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Due to the vast variety of possible uses for different displays, research in this field is needed for more complex shapes and glasses of various thicknesses. Bessel-Gaussian beams with their elongated, thin focus profile and self-healing nature are an excellent fit, even for glasses up to several millimeters. Additional development to more complex beam profiles allows precise tailoring with respect to the mandatory specifications of the cutting process such as process speed or the realization of inner contours. One concept for the latter is the use of tilted Bessel-Gaussian beams to achieve both high quality and easy separation. Further approaches include the usage of higher-order Bessel beams or modified Gauss-Bessel beams. We employ digital holographic techniques to create the various profiles with the desired absorption distribution.
Traditional microscopes fail to characterize these sensible changes in the interaction region, since they are limited to visualize permanent changes (ex situ) of the glass structure only. We take advantage of pump-probe microscopy to receive concise recordings of the extinction mechanisms of the beam-material-interaction. With both, high temporal and spatial resolution of in-situ diagnostics we gain access to the entire process window which enables us to develop optimized processing parameters for high-quality glass cuttings.
This is exemplified by results achieved for ablation, welding and modification cutting by elongated beam shapes. At a probe delay in the ns-range, pressure waves can be observed. Applying fluence near threshold, a remarkable influence of accumulation on the absorption becomes obvious even at repetition rates down to 10 kHz. Increasing the repetition rate results in thermal load on a zone by far extending the initial absorption region, as can be seen by pump-probe polarization microscopy. Pump-probe diagnostics support aberration correction for improved modification cutting by Bessel-like beams. The examples on processing results highlight the achievements enabled by thorough consideration of the plurality of relevant effects.
To pursue this approach and analyze various damage mechanisms in a subtractive micromachining process, we apply a novel pump probe microscopy setup, which enables us for the first time to examine an extended parameter range. We present in-situ data of the nonlinear interaction region in glass on a micrometer scale with a temporal resolution of approximately 200 fs comprising the laser material interaction from femtoseconds to microseconds. Our investigations are carried out for incubation and accumulation processing regimes up to a repetition rate of 1 MHz. Additionally, pump pulse durations between 300 fs to 20 ps, as well as several burst operation modes are accessible with our experimental setup. Our extensively automated pump probe setup enables us to reconstruct the material extinction response to analyze the complex absorption profiles. In this context, we report on flexible processing strategies and exemplarily processing results.
We applied pump probe technology and in situ stress birefringence microscopy for fundamental studies on the influence of energy and duration (100 fs – 20 ps), temporal and spatial spacing, focusing and beam shaping of the laser pulses.
Applying pump probe technique we are able to visualize differences of spatio-temporal build up of absorption, self focusing, shock wave generation for standard, multispot and beam shaped focusing. Incubation effects and disturbance of beam propagation due to modifications or ablation can be observed.
In-situ imaging of stress birefringence gained insight in transient build up of stress with and without translation. The results achieved so far, demonstrate that transient stress has to be taken into account in scaling the laser machining throughput of brittle materials. Furthermore it points out that transient stress birefringence is a good indicator for accumulation effects, supporting tailored processing strategies.
Cutting results achieved for selective laser etching by single pass laser modification exemplifies the benefits of process development supported by in situ diagnostics.
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