A loopback test is a hardware or software method which feeds a received signal or data back to the sender. It is actually used being an assist in debugging physical connection problems.
Fiber optic transceiver may be the fundamental part in every fiber optic communication network equipment. Its function is identical to your computer’s Ethernet Card. (although just about all modern computers integrate it on the motherboard, not as a different PCI card anymore)
Fiber optic transceiver has two ports, a transmitter port and a receiver port. The direct attach cable port sends out laser signal to a linked transceiver as well as the receiver port receives laser signal from the other transceiver.
On fiber optic transceiver manufacturing floors and in R&D labs, we usually work with a fiber optic loopback module to make sure that the transceiver is working perfectly as designed as opposed to using another transceiver as the partner.
Basically exactly what the loopback module does is directly routing the laser signal from the transmitter port back to the receiver port. We can compare the transmitted pattern together with the received pattern to make sure they are identical and have no error.
The most popular types of transceiver module are SC, LC and MTRJ connector types. But each connector type is divided again by fiber type, connector polish type and attenuation.
Each connector type can be obtained for three fiber types. They are 50/125um multimode, 62.5/125um multimode and 9/125um single mode.
Only PC polish is accessible for multimode type fibers. But also for single mode fiber, two connector polish types are available: UPC polish and APC polish.
Along with fiber type, working wavelength is definitely the other important aspect for selecting the proper fiber optic loopback module for your specific application. Available wavelengths are 850nm, 1310nm dexhpky91 1550nm. 850nm and 1310nm are for multimode applications and 1550nm is made for single mode applications.
It is possible to specify just how the signal power level must be reduced from the loopback path. It is because the receiver port cannot handle high power. And in the real world fiber optic cable, there are always attenuations designed by fibers, equipment and physical environment. And so the transmitter power is attenuated to some safe level from the network before reaching another receiver.