How to Read an OTDR Trace: Events, Splices, Connectors, Bends and Fiber End Explained
Quick answer: An OTDR trace plots reflected light power (dB, vertical) against distance (horizontal). A steady downward slope is normal fiber attenuation. A spike followed by a drop is a reflective event such as a connector or mechanical splice. A step down without a spike is a non-reflective event such as a fusion splice or bend. A large spike followed by noise marks the fiber end or a break.
The Parts of an OTDR Trace
- Initial reflection and dead zone: the front-connector spike at 0 m. Use a launch cable box to see past it.
- Backscatter slope: typical single-mode attenuation is around 0.35 dB/km at 1310 nm and 0.20 dB/km at 1550 nm.
- Events: connectors, splices, bends and splitters.
- End of fiber: a final reflection, then the noise floor.
Common Event Signatures
| Event | What you see | Typical cause |
|---|---|---|
| Connector | Spike + small drop | Mated connectors in a panel or adapter |
| Fusion splice | Small step down, no spike | Good splice, usually under 0.1 dB |
| Macrobend | Step down, larger at 1550 than 1310 nm | Tight bend in a tray or cabinet |
| Gainer | Apparent step up | Joining fibers with different backscatter; test both directions |
| Ghost | Echo reflection beyond a real event | Strong reflections; reduce range or pulse |
| Splitter | Large step (about 17 to 21 dB for 1x32) | PON network |
| Break / end | Spike then noise | Fiber end or cut |
Tips for Accurate Traces
- Clean connectors first. See our connector cleaning guide.
- Test at both 1310 and 1550 nm; bends show up more at 1550 nm.
- Use short pulses for short links and longer pulses for long links.
- Average both directions for true splice loss.
- Increase averaging time to reduce noise.
OTDRs That Make Trace Reading Easier
Event-map views convert the trace into simple icons. The HSV-110D and HSV-100D are great for FTTH techs, while the COMWAY MAX-600-M4 and EXFO MAX-715D suit professional work. For PON, read our PON OTDR guide.
FAQ
What does a gainer mean on an OTDR trace?
A gainer is an apparent gain at a splice caused by joining fibers with different backscatter properties. Test from both ends and average to get true loss.
How do I tell a bend from a splice?
Compare 1310 and 1550 nm traces. A bend shows much higher loss at 1550 nm, while a splice is similar at both.
Why is there a spike at the end of the trace?
That is the Fresnel reflection from the fiber end or a clean break.
What is a ghost event?
A false reflection caused by strong reflective events bouncing within the fiber. It usually appears at a multiple of a real event's distance.