Ultrashort pulsed laser processing is an effective technology for high-precision cutting, ablation, and drilling of almost all types of material, with low thermal input to the substrate. Polymers which are usually transparent for the laser pulses can be efficiently processed. The material is heated very fast resulting in direct evaporation of the irradiated material volume. On one hand, incubation effects occur due to changed surface roughness, voids, and chemical changes of the material, leading to a variation of energy absorption. On the other hand, if a high repetition rate and high pulse energy are applied to achieve high productivity at polymer cutting, heat accumulation occurs leading to melting and heat affected zones. An estimation of the pulse number dependent material modification and heat input which leads to these effects is not easily accessible for quantitative measurements. Furthermore, numerous process parameters influence the interaction making process analysis and optimization extensive. A versatile tool towards improved knowledge on ultrashort pulsed processes is pump-probe imaging. By using this, effects on a very short timescale such as nonlinear laser absorption, propagation of material waves and plasma generation can be visualized. The recording device in the setup which is applied for the analysis of polymer cutting and drilling in this paper is a high-speed camera with frame rates of up to 50 kHz. This enables recording video sequences contrary to taking a single picture. The authors utilized this to enable the observation of effects of heat accumulation which evolves after a large amount of pulses. Simultaneously, the authors were able to maintain the stated advantages of the pump-probe configuration. Therefore, under variation of several process parameters such as pulse repetition rate, pulse energy, and lateral pulse to pulse separation video sequences were recorded. By a variation of the pump-probe delays of 0 up to 5 ns, material wave propagation can be identified and optimized for evaluation. The points of origin of the observed waves are considered as the regions of highest nonlinear energy deposition; thus, the deposited energy distribution can be estimated. As every single pulse up to a pulse number of 1000 pulses is imaged, the transition from high penetration depth of the first incident pulses to strong surface absorption for an increasing pulse number due to incubation can be shown. Furthermore, strongly absorbing spots in the bulk volume caused by impurities and material modification lead to vapor generation under high pressure. Finally, the confined pressure is assumed to be a main reason for crack formation.
High-speed pump-probe imaging of ultrashort pulsed laser cutting of polymers
Johannes Heberle, Matthias Knoll, Ilya Alexeev, Tom Häfner, Michael Schmidt; High-speed pump-probe imaging of ultrashort pulsed laser cutting of polymers. J. Laser Appl. 1 May 2017; 29 (2): 022207. https://doi.org/10.2351/1.4983500
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