An optical amplifier assembly includes an optical amplifier stage including an active fiber with an active fiber core configured to guide and amplify a signal light; a pump laser configured to provide pump laser light; a wavelength-division multiplexing (WDM) coupler configured to couple the pump laser light into the active fiber core; and a gain-flattening fiber Bragg grating (FBG) arranged in the active fiber core. The optical amplifier stage is configured to provide a spectral gain profile corresponding to the active fiber core and the pump laser. The gain-flattening FBG is configured to at least partially flatten the spectral gain profile in order for the optical amplifier assembly to provide an amplified signal light with an at least partially flattened first spectral gain profile. The amplified signal light is derived from the signal light propagating through the active fiber core.