Optical Time Domain Reflectometer, an instrument that fires short laser pulses into one end of a fiber and records the backscattered and reflected light against time, which converts to distance. The resulting trace shows fiber attenuation as a slope, splices and bends as steps, connectors and breaks as spikes. Pulse width trades range against resolution, and the OTDR locates faults to within a few meters over tens of kilometers.
Bring the OTDR, customer says the link dropped and I want to know if the break is in our vault or on the pole line.
Use an OTDR to find and document events, but certify link loss with a light source and power meter: that is the measurement the network actually sees.
Do not mix it up with
- OLTS: An OLTS measures total end-to-end loss directly; an OTDR infers loss indirectly from backscatter and shows where the losses are.
Related terms
Trace
The graph an OTDR produces, plotting returned power in dB against distance along the fiber.
Event
Any point on an OTDR trace where the signal departs from the smooth fiber slope, such as a splice, connector or break.
Backscatter
The small fraction of light scattered in the fiber that travels back toward the source, which OTDRs measure.
Launch cable
A long reference fiber placed between the OTDR and the cable under test so the first connector can be measured.
Event dead zone
The distance after a reflective event during which an OTDR cannot detect a second, separate event.