3.8 OTDR Operation and Trace Reading

How an OTDR Works

An optical time-domain reflectometer (OTDR) works on the principle of Rayleigh backscatter. The instrument launches a short pulse of laser light into the fiber. As the pulse travels, microscopic density variations in the glass scatter a small fraction of the light in all directions; the fraction that travels back toward the OTDR is captured by an avalanche photodiode. The OTDR measures the intensity of that returning light and the time it took to return, then converts time to distance and plots the result as a logarithmic trace.

Because the speed of light in glass is about 2 × 10⁸ m/s (c / n, where n ≈ 1.46), 1 µs of round-trip time corresponds to roughly 100 m of fiber. The OTDR's distance calculation depends on the index of refraction (IOR) entered by the technician; setting the wrong IOR produces a proportional distance error. Typical IOR values: 1.468 for singlemode at 1310 nm, 1.467 at 1550 nm, 1.496 for multimode at 850 nm (values vary slightly by fiber type—always confirm against the cable datasheet).

Pulse Width and Dynamic Range

The OTDR's pulse width is a tradeoff between resolution and reach:

• Short pulse (5–30 ns) — high resolution, can resolve closely spaced events, but lower dynamic range (shorter reach).
• Long pulse (1 µs and up) — lower resolution (closer events blur together), but much higher dynamic range (longer reach into the fiber).

A Technician typically starts with a short pulse on a short premises link to resolve connectors, then switches to a longer pulse on an OSP run to see the far end. The instrument's dynamic range (in dB) is the maximum fiber attenuation it can see through; a 35 dB dynamic-range OTDR can probe roughly 80 km of OS2 singlemode at 1550 nm.

Dead Zones

Two dead zones matter:

• Event dead zone (EDZ) — the minimum distance between two reflective events for the OTDR to resolve them as separate. It is proportional to the pulse width and the receiver recovery time after a strong reflection.
• Attenuation dead zone (ADZ) — the distance after a reflective event before the OTDR can measure attenuation accurately again. ADZ is typically several times longer than EDZ.

A reflective connector at the start of the link blinds the OTDR for the dead-zone distance; this is why a launch cable (dead zone box) is mandatory. The launch cable places the first connector outside the dead zone so the OTDR can see the rest of the link. A standard launch cable is 1 km for singlemode.

Launch and Receive Cables

To measure the loss of the first connector, the OTDR needs a launch cable. To measure the loss of the last connector, the OTDR needs a receive cable attached at the far end. The launch and receive cables push the OTDR's blind spots outside the link under test.

The standard test setup is: OTDR → launch cable → link under test → receive cable. The OTDR then sees:

1. A reflection at the launch cable's far-end connector (the link's near-end connector) and can measure its loss.
2. The backscatter trace of the link.
3. A reflection at the link's far-end connector and can measure its loss.
4. A reflection at the far end of the receive cable, marking the true end of the test.

Without a receive cable, the OTDR cannot measure the loss of the link's far-end connector because the reflection from the cable end masks the connector event.

Wavelength Selection

Test at both wavelengths and compare. 1550 nm is more sensitive to bending loss than 1310 nm, so a trace that shows small loss at 1310 but large loss at 1550 indicates a macrobend.

Reading the Trace

An OTDR trace is a plot of returned power (dB scale, decreasing downward) versus distance (km, increasing rightward). Common features:

• Backscatter slope — the gentle downward slope of the trace is the fiber attenuation in dB/km. A steeper slope means higher attenuation.
• Reflective event (spike up) — a connector or mechanical splice, or the cable end. The spike's height indicates reflectance; the drop after the spike is the event's insertion loss.
• Non-reflective event (step drop, no spike) — a fusion splice or a macrobend. The size of the step is the splice or bend loss.
• Gainer — an apparent upward step, which is an artifact of mismatched fibers (e.g., a 50 µm-to-62.5 µm splice). Gain appears because the backscatter coefficient of the second fiber is higher; the actual loss must be measured from the other end.
• End of fiber — a sharp reflection that drops to noise. If the trace drops to noise without a reflection, the fiber end is shattered or bent—common after a break.
• Ghost — a periodic reflection that appears at twice the distance of a strong reflective event; it is not a real event and is removed by reducing reflectance at the strong event.

Hands-On and Written Exam Notes

On the TECH hands-on OTDR task, you will be asked to identify faults on a prepared trace and report the distance, type, and severity of each event. On the written exam, expect questions about IOR setting, the purpose of launch/receive cables, the difference between event and attenuation dead zones, and which wavelength to use to detect a bend. The most common field error is testing without a launch cable and misreporting the first connector's loss because it falls inside the dead zone.