Unlike optical isolators that block reflected light, a circulator routes optical signals in a specific order — typically Port 1 → Port 2 and Port 2 → Port 3 — while preventing unwanted back reflections. Thorlabs' Single Mode (SM) Optic Circulators are non-reciprocating, one directional, three-port devices that are used in a wide range of optical setups and for numerous applications. 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. Fiber optic circulators act as signal routers, transmitting light from an input fiber to an output fiber, but directing light that returns along that output fiber to a third port. 1 illustrates several possible circulator c nfigurations. The FC/PC and FC/APC connectors have a 2 mm narrow key.
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