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Fiber Polarity Basic

Polarity defines the direction of flow, such as the direction of a magnetic field or an electrical current. In fiber optics, polarity is directional; light signals travel through a fiber optic cable from one end to the other. A fiber optic link’s transmit signal (Tx) at end of the cable must match the corresponding receiver (Rx) at the other end.
So, what is fiber polarity? Fiber polarity could be defined as the direction light signals travel from one end to the other end of an optical fiber cable. While this seems obvious, fiber optic polarity is one area that seems to cause the most confusion among technicians. So, let’s break it down and start at the beginning.

What is Fiber Optics Polarity?

As optic cables become increasingly popular for transmitting high-speed data, fiber numbers also increase, making their connections more complex. Thus, users should take the time to understand how fiber optics polarity works. Consider the following factors when determining a solution to a polarity problem:
General Fiber Polarity: Is the polarity straight, crossed, flipped, or other?
Gender: Are the paired male and female components properly connected?
Orientation: Is the orientation KeyUp-KeyUp or KeyUp-KeyDown?
End-Face Alignment: Is it straight or angled? (in the case of MPOs, which are identical except for how the alignment pins are set).
All of these factors impact polarity. For now, let’s just concentrate on how to achieve the correct fiber polarity in a simple, repeatable, flexible way.

How Does Fiber Optics Polarity Affect the Installation of Fiber Optic Cables?

In duplex fibers, data transmission is bidirectional over two fibers: each fiber connects the transmitter on one end to the receiver on the other. The Tx should always connect to the Rx, no matter the number of cable segments or patch panel adapters in the circuit. The role of polarity is to maintain this connection, which is essential for data to flow.
 
These connections are relatively simple for duplex fibers, but multi-fiber push-on connectors, or MPOs, are more complex. Some MPOs can contain eight, 12, or 24 fibers for LAN applications. Three main deployment methods are depending on the types of MPO cables.

Easy to Understand Duplex

In duplex fiber applications, such as 10 Gig, bidirectional data transmission is carried over two fibers, where each fiber connects the transmitter on one end and to the receiver on the other end. The role of polarity is to maintain this connection.
 
In the graphic below, you can see that the Tx (B) should always connect to the Rx (A), regardless of how many patch panel adapters or cable segments are in the channel. If the polarity is not maintained—connecting a transmitter to a transmitter (B to B), for example—data will not flow. Obvious, right?
(Fiber optic cables are directional)
To help the industry select and install the right components to maintain proper polarity, TIA-568-C standards recommend the A-B polarity scenario for duplex patch cords. The A-B duplex patch cord is a straight-through connection that maintains the A-B polarity in a duplex channel. It’s also important to note that every fiber connector has a key that prevents the fiber from rotating when the connectors are being mated and maintain the correct Tx and Rx position.

Three methods to manage polarity in fiber connections using MPO

While duplex fiber cable polarity may seem straightforward, it all becomes a bit more complex when dealing with multi-fiber MPO-type cables and connectors. Industry standards call out three different polarity methods for MPOs, and each uses different types of MPO cables.

Method 1

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, so that the fiber located in Position 1 (Tx) arrives at Position 1 (Tx) at the other end.
(Type A MPO Polarity)
When using Method 1 for duplex applications, making the transceiver-receiver flip from Position 1 (Tx) to Position 2 (Rx) is required in a patch cord at one end. This is achieved with an A-A patch cord that shifts the fiber from Position 1 to Position 2 at the equipment interface.

Method 2

Uses key up connectors on both ends to achieve the transceiver-receiver flip, so that the fiber located in Position 1 (Tx) arrives at Position 12 (Rx) at the opposite end, and the fiber located in Position 2 (Rx) arrives at Position 11 (Tx) at the opposite end, and so on. For duplex applications, Method 2 uses straight A-B patch cords on both ends since there is no need for the transceiver-receiver flip. With the same type of patch cord on both ends, concern about which type of patch cord to use to which end is eliminated.
(Type B MPO Polarity)

Method 3

Method 3 uses a key up the connector on one end and a key down on the other end like Method 1—but the flip happens within the cable itself, where each pair of fibers is flipped so that the fiber in Position 1 (Tx) arrives at Position 2 (Rx) at the opposite end, and the fiber in Position 2 (Rx) arrives at Position 1 (Tx). While this method works well for duplex applications, it does not support parallel 8-fiber 40 and 100 Gig applications where Positions 1, 2, 3, and 4 of the MPO interface are transmitting and Positions 9, 10, 11, and 12 are receiving, and therefore it’s not recommended in those applications.
(Type C MPO Polarity)

Conclusion

Understanding fiber polarity and how to connect a system with absolute certainty is key to a successful installation. As we increasingly depend on fiber optic cables for high-speed data transmission, we must also understand the basics of polarity. Polarity is crucial in maintaining proper connections within fiber optic cables.

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