Testing, Measurement, and Fault Localisation (OTDR, Power Meter, VFL)
Testing, measurement and fault localisation are tested through instrument setup, OTDR trace interpretation, loss measurement and distance-to-fault reasoning.
Core rules
- OTDR sends pulses and measures Rayleigh backscatter and Fresnel reflections. Set IOR, wavelength, pulse width, range and averaging. Short pulse gives better spatial resolution and a shorter dead zone but less range; long pulse gives longer range but a larger dead zone.
- Launch and receive fibres move the first and last connectors out of the dead zone. Use bidirectional traces and average for splice loss.
- Event types: reflective connector (Fresnel reflection, e.g., −40 to −50 dB), fusion splice (non-reflective, low loss), mechanical splice (small reflection), bend (loss without reflection), end (strong reflection or loss of trace).
- Power meter: reference at source, then measure; loss (dB) = reference dBm − measured dBm. VFL at 650 nm finds breaks, tight bends and wrong patch-panel ports by visible red light.
- IEC 61300-3-35 endface acceptance and a clean mating interface are prerequisites for valid measurements.
Worked example
An OTDR trace at 1550 nm shows a reflective event at 2.450 km with 0.35 dB loss and −45 dB reflectance. A fusion splice would normally be non-reflective and ≤0.15 dB; a connector or mechanical splice is the likely cause. If the OTDR IOR is set to 1.4680 but the fibre is 1.4670, distance error ≈ (0.0010/1.4670) × 2,450 m ≈ 1.7 m, so do not dig on the OTDR distance alone - confirm with route markers and a VFL.
Traps
- Wrong IOR: distances shift and joints are missed.
- Ignoring dead zones near the launch connector; a bad first connector can hide a splice.
- Reading a negative splice loss as a gain; use bidirectional averaging.
- Using a VFL through a live system or looking into the port.