IEC 61290-4-3:2015(E) applies to output power controlled optically amplified, elementary sub-systems. It applies to optical fibre amplifiers (OFA) using active fibres containing rare-earth dopants, presently commercially available, as indicated in IEC 61291-1, as well as alternative optical amplifiers that can be used for single channel output power controlled operation, such as semiconductor optical amplifiers (SOA). The object of this standard is to provide the general background for optical amplifier (OA) power transients and its measurements and to indicate those IEC standard test methods for accurate and reliable measurements of the following transient parameters: - Transient power response; - Transient power overcompensation response; - Steady-state power offset; - Transient power response time. The stimulus and responses behaviours under consideration include: - Channel power increase (step transient); - Channel power reduction (inverse step transient); - Channel power increase/reduction (pulse transient); - Channel power reduction/increase (inverse pulse transient); - Channel power increase/reduction/increase (lightning bolt transient); - Channel power reduction/increase/reduction (inverse lightning bolt transient). These parameters have been included to provide a complete description of the transient behaviour of an output power transient controlled OA. The test definition defined here are applicable if the amplifier is an OFA or an alternative OA. However, the description in Annex A of this document concentrates on the physical performance of an OFA and provides a detailed description of the behaviour of OFA; it does not give a similar description of other OA types. Keywords: output power controlled optically amplified elementary sub-systems, optical fibre amplifiers, rare-earth dopants This publication is to be read in conjunction with IEC 61291-1:2012.