Fused fiber components for “eye-safe” spectral region around 2 \(\upmu \)m

Thulium-doped fiber lasers operating at wavelengths around 2 \(\upmu \)m are rapidly developing a new class of coherent light sources with a high slope efficiency reaching 70 %. The 2-\(\upmu \)m radiation sources have many advantages over the 1-\(\upmu \)m sources, e.g., better eye-safety, relaxed non-linear limits and often more efficient material processing. Particularly important application of 2-\(\upmu \)m fiber lasers is in a highly-efficient generation of wideband mid-infrared radiation through third order nonlinear effects in soft-glass fibers. In this paper we report on the development of passive components intended for fiber laser operation around 2 \(\upmu \)m, namely fiber couplers and wavelength division multiplexers for combination of 1.6- and 2-\(\upmu \)m radiation. Three commercially available fibers were used for the preparation of these components. The measured characteristics of the components are compared and the limitations are discussed, particularly the two-mode operation and high bend loss. Specific fiber designs are proposed in order to optimize the performance of the wavelength division multiplexer.