Knowledge base · Measurement
Dead zone: why the OTDR cannot see an event that is there
The dead zone is the stretch after a strong reflection where the instrument, for a while, tells nothing apart. Every OTDR has one, the most expensive included, because that is how the instrument works. It is also the main reason two connectors a couple of metres apart arrive in the file as a single row.
One word covers two different figures: the distance at which two events are still distinguishable as two, and the distance after an event at which loss can be measured again. A datasheet lists them side by side, and they differ several times over.
By Ilya (engineer, 6 years in fibre optics) ·
There are two dead zones
Both are defined by IEC 61746, both follow the pulse width, and they mean different things.
Event dead zone
The distance at which two neighbouring reflections are still distinguishable as two. For a reflective event it is measured across the peak, 1.5 dB down from its top. Inside it the instrument can still say that an event is there, but not how much each of the two cost.
Attenuation dead zone
The distance after an event at which the trace returns to undisturbed backscatter, usually within 0.1 or 0.5 dB. This is the minimum distance at which a splice can be measured after a connector. It is always longer than the event dead zone, and the stronger the reflection, the longer it gets.
Front-end dead zone
The same attenuation dead zone, but at the instrument itself: it starts at the OTDR's own connector. Because of it the first metres of the trace are not measured at all, and it is exactly what a launch cable moves out of your link.
Where its length comes from
The main control is the pulse width. With a long pulse the dead zone is roughly the pulse itself: while the instrument transmits, it does not listen. With a short pulse the pulse is no longer what limits it. The photodiode needs time to recover from a bright flash, and at short pulses the dead zone comes out several times longer than the pulse.
The pulse has to be chosen anew on every job. A short pulse gives a short dead zone and separates close events, but carries less energy, more noise and less reach. A long pulse reaches the far end, but close events merge into one. This is why one span is shot with several pulse widths: a long one to see the whole of it, a short one to resolve its start.
The second quantity behind the attenuation dead zone is the strength of the reflection itself. The more light comes back, the longer the trace takes to recover. A connector with an air gap blinds the instrument for a long while; a splice, which barely reflects at all, barely blinds it. That is why datasheet metres describe the instrument's best case: its shortest pulse and a single reflection of a stated strength.
Work it out for your pulse width
Pick the pulse width you test with and enter what you want to see. The calculator tells you whether the OTDR will see two close events separately and whether your launch cable is long enough.
How many times slower light travels in glass than in a vacuum. The OTDR uses it to turn the time a reflection takes to come back into metres. Choosing the fibre and wavelength fills in a typical value. If you know your own, from the cable datasheet or the OTDR settings, type it in.
From the OTDR datasheet
Pulse length on the trace
Event dead zone
Attenuation dead zone
The calculator on its own page: a table for every pulse width and how the zones were measured
What this looks like in the file
An event that falls inside another one's dead zone is usually not lost: the instrument reports it in one row together with its neighbour. Some instruments mark such a row, and we show that mark: the event reads "Merged". The loss beside it belongs to the group as a whole and the reflectance to the strongest component, and the row says so rather than printing them as an ordinary measurement of one event.
How close that is can be seen in our corpus: across the files that carry such rows, the distance between the merged components sits between 4 and 15 metres. These are connector pairs and short patch cords, exactly what stands in a cabinet and in a closure. Nothing can separate them after the fact: the file holds one row, and no processing will produce a second one. Separating them takes another acquisition at a shorter pulse.
Within the front-end dead zone, in the first metres of the trace, our own event analysis looks for nothing either. The instrument does not resolve events there, and drawing one where there is no data would pass a guess off as a measurement.
Next
A dead zone is not a stretch that is "not shown": the curve is there and is drawn in full. Only the measurement fails over that stretch.