Optical Time Domain Reflectometer (OTDR)

What an OTDR Measures

An OTDR fires a short pulse into the fibre and records what returns against time. Two mechanisms send light back: Rayleigh scattering, continuous along the glass and very weak, and Fresnel reflection at any abrupt change of refractive index — a connector, a mechanical splice, a break. Timing the returns converts them into distance.

It is single-ended, which is its great practical virtue: one engineer, one end of the fibre, a map of everything along it. It is also an inference — the trace shows how much light comes back, not how much arrives at the far end.

Reading the Trace

The gentle downward slope is fibre attenuation — roughly 0.35 dB/km at 1310 nm and 0.20 dB/km at 1550 nm for G.652.D. A step down with no spike is a non-reflective event: a fusion splice, a tight bend, a crushed cable. A spike followed by a step is a reflective event, almost always a mated connector. The drop at the end is the fibre end or a clean break.

Two artefacts are worth recognising. A splice between fibres of slightly different backscatter coefficient can display gain, an apparent step upward; it is an artefact, not free light, and averaging traces from both ends removes it. A bend loses far more at 1550 nm than at 1310 nm, which is how you tell a bend from a bad splice without leaving the instrument.

Pulse Width Is the Whole Trade-off

A short pulse resolves closely spaced events but carries little energy, so the trace is noisy and the reach short. A long pulse carries enough energy to see 100 km but smears events together. There is no correct setting, only one correct for the question.

The dead zone is the consequence. After any reflection the detector saturates and nothing can be distinguished until it recovers: the event dead zone is how close two reflections can be and still be told apart, and the attenuation dead zone — how far past a reflection a loss figure becomes trustworthy — runs two to three times longer. A launch cable moves the instrument's own connector clear of the region you care about.

When It Is the Wrong Tool

Inside a data centre most links are tens of metres, inside the dead zone of any usable pulse. An OTDR reports a good link as a featureless smear, and a broken one as the same. For those channels a source and power meter give the number that decides whether the link works — see Tier 1 and Tier 2 testing and the link budget calculator.

The OTDR earns its keep on outside plant and on any fault where the question is where rather than how much. Knowing a break is 1,847 m out is the difference between a targeted dig and a day walking the route.

Where You Meet It

Acceptance of new outside plant, splice-by-splice documentation for a warranty, and fault location after a cable strike. Always test at both 1310 and 1550 nm on single-mode: the pair separates bends from splices and gives wavelength-dependent context one trace cannot.

Related Terms

Insertion loss, return loss, Tier 1 and Tier 2 testing, G.652.D fibre, end-face inspection.