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