Calculate event dead zone and attenuation dead zone from OTDR pulse width. Get the recommended launch cable length to avoid near-end dead zone blind spots.
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:
Dead zones are specified by OTDR manufacturers and vary by instrument. The formulas here provide typical estimates.
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.
A 100ns pulse is a common mid-range setting for medium-haul testing. On SMF (n = 1.4682), the one-way pulse length works out to roughly 10.2 m — the physical length of fiber the pulse occupies as it travels.
From there: the event dead zone (1.5× pulse length) is about 15.3 m, the attenuation dead zone (4× pulse length) is about 40.9 m, and the recommended launch cable minimum (10% buffer beyond the ADZ, plus a fixed margin) comes out to roughly 55 m.
The practical takeaway: if you're testing a short jumper or a patch panel connection closer than ~55m from your OTDR using a 100ns pulse, use a shorter pulse width instead — otherwise the near-end connector's real loss is likely to be underestimated or missed entirely.
Why not always use the shortest pulse width to avoid dead zones entirely?
Shorter pulses put less energy into the fiber, which shortens the OTDR's maximum measurement range and raises the noise floor — you'll see a smaller dead zone but lose visibility further down the fiber, or lose events entirely under a rising noise floor. Pulse width is a range-vs-resolution tradeoff, not a setting to just minimize.
Do all OTDR vendors calculate dead zone the same way?
No — the underlying physics (pulse energy occupying a length of fiber) is universal, but exact multipliers and published dead-zone specs vary by instrument and manufacturer test conditions. This calculator uses standard estimation multipliers; always check your specific OTDR's datasheet for its measured EDZ/ADZ at each pulse width if you need certification-grade numbers.
What happens if I skip the launch cable?
The very first connector on the fiber under test sits inside the OTDR's own dead zone and effectively becomes invisible — a bad first connector (a very common failure point, since it's handled most) can go completely undetected, and everything downstream of it gets measured from the wrong reference point.