Femtosecond Laser Ablation in Burst Mode as a Function of Pulse Fluence and Intra-Burst Repetition RateWednesday (24.06.2020) 18:00 - 18:40 Room 1
We have measured ablation rates and HAZ for burst mode ablation on various materials as a function of pulse fluence, intra-burst repetition rate (60 MHz, 180 MHz, 360 MHz, 720 MHz, 1.44 GHz) and number of pulses per burst (1-20), using a 40 W, 1 MHz, 1035 nm Yb:Fiber MOPA with 300 fs pulse duration. Different geometries of the ablated areas were used, ranging from 1 x 3 mm rectangular cavities to one-dimensional cuts.
For metals and large area ablation, burst mode operation enabled access to the maximum ablation rate only at 60 MHz and usually for more than 5 pulses per burst (Fig.1a). Higher intra-burst repetition rates or less than 5 pulses usually leads to a decrease in ablation efficiency due to shielding by the ejected material/plasma (Fig. 1b). For materials with low thermal conductivity like glasses or plastics, thermal interaction between burst pulses can substantially increase the optimum ablation rate over the 1-pulse case, leading to more efficient ablation as the number of burst pulses and the intra-burst frequencies are increased (Fig. 1b). A similar behavior was observed for silicon. The efficiency of burst mode ablation also strongly depends on the geometry of the ablated area. In general, the aspect ratio (depth/width) has to be well below 1, or the ablated area has to be at least twice as wide as the spot diameter to allow an escape route for the plasma.
For all samples investigated, we found that the size of the heat-affected zone decreases with increasing ablation rate (in mm3/(W.min)). Operating at the optimum fluence, either by using burst mode or by increasing the process repetition rate, will automatically lead to reduced HAZ since a higher fraction of the incident pulse energy is used to remove material and less heat is accumulated in the work piece.
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