Single-mode Yb-free Er-doped all-fiber laser cladding-pumped at 976 nm with record efficiency of 40 % and output power of 75 W

The power scaling of Er:SiO₂ fiber lasers is more challenging than in their Yb counterparts. The reason is the low Er³⁺ absorption cross-section and the increase of unbleachable losses with increasing of Er³⁺ concentration due to clustering that impedes the manufacturing of efficient double-clad fibers. Optical-to-optical conversion efficiency does not exceed 24% for conventional single-mode fiber based lasers cladding-pumped at 976 nm. Several approaches can be used to increase the efficiency at 1.55 μm. But co-lasing at 1 μm in Er-Yb fibers and low electrical-to-optical efficiency of sources for in-band pumping prevents their widespreade use. In our previous paper slope efficiency of 32% was achieved in a power-scalable amplifier scheme owing to the use of the novel Al₂O₃-P₂O₅-SiO₂-glass matrix and the decrease of the first cladding diameter down to 95 μm.