Exceptional scientific achievements with high economic potential
Prof. Dr. Nolte, Friedrich Schiller University / Fraunhofer IOF
The shorter the pulse duration, the more precisely the laser functions as an industrial tool. Ultra-short laser pulses are now paving the way to new applications. Thanks to the short pulses lasting a few picoseconds – or billionths of a second – the material being processed hardly heats up at all. Dr. Stefan Nolte, professor of Experimental and Laser Physics at the Friedrich Schiller University and at the Fraunhofer Institute for Applied Optics and Precision Engineering, laid the scientific foundations by researching the basic physical properties involved.
1. Together with Dr. Jens König of Bosch and Dr. Dirk Sutter of TRUMPF you was awarded the Deutscher Zukunftspreis 2013 (German Future Prize). How long did it take to introduce the ultra-short pulse laser (USPL) into industrial mass production? What form did this kind of collaboration take?
Dr. Stefan Nolte: I have already been working on the topic for almost 20 years, and collaborating with Bosch and Trumpf since 1999. At that time, the German Federal Ministry of Education and Research created suitable research conditions by establishing funding initiatives for ultra-short pulse lasers. In the Primus and Promptus projects in particular, we worked together to develop very important foundations for industrial use of this technology.
The key benefits of ultra-short laser pulses in extremely precise, low-damage processing were already known at that time. However, there was no question of using USPLs in industrial mass production. Any studies in this direction were conducted in research laboratories. A lot more work was necessary to develop a productive and reliable process. In particular, we had to find out which pulse duration was really needed and which pulse energies could best be used.
While the experimental foundations were being developed at the University of Jena and at the Fraunhofer IOF, Bosch was working on the requirements and specifications relating to industrial application. Trumpf was concerned with making the ultra-short pulse lasers not only more and more powerful but, above all, more reliable and more robust. It was this shared work that enabled us to finally implement reliable industrial mass production with USPLs with all their attendant benefits.
2. Car making without laser technology is now inconceivable. But ultra-short pulse lasers are relative newcomers in this sector. Why is this?
Dr. Stefan Nolte: Ultra-short pulse lasers were highly complex systems at the outset and not suited to industrial use. And so, at that time, we faced a chicken-and-egg situation. The development of robust and reliable beam sources and associated plant engineering was expensive and laborious and involved high market risk. On the other hand, successful industrial use in the production process depended primarily on the beam sources and plant engineering. And, in turn, process development itself depended on the available laser parameters. Consequently, alliances combining basic research, laser development, and industrial implementation were, of course, essential to drive the topic to maturity. Courage was also needed to embark upon the risky development path associated with a new tool.
3. At how many watts do the USPLs currently used at Bosch operate and how is this output power achieved?
Dr. Stefan Nolte: Mainly laser sources with a mid-range output power of 50W are used in current production at Bosch although lasers with much greater power are already available. The Trumpf company has adopted the disk laser concept to achieve these levels of output power. Only a thin disk is used instead of a laser beam. This approach helps prevent thermo-optic problems in the laser. Power scaling is achieved by increasing the size of the laser spot on the disk.
4. Where are ultra-short pulse lasers currently in use? Are further applications planned?
Dr. Stefan Nolte: At Bosch, this technology is currently deployed to drill geometrically complex holes in fuel injection nozzles. As a result of these holes, gasoline is distributed more efficiently in the combustion chamber, thus contributing toward fuel savings of up to 20 percent. Further examples of industrial application relate to more efficient oil burners, ceramic exhaust-gas sensors with enhanced measuring accuracy, and drainage grooves in diesel injectors. But there are other applications in totally different fields such as in the cutting of hardened-glass cover screens for smartphones and in the manufacture of bioresorbable stents made of polymer.
Unfortunately, it's not always possible to talk freely about all applications. But there is already a whole spectrum of further applications and I am sure many more will follow in the future, simply because the range of materials that can be processed is enormous.
5. What are the advantages of the "ultra-short" technology?
Dr. Stefan Nolte: Ultra-short pulse lasers allow all materials to be processed with extremely high precision. For this purpose, skillful selection of pulse duration, pulse energy, cycle rate and the right focus are needed. The material is heated so quickly and so powerfully that it vaporizes without melting. This enables ablation in the very finest ranges of just a few millionths of a millimeter (i.e. nanometers) to take place – without any burr formation from residual melt and any heat transfer to the material that would impair its quality. This is why this is referred to as "cold processing".
6. Where is there still room for improvement?
Dr. Stefan Nolte: This technology represents a new tool. Many applications still wait to be explored. These will be accompanied by a further increase in the power of laser systems and corresponding ongoing development of process control can be expected in order to address new applications and make use of the laser power in efficient manufacturing processes. In other applications a lower pulse energy will suffice – potentially with a still higher cycle rate, as is possible with a small, new fiber laser based product by Trumpf.
7. Where is development of the ultra-short pulse laser heading?
Dr. Stefan Nolte: Firstly, ultra-short pulse lasers will become ever more powerful. But, depending on application, it can make sense to use even shorter pulse durations, especially when processing transparent materials. For example, at the last laser trade fair Trumpf presented a femtosecond variant that it was able to derive relatively easily from the established ytterbium-based industry lasers. However, it is not just a matter of shortening the pulse duration but also of supplying tailored temporal and spatial power by means of appropriately shaped pulses.
Thank you for the interview.
Dipl. Ing. Nachrichtentechnik