Femtosecond laser ablation by bibursts in the MHz and GHz pulse repetition rates

Abstract

Here, to the best of our knowledge, for the first time we report the in-depth study of extremely high ultrafast laser ablation efficiency for processing of copper and steel with single-pulses, MHz-, GHz-and burst in the burst (biburst) regime. The comparison of burst, biburst and single-pulse ablation efficiencies was performed for beam-size-optimised regimes, showing the real advantages and disadvantages of milling and drilling processing approaches. Highly-efficient ultrashort pulse laser processing was achieved for ~1 µm wavelength: 8.8 µm 3 /µJ for copper drilling, 5.6 µm 3 /µJ for copper milling, and 6.9 µm 3 /µJ for steel milling.We believe that the huge experimental data collected in this study will serve well for the better understanding of laser burst-matter interaction and theoretical modelling.

Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses

“…If and are short compared to the intra-burst delay and shows a value in the range of this delay, the energy specific volume obtained for a triple pulse only slightly differs from the one of single pulses, e.g., steel 1.4301 (AISI 304) shows such short delay times and was intensively investigated. Different reported maximum energy specific volumes for single, double, and triple pulses are summarized in Figure 8 a [ 14 , 19 , 23 , 24 , 102 , 107 ]. For pulses with a pulse duration of 10 ps, an intra-burst delay of 12 ns, and wavelengths of 1064 nm and 532 nm, the maximum energy specific volume slightly decreased for a triple pulse compared to a single pulse [ 14 , 102 , 107 ].…”

“…The situation completely changes for copper (having longer delay times , and ) as shown in Figure 9 a,b. For double pulses, the aforementioned valley of death with a reduction in the maximum energy specific volume to less than 50% of the value for single pulses due to shielding effects is observed [ 14 , 24 , 59 , 60 , 102 ]. On the other hand, for triple pulse processing, the value strongly increases again and may even exceed the value for single pulses [ 14 , 24 , 59 , 60 , 102 ].…”

“…For double pulses, the aforementioned valley of death with a reduction in the maximum energy specific volume to less than 50% of the value for single pulses due to shielding effects is observed [ 14 , 24 , 59 , 60 , 102 ]. On the other hand, for triple pulse processing, the value strongly increases again and may even exceed the value for single pulses [ 14 , 24 , 59 , 60 , 102 ]. As can be seen in Figure 9 a, the maximum energy specific volumes are functions of the pulse duration (an increase for shorter pulse durations is observed) and wavelength (for copper, green radiation is beneficial and results in higher maximum energy specific volumes, which is mainly due to higher absorptance [ 16 ]).…”

“…The results for multi-pulse bursts for copper, brass, and aluminum are summarized in Figure 11 a [ 19 , 23 , 24 , 60 ]. Copper, brass, and aluminum show an alternating behavior of the maximum energy specific volumes with high values for odd numbers of pulses in the burst and low values for even numbers of pulses.…”

“…Thus, the available energy per pulse is often much higher than the energy actually needed for single pulse ablation. In order to attain the highest possible ablated volume for a given amount of energy, the energy can be distributed among pulses within a pulse package, a so-called pulse burst consisting of a minimum of two pulses and up to several hundred or thousand pulses [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. Each of these burst packages then carries a sufficiently high amount of energy that can be used for laser machining, while the total energy of all pulses together is close to the maximum available pulse energy of the laser.…”