Zero-dispersion wavelength

Chromatic dispersion in fiber is the sum of two contributions with opposite trends: material dispersion (silica's index curvature, negative below ~1270 nm and positive above) and waveguide dispersion (geometry-dependent, negative in conventional designs). They cancel at the zero-dispersion wavelength $\lambda_0$.

For standard G.652 fiber, $\lambda_0$ falls between roughly 1300 and 1324 nm. That is the historical reason the O-band was first home to telecom, and the ongoing reason datacom's 1310 nm intensity-modulated links (where no EDFA is available anyway) enjoy essentially free dispersion. Around $\lambda_0$, dispersion grows linearly with the slope $S_0 \approx 0.09$ ps/(nm²·km); by 1550 nm, standard fiber has climbed to $D \approx +17$ ps/(nm·km).

Because waveguide dispersion is a design knob, $\lambda_0$ can be relocated. Dispersion-shifted fiber (G.653) dragged it to 1550 nm to align zero dispersion with lowest loss. It promptly taught the industry a lesson: zero is the worst possible dispersion for WDM. Four-wave mixing between channels is phase-matched precisely where dispersion vanishes; DWDM channels riding near $\lambda_0$ intermodulate catastrophically. The correction, non-zero dispersion-shifted fiber (G.655), parks a small but deliberately nonzero dispersion (~4–8 ps/nm/km) in the C-band: enough walk-off to spoil FWM phase matching, little enough to ease compensation. Coherent-era systems, able to digitally absorb any dispersion, closed the argument in the opposite direction: plain G.652 with its large 1550 nm dispersion is now the preferred long-haul medium, nonlinearity-suppressing and cheap.

Two practical notes. Fiber datasheets specify $\lambda_0$ and $S_0$ per fiber design. Dispersion at any wavelength follows from the standard interpolation formula, which is how link totals get computed. And in nonlinear work the sign convention flips vocabulary: operation just above $\lambda_0$ (anomalous dispersion) is where solitons and most Kerr-comb states live, making $\lambda_0$'s location a design input for nonlinear devices, not just links.