Abstract: A fiber-optic amplifier comprising a laser source which emits signal radiation in a narrow band on one or more wavelengths in a first end of an amplifier fiber, and amplified signal radiation can be coupled out at a second end of the amplifier fiber. The amplifier fiber is a double-core fiber with a pump core and a laser core and the latter is end pumped or side pumped. The amplifier fiber is a multimode double-core fiber at which or within which is arranged, in the area of its first end, an element for transverse mode selection which suppresses modes higher than the fundamental mode.