OTDR Pulse Width, Resolution and Range Calculator
Choosing the OTDR pulse width is a trade between seeing closely spaced events and seeing far down the fiber. This calculator shows the physical length of each pulse, the resolution it implies and how far your dynamic range will reach.
Your numbers
Index of refraction set on the OTDR. A wrong value scales every distance.
Distance between samples at the selected range and resolution setting.
From the OTDR datasheet or the one-way dynamic range at the selected pulse width.
Allowance for connectors, splices, and the noise floor so events can still be measured.
Typical 0.35 dB/km at 1310 nm and 0.21 at 1550 nm for G.652.
Your results
Rule-of-thumb estimates. Published dead zones in OTDR datasheets are measured at specified reflectance and loss thresholds and vary by model.
Spatial resolution vs pulse width (m)
Sweeps Pulse width from 3 to 1000, holding your other inputs fixed.
Link test records to assets
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How to use the calculator
Select the pulse width you plan to use, enter the group index set on the instrument, the sampling resolution and the dynamic range at that pulse width. The tool gives you the physical pulse length, the spatial resolution, a practical event separation and the longest fiber you can expect to test with that setting. Short pulses suit patch cords and short links, while long pulses are needed for long-haul spans and PON feeders.
- Use the shortest pulse that still gets the far end above the noise
- Use launch and receive cables to move the first and last connectors out of the dead zone
- Always set the group index from the cable datasheet
Formulas
A pulse of width t occupies L = c x t / n meters of fiber, with c = 0.2998 m/ns. Because the reflected light travels back, two events closer than half of that length merge into one, so spatial resolution is about c x t / (2 n), roughly 10 m per 100 ns at a group index of 1.468. The measurable one-way range is approximately the usable dynamic range, after reserving margin for events, divided by the fiber attenuation per km.
Worked example
With 100 ns at an index of 1.4682, the pulse is 20.4 m long, giving about 10.2 m of resolution and roughly 12.2 m once two 1 m samples are added. A 32 dB dynamic range with 8 dB reserved and 0.21 dB/km fiber reaches about 114 km. If you used 10 us instead, resolution degrades to about 1 km, so events such as a patch panel and a nearby splice would be reported as a single event.
Common mistakes
The common errors are testing a short link with a long pulse, which hides closely spaced events in the dead zone, and entering the wrong group index so every distance is off by the same ratio. Also remember that vendor-quoted dynamic range uses a specific averaging time and SNR=1 criterion, so the working range is shorter. Use bidirectional testing and average splice loss to avoid gainer artifacts.
Frequently Asked Questions
What is an OTDR dead zone?
It is the distance after a reflective event during which the OTDR cannot measure properly because the detector is recovering. Event dead zone is for detecting a second event, attenuation dead zone is for measuring loss after it.
Which pulse width should I choose?
Use the shortest pulse that gets the far end of the fiber out of the noise. For short enterprise links 3 to 30 ns is common, for metro routes 100 ns to 1 us, and for long haul 3 to 20 us.
Why does the group index matter?
The OTDR measures time and converts it to distance with the index. An error of 0.01 in index changes distance by about 0.7 percent, enough to point a locate crew at the wrong spot.
Is resolution the same as dead zone?
No, but they are related. Spatial resolution is about half the pulse length, while datasheet dead zones depend on detector recovery and the thresholds used and are usually longer than that.
Netray can help keep test results and as-built records tied to the assets and work orders in your ERP.
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