New design tackles heat challenges in high-power fiber lasers

Thulium fiber lasers, operating at a wavelength of 2 micrometers, are valued for applications in medicine, materials processing, and defense. Their longer wavelength makes stray light less damaging compared to the more common ytterbium lasers at 1 micrometer. Yet, despite this advantage, thulium lasers have been stuck at around 1 kilowatt of output power for more than a decade, limited by nonlinear effects and heat buildup. One promising route to break this barrier is inband pumping—switching from diode pumping at 793 nm to laser pumping at 1.9 µm. This approach improves efficiency and reduces heat, but it introduces new challenges for fiber components, especially the cladding light stripper (CLS).

CLS devices remove unwanted light traveling in the fiber’s outer cladding, which otherwise degrades beam quality and can damage components. For inband-pumped thulium lasers, CLS must handle high powers at long wavelengths. Conventional polymer-based CLS designs fail here: most polymers absorb strongly at 2 µm, causing intense localized heating and rapid burnout at just a few watts. Alternatives like etched or laser-processed fibers can withstand higher powers but struggle to remove low-angle light—a critical issue for pump lasers. Multimaterial CLS designs exist, aligning layers with increasing refractive index along the fiber to spread heat, but they are complex and hard to implement.

As reported in Advanced Photonics Nexus, researchers at Fraunhofer IOF in Germany have developed a simpler solution: a single-material CLS with self-adapting behavior. The material’s refractive index starts slightly above that of glass and decreases as temperature rises, thanks to a strongly negative thermo-optical coefficient. At low power, the CLS strips light efficiently. As power increases, the heated sections become less effective, passing remaining light to cooler regions. This spreads heat along the fiber length instead of concentrating it at the start, preventing catastrophic overheating. “This is a game-changer for quick lab experiments at medium powers,” says lead author Dr. Tilman Lühder.

Backed by simulations and experiments, the team demonstrated the concept on fibers of 125 µm and 400 µm diameter for all relevant thulium wavelengths. Results show over 20 W of stripped signal light at 2 µm and up to 675 W at 793 nm, setting a new record for single-material CLS designs. Bending the fiber further boosts performance, achieving stripping efficiencies above 40 dB. Although designed for thulium lasers, the approach is adaptable: by tuning the refractive index, it can serve other systems, including erbium (1.5 µm) and ytterbium (1 µm) lasers.

This technology could help overcome the long-standing power ceiling for thulium fiber lasers, especially in inband-pumped architectures. By enabling robust, high-efficiency stripping at challenging wavelengths, the Fraunhofer team’s design offers a practical, scalable path toward next-generation fiber laser systems.

For details, see the original Gold Open Access article by T. Lühder, T. Walbaum, and T. Schreiber, “Index-adapting cladding light stripper for high-power thulium fiber lasers," Adv. Photon. Nexus 4(6), 066005 (2205), doi: 10.1117/1.APN.4.6.066005