Ultrafast laser processing of transparent materials supported by in-situ diagnostics

Abstract

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.

Throughput scaling by spatial beam shaping and dynamic focusing

“…Especially under heat accumulation conditions process, development requires paying attention to effects on multiple scales in space and time. We already applied different setups for in-situ diagnostics 13 . The extension towards improved pump probe capabilities is in progress, will allow a sup-ps temporal resolution within a delay range spanning from sub-ps to ms at frame rates of more than 200 kHz under applying spatial and temporal beam shaping and highly dynamic focusing.…”

Structured light for ultrafast laser micro- and nanoprocessing

“…The combination of ultrashort pulsed volume modifications and subsequent selective chemical etching of dielectrics, known as selective laser etching (SLE), enables rapid fabrication of threedimensional (3D) glass structures of arbitrary shape with smallest structural features down to the 10 μm scale. [65][66][67] The remarkable 3D fused silica articles provided by, e.g., LightFab 68 convince with extreme geometries, including microtunnels, cavities, or mounted moving parts. Usually, the corresponding processing strategy is based on 3D-scanning a single Gaussian focus distribution into the transparent workpiece to spatially control the nonlinear energy deposition.…”

“…Since femtosecond laser pulse welding around 1 μm wavelength of transparent materials requires moderate pulse energies <10 μJ 71 concepts for throughput scaling are becoming increasingly important. 6,67,69 This becomes even clearer if one considers the recent progress in pulse energy and average power of today's ultrafast laser platforms. 26 We use a structured light concept based on a diffractive 3D-beam splitter to discuss the potential of parallel processing for particularly efficient welding of transparent materials.…”

“…Especially, the manufacturing of microstructures requires highest accuracy and minimal damage of the workpiece. Although the materials to be processed are very diverse-from highly absorbent to transparent 4,67,90,91 -the process strategy is usually based on a Gaussian focus (single spots or multiple copies 6,80 ) being directed over the workpiece using scanner optics or axis systems. Although, as mentioned, high quality machining results are Fig.…”

Throughput scaling by spatial beam shaping and dynamic focusing

“…Detrimental heat accumulation has to be avoided by choosing the distance between the individual spots properly 12 . For processing of materials induced by nonlinear absorption inside of the transparent volume, tight focusing is essential 13 . Therefore the volume processed by standard focusing with a single spot is hard to scale and throughput scaling is limited even at elevated repetition rates and even in case the processing does not suffer from but is based on heat accumulation.…”