In an example embodiment, an optical amplifier comprises a doped multi-core optical fiber and two optical couplers placed at the ends of the doped multi-core fiber, with each optical coupler having a respective plurality of optical waveguide cores optically coupled to the optical waveguide cores of the doped multi-core fiber. The spatial arrangement of the cores at the input end of the first optical coupler is configured for low-loss intake of the optical energy from the input transmission line. The spatial arrangement of the cores at the output end of the first optical coupler and the spatial arrangement of the cores at the input end of the second optical coupler match the spatial arrangement of the cores in the doped multi-core fiber. The spatial arrangement of the cores at the output end of the second optical coupler is configured for low-loss transfer of the optical energy into the output transmission line.