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

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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.