OTDR testing
This domain tests optical time-domain reflectometer setup and interpretation. The exam expects you to understand backscatter and Fresnel reflections, pulse-width tradeoffs, dead zones, index of refraction, launch and receive cables, bidirectional splice averaging, and the difference between an OTDR event table and a true insertion loss measurement.
How the OTDR works
An OTDR launches a pulse and measures returned light versus time. Fiber attenuation appears as a downward slope; splices and connectors appear as events, with reflective connectors producing Fresnel peaks. Short pulses improve resolution but reduce dynamic range; long pulses reach farther but enlarge event and attenuation dead zones. Set the index of refraction to the fiber’s specified value or distances will be wrong. Use a launch cable longer than the event dead zone and a receive cable when the far-end connector must be measured.
Reading events
Splice loss is the level difference across the event, but a splice can look like a gainer in one direction because of backscatter differences. Bidirectional averaging is the accepted way to report splice loss. Reflectance is read from the peak; APC connectors should show much lower reflectance than UPC. Ghosts are false events caused by strong reflections and can be recognized by their spacing and lack of a real loss step.
Worked example
An OTDR event table shows a splice at 8.00 km with 0.35 dB in one direction and 0.15 dB in the reverse direction. The bidirectional average is (0.35 + 0.15) / 2 = 0.25 dB. If the splice specification is 0.30 dB, it passes. A 10 ns pulse with a 1 m event dead zone and 4 m attenuation dead zone cannot resolve a connector only 2 m after the launch; use a 100 m launch cable instead.
Traps
- Leaving the wrong index of refraction, which shifts every distance.
- Using a pulse width that is too long and hiding close events in the dead zone.
- Reporting one-direction splice loss as final when bidirectional averaging is required.
- Mistaking a ghost reflection for a real event.
- Using a standard OTDR through a live PON splitter without the correct filtered PON OTDR and wavelength.