Knowledge base · Events
Why a connector reflects so much
There is a connector in the event table, and its reflection is markedly larger than its neighbours'. The fibre is usually intact. More often than not the cause is whatever ended up between the two end faces.
See the reflections in your own file
Reflectance belongs to the joint, not to the line: the next connector on the same cable may read quite differently.
By Ilya (engineer, 6 years in fibre optics) ·
What makes a reflection large
The first cause below is more common than the other two together.
A dirty end face
Dust, lint off a wipe, a fingerprint. A gap of another medium appears between the faces, and the light reflects where it should not. The core of a singlemode fibre is thinner than a human hair, so a particle you cannot see at all is enough to matter. Instrument manuals name end-face contamination as the number one cause of link failures, and it is the same reason a repeated measurement on the same fibre comes back with a different figure.
The polish of the end face
End faces come domed or angled. An angled face steers the reflected light out of the core, so at equal cleanliness it reflects markedly less than a straight one. Hence a common confusion: two connector types in one line reflect differently by construction, not by workmanship, and reading one against the other as "worse and better" makes no sense.
The mechanics of the joint
A worn or broken adapter, an incomplete mating, misalignment, an air gap from a tilt. Here reflection and loss grow together: part of the light came back, part of it went past the core.
What to do about it
Instrument vendors set the order, and it is worth keeping whole: disconnect the source and put safety glasses on, then look, then clean, then look again, and only then measure. The end face is inspected with a microscope, the one many instruments carry built in; contamination is not something the eye can judge.
Cleaning is dry or wet. Dry: the face is wiped perpendicularly across a lint-free wipe a few times, or run through a reel cleaner. Wet: the wipe is dampened with isopropyl alcohol, and the face must be dried afterwards, because a liquid residue attracts more dirt than the dust it replaced and is harder to remove. Wipes and swabs are never reused.
After cleaning, inspect again. If the reflection is unchanged, the cause lies elsewhere, and the adapter and the connector itself are next in line; replacing it blind, before inspection, buys nothing. As for dust caps: a connector left uncapped is dirtier than one that was just cleaned.
How to check this on our screen
A reflective row in the event table carries its own reflectance figure, the one the instrument recorded. A dash in its place is not a zero: the instrument recorded no such measurement, and we do not put ours there instead. Beside it, in the totals, stands ORL, the return loss of the whole line at once; a single dirty connector lifts that too, which is why on a line with nothing to clean the two figures usually worsen together.
The guess can be checked with the very thing files are opened side by side for: take a trace before cleaning and one after, open both and put the curves on one plot. Cleaning changes the reflection at one joint and leaves the shape of the rest of the trace alone. If everything moved at once, the acquisition conditions changed.
What we do not do: we do not tell you whether an end face is clean, and we do not weigh your figure against a limit. End-face inspection is a separate measurement with a separate instrument, and it does not reach the OTDR file in any form. It has an industry standard of its own with pass profiles, IEC 61300-3-35, and its values, too, reach you from your project rather than from us.
Next
Reflectance is written as a negative number, and it does not read the way it looks: the further from zero, the weaker the reflection. Of two connectors, the one that reflects more is the one whose figure is closer to zero.