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g/networking • Posted by u/fiber_splicer_faye •

OTDR shows a ghost event at 340m on a 280m span — what am I looking at? Fiber

Testing a new OS2 span, campus backhaul, 280 meters end to end confirmed by the as-builts and by the fiber's own length measurement.

OTDR trace shows a clean launch, one splice at 145m as expected, the end reflection at 280m, and then another reflective event at about 340m — 60 meters past the end of a fiber that does not go that far.

Trace is otherwise clean. Loss budget is fine. Everything works.

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u/fiber_fred •
✓ Accepted Solution

That is a ghost, and it is a reflection artifact, not a real event. The OTDR sent a pulse, it bounced off your strong end-face reflection at 280m, traveled back toward the launch, hit another reflective surface — usually the connector at the launch or your patch cord — and bounced forward again.

The instrument has no way to know the light took a round trip, so it computes distance from time of flight and puts the event at 280 + 60 = 340m.

Ghosts always appear at integer-ish multiples or sums of real reflective event distances. Add your event distances together and you will usually find the arithmetic that produced it.

Fix if it bothers you: reduce the end reflection. Angle-polished connectors, or an index-matching gel on the far end during test, or just a mandrel wrap. Or ignore it, because a ghost past the end of the fiber affects nothing.

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✓ Solution
u/fiber_splicer_faye OP •

280 minus 145 is 135, and 280 plus 60... the launch connector is about 2m in. It is the round trip off the end face back to the launch connector and forward again. The arithmetic works.

Been doing OSP for six years and this is the first ghost I have had to actually think about, mostly because I usually work on spans long enough that ghosts land past the trace window.

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u/cat6_carlos •

Short spans are where ghosts show up and short spans are exactly where people are least experienced with an OTDR, because on anything under about 500m most techs just do a light source and power meter test.

Which is arguably the right call. An LSPM tells you the loss, which is the thing you actually need to know. The OTDR tells you where the events are, which matters more on a long span with splices you might need to find later.

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u/rack_and_stack_rob •

The general principle worth remembering — an event past the physical end of the fiber is by definition not real. If the number is longer than the cable, stop looking for a cause in the cable.

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u/fiber_splicer_faye OP •

Stated that plainly it is obvious, and I still spent forty minutes on it. Sometimes the instrument's confidence is contagious.

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