OTDR and Power Meter Testing
OTDR and Power Meter Testing
This domain tests setting up and interpreting the two main field tests: OTDR traces for distance and events, and power-meter measurements for absolute optical power.
Core rules and concepts
- Before testing, clean all connectors and set the OTDR's wavelength, range, pulse width, refractive index (IOR) and averaging time. A wrong IOR shifts reported distance; a long pulse reaches farther but hides close events in its dead zone.
- The OTDR shows events: connectors, splices, bends and breaks. Reflectance spikes indicate connectors or air gaps; a non-reflective drop is usually a splice or bend. Use launch and receive fibers to see the near and far ends.
- A power meter must be zeroed with the source off, then connected with the correct adapter. Measure at the specified wavelength and record dBm. A stable source and known reference allow loss measurement.
- A VFL is only a continuity and fault-location aid; it does not replace loss testing.
Worked example
A 2 km single-mode link is tested at 1550 nm. The OTDR is set to IOR 1.468, range 5 km, and a 100 ns pulse. The trace shows a connector at 0.00 km, a splice at 1.20 km with 0.08 dB loss, and an end at 1.98 km. If the IOR is set to 1.500 instead, the same fiber events will be reported at slightly shorter distances, so the technician may mark the splice in the wrong place. Set IOR from the fiber manufacturer's data sheet.
Traps that cost marks
Do not use a pulse width so wide that two close splices merge into one event. Do not test without cleaning and inspecting connectors. Do not forget to zero the power meter. Do not report a one-way OTDR loss as final for a splice without checking the opposite direction.