1) Polarity
The term "polarity" refers, for example, to the direction in which an electric current flows, or in an optical fiber, the direction in which light flows from one end of the optical fiber cable to the other. Ensuring correct polarity is essential to ensure that the transmitted signal is properly directed to the receiving device and to ensure a continuous connection.
In fiber optic applications (e.g. 10G), data transmission is bidirectional through two optical fibers, each with one end connected to the transmitter and the other to the receiver. For multiple fibers, maintaining the correct polarity is more complicated, and the TIA-568 standard covers three polarity methods, each using a different type of MPO cable to ensure that each transmitter is properly connected to the receiver on the other end of the MPO cable. We'll cover these three approaches later in this article.
2) MPO connector
MPO connectors also meet specific physical interconnection standards. For example, they have a key on one side, and the direction of that key determines the polarity of the cable. In addition, a white dot on one side of the connector indicates fiber #1.
MPO connectors can also be male (two alignment pins) or female (two alignment holes). MPO connections are made between male and female connectors to ensure proper alignment.
The interface on an active MPO device, such as a transceiver, is usually male, so any MPO trunk cable connection needs to be female.
3) MPO connection method
As mentioned earlier, TIA-568 Polarity A, B, and C define three methods/polarity schemes for connecting MPO trunks and jumpers.
Method 1: Polarity A
Polarity A Use a direct cable, neutral polarity. This causes the fibers connecting the two ends to have the same fiber position, with the fiber at pin 1 position of one end (X) reaching pin 1 position of the other end (Y).
Method 2: Polarity B
The second method, or polarity B, uses "flip" or "reverse" polarity, which causes the fiber position to be reversed, with pin 1 on one end (X) mapping to pin 12 on the receiving end (Y), pin 2 mapping to pin 11, and so on, as shown below.
Method 3: Polarity C
This is one of the newer but less common criteria. It uses pair flips to send pin 1 on one end (X) to pin 2 on the other end (Y), and vice versa to send pin 2 (Y) to pin 1 on the other end (X). Connectors are used to maintain duplex integrity while still using standard boxes. This connection method can only be built using miniature distributed cables (not ribbon cables).
