Solution:

The major fiber manufacturers now market fiber specifically designed for WDM systems. Called “Non-zero dispersion-shifted fiber” (NZ-DSF), this fiber has a dispersion of around 4 ps/nm/km in the 1530-1570 nm band.

This low but positive dispersion figure minimizes the dispersion of the signal while avoiding the unwanted effects of four-wave mixing between WDM channels.

Fibers from different manufacturers have somewhat different characteristics. AT&T calls its dispersion-optimized fiber “Tru-Wave” and Corning uses the name SMF-LS.