What’s the most complicated topic in fiber optics? OTDR testing? Yeah, that’s right up there near the top. Cleaning connectors? You’d think that would be simple, but there are more opinions on how to do it than just about any process in fiber optics. My vote goes to fiber polarity—what seems like the simple process of making connections is actually one of the most complex subjects.

Fiber polarity is all about making connections from a transmitter to a receiver in a network link. Since most fiber links are bidirectional over two fibers, it means two fibers transmitting in opposite directions, so polarity is about making two connections: transmitter to receiver, and receiver to transmitter, to complete the link.

It starts with two fiber optic connectors such as SCs or LCs on each end of a fiber used to create a link between a transmitter and receiver. There must be another fiber connecting the transmitter at the far end to the receiver at the beginning. That’s easy enough, right?

Important bits to know about polarity

Polarity starts at the transceiver, the module that contains the transmitter and receiver. They are generally designed around a duplex connector, mostly two LC connectors clipped together in a pair. At the transceiver, the receiver input is on the right and the transmitter output on the left. If you have a two-fiber cable with duplex LCs on either end, the fiber in the cable must flip from one side to another to make the proper transmitter to receiver connection.

Now things get complicated. Rarely are two transceivers connected with a simple duplex cable. That cable may exist as a patchcord connecting the electronics to a patch panel, which is connected to another patch panel, maybe hundreds of meters or kilometers away, over one or more cables containing many fibers. When you plug in a patchcord to a panel at the other end, you must ensure that the path that starts at one transceiver is correctly connected to the transceiver on the far end.

This is when convention, standards and documentation become critical. Color codes are the first line of defense in polarity. When cables are connected or spliced, one must follow standard color codes while making the connections. Fibers and loose tubes in cables are color-coded. Bundles of fibers or tubes are also color-coded with colored binding tapes. There are standards for color coding that can handle cables with almost any number of fibers.

Concatenating (splicing together) two similar cables is simple—match the color codes of loose tubes or ribbons down to the very last fiber. Breakouts are more complicated. Splitting to two or more cables or dropping a few fibers off at a location from a higher fiber count cable can be done beginning with matching the fibers connected to the smaller cable, but recording which fibers and where becomes very important to the documentation.

Marking connections at patch panels is essential. The documentation should show the route of every fiber, and the labels at the ports at each end should be clear enough for a tech to know how to make the final connection with a patchcord.

That patchcord has a variable, too. Connectors are keyed and connect only one way. Duplex patchcords may have two variations—straight through and crossed fibers. Within each link, the fibers must cross to connect the transmitter to the receiver. That cross can occur anywhere in the link—in the permanently installed cable plant or in the patchcords on the ends. You must get it right to make the connections.

Multifiber cables turn up the heat

Where polarity goes from the difficult to the insane is with multifiber connectors. MPO connectors connect 12, 24, 16 or 32 fibers at a time. They are used in factory-terminated cabling systems for premises cabling applications. The complexity arises when you try to define polarity of 12, 24, 16 or 32 fibers on each end of a cable. That is why the TIA 568 standard has about 40 pages on polarity—half of the pages in the entire standard—about these multifiber connectors.

All of this illustrates the importance of clear documentation and labeling. And, of course, the tech making the connection must have a visual fault locator to trace fibers.

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