Er-Yb fibers are routinely used for generation of watt-level signal near 1.55 µm. Such fibers typically have core doped with a high P2O5 concentration (required for efficient energy transfer from Yb3+ to Er3+ ions) which result in a high core numerical aperture (NA) of about 0.2. As a results single-mode Er-Yb fibers has a small core diameter (~5 µm) and they are not suitable for high peak power applications. In the current communication we report on development of Er-Yb fibers with a low core NA based on highly fluorine doped P2O5-SiO2 glass.
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