OTDR Dead Zone Calculator
Event & Attenuation Dead Zone

What is OTDR Dead Zone?

An OTDR dead zone is the distance immediately after a reflective event (like a connector) where the detector is saturated and cannot reliably detect another event. There are two types:

• Event dead zone (EDZ): The minimum distance between two reflective events for the OTDR to detect both as separate events. Shorter, typically 1–10% of the pulse length.
• Attenuation dead zone (ADZ): The distance after a reflective event before the OTDR can accurately measure loss. Always longer than the event dead zone.

Dead zones are specified by OTDR manufacturers and vary by instrument. The formulas here provide typical estimates.

Why Launch Cables Matter

The near end of any fiber link is always inside the OTDR’s dead zone. The first connector of the fiber under test is invisible to the OTDR unless a launch cable is used.

A launch cable (also called a launch fiber, lead cable, or dead zone fiber) is a known good spool of fiber connected between the OTDR and the fiber under test. It pushes the near-end connector of the test cable out of the dead zone so the OTDR can see it and accurately measure its loss.

• Launch cable length must exceed the attenuation dead zone
• Add 10% buffer beyond the ADZ minimum
• For two-end measurements, also use a receive cable at the far end