OTDR Testing Explained: How to Read a Fibre Trace
An Optical Time-Domain Reflectometer (OTDR) sends pulses of light into a fibre and measures what comes back. The result is a graph of the fibre’s entire length — invaluable for acceptance and fault finding.
Reading the trace
- The sloping line is normal fibre attenuation; its slope shows loss per kilometre.
- A sudden drop without a spike is usually a fusion splice or a bend — the size of the drop is the loss.
- A spike followed by a drop is a reflective event such as a connector or mechanical splice.
- A large spike then noise marks the end of the fibre — or a break.
Why acceptance testing matters
Testing every fibre after construction, in both directions, proves the build meets the agreed loss limits and creates a baseline. When a fault happens later, comparing a new trace with the baseline pinpoints the distance to the problem.
Good practice
- Use launch and receive fibres so the first and last connectors are measured.
- Test at two wavelengths — typically 1310 and 1550 nm for single-mode — because bends show up more at 1550 nm.
- Average bidirectional results for accurate splice loss.
- Store traces with the as-built documentation.
Every route we build through our fibre services is handed over with OTDR records. Talk to our fibre team.
