An OTDR trace is a graph of distance vs. optical power — but to a trained eye, it’s also a diagnostic tool that reveals the nature of every fault before a crew is dispatched. This guide covers seven common fault signatures and what they indicate.
7 Fault Signatures
| Signature | Appearance on Trace | Likely Cause |
|---|---|---|
| 1. Sharp drop, high reflection | Vertical drop + large spike | Fiber break or open connector |
| 2. Gradual slope increase | Steepening over 20-50m | Macrobend (tight bend in fiber) |
| 3. Discrete event, low reflection | Small step, no spike | Splice with acceptable loss |
| 4. Gainer event (negative loss) | Trace steps UP | Mismatched backscatter coefficients — not a real fault; verify bidirectionally |
| 5. Noisy trace | Wide, fluctuating line | OTDR dynamic range limit — use longer pulse width |
| 6. Ghost reflection | Event at 2x distance of strong reflector | Second-order reflection — clean the connector causing it |
| 7. Water peak rise | Elevated attenuation at 1383 nm | Moisture ingress — check closures and jacket integrity |
Related Reading: OTDR fault location · Common fiber problems · Cable damage causes
Key Takeaways
- Always shoot OTDR bidirectionally to eliminate gainer masking.
- A gradual slope change = macrobend, not a splice. Don’t dispatch a crew to dig up a phantom splice.
- Save .sor trace files — the baseline trace at commissioning is your most valuable maintenance tool.
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