Thursday, February 20, 2020

"Ghosts" of OTDR

If you are testing short cables with highly reflective connectors, you may find ghosts. These are caused by the light reflected from the end of the connector that is reflected back and forth in the fiber until the noise level is attenuated. Ghosts arouse many confusions, since they appear to be real reflective events as connectors, but show no loss. The best way to determine if a reflection is real or a ghost is to compare it with the documentation of the cable network. You can eliminate ghosts by reducing reflections, for example,


OTDR limitations

The resolution of the limited distance of the OTDR makes its use very difficult in establishments or buildings where the cables usually have a length of a few hundred meters. Most OTDRs have many difficulties in solving characteristics in the typical short cables of an internal plant cable network, and it is likely to show "ghosts" of reflections in the connectors, and thus confuse the user of the OTDR. On very long cables, the OTDR will show a high noise farther from the instrument. If wider test pulses and more signal averages are used, the distance capacity of the OTDR will increase.

How to use OTDR correctly

There are certain precautions that will make the test easier to do and understand when using an OTDR. Always use a long launch cable, which allows the OTDR to stabilize after the initial pulse and provides a reference cable to test the first connector on the cable. If you want to test the end connector on the cable, a receive cable is needed at the end of the cable network.
Fiber optic jobs
The OTDR operator must configure the instrument carefully for each cable. Again, good documentation will help configure the test parameters. Always start with the OTDR set for the shortest pulse width for the best resolution and a range of at least 2 times the length of the cable you are testing. Perform an initial plot and see how you need to change the test parameters for best results. Some users are tempted to use the self-test function of the OTDR. Most problems are caused by newbies who use the self-test function, more than any other issue that may arise when using OTDRs. Never use the self-test function until an expert technician has configured the OTDR correctly and verified that it provides acceptable results.

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