A 3-port optical circulator is a non-reciprocal passive device that directs light sequentially from Port 1 to Port 2, Port 2 to Port 3, and Port 3 to Port 1. This means that if light enters port 1 it is emitted from port 2, but if some of the emitted light is reflected back to the circulator, it does not come out of port 1 but. Thorlabs' Optical Circulators are non-reciprocating, one-directional, three port devices which are great for bidirectional propagation of light in a single fiber. 1 illustrates several possible circulator c nfigurations.