This final draft European Telecommunication Standard (ETS) applies to Optical Amplifier (OA) devices and sub-systems to be used in transmission networks. It covers both Optical Fibre Amplifiers (OFAs) and Semiconductor Optical Amplifiers (SOAs). This ETS specifies those generic characteristics relevant for the use of OA devices (as power amplifiers, pre-amplifiers or line amplifiers) and OA sub-systems (as optically amplified transmitters or optically amplified receivers), primarily for applications in digital transmission. NOTE 1: As far as OFAs are concerned, the present ETS has been prepared from experience with Erbium-doped, silica-based fibre amplifiers, operating in the 1 550 nm wavelength region. Future OFAs, based on different active fibres and possibly operating in different wavelength regions, are not intended to be excluded from this ETS and may lead to additional specifications, as well as to modifications of the existing ones. NOTE 2: The OFA aspects of the present ETS are mainly based on ITU-T Recommendations G.661 and G.662.