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On long single-mode spans the link often stops working because the pulses have smeared into each other, not because the light ran out. Enter the fibre, the source linewidth and the bit rate and this gives you the accumulated dispersion and the reach it allows.
What chromatic dispersion is. Different wavelengths travel at slightly different speeds in glass. Every real light source emits a band of wavelengths rather than a single one, so a pulse that starts sharp arrives smeared. The spread is Δt = D · L · Δλ — the dispersion coefficient times the length times the source width.
Two different limits, and which one binds. For a broad source — a Fabry-Perot laser or a VCSEL — the limit really is pulse spread against the bit period, and Δt = D·L·Δλ is the right model. For a narrow DFB or externally modulated source the linewidth is so small that the binding constraint is instead the module’s own dispersion tolerance, quoted on the datasheet in ps/nm. This calculator checks both and tells you which runs out first — because applying only the pulse-spread rule to a narrow source gives an answer that is far too pessimistic. It would tell you a 10G ZR link cannot reach 80 km, when in practice it comfortably does.
Why 1310 nm is special. Standard G.652 fibre has its zero-dispersion point near 1310 nm, which is why the O-band is used for short-reach parallel optics and for LAN WDM. The trade-off is attenuation: 0.35 dB/km at 1310 nm against 0.22 dB/km at 1550 nm. Short and clean, or long and smeared — that is the choice the band forces.
Rate matters more than you would think. The bit period shrinks in proportion to the rate, so the tolerable spread shrinks with it — but the dispersion itself does not change. Going from 10G to 25G on the same fibre cuts the dispersion-limited reach by roughly a factor of four, because the module's dispersion tolerance falls faster than the rate rises. This is why a 10G DWDM link that ran 200 km unaided will not simply be upgraded to 25G over the same span.
With a narrow externally modulated source it is roughly 80–100 km on standard G.652 fibre before you need compensation, which is exactly why 80 km is such a common ZR-class reach figure. A directly modulated laser with a wider linewidth hits the limit much sooner.
Three ways. Use a narrower source — an externally modulated laser instead of a directly modulated one is often enough. Add a dispersion compensation module, which is a length of fibre with the opposite sign of D. Or move to a coherent module, which compensates for dispersion in DSP and can handle thousands of ps/nm without any optical compensation at all.
Chromatic dispersion is minor there; modal dispersion dominates. That is what the effective modal bandwidth figure on an OM3/OM4/OM5 datasheet describes, and it is why multimode reach is quoted as a fixed distance per standard rather than calculated from a loss budget.
Total channel loss, power budget and pass/fail margin for single-mode and multimode links
Open tool →nm ⇄ THz ⇄ GHz, plus photon energy and the optical band each wavelength falls in
Open tool →One-way and round-trip delay from route length and fibre refractive index
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.