This International Standard applies to circulators used in the field of fibre optics bearing all of the following features: ?C they are non-reciprocal optical devices@ in which each port is either an optical fibre or fibre optic connector; ?C they are passive devices in accordance with the categorization and definition provided in IEC TS 62538; ?C they have three or more ports for directionally transmitting optical power. An example of optical circulator technology is described in Annex A.