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What is a loopback test?
A loopback test is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems.
Fiber optic transceiver loopback test
Fiber optic transceiver is the fundamental part in any fiber optic communication network equipment. Its function is exactly the same as your computer’s Ethernet Card. (although almost all modern computers integrate it on the motherboard, not as a separate PCI card anymore)
Fiber optic transceiver has two ports, a transmitter port and a receiver port. The transmitter port sends out laser signal to a linked transceiver and the receiver port receives laser signal from the other transceiver.
On fiber optic transceiver manufacturing floors and in R&D labs, we usually use a fiber optic loopback module to verify the transceiver is working perfectly as designed instead of using another transceiver as its partner.
Basically what the loopback module does is directly routing the laser signal from the transmitter port back to the receiver port. Then we can compare the transmitted pattern with the received pattern to make sure they are identical and have no error.
What types of loopback modules are available?
The most popular types of fiber optic loopback modules are SC, LC and MTRJ connector types. But each connector type is divided again by fiber type, connector polish type and attenuation.
Fiber Types
Each connector type is available for three fiber types. They are 50/125um multimode, 62.5/125um multimode and 9/125um single mode.
Connector Polish Types
Only PC polish is available for multimode type fibers. But for single mode fiber, two connector polish types are available: UPC polish and APC polish.
Working Wavelength
In addition to fiber type, working wavelength is the other important factor for choosing the correct fiber optic loopback module for your specific application. Available wavelengths are 850nm, 1310nm and 1550nm. 850nm and 1310nm are for multimode applications and 1550nm is for single mode applications.
Attenuation
You can specify how the signal power level should be reduced in the loopback path. This is because the receiver port cannot handle very high power. And in real life fiber optic network, there are always attenuations introduced by fibers, equipment and physical environment. So the transmitter power is attenuated to a safe level through the network before reaching the other receiver.
However since you are routing the transmitter directly back to the receiver, attenuation usually must be added to protect the receiver.
How much attenuation should be added depends on your transceiver type and you must decide it by yourself. Fortunately, there are a wide range of attenuation options available from 0dB, 1dB, 2dB to 24dB or even more.
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Source: http://ezinearticles.com/?Loopback-Test—The-Easiest-Way-to-Ensure-Your-Fiber-Optic-Transceiver-Is-Working-Faultlessly&id=996293