This part of IEC 61290-4 applies to optical amplifiers (OAs) and optically amplified elementary sub-systems. More specifically, it applies to OAs using active fibres (optical fibre amplifiers, OFAs) containing rare-earth dopants, such as erbium doped fibre amplifiers (EDFAs), presently commercially available, as indicated in IEC 61291-1. The object of this part of IEC 61290-4 is to establish uniform requirements for accurate and reliable measurements, by means of the broadband source method, of the transient response of OFAs to dynamic changes in their input power, as defined in IEC 61290-4-1:2011. The broadband source method is different from the two-wavelength method described in IEC 61290-4-1 in that the saturating signal is not located at a single wavelength, but is rather spread out across the entire specified DWDM transmission band of the OFA-under-test (e.g. the C-Band, 1 525 nm to 1 565 nm). Thus, this method may be relevant to the characterization of transient events where the DWDM signals that are added or dropped are more or less uniformly spread across the transmission band. The difference between the two measurement methods is discussed in more detail in Annex A.