Thulium fiber lasers, operating at a wavelength of 2 μm, 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 μm. Yet, despite this advantage, thulium lasers have been stuck at around 1 kW of output power for more than a decade, limited by nonlinear effects and heat buildup. One promising route to break this barrier is in-band 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 in-band-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. Cladding light stripper (CLS) technology addresses a major challenge in scaling thulium fiber lasers beyond their long-standing 1-kW power limit. A self-adapting CLS design distributes heat along the CLS as input power increases (top to bottom in the thermal image), without meaningfully increasing in maximum temperature. By spreading heat along the fiber, the design prevents damage and enables record performance: over 20 W of stripped signal light at 2 µm and 675 W at 793 nm. Courtesy Fraunhofer Institute for Applied Optics and Precision Engineering IOF (Fraunhofer IOF), Jena/T. Lühder,. Researchers at Fraunhofer Institute for Applied Optics and Precision Engineering IOF (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,” said study lead author 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 >20 W of stripped signal light at 2 µm and up to 675 W at 793 nm, setting what the researchers call a 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. The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.4.6.066005).
