Pulsed laser cutting of bright metals

Numerical investigation on laser transformation hardening with different temporal pulse shapes

Scaling the pulse energy and peak power in high-power, narrow-linewidth Thulium-doped fiber lasers (TDFLs) is fundamentally limited by parasitic lasing, amplified spontaneous emission (ASE), and nonlinear effects. We present a nanosecond-pulsed Tm-doped photonic crystal fiber MOPA that successfully pushes these performance boundaries, delivering 2.07 mJ nearly rectangular-shaped pulses, with 52 kW peak power at 207 W average power, linear polarization, a narrow 390 pm linewidth at 2048 nm and excellent beam quality (M²<1.3). Based on these results, we provide a detailed analysis of these limiting mechanisms, supported by theory and experiment. We examine the impact of fiber geometry (core diameter, fiber length) and operational parameters of the TDFL (pump and signal wavelength, temporal pulse shape, pulse repetition frequency) on the spatial, temporal and spectral output characteristics. Thus, this work provides a comprehensive framework for understanding and optimizing high-energy pulsed fiber lasers and aims to clarify common misconceptions in the field.

The bias-pumped gain-switched fiber laser proposed by us is considered to be a novel pulsed fiber laser based on a new pulsing mechanism. With a certain signal power seeding, synchronization of the temporal evolution can be maintained between the output signal laser and the pump. The seed laser can conveniently be supplied with power by a CW pump; this is called bias-pump power. A pulsed pump is responsible for shaping the output pulse. Stable pulsed lasers with a tunable duration can be achieved under a bias pump combined with a pulsed pump. In addition, the temporal shape of the output pulses can be controlled based on this new pulsing mechanism. Compared with conventional gain-switched fiber lasers, a much simpler pulsed laser design can be provided by this novel pulsed fiber laser because there is no need to add a control unit to realize fast gain-switching.

To investigate the effects of laser temporal pulse shaping of the super-pulse thulium fibre laser (SPTFL) and to compare these in controlled in vitro conditions with various holmium: yttrium aluminium garnet (Ho:YAG) pulse delivery modes. The SPTFL (Urolase SP, IRE-Polus, Fryazino, Russia), with an emission wavelength of 1.94μm, and a Ho:YAG laser (P120H; Lumenis, Yokneam, Israel) with Moses technology were compared. Pulse shape, stone retropulsion and ablation efficiency were evaluated using BegoStones and compared for each laser mode: short (SP), long (LP), and Moses pulse (MP) for Ho:YAG, regular pulse (RP) and dual pulse (DP) for SPTFL. The Ho:YAG SP mode exhibited an asymmetrical pulse shape, with a steep leading slope and a much more gradual trailing slope, without any flat section. Pulses generated by the SPTFL were significantly longer and therefore had lower peak power than those generated by the Ho:YAG laser at equivalent energy settings. Retropulsion for the holmium:YAG LP and MP modes was similar and lower than that for the SP mode, but higher than for the SPTFL (all P ≤ 0.02), with an average stone displacement approximately four times and two times lower for SPTFL as compared to the Ho:YAG laser. Comparison of ablation volumes indicated that the SPTFL induced significantly higher (twofold) ablation than the Ho:YAG laser. The magnitude and initial velocity of stone retropulsion decreased with longer pulse duration and lower pulse peak power, without sacrificing ablation efficiency. These observations are manifest when comparing the Ho:YAG laser with the SPTFL. The novel SPTFL provides greater versatility and control of pulse variables than the Ho:YAG laser. Further clinical investigation of practical benefits achievable with pulse-shaping SPTFL modes is warranted.

The plasma-facing components (PFCs) in future fusion devices such as ITER will receive intense transient heat fluxes from plasma instabilities, such as edge localized modes (ELMs) and disruptions, which will limit material lifetime. The energy density and transient pulse duration are typically used to characterize the PFC damage threshold. However, these parameters are not sufficient to define a damage threshold, because different transient pulse shapes with the same energy density and same pulse duration produce different peak surface temperatures (and thus stresses, which ultimately determine material damage). The surface temperature and damage of tungsten (the material to be used for the ITER divertor target armor) in the form of surface roughening and melting are investigated using various temporal pulse shapes from an Nd:YAG laser in the PISCES-B facility. The heat flux factor is examined and shown to be an inadequate parameter to characterize the temperature rise except for square temporal pulse shapes. For ELM-like heat pulses, the long tail in the temporal shape results in a lower peak surface temperature and less damage compared to a symmetric triangle pulse with equal energy density.

In this research, we present the use of a solid-state laser for the paint removal on an aluminum AA6061 sheet. The laser utilized, was operating at a wavelength of 1064 nm and used to remove paint applied from a hand-held spray paint can. The pulse duration of the laser was 100 ns at 50 kHz of repetition rate, maximum pulse energy being 1 mJ. The laser pulse frequencies were varied from 50 kHz to 200 kHz and in combination with varying the features of pulse overlapping, an infinite array of paint removal parameters can be achieved. The cleaning results are shown in the images by the SEM and the spectrums from the EDX operating mode, used to determine the cleaning result of painted layer. This study shows that this laser technology is a contactless cleaning method to remove paint on an Aluminum AA6061 sheet. This is an alternative cleaning method, environmentally friendly and contributes to reduce pollution on the atmosphere.

The commercial availability of fiber lasers based on MOPA architectures with arbitrary temporal pulse shaping capabilities offers completely new possibilities for laser material processing. In this study, based on numerical modeling results in the nanosecond regime for the case of silicon at 1064 nm wavelength, we show that not only the single pulse laser ablation efficiency depends on the temporal pulse shape but, we also demonstrate how a stochastic approach can be applied in order to reach an optimized pulse shape maximizing the material vaporization rate for given laser pulse energy and duration. Experimental results are compared to the numerical modeling results, and the discrepancies are discussed in terms of the role played by plasma shielding effects and melt ejection at high intensity.

We analyze the transfer function of a three-dimensional atomic Bragg beamsplitter formed by two counterpropagating pulsed Gaussian laser beams. Even for ultracold atomic ensembles, the transfer efficiency depends significantly on the residual velocity of the particles as well as on losses into higher diffraction orders. Additional aberrations are caused by the spatial intensity variation and wavefront curvature of the Gaussian beam envelope, studied with (3+1)D numerical simulations. The temporal pulse shape also affects the transfer efficiency significantly. Thus, we consider the practically important rectangular-, Gaussian-, Blackman- and hyperbolic secant pulses. For the latter, we can describe the time-dependent response analytically with the Demkov-Kunike method. The experimentally observed stretching of the $\pi$-pulse time is explained from a renormalization of the simple Pendell\"osung frequency. Finally, we compare the analytical predictions for the velocity-dependent transfer function with effective (1+1)D numerical simulations for pulsed Gaussian beams, as well as experimental data and find very good agreement, considering a mixture of Bose-Einstein condensate and thermal cloud.

Process parameters determine the quality characteristics of additively manufactured (AMed) parts. During Laser-based Directed Energy Deposition (DED-LB), an elaborated parametric analysis is required to control the materials’ behaviour precisely. This work deals with the effect of process parameters on process stability in terms of consistent melt pool geometrical features, such as aspect ratio of the bead and dilution, formation of pores, and ductility of super duplex stainless steel (SDSS) during pulsed laser and powder DED-LB. Independent variables including laser power, deposition speed, pulse frequency, and flow rate of the shielding gas were selected as controlling factors to deposit single tracks and plates of SDSS. Experiments have been designed and conducted according to L16 orthogonal array of Taguchi methodology. Relative density was estimated using Archimedes method. Ductility was estimated by tensile testing of the builds. For geometrical evaluation, phase characterization, and microstructural assessment, specimens were subjected to optical and Scanning Electron Microscopies, and Electron Backscattered Diffraction. Overall results were interpreted and explained using the Taguchi approach and analysis of signal-to-noise ratio. Single objective parameter-property relationships were achieved and discussed. Conclusive parametric remarks under the multi-attribute optimization were derived using weighted Grey Relational Analysis. A desirable parametric combination was extracted, and the associated quality responses were verified by the satisfying confirmatory deposition. The key role of parameters on the process stability, microstructure, and mechanical behaviour was further discussed. This work offers a practical solution to the multi-response optimization problems that can be used in AM and surface treatment of SDSS. Graphical Abstract

Pulsed lasers were used extensively in material processing including cutting, drilling and marking. The temporal shape and duration of the pulse were important in optimizing the material processing quality. The flexible parameters of MOPA pulsed fiber laser can offer a broad range of material response characteristics. In this report, a MOPA pulsed fiber laser was configured with pulse duration from 2 to 500 ns and PRF that ranged from single shot to 4000kHz.Pulse energy up to 1mJ to investigate independently the contributions of each of these parameters on the marking process of Si wafer. Various experiments were presented to require the high quality marking on Si wafer. Topography evolution pictures of dot were taken and the depths were measured. Experimental results showed that single pulse energy had a significant influence on the threshold of the visible mark. Pulse numbers had more impact on the dot topography. Pulse energy and pulse duration had more influence on the mark depth. In order to control finely the mark depth, single pulse energy and pulse numbers should be optimized.

CO 2 - pulsed laser induced surface grafting of acrylamide onto ethylene - propylene rubber (EPR). II

The use of laser beams made up of ultrafast pulses for the processing of materials can be bothered by the consequences of optical Kerr effect (OKE) cumulated by the propagation through optical devices (windows, laser crystal, prisms, lenses, …). The latter are mainly a reduction of the intensity in the focal plane accompanied by a distortion of the temporal and spatial pulse shape. We present a comparative study on such distortions for Gaussian, super-Gaussian and LG10 (one central peak surrounded by a ring) beams. It is demonstrated that the LG10 beam shows sensitivity to OKE which is smaller than that of the Gaussian LG00, and super-Gaussian beams. As a result, the focusing performances of the LG10 beam are quite superior to that observed with Gaussian or super-Gaussian beam: a higher on-axis intensity, a narrower intensity pattern, and a temporal shape and an energy fluence almost undistorted.

Laser ablation of metals can be performed using cost-effective and efficient continuous wave fiber lasers using the fast power modulation capabilities of these beam sources. Single-pulse laser ablation of steel with pulse durations in the microsecond to millisecond time scale is observed using a high-speed camera setup to investigate the dynamic interactions between the generated melt pool and the vapor phase. Distinct processing regimes were identified depending on the pulse duration, temporal pulse shape, and power density. Pulse durations in the microsecond range can lead to beneficial processing conditions for ablation processes. In this regime, phase explosion and dynamic melt expulsion by melt uplift and coaxial droplet ejection due to the rapid drop in vapor pressure after the laser pulses were observed as desirable mechanisms, leading to effective material removal and high processing quality with little melt adhesion.

Temporal pulse profile characterization is necessary to ensure and quantify the quality of short pulse laser systems. Yet it remains challenging to measure the temporal behavior of a pulse in all of its comprehensiveness. In this manuscript we present results which encourage to perform more ambitious pulse characterizations with optimized scanning cross-correlators. Several temporal laser pulse profile measurements in multiple nanosecond time scale with high dynamic range are shown. The measurements were taken by our in-house third-order cross-correlator EICHEL (Schanz et al. in Opt Express 25:9252, 2017), which is able to resolve the intensity dynamics down to the level of amplified spontaneous emission. With this device we show for the first time the onset of the plateau of the amplified spontaneous emission in the laser profile and investigate the origin of several side-pulses created early in the laser system.

According to new environmental policies announced by European Union, car manufactures have been asked to avoid the unnecessary waste of energy and reduce pollution levels. Replacing steel with lightweight materials on car bodies is considered as one method to reduce the vehicle weight to achieve these requirements. In this study, lap welds of Zn-coated steel on Al alloy, which have recently been used in the automotive industry, were carried out using a single mode fibre laser. The interplay of processing parameters was investigated by means of changing processing parameters such as laser power, welding speed and laser pulse frequency. Metallurgical observations and mechanical tests were undertaken to evaluate the weld quality. Results show that good quality parallel welds with small fusion zones could be obtained even though the processing window to produce defect-free welds is extremely narrow. The amount of heat input and the laser pulse frequency play more important roles than others in pulsed wave fibre laser welding of Zn-coated steel on Al alloy.According to new environmental policies announced by European Union, car manufactures have been asked to avoid the unnecessary waste of energy and reduce pollution levels. Replacing steel with lightweight materials on car bodies is considered as one method to reduce the vehicle weight to achieve these requirements. In this study, lap welds of Zn-coated steel on Al alloy, which have recently been used in the automotive industry, were carried out using a single mode fibre laser. The interplay of processing parameters was investigated by means of changing processing parameters such as laser power, welding speed and laser pulse frequency. Metallurgical observations and mechanical tests were undertaken to evaluate the weld quality. Results show that good quality parallel welds with small fusion zones could be obtained even though the processing window to produce defect-free welds is extremely narrow. The amount of heat input and the laser pulse frequency play more important roles than others in pulsed wave fib...