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Laser powder bed fusion of glass: a comparative study between CO2 lasers and ultrashort laser pulses
Photonics-based mid-infrared interferometry: 4-year results of the ALSI project and future prospects
Here, we present the selective laser melting of pure copper using ultrashort laser pulses. This work involves a comparative study using 500 fs pulses at processing wavelengths of 515 nm and 1030 nm. The repetition rate of the applied laser system was varied within the MHz range in order to exploit heat accumulation. By using the ultrashort interaction times and tailoring the repetition rate, the induced melt pool can be significantly optimized yielding robust copper parts revealing thin-wall structures in the range below 100 μm.
This is exemplified by results achieved for ablation, welding and modification cutting by elongated beam shapes. At a probe delay in the ns-range, pressure waves can be observed. Applying fluence near threshold, a remarkable influence of accumulation on the absorption becomes obvious even at repetition rates down to 10 kHz. Increasing the repetition rate results in thermal load on a zone by far extending the initial absorption region, as can be seen by pump-probe polarization microscopy. Pump-probe diagnostics support aberration correction for improved modification cutting by Bessel-like beams. The examples on processing results highlight the achievements enabled by thorough consideration of the plurality of relevant effects.
Due to the vast variety of possible uses for different displays, research in this field is needed for more complex shapes and glasses of various thicknesses. Bessel-Gaussian beams with their elongated, thin focus profile and self-healing nature are an excellent fit, even for glasses up to several millimeters. Additional development to more complex beam profiles allows precise tailoring with respect to the mandatory specifications of the cutting process such as process speed or the realization of inner contours. One concept for the latter is the use of tilted Bessel-Gaussian beams to achieve both high quality and easy separation. Further approaches include the usage of higher-order Bessel beams or modified Gauss-Bessel beams. We employ digital holographic techniques to create the various profiles with the desired absorption distribution.
Traditional microscopes fail to characterize these sensible changes in the interaction region, since they are limited to visualize permanent changes (ex situ) of the glass structure only. We take advantage of pump-probe microscopy to receive concise recordings of the extinction mechanisms of the beam-material-interaction. With both, high temporal and spatial resolution of in-situ diagnostics we gain access to the entire process window which enables us to develop optimized processing parameters for high-quality glass cuttings.
Single laser pulses with 1026nm wavelength, 6ps (FWHM) pulse duration and 200μJ pulse energy were applied to fused silica, Borofloat 33 and Gorilla glass. Electron densities around 1 x 1020cm-3 in the focal plane and 1 x 1019cm-3 in front of the focus are obtained, independent from the glass type used.
The free carriers slowly decay within several ns, while the decay time depends on both the maximum electron densities reached and glass species. In this process a part of the excited electrons relax within several 10ps into a long-living stage where a transient effect is observed. Here, various probe wavelengths show differences in the recorded signal.
A further carrier relaxation leads to permanent (stress, voids) and non-stable (color center) modifications crucial for precise glass dicing applications.
In this work, we present the application of ultra-short laser pulses as a tool to reproduce space weathering, with focus on micrometeoroid impacts. In our experiments, slices of single-crystal olivine were irradiated under vacuum condition using 100 fs single-shot laser pulses. In order to perform spectral measurements, the laser-damaged regions were distributed over the sample surface within a grid geometry. After laser processing, a comprehensive study was performed by using spectroscopic measurements in the NUV-vis-NIR range, white light interferometry, SEM and TEM analysis. The cross-sections of the laser-generated craters reveal different layers including from the top to the bottom: an amorphous layer, two polycrystalline layers with different textures, and a defect-rich olivine substrate. Moreover, iron nanoparticles occur within the lower part of the amorphous layer and the polycrystalline layer. We can reproduce microcraters whose morphology, microstructure, and distribution of iron nanoparticles are similar to those found in the soil samples of the Moon or of the asteroid 25143 Itokawa.
We also determined the breaking strength with a three-point bending test. The determined maximal value of 73 MPa is equivalent to 85% of the stability of the pristine bulk material.
Pulse separation times within the burst from 1 ps to 8 ps result in deeper holes with a larger silhouette area, however equal or reduced hole quality and reproducibility compared to drilling with individual pulses. In contrast with pulse separation times from 510 ps to 4 ns a quality and reproducibility improvement is visible. For these delay times the achieved depth was equal or higher compared to micromachining without bursts.
We applied pump probe technology and in situ stress birefringence microscopy for fundamental studies on the influence of energy and duration (100 fs – 20 ps), temporal and spatial spacing, focusing and beam shaping of the laser pulses.
Applying pump probe technique we are able to visualize differences of spatio-temporal build up of absorption, self focusing, shock wave generation for standard, multispot and beam shaped focusing. Incubation effects and disturbance of beam propagation due to modifications or ablation can be observed.
In-situ imaging of stress birefringence gained insight in transient build up of stress with and without translation. The results achieved so far, demonstrate that transient stress has to be taken into account in scaling the laser machining throughput of brittle materials. Furthermore it points out that transient stress birefringence is a good indicator for accumulation effects, supporting tailored processing strategies.
Cutting results achieved for selective laser etching by single pass laser modification exemplifies the benefits of process development supported by in situ diagnostics.
In this paper, we review the fundamental concepts of this technology and provide an overview of its applications for purposes of multiphoton microscopy and laser materials processing. Moreover, numerical simulations on the nonlinear pulse propagation within transparent media illustrate the linear and nonlinear pulse propagation, highlighting the differences between conventional focusing and SSTF. Finally, fs-laser induced modifications in gelatine are presented to compare nonlinear side-effects caused by conventional focusing and SSTF. With conventional focusing the complex interplay of self-focusing and filamentation induces strongly inhomogeneous, elongated disruptions. In contrast, disruptions induced by SSTF are homogeneously located at the focal plane and reduced in length by a factor >2, which is in excellent agreement with the numerical simulations of the nonlinear pulse propagation and might favor SSTF for demanding applications such as intraocular fs-laser surgery.
Supersymmetric partners of multimode waveguides are characterized by the fact that they share all of their effective indices with the original waveguide. The crucial exception is the fundamental mode, which is absent from the spectrum of the partner waveguide. Here, we demonstrate experimentally how this global phase-matching property can be exploited for efficient mode conversion. Multimode structures and their superpartners are experimentally realized in coupled networks of femtosecond laser-written waveguides, and the corresponding light dynamics are directly observed by means of fluorescence microscopy. We show that SUSY transformations can readily facilitate the removal of the fundamental mode from multimode optical structures. In turn, hierarchical sequences of such SUSY partners naturally implement the conversion between modes of adjacent order. Our experiments illustrate just one of the many possibilities of how SUSY may serve as a building block for integrated mode-division multiplexing arrangements. Supersymmetric notions may enrich and expand integrated photonics by versatile optical components and desirable, yet previously unattainable, functionalities.
Different coating options were evaluated in order to provide the necessary high reflectivity and a satisfactory laser damage threshold for ultrashort laser pulses in the few ps to fs regime at λ = 1030 nm. High-reflective metal layers enhanced by dielectric HfO2/SiO2 stacks were found to be the most advantageous coating option due to their comparatively small thickness and measured damage thresholds above 1 J/cm2@8ps.
Spatial and temporal temperature distribution of ultrashort pulse induced heat accumulation in glass
In borosilicate glass, the maximal temperature directly after the excitation (pulse energy of 1100 nJ, repetition rate of 1 MHz, wavelength of 1044 nm, pulse duration of 600 fs, 2000 pulses per laser spot) is more than 5000 K and rapidly cools down within several hundreds of ns.
Our results reveal that only few laser pulses already lead to an erasure of nanometric pores which is mapped by the total (X-ray) scattering volume as well as by the strong reduction of the initial form birefringence. Simultaneously, new nanostructures form which arrange in individual grating planes with ongoing irradiation. However, since the rewrite process is no ideal mechanism some of the old sheets remain, which perturb the quality of the new nanograting. When rewriting multiple times the glass becomes even more porous due to repetitive annealing and quenching. This promotes the formation of new inhomogeneities and in turn leads to an increase in optical retardance.
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This course provides attendees with the knowledge necessary to understand and apply femtosecond laser pulses for micromachining tasks in a variety of materials. Emphasis will be placed on developing a fundamental understanding of how femtosecond pulses interact with the sample. From this knowledge, the advantages and limitations of femtosecond lasers for various micromachining tasks can be readily understood. Examples will be given in the micromachining of the surface of metals, semiconductors, and transparent materials, as well as the formation of photonic and microfluidic devices in the bulk of transparent materials.
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