Knowledge base · Events
What the events on an OTDR trace mean
The curve drops in a step, jumps in a spike, or falls away into noise. Each of those shapes is an event, and the shape tells you what actually sits at that point of the fiber.
See the events in your own file
An event is a place, not a verdict: what to do about it is decided by your project's threshold.
Three kinds worth telling apart
What tells them apart is the shape of the curve, not the label in the table.
Reflective
A narrow spike upward, after which the level continues lower than before. That is the boundary between two media: a connector, a mechanical splice, an end face. The spike can be narrow enough that decimating the curve swallows it — which is why we draw the trace in a way that never can.
Non-reflective
A step down with no spike. A fusion splice or a bend: the medium does not change, but some of the light leaves. One trace cannot tell a splice from a bend — a bend costs more at the longer wavelength, and that only shows if 1310 and 1550 nm are laid side by side.
A ghost
A reflection that is not in the fiber at all: the light reflected twice, and the instrument drew an event at double the distance. The sign is a distance that is a multiple of the distance to a strong reflector, with no loss at it.
The fiber end, and what lies past it
The end is where the curve breaks into noise, usually with a strong reflection off the end face. Everything drawn beyond it is not fiber but receiver noise, and events there mean nothing. There is exactly one reason to look: if the end turned up well short of the designed length, the measurement caught the wrong span — or a break.
Why our list differs from the instrument's
The instrument stores the events its own detector found, at its own threshold settings. We search again and show both lists, marking who found each event — the instrument, us, or both. They often disagree, and that is normal: the thresholds differ. Neither list overrules the other; comparing them is the work by eye the file was opened for.
Next
We run one detector for every instrument's files: behaviour never branches on the model name in the header — that field is edited by hand, and a branch on it would send a file down the wrong path.