Reading a trace
A reflectometer shows you the length and the losses along it, and where the fault is.
01What the line is telling you
An optical time-domain reflectometer fires a pulse of light down a fibre and listens to what comes back. Rayleigh backscatter — light scattered by microscopic imperfections in the glass — returns continuously along the length, creating a gently sloping line on the display. The slope is the fibre's attenuation coefficient: steeper means lossier. A healthy single-mode run loses roughly 0.35 dB/km at 1310 nm (O band) and about 0.2 dB/km at 1550 nm (C band), sometimes less on premium glass; if your slope is noticeably steeper, you have a problem before you have found a fault.
Events interrupt that slope. A connector or mechanical splice shows as a step down — the loss happens at one point and the trace resumes at the new level. A fusion splice done well is nearly invisible: sub-0.1 dB is routine, and a good technician expects under 0.05 dB. A reflective event — an open connector end-face, an air gap, a break — produces a spike above the baseline before the step down, because some light bounces straight back. A non-reflective event, like a tight bend radius or a poorly cleaved splice, drops the trace without the spike. That distinction tells you immediately what you are looking at before you even pull a panel.
What to watch for on the trace
- Rayleigh backscatter slope — the continuous return that reveals attenuation along the length; steeper is worse
- Reflective event — a spike on the trace, indicating a connector, air gap or clean break
- Non-reflective event — a step down without a spike; bends, bad splices, water ingress
- Fresnel reflection — the spike at the far end; absent means the fibre is broken before it gets there
- Refractive index setting — must match the fibre type; wrong value shifts every distance reading proportionally
- Bidirectional measurement — splice loss averaged from both ends corrects for mode-field mismatch
The far end shows as a large reflective spike followed by noise: the Fresnel reflection off the glass-air interface at the fibre's termination. If you do not see this, the fibre has no far end in optical terms — it is broken somewhere, and the last event before the noise floor is your fault location. The OTDR measures distance as round-trip time divided by two, corrected for the glass's refractive index. Set the wrong index and every distance reading is wrong by a consistent percentage — a simple mistake that wastes hours in the field.
Interpreting a trace is not difficult, but it rewards attention. A large splice loss that reads smaller from the far end — or vice versa — is a real phenomenon caused by mode-field mismatch; the true loss is the average of both directions. Always measure from both ends.
| Rayleigh backscatter slope | the continuous return that reveals attenuation along the length; steeper is worse |
| Reflective event | a spike on the trace, indicating a connector, air gap or clean break |
| Non-reflective event | a step down without a spike; bends, bad splices, water ingress |
| Fresnel reflection | the spike at the far end; absent means the fibre is broken before it gets there |
| Refractive index setting | must match the fibre type; wrong value shifts every distance reading proportionally |
| Bidirectional measurement | splice loss averaged from both ends corrects for mode-field mismatch |
