This Recommendation provides optical parameter values for physical layer interfaces of dense wavelength division multiplexing (DWDM) systems primarily intended for metro applications which include optical amplifiers. Applications are defined using optical interface parameters at the single-channel connection points between optical transmitters and the optical multiplexer, as well as between optical receivers and the optical demultiplexer in the DWDM system. This Recommendation uses a methodology which does not specify the details of the optical link, e.g., the maximum fibre length, explicitly. This version of this Recommendation includes unidirectional DWDM applications at 2.5 and 10 Gbit/s with 100 GHz channel frequency spacing as well as applications at 10 Gbit/s with 50 GHz channel frequency spacing.