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We propose an engineering model to estimate the dimensions of ablation, taking into account on the one hand the material properties such as the ablation threshold, penetration depth and the refractive index and, on the other hand, the processing parameters namely the pulse energy and beam diameter, scanning speed, repetition rate and angle of incidence. The model considers as well the effects of incubation, changes of topography during multi-pulse irradiation, surface reflectivity and Gaussian beam diameter variation with the distance to the focal plane.
The model is able to simulate the profiles of ablation surfaces produced by normal or tilted laser beam, either for spot, line and area processing. The results obtained are validated by comparison to the ones obtained experimentally. Both the model and the experiments focus on stainless steel. The predictions of the model also allow for the optimization of the micromachining process, both energy and time wise.
We present a simple engineering model for ultrafast laser processing, applied in various real life applications: percussion drilling, line engraving, and non normal incidence trepanning. The model requires only two global parameters. Analytical results are derived for single pulse percussion drilling or simple pass engraving. Simple assumptions allow to predict the effect of non normal incident beams to obtain key parameters for trepanning drilling. The model is compared to experimental data on stainless steel with a wide range of laser characteristics (time duration, repetition rate, pulse energy) and machining conditions (sample or beam speed). Ablation depth and volume ablation rate are modeled for pulse durations from 100 fs to 1 ps. Trepanning time of 5.4 s with a conicity of 0.15° is obtained for a hole of 900 μm depth and 100 μm diameter.
Because the stretched-pulse duration is of the order of hundreds of picoseconds, the DPA delay is induced using a freespace interferometer with reasonable arm lengths of few tens of centimeters. The use of a common interferometer to divide and recombine temporal pulse replicas, together with the Sagnac geometry, results in an identical optical path for all four replicas. Therefore, the whole spatio-temporal combining architecture is passive, avoiding the need for active electronic stabilization systems. Because we only use two temporal replicas, the system is immune to differential saturation levels or B-integrals between successive pulses: this is compensated by controlling the amplitude of both pulses at the input of the amplifying setup.
This setup allows the generation of 1 mJ, 300 fs compressed pulses at 50 kHz repetition rate, corresponding to 50 W output average power, with a combining efficiency above 90% at all power levels.
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