We present an overview on the development and characterization of multiscale laser processing optics for versatile material modifications across more than six orders of magnitude. Starting with solutions for micromachining we present high-NA microscope objectives creating sub-wavelength material modifications on macroscopic scales with highest peak intensities. Moving on to the millimeter range, the adaptability and scalability of scanning optics is examined for large-area machining. Finally, we explore line beam optics in the meter range, evaluating their use in uniform material processing using average powers above 100 kW. This study provides an insight into the design and performance characteristics of such optics and demonstrates their potential in advanced laser processing.
Cleaving of glass substrates with shaped edges using a laser-only concept is presented. In a first laser process shaped ultrashort laser pulses modify in a single pass the entire material thickness with arbitrary edge shape geometries. Afterwards in a second pass CO2 laser radiation is absorbed in the modified area and resulting stresses lead to the separation of the glass. We investigate the quality of the achieved edges and corresponding mechanical properties. The cutting strategy, so far conducted on straight contours, is successfully transferred to curved contours maintaining edge qualities.
We introduce ultrafast laser machining concepts where microscopic modifications can be produced at macroscopic levels. The efficient use of the source’s energy and power performance is enabled by optical tools that distribute the radiation onto large surfaces or into large volumes. Here, holographic beam splitters are used to create focus copies at arbitrary positions in the working volume of a focusing unit. This allows, for example, to cut transparent materials with customized edges in a single laser pass or to high-speed texture articles with curved surfaces without adjusting the focus position.
Simulations and measurements on selective laser etching of display glasses are reported. By means of a holographic 3D beam splitter, ultrashort laser pulses are focused inside the volume of a glass sample creating type III modifications along a specific trajectory like pearls on a string. Superimposed by a feed of the glass sample a full 3D area of modifications is achieved building the cornerstone for subsequent etch processes. Based on KOH the modifications are selectively etched at a much higher rate compared to unmodified regions resulting in a separation of the glass along the trajectory of modifications. For gaining further insight into the etch process, we perform simulations on this wet chemical process and compare it to our experimental results.
Using lasers for permanent markings is a well-known method and standard in many areas in industrial manufacturing. Various processes on all kinds of materials are used to achieve durable markings, where the content is often serial numbers, codes and company logos. Whereas laser marking processes are mostly easy to handle it becomes challenging if the markings need to fulfill the requirements of the medical industry. The demand for markings in the medical industry is increasing because of the regulations in this sector. The labelling of medical devices with a traceable Unique Device Identification (UDI) code has become mandatory. Laser marking of medical steel needs to fulfill certain criteria where corrosion resistance, a toxicologically uncritical surface and good contrast, are key factors. Standard laser marking systems are reaching their limits to fulfill these criteria. The best choice to accomplish this challenge is the usage of ultrashort pulsed (USP) lasers. The process using USP lasers, fulfilling the criteria of medical industry, is often called “black marking”. We use the TruMicro Mark 2030 G2 S. This laser has the advantage of full flexibility of the important parameters, such as pulse energy, pulse frequency and pulse duration. Due to the great necessity for reliable medical devices, we investigate the effect of varying different laser parameters on the resulting structures of the black marking process on stainless steel in an experimental study. Analyzed is the dependency of the resulting structures on the energy density by varying the laser spotsize. This is adjusted by a parameter in the marking software and can be set continuously. The dependency of the resulting structures on the pulse duration is investigated by varying the pulse duration. The pulse duration is adjusted by a parameter in the marking software and can be tuned in the range of 400 fs to 20 ps at 100 fs increments. The fundamental parameters like beam quality, beam pointing or energy stability are not affected by changing the pulse duration. The fast switching time of < 800 ms enables for intra-process tuning of the temporal energy deposition. Analyzing the results by a Scanning Electron Microscope (SEM) reveals different surface structures. The structures change in its appearance, periodicity, and groove depth. Various of the described structures can give a permanent black contrast and fulfill the requirements of medical industry.
The laser-based fabrication of glass substrates with chamfered edges and arbitrary contour geometries is reported. To achieve energy deposition along any edge geometry, a holographic beam splitter is used generating a large number of focus copies in the bulk of transparent materials. If an adapted rotation of the focus distribution is additionally used during machining, the production of substrates with arbitrary contours, such as circular or display geometries, is enabled. After the laser modification step, separation is achieved using a selective laser etching strategy. The advantages of our process are discussed using selected process results and associated laser parameters.
We present an optical concept and adapted ultrashort pulsed laser parameters for a precise cleaving process of flexible ultrathin glasses. To this end, non-diffracting beams with tailored transverse intensity profiles generate asymmetric type-III-regime modifications along the entire substrate thickness. These laser-induced material changes not only show advantages in cutting but can also improve the bending properties of these flexible glasses when arranged in a specific manner. During the relative movement of the workpiece to the processing optics, crack connection occurs between the specifically aligned modifications only, which considerably facilitates glass separation and increases yield.
A two-step laser-based concept is presented for cleaving glass substrates with tailored edges. In a first step beam shaped ultrashort laser pulses are used to modify the transparent material along chamfered or C-shaped edges. Secondly, thermal stress is applied close to the modified area by absorbing the radiation from CO2 laser. The tensile stress thus induced on the upper side of the glass leads to the actual release. The efficacy of our approach is demonstrated by presenting selected samples with tailored shaped edges and discussing corresponding edge qualities.
We report on the separation of glass substrates with customized edge contours including C-shapes. To achieve single-pass laser modifications along the entire contour geometry a processing optics is presented where a multitude of foci are simultaneously distributed inside a specific volume using a large-working-volume focusing unit. Tangential angles of the focus trajectory to the surface can be almost arbitrarily chosen and amount to even less than 45-deg in case of aiming for chamfered edges. After having induced laser modifications along the desired edge geometry, separation is done chemically in the present case. The glass articles, thus fabricated, meet the demands of the display industry in terms of bending strength and surface quality.
The separation of complex inner and outer contours of glass articles with curved surfaces using ultrashort pulsed lasers is reported. Single-pass, full-thickness modifications along the entire substrate are achieved using a processing optics that allows for beam shaping of non-diffracting beams and, additionally, for aberration compensation of phase distortions occurring at the curved interface. The glass articles finally separated by thermal stress or via selective etching meet the demands of the medical industry in terms of micro-debris, surface quality, and processing speed.
We report on the separation of complex inner and outer contours from glass articles with curved surfaces using ultrashort pulsed lasers. To achieve single-pass, full-thickness modifications along the entire substrate a processing optics is presented that allows for beam shaping of non-diffracting beams and, additionally, for aberration compensation of phase distortions occurring at the curved interface. The glass articles finally separated by thermal stress or via selective laser etching meet the demands of the medical industry in terms of micro-debris, surface quality and processing speed.
We report on ultrashort pulsed laser fabrication strategies for glass articles with customized edges and curved surfaces. To achieve single-pass, full-thickness modifications along the entire substrate processing optics are presented that allows for beam shaping of non-diffracting beams and, additionally, for aberration compensation of phase distortions occurring at the tilted or curved interfaces. The efficacy of our concepts is presented by evaluating the surface and edge qualities of separated glass tubes with complex inner and outer contours as well as glass chamfer structures.
We report on our recent developments in the field of ultrashort pulse welding of transparent and transparent to opaque materials. Based on recent trends in diverse branches such as biomedicine or consumer electronics we obtain a demand of reliable and sustainable joining technologies. This can be addressed by the adhesive free and localized laser joining technique using ultrashort laser pulses. Nonlinear absorption as well as heat accumulation within the focal region generate localized joints that are long term stable. On the other hand, the short focal tolerance and small gap size that can be bridged leads to high requirements for the surface quality of the weld partners hindering a cost-effective industrial usage. To overcome these limitations, spatial and temporal beam shaping of the ultrashort laser pulses is used. Based on temporal energy modulation together with the world´s first optics for ultrashort pulse welding (Top Weld optics) much better weld performance in terms of focal tolerance and gap bridging is achieved. The process allows for the bridging of gaps up to 10μm and a focal tolerance of up to 300μm which is several times higher compared with Gaussian focusing (4μm gap size, 80μm focal tolerance). This enables a highly reproduceable welding process even for larger sample dimensions e.g. in the field of architecture. Furthermore, to ensure welding in industrial environment with high throughput a simple process diagnostic based on monitoring the process illumination is presented.
The industrial maturity of ultrashort pulsed lasers has triggered the development of a plethora of material processing strategies. Recently, the combination of these remarkable temporal pulse properties with advanced structured light concepts has led to breakthroughs in the development of laser application methods, which will now gradually reach industrial environments. We review the efficient generation of customized focus distributions from the near-infrared down to the deep ultraviolet, e.g., based on nondiffracting beams and three-dimensional-beam splitters, and demonstrate their impact for micro- and nanomachining of a wide range of materials. In the beam shaping concepts presented, special attention was paid to suitability for both high energies and high powers.
Ultrashort pulsed lasers represent unique tools for the processing of micro-optical components. Pulse durations around 1 ps and corresponding extreme peak intensities lead to interaction processes with all conceivable materials. As parts of almost every optoelectronic device, transparent materials represent a particularly challenging example for processing. Here, a controlled energy deposition at the surface or inside the volume is required while maintaining optical properties or implemented functionalities of adjacent areas. The talk will review strategies for the micro-processing of transparent materials that become possible by spatiotemporal beam shaping. Here, the beneficial use of non-diffracting beams is discussed as well as 3D-beam splitting approaches.
During the last years processing of transparent materials by ultrashort laser pulses has gained interest. Spatial and temporal pulse shaping has already proven its potential for advances, widening existing and opening new application fields. The paper focuses on supporting the application development by extending pump-probe diagnostics via combining pulse shaping capabilities, dynamical beam positioning, processing at elevated repetition rates, energy modulation and high temporal resolution over an essentially infinite range of delay. Applying these capabilities gives inside into effects resulting from spatial and temporal shaping, on the laser matter interaction of individual and of a multitude of pulses, the latter typically effective on larger scales due to accumulation. The influence of beam shaping and processing parameters on the dynamic development of the interaction zone in multi pulse exposure highlights the potential of such diagnostic tools. Observations relevant for development of transparent materials processing ultrafast lasers by absorption induced inside of the workpiece are presented. Initiation and development of cracks as a major aspect in brittle materials processing can be analyzed in detail. Pump-probe polarization microscopy for transient stress birefringence observation reveals that both, pressure waves and temperature gradients from accumulation, are of major importance in scaling industrial processing. Considering these findings facilitates addressing different application fields, illustrated by examples on ultrafast welding and selective etching by shaped beams.
Daniel Flamm, Daniel Günther Grossmann, Michael Jenne, Felix Zimmermann, Jonas Kleiner, Myriam Kaiser, Julian Hellstern, Christoph Tillkorn, Malte Kumkar
The remarkable temporal properties of ultra-short pulsed lasers in combination with novel beam shaping concepts enable the development of completely new material processing strategies. We demonstrate the benefit of employing focus distributions being tailored in all three spatial dimensions. As example advanced Bessel-like beam profiles, 3D-beam splitting concepts and flat-top focus distributions are used to achieve high-quality and efficient results for cutting, welding and drilling applications. Spatial and temporal in situ diagnostics is employed to analyze light-matter interaction and, in combination with flexible digital-holographic beam shaping techniques, to find the optimal beam shape for the respective laser application.
The high peak power of ultrashort laser pulses enables the processing of transparent materials by inducing absorption nonlinearly. There are already a variety of applications in the field based on volume or surface absorption. Spatial beam shaping offers high potential, for example by applying Bessel-like beams for single pulse full thickness modification in cutting applications. Temporal shaping the pulse or applying bursts of pulses adapted in amplitude and interval is a further option to localize and dose the energy deposition. An alternative option for scaling is processing at elevated repetition rates. This typically results in accumulation effects, often not desired, sometimes useful or even necessary for several applications. Learning about the complex interplay of the effects relevant for ultrashort pulse laser processing of transparent materials is crucial for the development of advanced industrial applications. Pump-probe diagnostics have proven to be a powerful tool for analyzing the laser matter interaction of spatially shaped beams with high temporal resolution. By extending this to broader range of temporal parameters of the pump, including flexible burst operation, combined with unlimited delay range of the probe and integrated optional polarization microscopy, high speed camera and observation during translation of the workpiece, the setup is suitable to analyze effects on different temporal and spatial scales in a single setup. The potential of this modular experimental system is demonstrated by analyzing multi pulse focusing of Gaussian and Bessel-like beams into glass.
Due to its hardness and scratch resistance sapphire is a favorite material for various high-quality applications e.g. in consumer electronics. Because of those excellent properties sapphire is a demanding material regarding processing. Using ultrashort pulses in combination with beam shaping offers the possibility to deposit energy precisely into the material and modify in a controlled manner reducing thermally induced stress and avoiding microcracks. Separation along modified paths especially for inner contours is still an open task. Selective etching of laser modified sapphire is a promising technology to release outer contours as well as inner contours and even smallest through holes. By using Bessel-like beam profiles an amorphized elongated modification in the monocrystalline bulk along the whole material thickness can be achieved by a single pulse. The amorphous phase in contrast to the monocrystalline sapphire is etchable in 30 wt.-% KOH solution. For a successful process development, a fundamental comparison of different types of modification and its etching behavior depending on pulse duration, pulse energy, number of pulses, spatial and temporal distances of modifications at a wavelength of 1030 nm is carried out. The etching rate depends on the processing and etch solution parameters and is optimized to 10 μm/min. Besides the contours a nanosieve consisting of two-dimensional arranged crack free nanoholes (200 nm in diameter, 5 μm in distance) is realized with an aspect ratio of 1:1500.
With availability of high power ultra short pulsed lasers, one prerequisite towards throughput scaling demanded for industrial ultrafast laser processing was recently achieved. We will present different scaling approaches for ultrafast machining, including raster and vector based concepts. The main attention is on beam shaping for enlarged, tailored processed volume per pulse. Some aspects on vector based machining using beam shaping are discussed. With engraving of steel and full thickness modification of transparent materials, two different approaches for throughput scaling by confined interaction volume, avoiding detrimental heat accumulation, are exemplified. In Contrast, welding of transparent materials based on nonlinear absorption benefits from ultra short pulse processing in heat accumulation regime. Results on in-situ stress birefringence microscopy demonstrate the complex interplay of processing parameters on heat accumulation. With respect to process development, the potential of in-in-situ diagnostics, extended to high power ultrafast lasers and diagnostics allowing for multi-scale resolution in space and time is addressed.
The suitability of materials for deep ultraviolet (DUV) waveguides concerning transmittance, fabrication, and coupling properties is investigated and a fused silica core/ambient air cladding waveguide system is presented. This high refractive index contrast system has far better coupling efficiency especially for divergent light sources like LEDs and also a significantly smaller critical bending radius compared to conventional waveguide systems, as simulated by ray-tracing simulations. For the fabrication of 300-ffm-thick multimode waveguides a hydrouoric (HF) acid based wet etch process is compared to selective laser etching (SLE). In order to fabricate thick waveguides out of 300-ffm-thick silica wafers by HF etching, two masking materials, LPCVD silicon nitride and LPCVD poly silicon, are investigated. Due to thermal stress, the silicon nitride deposited wafers show cracks and even break. Using poly silicon as a masking material, no cracks are observed and deep etching in 50 wt% HF acid up to 180 min is performed. While the masked and unmasked silica surface is almost unchanged in terms of roughness, notching defects occur at the remaining polysilicon edge leading to jagged sidewalls. Using SLE, waveguides with high contour accuracy are fabricated and the DUV guiding properties are successfully demonstrated with propagation losses between 0.6 and 0:8 dB=mm. These values are currently limited by sidewall scattering losses.
The present work investigates the influence of the pulse duration and the temporal spacing between pulses on the ablation of aluminosilicate glass by comparing the results obtained with pulse durations of 0.4 ps and 6 ps. We found that surface modifications occur already at fluences below the single pulse ablation threshold and that laser-induced periodic surface structures (LIPSS) emerge as a result of those surface modifications. For 0.4 ps the ablation threshold fluences is lower than for 6 ps. Scanning electron micrographs of LIPSS generated with 0.4 ps exhibit a more periodic and less coarse structure as compared to structures generated with 6 ps. Furthermore we report on the influence of temporal spacing between the pulses on the occurrence of LIPSS and the impact on the quality of the cutting edge. Keywords: LIPSS,
For the development of industrial NIR ultrafast laser processing of transparent materials, the absorption inside the bulk material has to be controlled. Applications we aim for are front and rear side ablation, drilling and inscription of modifications for cleaving and selective laser etching of glass and sapphire in sheet geometry.
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.
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