What the graph is showing
An OTDR fires pulses of light down the fiber and measures the light that scatters back. It plots that on two axes:
- Left to right = distance down the fiber, from the OTDR at 0 to the far end.
- Bottom to top = returned power in dB. Higher on the graph means more light coming back.
A healthy fiber shows a straight line sloping gently down from left to right. That downward slope is the fiber's natural attenuation: the longer the light travels, the less comes back. Everything interesting is a break in that smooth slope.
The four things you'll see
Almost everything on a trace is one of these:
1. A gradual downward slope (the fiber itself)
The steady diagonal line is the bare fiber. Its steepness is the attenuation in dB/km. A steeper-than-normal slope over a stretch can mean a stressed or bad section of cable.
2. A small step down (a splice)
A fusion splice shows as a small drop in the line with no spike. It loses a little light but doesn't reflect much. Good splices are tiny steps; a big step down is a high-loss splice worth investigating.
3. A spike up, then a step down (a connector)
A connector (or any glass-to-glass gap, like a mechanical splice) reflects light, so it shows a sharp spike up, then the line resumes lower. The spike height is the reflectance; the drop across it is the connector loss. A tall spike means a dirty or bad connector.
4. A big drop to the noise floor (the end of the fiber)
The fiber end is where the line falls off a cliff into the noise at the bottom. An open connector or a clean break shows a reflective spike there; a shattered or bent end may just drop with no spike.
What it looks like on a real span
Here's the actual output from the free OTDR Trace Viewer on a sample 40 km span at 1310 / 1550 nm. The readout up top is the headline result; the table is every event it found, and you can see all four types in it: the launch, the splices, the fiber sections between them, and the end of fiber.
| # | Distance | Section length | Type | Loss @ 1310 nm | Loss @ 1550 nm | Reflectance |
|---|---|---|---|---|---|---|
| 0 | 0.000 km | — | Launch | — | — | -42.46 dB |
| — | — | 9.998 km | Fiber section | — | — | — |
| 1 | 9.998 km | — | Splice | 0.199 dB | 0.198 dB | — |
| — | — | 10.003 km | Fiber section | — | — | — |
| 2 | 20.001 km | — | Splice | 0.298 dB | 0.299 dB | — |
| — | — | 5.000 km | Fiber section | — | — | — |
| 3 | 25.001 km | — | Splice | 0.398 dB | 0.401 dB | — |
| — | — | 5.001 km | Fiber section | — | — | — |
| 4 | 30.002 km | — | Splice | 0.198 dB | — | — |
| — | — | 0.032 km | Fiber section | — | — | — |
| 5 | 30.034 km | — | Splice | — | 0.215 dB | — |
| — | — | 7.039 km | Fiber section | — | — | — |
| — | 37.073 km | — | End of fiber | — | — | -45.58 dB |
Dead zones: why the start looks blind
Right after a strong reflection (like the connector at the OTDR), the receiver is briefly overwhelmed and can't measure. That stretch is the dead zone, typically one to a few meters. It's why an OTDR struggles with very short links and why techs use a launch cable: it moves the dead zone off the fiber under test so the first real connector is visible.
Reading loss the right way: markers, not eyeballs
To measure the loss of a section, you place two markers (usually called A and B) on either side of it and read the dB difference. The OTDR (or a viewer) does the math. Loss across a single event, attenuation per km over a stretch, and total end-to-end loss are all just marker placements.
One thing that trips people up: pulse width. A long pulse sees far but blurs close events together; a short pulse resolves close events but doesn't reach as far. If two events look merged on a long-pulse shot, re-check them on a short-pulse (low ns) shot before calling it.
Read your own trace now
The fastest way to learn this is on a real trace. Open the free OTDR Trace Viewer, drop a .sor file in, and you'll see the waveform with every event marked, plus a table listing each one with its distance, loss, and reflectance. Drag the A/B markers to measure any section yourself. (New to the files? See how to open a .sor file.)
See it on your own span. Drop a .sor or .trc into the viewer and read the trace with every event called out, free and in your browser.
Open the OTDR Trace Viewer →When you have a whole job
Reading one trace is one thing; a build leaves you hundreds, in both directions and multiple wavelengths. To get a single pass/fail verdict per fiber across the whole job, drop the lot into Fiber Analysis instead of opening them one by one.
Related
- How to open a .SOR file online
- OTDR Trace Viewer & Analyzer
- Fiber Analysis, for a whole job at once
- Fiber Testing Field Guide
- All field guides