CWDM vs DWDM
The two wavelength-multiplexing grids: CWDM spaces channels 20 nm apart so lasers can run uncooled; DWDM packs channels at 100/50 GHz (or tighter) in the C-band where amplifiers and coherent systems live.
Both schemes carry independent channels on separate wavelengths through one fiber. The grid spacing determines everything downstream: laser cost, channel count, amplification, and reach.
CWDM (ITU-T G.694.2) defines 18 channels on a 20 nm grid from 1271 to 1611 nm. Twenty nanometers is generous by design: it swallows the ~0.1 nm/°C drift of an uncooled DFB across its whole temperature range, so CWDM transmitters need no TEC and no wavelength locker, just a binned laser. The result is the cheapest form of fiber-pair multiplication, standard in metro access, mobile fronthaul, and campus links. The constraints follow from the same breadth: channels span the water-peak and high-loss regions (older G.652 fiber loses several of the 18), and, decisively, most of the grid lies outside the erbium window, so CWDM links are unamplified, budget-limited to tens of kilometers, typically ≤8 or ≤16 channels in practice.
DWDM (G.694.1) packs channels at 100 GHz (~0.8 nm), 50 GHz, or on modern flexible grids down to 12.5 GHz granularity, almost always inside the C-band (~1530–1565 nm; L-band for expansion). Forty to ninety-six channels ride one fiber through shared EDFAs: one amplifier per span for every channel simultaneously, which is the economic engine of long-haul transport. The price of density: temperature-stabilized, wavelength-locked transmitters (or tunables), tighter filters (AWGs, WSS), and management of the nonlinear interactions that close channel spacing invites (XPM, FWM). The DWDM Channel Grid Calculator tabulates both grids.
Selection heuristic: no amplification needed, short reach, cost-driven → CWDM. Amplified reach, high aggregate capacity, coherent formats, ROADM-based networks → DWDM. Hybrid deployments layer a few DWDM channels inside one CWDM window (the 1531/1551 nm slots) when an access network outgrows its grid.
References: ITU-T Recommendation G.694.2, Spectral grids for WDM applications: CWDM wavelength grid. ITU-T Recommendation G.694.1, Spectral grids for WDM applications: DWDM frequency grid. G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed. (Wiley, 2021).