Fraunhofer ILT utilizes short pulse lasers to achieve high-speed optical stamping

At the Fraunhofer Institute for Laser Technology (ILT), researchers in collaboration with RWTH Aachen University – Chair for Technology of Optical Systems (RWTH-TOS) are using a spatial light modulator (SLM) to shape the beam of an ultrashort pulse laser precisely into the desired pattern to apply to the surface of a workpiece.
The developers say that this approach “significantly speeds up processing and opens up new possibilities for, among others, the steel and metalworking industries or the glass industry.”

Initial tests have show that process times can be reduced by at least 80 percent. Such a surface treatment process offers advantages over wet chemical etching and electrical discharge machining.

Wet chemical etching not only produces waste that is harmful to health and the environment, but the process is also inflexible because it requires masks. Electrical discharge machining (EDM) also has its disadvantages: It consumes a great deal of energy, produces toxic sludge, and only delivers random, stochastic microstructures. Unlike the laser process, the surface properties cannot be specifically tailored to subsequent process steps.

Optical stamping process. Click for info

Precision patterning

“The optical stamping process allows this problem to be circumvented,” said Sönke Vogel from the Micro and Nano Structuring Group at ILT. Vogel and his team use an SLM to precisely shape the beam of a USP laser into the desired pattern and apply it to the workpiece surface in a single step.

“This creates microstructures that are precise, reproducible, and made in a fraction of the time previously required, with significantly less wear and tear compared to mechanical processes and without the need to retool the optics,” he said.
In optical stamping, the laser beam is not guided across the surface in a vector-based manner using scanner mirrors, but is shaped into the desired structural pattern in a single step and transferred directly to the workpiece. The core component is an SLM with Liquid Crystal on Silicon technology.

Paul Buske, Computational Optics at RWTH-TOS, develops phase masks for the SLM using optical neural networks. Each phase mask corresponds to an optically realized plane, and wave optics methods are used to calculate the connections between these planes.

“Thanks to established AI training methods, optical neural networks enable unprecedented flexibility in beam shaping,” said Buske. “Pattern sizes and geometries can be varied, expanded, or completely replaced.”

Thanks to this innovation, industry can generate deterministic microstructures with precisely reproducible geometry, reduce processing times significantly, and adapt structures to the specific requirements of individual components or subsequent processes.

Optical stamping allows a pattern to be flexibly adjusted

Targeted microstructures for steel
In flat steel production, for example, the surfaces of embossing rolls have so far mostly been microstructured stochastically using EDM. Although the structures embossed in this way improve properties such as the bendability or adhesion of coatings, they are not tailored to specific subsequent processes.

In the EU project METAMORPHA, ILT and RWTH-TOS are pursuing a different approach together with project partners such as Thyssenkrupp Steel Europe. The project aims to develop innovative surfaces and thus sustainably improve the quality of European flat steel products. The partners have demonstrated an 81 percent reduction in process time.

“The collaboration in the METAMORPHA project shows us how laser processes can be transferred directly into industrial practice," said Benjamin Lauer, project manager at Thyssen Krupp Steel Europe.

3D surface profile of an optically stamped microstructure

Fast structuring for signal transmission

Another technology demonstration involves low emissivity glass, an ultra-thin metal layer on glass that reflects heat radiation. However, this coating also blocks mobile phone signals. To enable reception, the metal layer must be partially removed – usually a time-consuming process with a scanning single-beam USP laser. Optical stamping allows precise openings to be made in the coating in a single pulse without subjecting the glass to thermal stress. The USP laser removes the layer with pinpoint accuracy while leaving the substrate undamaged.

In tests at Fraunhofer ILT, the beam was shaped into a circular pattern with a diameter of 450 µm using an SLM and applied at a feed rate of 9 m/s, a pulse energy of 200 µJ, and a repetition rate of 20 kHz. The result: clear, sharply defined structures that allow radio waves to pass through without significantly impairing the thermal insulation. Compared to conventional scanning at 3 m/s, 600 kHz, and 4 µJ per pulse, the processing speed and area rate were dramatically increased by a factor of 30.

Source: optics.org