The ABC's of Fiber Polarity

Polarity defines the direction of flow, such as the direction of a magnetic field or an electrical current. In fiber optics, polarity defines the direction in which light signals travel through an optical fiber. For everything to work properly and for data to be sent correctly with light signals, a fiber optic link's transmit signal (Tx) at one end of the cable must match the corresponding receiver (Rx) at the other end of the cable. Polarity can be a confusing topic, so let's take a deeper look at how it works. 

Easy to Understand Duplex

In duplex fiber applications, such as 10 Gig, data transmission is bidirectional over two fibers - each fiber connects to the transmitter on one end and to the receiver on the other end. Polarity ensures this connection is maintained. 

TIA-568-C standards recommend an A-B polarity scenario for duplex patch cords. In this scenario, the Tx (B) should always connect to the Rx (A), regardless of how many patch panel adapters or cable segments are in the channel. The A-B duplex patch cord is a straight-through connection that maintains the A-B polarity in a duplex channel. 

More Complex Multiples

Multi-fiber MPO type cables and connectors can add more variables to the mix. Industry standards have identified three polarity methods for MPOs - Method A, Method B and Method C. Each method uses different types of MPO cables.

Method A uses Type A straight-through MPO trunk cables with a key up connector on one end and a key down connector on the other end. Method B uses key up connectors on both ends to achieve the transceiver-receiver flip. Method C is very similar to Method A, but each pair of fibers is flipped. 

With three different polarity methods and the need to use the correct type of patch cords for each, deployment mistakes can be common. The Fluke Networks MultiFiber™ Pro allows you to test individual patch cords, permanent links and channels for correct polarity. 

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