An OTDR doesn't measure your fibre directly — it measures light, and asks you to infer the fibre's condition from how that light behaves. Reading a trace correctly is the difference between a useful diagnostic and a stack of graphs that don't tell you anything actionable.
How an OTDR Builds Its Picture
Rayleigh backscattering happens continuously along the fibre's entire length, producing the steadily sloping baseline representing normal attenuation per kilometre — no events, just fundamental scattering loss.
Fresnel reflection happens at discrete points where the fibre encounters a sudden refractive index change — a connector, mechanical splice, or break — producing sharp upward spikes often thousands of times stronger than background scatter. A clean fusion splice typically shows as a step down rather than a spike.
Reading the Trace: What Each Feature Means
| Trace feature | What it usually indicates |
|---|---|
| Steady downward slope | Healthy fibre, attenuation per km |
| Sharp upward spike | Connector, mechanical splice, open end, or break |
| Step down, no spike | Good fusion splice or bend |
| Sudden vertical drop to noise floor | Fibre end, catastrophic break, or far-end connector |
| Gradual upward bow before a step | Gainer / apparent gain — backscatter mismatch, not real gain |
An apparent "gain" event isn't the splice adding power — it happens when two fibres with different backscatter coefficients are spliced together. Testing from both ends and averaging removes this artefact, which is why certification standards typically require bidirectional testing.
Dead Zones: The OTDR's Blind Spots
Every Fresnel reflection temporarily saturates the receiver, needing a recovery period before accurate measurement resumes.
Event dead zone (EDZ): the minimum distance after a reflective event at which the OTDR can detect a second, separate event exists.
Attenuation dead zone (ADZ): longer than EDZ, the minimum distance before an accurate loss measurement can be made.
Dead zone length depends on pulse width: shorter pulses give shorter dead zones but lower dynamic range, while longer pulses extend reach at the cost of larger blind spots.
Practical Technique: Getting Useful Results
Use a launch cable. Connecting the OTDR directly to the fibre under test means the OTDR port's own reflection dead zone obscures the first connector. A launch cable (typically 100m–1km) absorbs that initial dead zone; a tail cable does the same at the far end.
Match pulse width to the job. Short pulses for short links and close-spaced events; longer pulses where dynamic range matters more than dead zone size.
Test bidirectionally for anything that matters. Single-direction traces are vulnerable to gainer artefacts.
Increase averaging time for noisy traces, within limits. Longer acquisition improves signal-to-noise ratio, though returns diminish past a certain point.
What an OTDR Tells You That a Power Meter Doesn't
A power meter gives you one number: total end-to-end loss. An OTDR's trace breaks that down into a map — this connector contributes 0.3 dB, this splice 0.05 dB, there's an unexpected bend at 1.2km. For installation verification and troubleshooting, the trace is the diagnostic tool a power meter can't replace.