Fiber Optic Chromatic Dispersion Penalty Calculator
Chromatic dispersion spreads a pulse in time because different wavelengths in the source travel at different speeds in glass. This calculator turns fiber length, source spectral width and dispersion coefficient into picoseconds of broadening and compares it with the bit period.
Your numbers
Signalling rate of the optical channel, for example 10 for 10GBASE-LR.
About 17 at 1550 nm and near 0 at 1310 nm for G.652 fiber. Use the value on the datasheet.
RMS or FWHM width of the laser as given by the transceiver datasheet. A DFB laser is a few tenths of a nm or less.
Your results
Planning estimate for a Gaussian source and a single dispersion value. Use the transceiver vendor's dispersion tolerance (ps/nm) and ITU-T G.652 data for design sign-off.
Pulse broadening vs fiber length (ps)
Sweeps Fiber length from 1 to 120, holding your other inputs fixed.
Automate link checks from your records
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How to use the calculator
Enter the line rate, the dispersion coefficient D from the fiber datasheet, the length of the link and the spectral width of the transmitter. The result shows broadening in picoseconds, what share of one bit period it consumes and the length at which it reaches the planning limit you chose. The chart sweeps fiber length so you can see where the broadening line crosses the allowed line.
- Use D near 17 ps/(nm km) at 1550 nm for standard G.652 fiber
- Use a small D (below about 3) for 1310 nm links, with the exact figure from the datasheet
- Pick the allowed share from your vendor's dispersion penalty guidance
The formula behind it
Pulse broadening is delta tau = |D| x L x delta lambda, with D in ps/(nm km), L in km and delta lambda in nm. The bit period is 1000 divided by the rate in Gb/s, in picoseconds. A common planning rule keeps the broadening below roughly a quarter to a third of the bit period to hold the power penalty near 1 to 2 dB. Narrow DFB lasers and external modulators make long 10G reach possible at 1550 nm, while wide Fabry-Perot lasers are limited to short links.
Worked example and how to read it
A 10 Gb/s link over 40 km of G.652 at 1550 nm with a 0.1 nm source gives 17 x 40 x 0.1 = 68 ps. The bit period is 100 ps, so the link uses 68 percent of the bit period and exceeds a 30 percent limit; the calculated reach at that limit is about 17.6 km. In practice vendors specify a dispersion tolerance in ps/nm for the transceiver, for example 800 ps/nm for 40 km optics, and that figure is what you compare to D x L.
Common mistakes
The most frequent error is using the 1550 nm coefficient for a 1310 nm link, which overstates dispersion, or the reverse. Another is ignoring the chirp of directly modulated lasers, which makes the effective linewidth wider than the datasheet figure. Remember that dispersion compensation modules, coherent receivers and dispersion-shifted fiber change the picture entirely, and that polarization mode dispersion is a separate limit that matters mostly at 40G and above on long routes.
Frequently Asked Questions
What dispersion value should I use for G.652 fiber?
About 17 ps/(nm km) at 1550 nm and roughly 0 to 3 ps/(nm km) around 1310 nm, depending on how far the source is from the zero-dispersion wavelength. Take the exact figure from the cable datasheet or the fiber test report.
Why is the result different from my transceiver's reach?
Vendors specify a dispersion tolerance in ps/nm that already includes the actual transmitter chirp and receiver tolerance. Multiply D by L and compare that with the tolerance. This calculator is a simplified Gaussian estimate.
Does dispersion matter at 1 Gb/s?
Rarely on typical enterprise links. The bit period is 1000 ps, so even tens of km at 1550 nm with a narrow laser stay well inside the limit. It becomes the dominant limit at 10 Gb/s and above on long 1550 nm routes.
Is polarization mode dispersion included?
No. PMD scales with the square root of length and is specified in ps per root km. It is checked separately, mainly for high-rate coherent and long-haul links.
Netray can pull link records from your ERP or asset system and run these checks across every route you have installed.
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