Cladding-pumped Er-doped (Yb-free) fibres with Al 2 O 3 –SiO 2 core highly doped with fluorine

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Double-clad Er-doped fibers highly doped with aluminum (up to 3 mol.%) and fluorine (up to 1.5 wt.%) have been fabricated and studied. The fibers have a core numerical aperture of approximately 0.055, which allowed us to achieve a single-mode propagation regime for a fiber core diameter of 35 µ m. The possibility of increasing the pump-to-signal conversion efficiency in the L-band using the developed double-clad Er-doped fibers was demonstrated. Utilization of the F–GeO 2 –Al 2 O 3 –SiO 2 –Er fiber together with a P 2 O 5 –Al 2 O 3 –SiO 2 –Er fiber was found to be promising for single-frequency amplifiers because the fibers have negligible overlap of the stimulated Brillouin scattering gain. This feature allowed us to build a cascaded double-clad Er-doped fiber amplifier (two Er-doped fibers of different types were spliced one by one) with a threshold of stimulated Brillouin scattering greater than that of an amplifier based on a single Er-doped fiber with the same net length and pump-to-signal conversion efficiency.