到功率波动的影响，这可能会破坏光学元件。此外，非线性晶体内的最佳光束半径强烈地依赖于输出功率。在该项目中，将研究腔内自适应光学器件的使用，以实现稳定的激光器操作，输出功率高达1 kW，具有衍射限制光束质量，同时具有与输出功率无关的恒定高光学效率（> 45%）。此外，自适应光学器件应允许在激光器操作期间在M2 = 1和M2 = 10之间准静态调整光束质量。变形镜的技术局限性、相关激光量的获取的挑战性以及激光特性之间的非线性耦合导致了复杂的控制问题，这需要对已知的控制方法进行扩展和修改。只有通过激光技术与系统和控制工程之间的跨学科合作才能实现上述目标。

In laser-based manufacturing, many applications benefit from lasers emitting visible radiation. The visible laser radiation leads to an increased absorptivity and hence to an improved process stability when compared to the commonly used near-infrared radiation with a wavelength of 1.03 µm. Currently, no laser-active materials are known that allow the direct generation of visible radiation with high average power in the kW range. However, such powers in continuous-wave operation can be achieved in the green spectral range with intra-cavity frequency doubled thin-disk lasers emitting at a wavelength of 1.03 µm.In the case of intra-cavity frequency doubling, the degree of output coupling, the intra-cavity power and the phase matching between the infrared and green waves inside the crystal are non-linearly coupled with each other. This makes such lasers very susceptible to power fluctuations, which can destroy optical components. In addition, the optimum beam radius within the non-linear crystal strongly depends on the output power. Within this project, the use of intra-cavity adaptive optics will be investigated in order to achieve stable laser operation with an output power of up to 1 kW with diffraction-limited beam quality and, at the same time, constantly high optical efficiency (> 45 %) independent of the output power. In addition, the adaptive optics should allow quasi-static adjustment of the beam quality between M2 = 1 and M2 = 10 during laser operation. The technical limitations of the deformable mirrors, the challenging aquisition of the relevant laser quantities and the non-linear coupling between the laser properties lead to a complex control problem, which requires the extension and modification of known control methods. The aforementioned goals can only be achieved through interdisciplinary cooperation between laser technology and systems and control engineering.