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Microprocessing with high-power USP-lasers: multi-beam technology boosts cost-efficiency

The Fraunhofer Institute for Laser Technology ILT has developed a new optics system for the manufacture of periodic microstructures, which increases the processing speed when working with ultra-short-pulse (USP) lasers to many times that yielded by conventional systems. By splitting the laser beam into several beamlets, the output of high-power ultra-short-pulse lasers can be fully utilized for microprocessing, which delivers a significant reduction in process costs. At the 2nd USP Workshop, which will take place in the German city of Aachen from April 17 to 18, 2013, an expert will present the process underlying this innovative development.

Whether for manufacturing masks and microsieves or creating functional surfaces for components or stamping tools that are subject to high tribological stresses, more and more applications require a microstructured surface and require one over ever greater areas. The fabrication of micrometer-scale structures calls for especially precise processing of the workpieces, and the ultra-short-pulse laser is the ideal tool for this purpose. Alongside mechanical processes, nanosecond lasers are currently the most popular choice for this type of work, with their greater cost-efficiency making them better established than USP lasers. However, there is the drawback with nanosecond processing that beading occurs as a result of melting effects, which means that the workpiece often needs extensive post-processing. In addition, the melting effects limit the resolution of the microstructuring.

By contrast, functional surface structures can be created with ultra-short-pulse lasers such that zero post-processing is required. On account of the strong localization of the laser energy applied to the workpiece and the very high beam intensities, no beading occurs during processing. Furthermore, processing with a USP laser delivers extremely high precision within a tolerance range of just a few micrometers along with very high depth resolution in the region of a hundred nanometers. However, ablation rates are relatively low, which means that processing times are very long compared with those of pulsed lasers operating in the nanosecond range. For the time being, therefore, microstructuring with USP lasers is only attractive from an economic perspective for high-end products or tools for mass replication. Moreover, in many microstructuring applications, only a fraction of the available laser output can usually be used with the USP laser systems in the power range of 50 to 100 watts which are commonly used in industry today, as only a limited maximum power can be applied to every processing point. Injecting too much power into a processing point, especially with small focus diameters in the micrometer range, leads to plasma formation as well as to thermal effects together with beading caused by melting; the result is poor processing results.

Researchers at Fraunhofer ILT have now addressed the question as to how optimum use can be made of high laser power for USP microstructuring while ensuring perfect processing results.

How can we get the most out of the available laser power?

High-speed beam deflection is one option for fully utilizing the available laser power for USP microstructuring. To ensure high ablation quality with this method, the pulse energy is kept low. However, a high surface rate is obtained by virtue of a high pulse frequency coupled with a high scan speed. With a polygon scanner system developed at Fraunhofer ILT, it is possible to reach scan speeds of up to 350 m/s. This enables high-frequency laser pulses to be rapidly distributed on to large surfaces. Another option currently being pursued at Fraunhofer ILT is the parallelization of laser ablation. By splitting the laser beam into several beamlets, significantly greater laser power can be utilized. This beam splitting is made possible by a diffractive optical element (DOE). The DOE consists of an array of microstructures, which are capable of producing virtually any type of intensity distribution behind the element by means of diffraction - depending on the design. The research team at Fraunhofer ILT fitted the DOE between the beam source and a galvanometer scanner in such a way that the split laser beams are imaged in the galvanometer scanner. Focusing the beams using an f-theta lens eventually produces a periodic array of processing points, which can then be moved over the workpiece. This makes it possible to ablate patterns of any complexity.

The splitting of a laser beam into 16 beamlets has already been successfully demonstrated at Fraunhofer ILT. This beam parallelization enables the workpiece to be processed at 16 periodically arranged points simultaneously, resulting in a 16-fold increase in processing speed. In a laboratory experiment, experts have already successfully tested processing with 144 beamlets, and further scaling is possible.

USP microstructuring on large surfaces is becoming cost-effective

In future, this technology will permit the output reserves of current high-power ultra-short-pulse laser systems to be fully utilized on the workpiece for laser processing. Processing times will drop accordingly, leading to a significant reduction in overall process costs. This will make USP lasers significantly more attractive to users from an economic point of view for manufacturing periodic microstructures. With this approach, it becomes economically feasible to structure even large surfaces. Based on this technology, Fraunhofer ILT has developed a prototype machine for producing microstructures with USP lasers, which is to be further developed in future projects to make it suitable for industrial applications. The long-term goal is to use multi-hundred-watt lasers for microstructuring before too long.

2nd USP workshop from April 17 to 18, 2013 in Aachen

One of the topics of the 2nd ultra-short-pulse laser workshop is the use of USP lasers for producing resource-saving microstructures for highly stressed components. As part of the "SmartSurf" project, Fraunhofer ILT is collaborating with well-known partners from the industrial sector to investigate how to minimize friction and wear in surfaces that come into contact with each other by means of introducing tiny depressions a few micrometers deep into the surfaces. One example of a component being structured in this way is the piston rings used in cylinder liners. The project is embedded in the German Federal Ministry of Education and Research's "Efficiency Factory" initiative for promoting resource-efficient production technologies. Following the presentation, our expert Stephan Eifel will be available for a discussion with the audience. You can register for the 2nd USP workshop as of immediately at www.ultrakurzpulslaser.de.

For further information

Dipl.-Phys. Stephan Eifel Micro and Nano Structuring Group Phone +49 241 8906-311 stephan.eifel@ilt.fraunhofer.de

Dr. Jens Holtkamp Head of Micro and Nano Structuring Group Phone +49 241 8906-273 jens.holtkamp@ilt.fraunhofer.de

Fraunhofer Institute for Laser Technology ILT Steinbachstraße 15 52074 Aachen Phone: +49 241 8906-0 Fax: +49 241 8906-121

idw :: 29.01.2013