a method is provided for locating a fault arising in one or more optical amplifiers from among a plurality of optical amplifiers operating in saturation and located along an optical transmission path . the method begins by generating a cotdr trace representing a backscattered and / or reflected optical power level along the transmission path and comparing the cotdr trace to a previously obtained reference cotdr trace to generate a difference trace that represents a change in gain . the change in gain is assigned to at least one of the plurality of optical amplifiers based on the difference trace .