Detailed explanation of MPO fiber link polarity

With the continuous migration of 10G networks to 40G/100G networks, MPO multi-core cables have also continued to develop, and many network engineers will use MPO trunk cables as the preferred solution. Generally speaking, an optical link requires two fibers to complete the entire transmission process. For example, an optical module includes a receiving end and a transmitting end. When in use, in order to make the optical signal transmit data correctly, it must be ensured that the receiving end and the transmitting end are in an interconnected state, and the optical fiber link at one end of the cable transmits the signal (Tx) The signal must be connected to the other end. Corresponding receivers (Rx) are matched, and this matching is called polarity, which defines the direction in which the optical signal propagates through the fiber.

Part1

In order to ensure the accuracy of the polarity in the MPO network system, we must first understand the performance of each MPO product. This part will briefly introduce the MPO products in the MPO network system.

1. MPO connector

An MPO connector can support 12-core, 24-core, 36-core and 72-core fiber optic connections. Each MPO connector has a keyway. When the keyway is on the upper half of the connector, it is called the keyway facing up (Key up) MPO connector, when the keyway is in the lower half of the connector, it is called a keyway down (Key down) MPO connector. There is a dot on one side of the MPO connector to mark the location of the 1st core hole.

MPO fiber link

2. MPO adapter

MPO connectors (male) and MPO connectors (female) are paired with an MPO adapter. MPO adapters are divided into two types: A-type and B-type: A-type—the keyway on one side of the A-type adapter is upward, and the keyway on the other side is downward. Therefore, when connecting two MPO connectors, the keyways of the two connectors are on parallel lines. Type B—The keyways on both sides of the Type B adapter face up, so when connecting two MPO connectors, the keyways of the two connectors are in a straight line.

3. MPO cable

Both ends of the MPO trunk cable are pre-terminated with (male/female) MPO connectors, which can support 12-fiber, 24-fiber, 48-fiber and 72-fiber connections. MPO branch cable: One end of the MPO branch cable is pre-terminated with a (male/female) MPO connector, and the other end is pre-terminated with multiple duplex LC/SC connectors, supporting multi-core cables to single-core or dual-core Conversion of fiber optic cables. MPO transfer module box: The MPO transfer module box adopts a closed box structure, which can accommodate 12-core or 24-core pre-terminated MPO branch optical cables. The picture below shows the MPO adapter module box with 2 MPO connectors and 12 duplex LC interfaces.

Part2

Polarity classification of MPO trunk cables: MPO trunk cables are divided into three types: straight-through, complete crossover and pair crossover, namely, Type A (key up-key down straight-through), Type B (key up-key up/key down-key down) Full cross), Type C (key up - key down line pair cross).

1. Type A straight-through MPO trunk cable

The straight-through MPO trunk optical cable uses straight-through cables. The two ends are pre-terminated with the MPO connector with the keyway facing up and the MPO connector with the keyway facing down. Therefore, the corresponding positions of the fibers at both ends of the cable are the same, that is to say, the left connector The position of the first core hole corresponds to the position of the first core hole of the right connector.

2. Type B fully crossed MPO trunk cable

The fully crossed MPO trunk cable uses a reversed cable. Both ends are pre-terminated with MPO connectors with the keyway facing up. In this cable, the corresponding positions of the fibers at both ends of the cable are opposite. The position of the first core hole of the machine corresponds to the position of the 12th core hole of the right connector.

3. Wire-to-cross MPO trunk optical cable

The line-to-cross MPO trunk cable is the same as the straight-through MPO trunk cable. The two ends are pre-terminated with the MPO connector with the keyway up and the MPO connector with the keyway facing down. However, in the line-to-cross MPO trunk cable, one end of the optical cable is adjacent to each other. The corresponding positions of the two fibers are opposite to those of the two adjacent fibers at the other end, that is to say, the position of the first core hole of the left connector corresponds to the position of the second core hole of the right connector, while the position of the first core hole of the left connector corresponds to the position of the second core hole of the left connector. The position of the two core holes corresponds to the position of the first core hole of the right connector.

4. Duplex jumper type

The three polarities of connection methods and the different polar methods use different types of MPO trunk cables. However, all methods utilize duplex patch cords to form fiber optic links. The TIA standard also defines two different types of LC or SC duplex fiber optic patch cords to complete end-to-end duplex connections: A-A (crossover) patch cords and A-B (straight-through) patch cords.

Part3

How to Manage Polarity

This section will explain how to ensure the correctness of the connection polarity of the MPO optical device under the TIA standard. The three polarity methods specified in the TIA568 standard are called Type A, Type B, and Type C.

1. Type A method

A straight-through wiring module and two different jumpers are used. One end of the jumper is a straight-through pair, and the other end is a reversed pair. The figure below (Rx means receiving, Tx means transmitting) reflects the type A connection method. This connection method uses a straight-through MPO trunk optical cable. In order to ensure the accuracy of the polarity, the left side of the optical fiber link uses standard duplex. A-B type jumper, A-A type jumper is used on the right side.

2. Type B method

A straight-through wiring module and a straight-through pair of jumpers are used. The figure below reflects the type B connection. This connection method uses a fully crossed MPO trunk optical cable. Since the corresponding positions of the optical fibers at both ends of the fully crossed MPO trunk optical cable are opposite, both ends of the optical fiber link use standard A-B type jumpers.

3. Class C method

The polarity is corrected by flipping the pair in the trunk cable, using the same module and the same jumper. The figure below reflects the C-type connection. The C-type connection method uses a pair of crossover MPO trunk cables, and both ends of the optical fiber link use standard A-B type jumpers.

Summarize

When there are two or more optical fibers in a link, especially when multi-core MPO products are used for high-speed transmission, it becomes more complicated to ensure the accuracy of the polarity of the optical fiber network, and there will be polarity problems , so we need to pay attention in the wiring.

The polarity problem faced by network engineers when using MPO/MTP products to meet the increasing demand for high-speed transmission can be solved by selecting appropriate MPO optical cables, MPO connectors, and MPO transfer module boxes and jumpers. For a reliable high-density 40/100G transmission solution, first select the desired polarity method, and then select the appropriate MPO optics to support this polarity method.