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.
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.
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.
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.
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.
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.
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.
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,