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-wavelen