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The ITU-T G.694.1 DWDM grid is anchored at 193.1 THz; G.694.2 CWDM runs 1271 to 1611 nm on 20 nm spacing. Enter a channel number, a frequency or a wavelength and this returns the other two, plus the neighbouring channels so you can see where you are in the band.
The DWDM grid. ITU-T G.694.1 anchors the dense grid at 193.1 THz and allows spacings of 100, 50, 25 and 12.5 GHz. Channel numbering in the C-band follows f = 190.0 + n × 0.1 THz, so C34 is 193.4 THz — 1550.12 nm. Coherent pluggables tune anywhere on the grid; direct-detect DWDM optics are ordered per channel.
The CWDM grid. ITU-T G.694.2 defines 18 channels from 1271 to 1611 nm on 20 nm spacing. The wide spacing is what lets CWDM use uncooled lasers, which is why it costs a fraction of DWDM. The catch is the six channels between 1351 and 1451 nm, which sit in or near the fibre water peak — on older G.652.B fibre those are unusable, which is why so many CWDM systems only use the eight channels from 1471 to 1611 nm.
Choosing a spacing. 100 GHz is the workhorse: 48 channels in the C-band, filters that are cheap and stable, and it is what nearly every coherent module expects. 50 GHz doubles the count at the cost of tighter filters and less wavelength-drift tolerance. 200 GHz — not on this list because it is simply every other 100 GHz channel — buys wider passbands and about a decibel less insertion loss when you only need eight channels.
193.4 THz, which is 1550.12 nm. The C-band channel numbers run from about C17 (191.7 THz, 1563.86 nm) to C61 (196.1 THz, 1528.77 nm) on the 100 GHz grid, giving the 44–48 usable channels most muxes are built for.
Eighteen on modern G.652.D fibre. On older non-zero-water-peak fibre, the 1351–1451 nm channels are lossy enough to be unusable, so most deployments use the eight channels from 1471 to 1611 nm. Check what fibre you have before you buy an 18-channel mux.
Yes, and it is a common upgrade path. A CWDM channel is 20 nm wide, which comfortably contains a whole block of DWDM channels — you drop one CWDM wavelength with an OADM and feed a DWDM mux from it. Watch the cumulative insertion loss; you are now paying for two passive layers.
nm ⇄ THz ⇄ GHz, plus photon energy and the optical band each wavelength falls in
Open tool →Total channel loss, power budget and pass/fail margin for single-mode and multimode links
Open tool →Pulse spread from Δt = D · L · Δλ, and the distance at which dispersion — not loss — ends the link
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.