Fraunhofer ILT improves diode lasers thanks to fiber Bragg gratings...

06 Mar 2025

...and Fraunhofer IWS spinoff Fusion Bionic wins Fraunhofer Founder Award 2024.

Whether for applications in medical technology, telecommunications or aerospace, demand for high-power lasers is increasing in many industrial sectors. Users are focusing on how cost-effective and stable the systems are.

Now, Germany’s Fraunhofer Institute for Laser Technology (ILT) says it has made significant progress in high-power diode lasers. It has transferred the writing of fiber Bragg gratings from the world of fiber lasers to that of diode lasers. Dr. Sarah Klein developed the process as part of her dissertation and recently won third place in the prestigious Hugo Geiger Prize.

The complexity of fiber laser systems can be reduced enormously with fiber Bragg gratings. If the optical gratings are written directly into the fiber, they can replace external resonator mirrors. In 2019, within the BMBF-funded EKOLAS project, ILT took part in developing a process that had been established for inserting FBGs into the interior of single-mode optical fibers with a core diameter of 6 µm.

Coordinated by Laserline, the consortium succeeded in writing the fiber Bragg gratings into quartz fibers with a core diameter of 100 µm using USP lasers. The material melts briefly under the influence of the ultrashort laser pulses, cools down again very quickly and changes its optical properties in the bulk material processed in this way.

A single FBG with a diameter of 100 µm is sufficient to relocate the previously external resonator mirrors into the fiber and optimize multimode fiber lasers in many respects. As part of her doctoral thesis, the Fraunhofer researcher also transferred this process to fiber-coupled diode lasers.

Same concept - new objective

In her work, Klein focused not only on multimode fiber lasers, but also on optimizing diode lasers, which are needed to pump solid-state lasers. This changes the objective. In contrast to fiber lasers, the FBGs are used in this application to improve the spectral properties of the diode laser radiation.

However, diode lasers emit broadband radiation. For this reason, the researcher developed a concept to reduce the bandwidth and stabilize the wavelength of the laser radiation. Once again, a directly inscribed fiber Bragg grating is central to this approach.

Thanks to this increase in brilliance, the energy input into the solid-state laser is many times more efficient and, therefore, more cost-effective: a significant advantage for industrial applications.

Klein continued to develop the process as part of an in-house project at the Fraunhofer-Gesellschaft. Here too, as in the EKOLAS project, her aim was to inscribe the optical gratings in multimode fibers used as waveguides for diode lasers. “Normally, laser technology is all about miniaturization. In my research work, it was exactly the opposite,” she said. She had to transfer the USP process, developed for a core diameter of six micrometers, to a diameter of 100 µm.

It was extremely complicated to seamlessly and precisely align the FBG segments; furthermore, energy management was very challenging. In order to inscribe the many gratings in the much larger multimode fibers in a single step, she would theoretically have had to multiply the energy input. However, this option was ruled out from the very beginning.

Klein mastered the challenge by lining up over a dozen of the FBGs, which are only 6 µm in size, in several exposure processes. It was important to work seamlessly. “The writing process would have been much easier with an angular core geometry,” she said. Writing the FBG up to the outermost edges was enormously complicated in terms of the required precision.

Spin-off Fusion Bionic Wins Fraunhofer Founder Award

On February 19, 2025, the Fraunhofer Founder Award, endowed with 50,000 euros, was presented to Fusion Bionic, Dresden, Germany. The Fraunhofer Institute for Material and Beam Technology (IWS) spin-off impressed the judges with its bio-inspired laser technology.

Fusion Bionic applies bio-inspired principles, such as the lotus (surface) effect, to technical surfaces using an ultra-fast laser process. The resulting bionic effects are based on extremely fine micro- and nanotextures that significantly improve the performance of surfaces.

Fusion Bionic’s solutions are based on direct laser interference patterning (DLIP), a high-speed laser technology up to 100 times faster than established methods. Fusion Bionic offers services spanning application development, technology training, contract manufacturing, and hardware solutions based on DLIP technology. In April 2021, Fusion Bionic GmbH was spun off from Fraunhofer IWS with a team of researchers led by Dr. Tim Kunze and experts from the industry.