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Wavelength and frequency are two views of the same thing, tied together by the speed of light. DWDM planning happens in frequency, transceiver datasheets are written in nanometres, and this converts between them — with photon energy and the ITU band name thrown in.
The relationship. Frequency times wavelength equals the speed of light: f = c ÷ λ. In vacuum c is 299,792,458 m/s, so 1550 nm is 193.414 THz and 1310 nm is 228.849 THz. Because it is a reciprocal relationship, equal steps in frequency are not equal steps in wavelength — which is exactly why the DWDM grid is defined in frequency.
Why the grid is defined in frequency. A laser's stability is naturally described in frequency, and the ITU grid needs constant spacing so that filters can be built to a single design. 100 GHz is about 0.8 nm near 1550 nm but about 0.57 nm near 1310 nm — if the grid were defined in nanometres, the frequency spacing would drift across the band.
Wavelength in fibre. Light travels slower in glass, so the wavelength inside the fibre is shorter than in vacuum by the group index — about 1.4682 for standard single-mode fibre. Datasheets and the ITU grid always quote the vacuum wavelength, so leave the medium set to vacuum unless you specifically need the in-fibre figure.
193.414 THz in vacuum. The ITU C-band grid is anchored at 193.1 THz, which is 1552.52 nm — close to but not the same as the round number 1550 nm that datasheets use as shorthand for the whole C-band.
About 0.80 nm at 1550 nm, 0.86 nm at 1610 nm and 0.57 nm at 1310 nm. The spacing is constant in frequency, so it stretches in wavelength terms as you move to longer wavelengths.
Mostly for sanity-checking detector and laser physics — a 1550 nm photon carries about 0.80 eV, and a 1310 nm photon about 0.95 eV. It explains why germanium and InGaAs detectors work in the telecom bands while silicon does not.
ITU channel number ⇄ frequency ⇄ wavelength, for 100/50/25/12.5 GHz DWDM and the 20 nm CWDM grid
Open tool →Pulse spread from Δt = D · L · Δλ, and the distance at which dispersion — not loss — ends the link
Open tool →Convert optical power between logarithmic and linear units, in both directions
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.