Tip of the Week – 16 Oct 2013

Tip of the Week – Are You Seeing Ghosts

Are you seeing ghosts in your OTDR traces? Don’t worry, you do not have to buy a new one, even the most expensive OTDRs can suffer from ghostly events. A ghost event in an OTDR trace is a connector event that appears in a trace but do not exist at the location where the OTDR shows it. They are caused by multiple OTDR pulse reflections of a real connector event causing the OTDR to mistakenly show this event again further down in the trace. To identify a ghost event, first see if the distance of the event is an exact multiple of a previous event. For example, if the first connection event is at 11.31 km and you see another connection event in your trace at exactly 22.62 km, you may want to check to be sure its not a ghost. Next reduce the OTDR’s pulse width and range to see if the ghost disappears. If you are still not sure, test the fiber span from the other end and see if the event still appears in the same place. Here are a few pointers to help prevent future ghosts from haunting your OTDR traces.
  • Check to see that all fiber optic connectors are clean. Dirty connectors can increase connector reflection power and cause ghosts.
  • Use minimum required range and pulse widths to see the entire trace.
  • Ghosts can often occur after the end of the cable, so check with your records to know the length of cable you are measuring.
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