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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 P 2 O 5 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 P 2 O 5 -SiO 2 glass.

Ultrashort pulse fiber laser systems with both high average and peak power are promising tools for a wide range of applications. Usually pulsed fiber laser systems exhibit either high average power [1] or high peak power [2]. Only utilization of rod-photonics-crystal-fiber technology [3] allows one to achieve both high average (130 W) and peak (870 kW) power with a price of non-monolithic design (mainly bulk elements are used in [3], which lead to the laser's high cost and lose of reliability). Moreover not perfectly single-mode output (M2 = 1.30) results in limitation of maximum average power caused by mode instability. Recently sub-MW peak power in monolithic all-fiber laser scheme was demonstrated in [4] using tapered fiber approach. Scalability of average power in tapered fiber to 28 W at peak power of 292 kW was demonstrated in [5]. In the current communication we demonstrated further progress in average and peak power scaling using this approach.