Chromatic Dispersion

Why Glass Has More Than One Speed

The refractive index of silica varies slightly with wavelength, so components of a pulse at different wavelengths travel at different group velocities. No real source is monochromatic, so a pulse that left sharp arrives spread. Enough spreading and adjacent symbols overlap into each other.

The dispersion coefficient D captures the rate: about 17 ps/(nm·km) on standard G.652 fibre at 1550 nm, with the recommendation permitting anywhere from 13.3 to 18.6. It passes through zero between 1300 and 1324 nm and goes negative below that, which is why the O-band is where short-reach single-mode optics live.

Two Different Limits, and Picking the Wrong One

With a broad source — a Fabry-Perot laser, a VCSEL — the spectral width comes from the laser and the limit is pulse spread against the bit period: Δt = D × L × Δλ. With a narrow DFB or EML the laser linewidth is negligible and the spectrum is set by the modulation itself.

In that second case the module datasheet quotes a dispersion tolerance directly in ps/nm, and that number falls roughly with the square of the symbol rate, because the modulation sidebands widen while the symbol period shrinks. Applying the broad-source formula to a narrow source gives a badly pessimistic answer; the dispersion calculator evaluates both and reports which one binds.

The Numbers on a Real Span

Eighty kilometres at 1550 nm accumulates about 1360 ps/nm. A 10G DWDM ZR module is generally specified to cope with that; a 25G one on the same span usually is not, which is the whole reason 10G reaches further than 25G on identical fibre.

Coherent changed the scale of the problem rather than the physics. A 400ZR pluggable tolerates 2400 ps/nm and OpenZR+ modes reach 20,000 ps/nm and beyond, because the coherent receiver has the full optical field and can apply the inverse transfer function in the DSP.

Where You Meet It

Any single-mode link past about 40 km at 1550 nm, and as the reason LAN-WDM packs its four LR4 lanes within a few nanometres of the dispersion zero instead of spreading them across the O-band. It is also why 1310 nm serves the short-reach parallel and duplex classes compared in DR4 vs FR4 vs LR4.

The classic symptom is a link with ample receive power and a stubborn error floor that gets worse when the rate goes up and better when the wavelength moves toward 1310 nm. Before reaching for a compensation module, check whether a longer-reach optic with a stated ps/nm figure already covers the span.

Related Terms

PMD, coherent optics, EML, G.652.D and DWDM.