Recently we have demonstrated perspectives of application double clad (DC) highly Er-doped (Yb-free) fibers for pulse amplification to a record-high energy and peak power . Single-mode propagation regime, high pump-to-signal conversion efficiency (PCE) of 25% in L-band (near 1575 nm) and a relatively short fiber length (~ 6 m) was achieved by optimization of fiber geometry and core compound. Core-to-cladding ratio was chosen as high as possible (core and cladding diameters were 35 μm and 125 μm correspondingly) to increase pump absorption from the cladding. Utilization of P2O5-Al2O3-SiO2 (PAS) glass matrix  allowed us to achieve a low core numerical aperture (NA) and, simultaneously, high PCE even for heavily Er-doped fibers.
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