7.4.2 Types Of WDM Systems

WDM systems are commonly classified according to closeness of the channel spacing.

Coarse WDM (CWDM) employs relatively wide wavelength spacing (typically 20 nm intervals) across approximately 1270–1610 nm. Because the spacing is wide, uncooled lasers and simpler filters can be used, reducing cost. CWDM is therefore common in metropolitan and access networks. Channel counts are typically up to 18 wavelengths.
Dense wavelength-division multiplexing (DWDM) uses closely spaced optical channels, typically defined in frequency spacing rather than wavelength spacing. Standard channel spacings include: 100 GHz, 50 GHz, 25 GHz (and below in advanced systems).

In practice, DWDM channel frequencies are standardized according to the ITU-T frequency grid (Recommendation ITU-T G.694.1), which defines channels on a reference grid centered at 193.1 THz (corresponding approximately to 1552.52 nm). Channel spacing is specified in frequency units (e.g., 100 GHz or 50 GHz) to ensure interoperability between equipment from different manufacturers.

More recent systems employ a flexible frequency grid (“flex-grid”), in which channel spacing is not fixed but can be allocated in smaller increments (typically 12.5 GHz steps). Flex-grid operation enables variable channel bandwidths to accommodate advanced modulation formats and improves spectral efficiency in high-capacity optical transport networks.

Since optical frequency and wavelength are related by f=c/l, equal frequency spacing corresponds to non-uniform wavelength spacing. DWDM systems typically operate within C-band (1530–1565 nm) and often L-band (1565–1625 nm).

Because many more channels can be accommodated within a narrow spectral band, DWDM enables extremely high aggregate capacities, often multiple terabits per second per fiber. As channel spacing narrows, system complexity increases due to tighter laser stability requirements, dispersion sensitivity, and nonlinear optical effects.