Introduction of Loopback Cable

What is Loopback Cable?

When testing the transmission equipment, fiber optic loopback device is often used as the testing tool. It is known as the routing of electronic signals, digital data streams, or flows of items back to their source without intentional processing or modification. Fiber optic loopback is widely used for various applications. In terms of telecommunication, loopback is a hardware or software method to feed a received signal or data back to the sender. It is very useful for solving physical connection problems.This post will be a guide on how to choose a right loopback cablefor specific transceiver module.


Types of Fiber Optic Loopback

So, what is exactly the fiber optic loopback? Before deciding which loopback cable to use, we should firstly know the structure and classification of fiber loopback cable.By diagnosing the problems of optical equipment, fiber optic loopback provides an easy way to test the performance of the optical network devices. Generally, fiber optic loopback cable and fiber optic loopback module are both fiber optic loopbacks. Fiber optic loopback cable is the traditional fiber optic loopback with a visible cable. It is equipped with two fiber optic connectors on each end of the cable. When sticking the connectors together, the cable will shape like a loop. As for fiber optic loopback module, the biggest difference is that it has a enclosure to protect the inside cable. And the looped space is reduced for an easier usage and economic package.According to the optical connector type of the loopback, fiber loopback cables can be divided to LC, SC, FC, ST, MTP/MPO, E2000, etc. In testing fiber optic transceiver modules, the most commonly used are LC, SC and MTP/MPO loopback cables.


Figure 1: LC & SC Loopback Cable

The LC and SC loopbacks are made with simplex fiber cable and common connectors; it’s not difficult to understand their configurations. As for the MTP/MPO loopback, it is mainly used for testing parallel optics, such as 40G and 100G transceivers. Its configuration varies since the fiber count is not always the same in different applications.

8 Fibers MTP/MPO Loopback Cable Configuration

In a 8 fibers MTP/MPO loopback, eight fibers are aligned on two sides of the connector, leaving the central four channels empty. And the fibers adopt a straight configuration of 1-12, 2-11, 5-8, 6-7. The polarity channel alignment is illustrated in the following figure.


Figure 2: 8 Fibers Loopback Polarity Channel Alignment

12 Fibers MTP/MPO Loopback Cable Configuration

The only difference between the 12-fiber MTP loopback and the 8-fiber loopback is that the central four channels are not empty. Its alignment is 1-12, 2-11, 3-10, 4-9, 5-8, 6-7.


Figure 3: 12 Fibers Loopback Polarity Channel Alignment

24 Fibers MTP/MPO Loopback Cable Configuration

The 24 fibers MTP loopback also adopts type 1 polarity. Its alignment design is shown below.


Figure 4: 24 Fibers Loopback Polarity Channel Alignment

How to Use Loopback Cable?

The loopback cables are often used in conjunction with testing software to “loop” traffic right back into the port. If the data sent out into the loopback plug is identical to the data received from the loopback plug, you can assume that the basic communication functions of the port are working properly. So the common application of loopback cable is Loopback Test.


Loopback Test

Fiber loopback cable is often utilized to check whether the transceiver module is working perfectly as designed. As we all know, transceiver module has two ports, a transmitter port and a receiver port. The former one is to send out laser signals and the latter is to receive signals. When performing the loopback test, the fiber loopback cable can be directly plugged into the output and input port of transceiver module respectively (the ports at the ends of the connection must be compatible). Thus, during the testing process, the loopback cable directly routes the laser signal from the transmitter port back to the receiver port. Then we can compare the transmitted pattern with the received pattern to troubleshoot a defective node in the network. Fiber optic loopback testing is the easiest way to ensure that the transceiver works faultlessly. When selecting a suitable fiber loopback for the transceiver, we should consider the connector type, polish type, and cable type.


When it comes to practical application, fiber optic loopback test is often employed for checking fiber optic transceivers. Since transceiver has two ports for receiving and transmitting the light signal, it is necessary to test the ports to see whether they are still under operation. Thus, fiber optic loopback test is the most convenient way for transceiver maintenance. The testing process is by routing the laser signal from the transmitter port back to the receiver port. Then the transmitted pattern is compared with the received pattern to make sure they are identical and have no error.


All in all, loopback cables play an important role in troubleshooting in laboratories and manufacturing environments. They facilitate the testing of simple networking issues and are available at very low costs.Similar to other cables, it also has multiple classifications of fiber types, connector types for different needs. The deployment of fiber optic loopback components has greatly saved the trouble for device checking. There is no doubt that using fiber optic loopback is an effective method in fiber optic communication. There are many loopback cable manufactures on the market, providing single mode and multimode fiber optic loopback plugs available with FC, LC, MT-RJ, SC connectors. Fiber-Mart is one of the fiber loopback cable providers, all loopback cables are precision terminated and feature extremely low loss characteristics for transparent operation in the test environment.I believe you can find a suitable products for your devices in Fiber-Mart. please contact us:


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