An echo of a real reflective event that shows up on the trace where there is nothing in the fiber, usually at twice the distance of a strong reflection or out past the end of the fiber in the noise. A ghost shows no loss across it, which gives it away. Shorter pulses, a lower-reflectance far end, index-matching gel or cleaning the highly reflective connector make it disappear.
That spike at 2 km on a 1 km cable is a ghost, it's sitting at exactly twice the panel distance.
Check the distance: if the mystery event sits at an exact multiple of a strong reflection, it is a ghost.
Also heard as
- echo
- ghost event
Related terms
OTDR
Optical Time Domain Reflectometer: a tester that maps a fiber's loss and events by timing light reflected back along it.
Reflectance
The fraction of light reflected by a single event such as a connector, in dB, where more negative is better.
Event
Any point on an OTDR trace where the signal departs from the smooth fiber slope, such as a splice, connector or break.
Index-matching gel
A clear gel with nearly the same refractive index as fiber, used to fill gaps between fiber ends.
Trace
The graph an OTDR produces, plotting returned power in dB against distance along the fiber.