In this paper, we studied high-energy single frequency transform-limited Er-doped amplifiers core pumped by a specially developed Yb-doped fiber laser at 980 nm and a Raman laser at 1480 nm. It was demonstrated that pumping at 1480 nm allows achieving slightly higher maximum pulse energy for long pulses in the absence of nonlinear effects. At the same time, amplifiers with a pump at 980 nm have a higher threshold of stimulated Brillouin scattering (SBS), and higher peak power of shorter pulses can be achieved in such a scheme. Lasers with more than 730 μJ pulse energy and with higher than 3.5 kW peak power were demonstrated.

The technical process of the synthesis of a fluorophosphosilicate (FPS) glass core was thoroughly investigated for the first time utilizing a modified chemical vapor deposition (MCVD) all-gas-phase fabrication method. It was discovered that the limiting doping level of the silica glass simultaneously co-doped with phosphorus (P) and fluorine (F) was found to be confined by the formation of POF₃ gas. The dopants content was achieved as high as 4.7 at% of P and 1.1 at% of F in a glass core, respectively. A developed “in-house” manufacturing method makes it possible to fabricate a large mode area (LMA) purely single-mode Er–Yb co-doped optical fibers with a core diameter of 20 μm and with a lasing efficiency comparable to commercially available LMA Er–Yb optical fibers.