If you've had fiber installed and the contractor didn't hand you a test report at the end, you don't actually know if the job was done correctly. You know the connectors are plugged in and the lights on the switch turned on. You don't know how much signal loss is in that run, whether a splice is marginal and likely to fail in six months, or whether there's a hidden bend or crack a few hundred feet down the line. That's what OTDR testing is for.
What an OTDR actually does
An Optical Time-Domain Reflectometer (OTDR) sends a light pulse down the fiber and measures the light that reflects back over time. Every splice, connector, bend, and imperfection reflects a small amount of light, and the timing of that reflection tells you exactly how far down the cable it occurred. The strength of the reflection tells you how much loss is happening at that point.
The output is a trace — a graph of signal loss plotted against distance — plus a report showing:
- Total link loss, measured in decibels (dB)
- Loss at each individual splice and connector point
- The exact distance to any break, if one exists
- A pass/fail result against the loss budget for that type of run
Why "it's plugged in and working" isn't enough
A fiber run can pass a basic light-source-and-power-meter check (does light get from one end to the other) while still being out of spec. A marginal splice with slightly too much loss won't necessarily cause an outage on day one — it'll show up six months later as intermittent packet loss, a data center uplink that mysteriously underperforms, or a security camera feed that drops during high-bandwidth hours. By then, the run is buried, ceiling tiles are back in place, and finding the problem means re-testing the whole path from scratch.
OTDR testing at the time of installation catches this before it's a problem. If a splice comes back marginal, it gets re-spliced right then, while the crew and equipment are still on site.
What to ask for when a job is "done"
If you're having fiber installed — whether it's a single run between two rooms or a full outside-plant build — ask for the OTDR test report before you sign off. A legitimate report should show:
- Every strand tested individually (not just "the fiber passed")
- Loss values at each splice/connector, not just a single total number
- The reference standard used (this varies by application — a data center backbone and a campus interconnect don't have the same acceptable loss budget)
- Bidirectional testing where required, since loss can read differently depending on direction
If a contractor can't produce this, ask why. It's a five-minute test per strand with the right equipment, and it's the only real evidence that a fiber run was actually done to spec rather than just installed and hoped for.
At MD Fiber Optic, every splice and every completed run — new construction or repair — gets OTDR tested and the report handed over as part of the job, not as an upsell. If you want to see a sample report format before your project starts, get in touch and we'll walk you through it.