Theoretical and experimental investigations on pulse front deformations for laser material processing using ultra-short laser pulses

在过去的几年里，超短激光脉冲已经被建立在几个应用领域，以高精度地加工不同的材料。通过减小脉冲宽度，可以在恒定的平均激光功率下获得更高的功率密度，从而能够激发非线性过程，从而有效地吸收激光辐射。与扫描仪系统相结合，可以在较高的加工速度下加工大型零件，因为聚焦的激光光束相对于工件是可能的高速和加速的。对于超短激光脉冲通过色散光学元件的传输，群速度和相位速度之间的差异导致脉冲和相位前沿之间的延迟(传播时间差或PTD)。对于通过正透镜平行于光轴传播的脉冲，PTD最大值将沿着主射线出现，而PTD则由于透镜的较小玻璃厚度而沿边缘射线达到最小值。该效应导致焦点区域中的脉冲持续时间增加。在文献中讨论了对平行光轴入射系统的PTD效应进行补偿的一些原理。模拟和实验初步研究表明，PTD对光场角度有很强的依赖性。对于激光扫描仪的光学系统，这会导致脉冲变形和持续时间取决于扫描角度。首次模拟表明，对于传统的扫描器光学系统而言，相对较小的6°扫描角导致了大约100ps的时间脉冲扩展。对于敏感的过程，例如处理脉冲持续时间约为500 fs的透明介质，可以预期扫描角度相关的结果。目标是开发新的光学设计概念和方法，以设计在整个扫描范围内具有均匀脉冲特性的激光扫描器的聚焦透镜。工作时间表包括领域建模/仿真、概念/设计方法和实验验证。场模拟包括对不同几何光学方法和波动光学方法的研究，以及它们对描述聚焦透镜引起的脉冲变形的有效性的分析。建立了测量方法，研究了不同视场角度下聚焦透镜后的脉冲前沿，并与模拟结果进行了比较。根据仿真和测量结果，将寻求不同的设计方法，并结合适用的仿真技术。总之，我们打算设计不同的示例性目标，以及通过测量和加工玻璃样品的实验来进行实验表征。

During the last years ultra-short laser pulses have been established in several application areas, to process different materials with high accuracy. By reducing the pulse duration a higher power density can be achieved at a constant average laser power which enables the excitation of nonlinear processes and thus an efficient absorption of the laser radiation.In combination with scanner systems large work pieces can be machined at high process velocities, because high velocities and accelerations of the focused laser beam with respect to the work piece are feasible.For the propagation of ultra-short laser pulses through dispersive optical elements the difference between group and phase velocity leads to delay (propagation time difference or PTD) between pulse and phase front. For a pulse which propagates parallel to the optical axis through a positive lens the PTD maximum will occur along the chief ray, whereas the PTD reaches a minimum along the marginal rays, due to the minor glass thickness of the lens. This effect leads to an increase of the pulse duration in the focal region. Some principles to compensate the PTD effect for systems with light incidence parallel to the optical axis are discussed in the literature.Both simulative and experimental preliminary studies demonstrate a strong dependency of the PTD on the field angle. For optical systems for laser scanners this causes pulse deformations and durations depending on the scan angle. First simulations show that comparatively small scan angles of 6° lead to a temporal pulse expansion of about 100 fs for conventional scanner optics. For sensitive processes as for example the processing of transparent dielectrics with pulse durations of about 500 fs, scan angle depending results can be expected.Goal is the development of new optic design concepts and methods to design focusing lenses for laser scanners with homogeneous pulse properties all over the scan range. The working schedule contains the fields modeling/simulation, concepts/design methods and experimental validation. The field modeling/simulation contains investigations of different geometrical and wave optical methods and the analysis of theirs validity for the description of pulse deformations caused by focusing lenses. At once a measurement method shall be set up, to investigate the pulse front behind focusing lenses for different field angles and compare the results with the simulation. Based on the simulation and the measurement results, different design methods will be pursued and combined with applicable simulation techniques. In conclusion the design of different exemplary objectives is intended, as well as the experimental characterization by measurements and also experiments of processing glass samples.