Thulium fiber laser hits record-breaking 1.9kW

10 Apr 2025

Latest developments in spectral beam combining at Fraunhofer IOF almost double typical power output.

Combination gratings

Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) say that they have built a thulium fiber laser capable of emitting 1.91 kW - nearly double the typical output.

The Jena-based team, led by Till Walbaum, have used spectral beam combining - a technique also deployed in laser weapons development - to raise the output power of systems emitting light at wavelengths between 2030 and 2050 nm.

While thulium lasers are already used in medical applications, in particular urology (IPG Photonics has developed a thulium fiber laser for treatment of kidney stones), those wavelengths are expected to prove especially suitable for future free-space optical satellite links taking advantage of low atmospheric attenuation and better eye safety.

“Our goal is to optimize the technological basis so that we can reach the next level of performance with reliable individual sources,” commented Walbaum.

Combination grating
In the spectral beam technique, slightly different laser wavelengths are directed onto gratings at subtly different angles, so that diffraction combines each of the sources into a single beam of light.

“This not only increases the performance of the fiber laser system, but also preserves the beam quality and thus the good focusing ability of the laser beam,” explains the Fraunhofer team.

They point out that previous systems have reached their physical limits at high power levels, largely because of overheating from low combination and laser efficiencies.

To get around that, Walbaum and colleagues devised more efficient individual sources and improved cooling systems, for example using “cold splicing” between fibers to enable low-loss coupling and better temperature regulation.

Another important development is a customized diffraction grating said to have an efficiency of more than 95 per cent as well as excellent thermal performance.

20kW target
Friedrich Möller from the Jena group said: “The combination grating is the heart of our system. Up to now, optical combining elements such as gratings and dichroic mirrors for wavelengths around 2 µm were only available for laser powers of a few hundred watts.

“However, our colleagues at the institute have developed a special diffraction grating that also works excellently in the multi-kilowatt region under challenging parameters. It enables a low-loss beam combination with overall efficiencies greater than 90 per cent and is the baseline for our next leaps in performance.”

Walbaum added: “We have created the technological prerequisites for realizing laser systems with even higher performance and reliability. The next big challenge is now to reach the 20 kW level.”

Aside from urology and free-space communications, high-performance thulium fiber lasers could find use in polymer material processing.

Compared with conventional fiber lasers operating at around 1 µm the sources offer improved eye safety, because scattered light at 2 µm is readily absorbed by the cornea and does not hit the retina.