Driving Fraunhofer ILT Fiber Laser Breakthroughs with Holmium-Doped Fibers 

Bringing cutting-edge fiber laser technology to industry using COTS components

Fraunhofer is German’s leading research organization for applied science, developing technologies originating from academic research and transferring to the industry. The Fraunhofer Institute for Laser Technology ILT, based in Aachen, has 40 years of expertise in laser technology. With more than 530 employees, state-of-the-art facilities and international references, it is one of the most important contract R&D institutes in its field worldwide.

Companies approach Fraunhofer ILT to explore new joint developments or to qualify products. And the Institute can also investigate new state-of-the-art developments that could lead to cutting-edge technologies raising the attention of companies for potential industrialization. 

Dr. Patrick Baer, Fiber Lasers Group leader at Fraunhofer ILT, explores new architectures for fiber laser systems. His team develops turn-key solutions based on industry-grade optical components, bringing potential technological breakthroughs across a wide variety of demanding applications.

Pushing fiber laser technology to the limits - for astrophysics and beyond…

Dr. Baer’s group has been working for several years on a laser architecture based on holmium (Ho)-doped fibers manufactured by Exail. Fiber lasers operating in the 2 µm range could be a future solution for the Einstein Telescope, a third-generation gravitational-wave detector planned for realization by 2035. One current site candidate is the Euregion Meuse-Rhine in the border area of Germany, Belgium and the Netherlands. 

Gravitational-wave detectors provide a unique window into interstellar processes, such as supernovae and the collision of compact objects like neutron stars and black holes, which can be identified by their characteristic gravitational wave signatures. In previous and current international projects, such as “E-TEST”, multiple technologies are being pushed beyond today’s state of the art, including fiber lasers. 

The Einstein Telescope will be built around 250 meters underground, potentially in the Euregion Meuse-Rhine. With interferometers in the three tunnels, each ten kilometers long, it will measure collisions of black holes in the early universe. | Copyright: © NIKHEF.

The challenge is extraordinary: the detector of the Einstein Telescope shall be at least ten times more accurate in its measurement than current detectors. With its interferometer arm length of approximately 10 kilometers, it shall detect differences in distance ten thousand times smaller than the size of a proton. Achieving this requires an extremely stable, low-noise interferometric setup, based on components such as a laser system, photodiodes, and optical components, for example beam splitter and mirrors. The system must be highly stable since any fluctuation in its output signal could be misinterpreted as a gravitational-wave signal.

The current requirement for a potential laser system for the low-frequency interferometer (ET-LF) of the Einstein Telescope is to deliver up to 10 W of output power with a combination of exceptional power stability, narrow linewidth, and polarization purity. Meeting such extreme requirements for a fiber laser emitting at approx. 2 µm also makes the resulting technology platform promising for other fields, such as quantum technology or high-precision metrology applications, and Dr. Baer’s team is building valuable expertise through this development.

Unlocking holmium-doped fiber amplifier at 2095 nm with industry-grade components

For the Einstein Telescope, Fraunhofer ILT aims to deliver a fully integrated holmium fiber amplifier in the 2090 nm range as a candidate technology for the final system. The fiber laser group’s philosophy is to develop innovative prototypes using commercially available components with proven industrial track records, making later reproduction and industrialization easier and more cost-effective.

A prototype of highly stable Ho-doped fiber amplifier is currently being developed at the Fraunhofer ILT. The technology also shows potential for quantum and high-precision metrology applications. | Copyright: © Fraunhofer ILT, Aachen, Germany

“Our goal is to build a laser and push each component of the architecture to its limits to understand both the technical and physical effects,” explains Dr. Baer. “Our expertise combines technological know-how and deep physical understanding.”

Since 2022, his team has tested several laser architectures using different versions of Ho-doped fibers manufactured at Exail’s Lannion site. Initial versions showed the functionality of the fibers but had some downsides — notably in efficiency — although they validated the overall architecture. In 2025, Exail delivered a new Ho-doped fiber with significantly higher efficiency and polarization-maintaining capability (IXF-HDF-PM-20-250-0.08-V1), to further improve the fiber laser system. For pumping of the Ho-doped fibers, Exail’s thulium (Tm)-doped fibers are being used in the system.

“It was really kind of Exail to send us the newest prototype of Ho-doped fiber.” Says Dr. Baer “We tested it, and we are now gathering as many results as possible before Photonics West 2026. The new fiber allows us to improve our system dramatically — efficiency has doubled from 32%, and we expect even better stability and further improvements."

With this new fiber, the laser setup now achieves outstanding laser parameters, particularly in output power and efficiency in combination with a high polarization extinction ratio and narrow linewidth. This progress allows Baer’s team to return to its preferred, simpler system architecture, which requires fewer components and less pump power.

Laser architecture using an in-line Tm-doped fiber laser for generating the pump radiation for the initial version of Exail’s Ho-doped fiber, achieving up to 25 W of output power. With this setup, more than 40 W of pump power can be used.  Such an in-line architecture was setup instead of a typical architecture (see lower picture) since commercially available wavelength-division multiplexers (WDM) are only functional for up to approx. 20 W pump power as the technology is not mature yet at 2 µm – which only resulted in approx. 6 W achievable output power.

Expected architecture by Dr. Baer: Since the efficiency of the new Exail’s Ho-doped fiber is now higher (approx. 60%), it is expected that 20 W pump power are sufficient to achieve 10 W output power. Therefore, now the commercially available WDM can be used, resulting in an improved system architecture. With the Tm-doped pump laser now acting as an independent actuator, it is expected to further improve the achieved laser parameters.

Some final refinements are still needed to achieve optimal performance. Once the final setup is confirmed, the team will design a compact, fully integrated module suitable for end-users — such as Einstein Telescope scientists — who may not have laser engineering expertise.

A constructive and collaborative partnership

Dr. Baer emphasizes the quality of the collaboration with Exail. He highlights the productive technical exchanges around new products, the ability to test prototypes early, and the openness to custom developments — as well as the freedom to publish results mentioning the fiber and its performance. He sees the collaboration as a mutually beneficial relationship between supplier and customer.