OTDR Ghost at 1.1 km on a 900 m Link
A technician is testing a 900 m OS2 building backbone with a 1310 nm OTDR. The test setup uses a 10 m launch cable and a 10 m receive cable. The OTDR trace shows the fiber end at about 0.90 km, but also a strong reflective event at 1.10 km with an apparent loss of 3.5 dB. The event does not appear when the trace is run from the far end. A power meter and light source measure 1.2 dB total loss for the link, which agrees with the design. The technician is about to order a repair of the cable at the 1.10 km point, but the cable route is only 900 m long and has no splice at that distance. The OTDR pulse width is 100 ns, and the near-end connector has visible dust.
1. What is the 1.10 km event, and how does the test setup create it?
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The 1.10 km event is almost certainly a ghost, not a real fiber feature. A strong Fresnel reflection at the dirty near-end connector sends a large pulse back to the OTDR. Part of that pulse can reflect again at the OTDR output connector and travel back down the fiber, creating a second displayed event at an apparent distance related to twice the original reflection distance or an instrument offset. The apparent loss is not a true splice or bend loss.
The clues are that the event lies beyond the actual fiber end, it appears from only one direction, and the power meter shows only 1.2 dB total loss. Dust on the connector increases the reflection and makes the ghost more likely. A shorter pulse or different launch condition may make it move or disappear, which further confirms it is an artifact.
2. What should the technician do before ordering any repair?
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Stop the repair order. Inspect and clean both near-end connectors, then repeat the OTDR test with the launch and receive cables in place. Use a shorter pulse width and, if available, a longer launch cable to separate the OTDR port reflection from the link. Compare bidirectional traces; a real event should appear from both directions with consistent loss, while a ghost usually does not.
Confirm the link with an independent power meter and light source, and verify the fiber route length against records. If the ghost persists after cleaning and changing test parameters, check the OTDR port connector and the launch cable for damage. Only a consistent bidirectional event with physical evidence should be scheduled for repair.