Side-fused signal–pump combiner and microstructured cladding light stripper for triple-clad fibers

Triple-clad fibers are, in particular, used in thulium- and holmium-doped fiber lasers and amplifiers to increase the threshold for critical fiber damage, e.g., caused by absorption in the coating. This would allow for further power scaling. However, essential fiber-based components such as side-fused signal–pump combiners and cladding light strippers are not available for this fiber type. As the fiber design prevents the required access to the inner pump cladding, it hinders efficient component manufacturing. Here, we present manufacturing processes involving CO

₂ laser-based ablation that enable the production of these components. For the manufactured signal–pump combiner with a 14/400/480-μm triple-clad fiber and up to four 135/155-μm pump input fibers, a pump coupling efficiency of 95.5%, a signal loss of 0.2 dB, and a signal-to-pump isolation of 34 dB were achieved. We confirm by ray tracing simulation that most of the pump power is coupled into the inner cladding of the triple-clad fiber. The cladding light stripper, manufactured out of the same fiber sample, achieved a cladding light attenuation of 20.8 dB, whereas no significant signal loss was detected. The availability of these components enables monolithic laser and amplifier setups with triple-clad fibers and supports their power scaling with high beam quality.