Ultrafast laser machining: process optimization and applications

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

“…An almost identical behavior was observed for a 2 ps pulse duration but with higher ablated volumes due to the usage of shorter pulses [ 19 ]. Similar results with even higher energy specific volumes have been reported for 300 fs and 350 fs pulse duration in the near infrared (NIR) [ 23 , 60 ]. In contrast to the rest of the literature, in [ 24 ], a significant drop by a factor of two of the maximum energy specific volume for steel (from about 0.4 mm 3 /min/W for single pulses down to less than 0.2 mm 3 /min/W, Figure 8 a, blue empty squares) for a triple pulse burst was reported for a 210 fs pulse duration for a wavelength of 1030 nm.…”

“…The most important measure to distinguish between the ultra-short pulse regime and classical thermal heating in equilibrium is the electron–phonon coupling time , which is a characteristic time for heating the ionic lattice. In first approximation, this value is a constant, but, in principle, it depends on the deposited energy, and the time of relaxation can be multiples of that characteristic time [ 23 ]. It can be estimated by , with being the heat capacity of the ionic system and being the electron-phonon coupling constant [ 37 ].…”

“…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 ].…”

“…Figure 10 a shows the maximum energy specific volumes for brass [ 60 ], aluminum [ 23 , 107 ], silver [ 59 ], gold [ 59 ], and molybdenum [ 23 ]. Except for molybdenum, a strong shielding effect for the double pulse burst followed by a significant increase in the maximum energy specific volume for the triple burst pulse is observed.…”

“…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.…”