Index-adapting cladding light stripper for high-power thulium fiber lasers

Cladding light strippers (CLSs) are essential components for high-power monolithic fiber laser systems. Because they allow for bending of the fiber, which leads to an excellent stripping efficiency of light with a low ray angle, refractive index-based CLSs have an advantage over the commonly used alternative approaches. However, conventional high-index CLSs overheat at relatively low input power as the maximum temperature, located in a hot-spot, increases linearly with the input power. This applies particularly to CLSs in thulium-based fiber systems, where very low power can already lead to extreme heat generation due to the high cladding material absorption around 2 μm. Here, we investigate materials with a highly negative thermo-optical coefficient combined with a refractive index closely above glass to distribute the stripped power and heat uniformly along an increased fiber length. Analyzing multiple CLS geometries for fiber diameters of 125 and 400 μm, we show record-high maximum input powers for single-material CLSs of 21.8 W for the signal (2039 nm) and 675 W for the pump wavelength (793 nm). Transmitting excess light instead of overheating, this wavelength-adaptable self-protecting CLS concept is fast to apply onsite in the lab and reaches stripping efficiencies of >40 dB in the bent version.