Stability of a fiber amplifier by eliminating optical feedback into its pumb light sources

An optical amplifier including a gain fiber, first and second sources of pump light for providing pump light through the gain fiber, first and second optical couplers coupling the first and second sources of pump light to different points along the gain fiber, and a cut-off filter coupled to one or the other of the first and second optical connections for preventing pump light generated by one of the sources from reflecting off the second of the sources and reentering the first source. The gain fiber also includes dopant atoms such as erbium. The cut-off filter is preferably one of a wavelength division multiplexer and an optical isolator.