An optical amplifying unit (100; 100') for amplifying optical signals in an optical transmission system has an amplification wavelength band with a lower wavelength limit greater than 1570 nm and includes an input (101) for the input of optical signals, an output (102) for the output of optical signals and an optical amplifier (104) interposed between the input (101) and the output (102) to amplify the optical signals. The optical amplifier (104) includes an amplification fiber (108) co-doped with erbium and ytterbium, at least a pump source (109, 110) to generate pump radiation and at least an optical coupler (111, 112) optically coupling the pump sources (109, 110) to the amplification fiber (108). The optical amplifying unit has a power gain of at least 31 dB when the optical signals have an input power of at least -10.5 dBm and wavelengths within the amplification wavelength band.