Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF have developed thulium fiber laser systems that almost double the previous performance world record. The system is made up of three high-power thulium fiber lasers that emit light in the spectral range of 2030 to 2050 nm and achieves an outpower of up to 1.91 kW, close to double that of conventional systems at ~1.1 kW. High-power fiber lasers are a versatile tool for numerous technological applications, such as material processing or free space communication. The choice of the right spectral range plays a decisive role, especially over long distances — from earth to satellites, for example. The spectral region above 2030 nm is considered particularly suitable, as atmospheric losses are low there and the laser light is relatively eye-safe. The researchers in Jena are now building on their previous record and consistently developing the technology further. “Our goal is to optimize the technological basis so that we can reach the next level of performance with reliable individual sources,” said Till Walbaum, group leader for laser technology at Fraunhofer IOF. The principle of spectral beam combining is central to this. In this process, laser beams of different wavelengths are directed onto special optical reflection gratings at adapted angles. Diffraction combines the laser beams into a single beam. This not only increases the performance of the fiber laser system but also preserves the beam quality and thus the focusing ability of the laser beam. Three parallel individual beams are combined into a laser beam with a record output of 1.91 kW by means of height-shifted reflection gratings. Courtesy of Fraunhofer IOF. Previous systems reach their physical limits at high power levels, in particular due to overheating caused by low combination and laser efficiencies. The Fraunhofer IOF team has addressed these challenges with new, more efficient individual sources and improved cooling systems. In particular, a special connection technique for fibers, known as “cold splicing,” enables low-loss fiber-to-fiber coupling and effective temperature regulation. Another key component is a specially developed diffraction grating with an efficiency of over 95% and excellent thermal performance. “The combination grating is the heart of our system,” said Friedrich Möller, scientist in the Laser Technology department at Fraunhofer IOF. “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-kW region under challenging parameters. It enables a low-loss beam combination with overall efficiencies greater than 90 percent and is the baseline for our next leaps in performance.” “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,” said Walbaum on the future potential of the technology. The high-performance thulium fiber lasers open a wide field of applications, including medical procedures, polymer processing, and optical data transmission. An important advantage of the lasers is their improved eye safety. Scattered light with a wavelength of 2 µm is absorbed by the cornea and does not reach the retina, which enables safer use in industrial and medical applications.