Knowledge base · Calculator
OTDR dead zone calculator
Pick the pulse width you test with and enter what you want to see: two connectors a few metres apart, or the first connector of the link behind a launch cable. The calculator tells you whether the OTDR will see them separately.
By Ilya (engineer, 6 years in fibre optics) ·
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
Every pulse width at once
The same arithmetic for the pulse widths OTDRs offer, in G.652 fibre at 1310 nm. Click a row to pick that pulse width in the calculator.
| Pulse width | On the trace | Event dead zone | Attenuation dead zone |
|---|---|---|---|
| 3 ns | 0.3 m | 0.6–1.1 m | 2.8–14 m |
| 5 ns | 0.5 m | 1.0–1.8 m | 4.7–23 m |
| 10 ns | 1.0 m | 1.2–3.8 m | 4.7–23 m |
| 20 ns | 2.0 m | 1.7–3.8 m | 4.7–23 m |
| 30 ns | 3.1 m | 2.5–3.8 m | 4.8–23 m |
| 50 ns | 5.1 m | 5.4–7.7 m | 11–24 m |
| 100 ns | 10 m | 9.0–12 m | 14–24 m |
| 300 ns | 31 m | 28–33 m | 51–100 m |
| 500 ns | 51 m | 46–51 m | 55–123 m |
| 1 µs | 102 m | 93–101 m | 109–245 m |
| 3 µs | 307 m | 279–303 m | 328–736 m |
| 10 µs | 1.02 km | 930 m – 1.01 km | 1.09–2.45 km |
| 20 µs | 2.04 km | 1.86–2.02 km | 2.19–4.90 km |
The range is the middle half of the measurement: a quarter of instruments have a shorter zone than the lower end, a quarter a longer one than the upper end.
How it is calculated
The pulse length on the trace is exact: c · τ / (2n), where c is the speed of light, τ the pulse width and n the index of refraction. The 2 is there because light travels to the event and back. A 100 ns pulse in G.652 fibre at 1310 nm (n = 1.4670) takes up 10.2 m of trace.
The zones are not derived from a formula. They are measured on files from real instruments in our corpus. The event dead zone is the width of the peak 1.5 dB below its top, as IEC 61746 defines it. The attenuation dead zone runs from the start of the peak to where the trace is back within 0.5 dB of the backscatter level. Every manufacturer weighs the same in the measurement, however many of its files we hold.
From 400 ns up, the event dead zone is close to the pulse itself: a ratio of 0.9–1. Short pulses behave differently. Below 8 ns it is 2–3.5 times the pulse length, from 8 to 15 ns 1.2–3.7 times. The receiver is the reason: after a bright flash it needs time to recover, and that time no longer depends on the pulse.
The attenuation dead zone is longer and depends more on the reflection itself. At long pulses it is 1.1–2.4 times the pulse length, at the shortest 9.2–45.2 times. A connector with an air gap blinds the OTDR for a long time; a fusion splice hardly does.
The exact figure for your model is in its datasheet, but only for the shortest pulse and one reflection of a given strength. Enter it under “From the OTDR datasheet” and the calculator will work from your instrument.
Questions
Two connectors have already merged into one row. Can I separate them in the saved file?
No. The file holds one row, and no processing will produce a second one from it. You need a second measurement with a shorter pulse, and the calculator tells you which.
Why is the zone in my OTDR's datasheet shorter than here?
The datasheet quotes the best case: the shortest pulse and a reflection of a given strength. At the pulse width used for a long link the zone is longer, and that is what the calculator works out.
How long should my launch cable be?
Longer than the attenuation dead zone at the pulse width you test with. Enter the length of your launch cable and the calculator tells you whether it is enough.