Tool · Fiber and telecom

DWDM Channel Grid Calculator

Which ITU channel is a laser at 1550 nm closest to, and how many channels fit in the C-band at 50 GHz? The calculator lays out the ITU-T G.694.1 frequency grid across any band, lists every channel with its frequency and wavelength, finds the channel nearest a wavelength, and shows how flexible-grid slots pack. Background: wavelength-division multiplexing, CWDM and DWDM, and ROADM.

Grid and band

Grid

Flexible slot width m (× 12.5 GHz)

Band

Range from (nm)

Range to (nm)

Look up wavelength (nm)

Channels follow ITU-T G.694.1: 193.1 THz plus whole multiples of the spacing. The band edges are the usual ITU band names; an amplifier’s own gain band is usually narrower.

Presets

CWDM grid, ITU-T G.694.2

λ (nm)	f (THz)	Band

Coarse WDM uses 18 wavelengths 20 nm apart, wide enough for uncooled lasers that drift with temperature.

Readouts

Channels across the range

Each line is a channel’s central frequency; the highlighted one is nearest the wavelength looked up, marked by the triangle.

Channel table

n	f (THz)	λ (nm)

Learn with it

Three short experiments. Each one sets the inputs, says where to look, and asks for a prediction before it shows the result.

Checked against

These checks run in your browser on every load. Grid frequencies and wavelengths are compared with values worked out by hand from c = 299 792 458 m/s, and the channel count is compared with a count made channel by channel.

Check	Expected	Computed	Tolerance

The grid definitions are those of ITU-T Recommendations G.694.1 (DWDM frequency grid) and G.694.2 (CWDM wavelength grid). The tolerance is the largest relative difference from Expected that still passes.

The model

The DWDM grid of ITU-T G.694.1 is defined in frequency. Its channels sit at

f_n = 193.1\ \mathrm{THz} + n\,\Delta f, \qquad \lambda_n = \frac{c}{f_n}

with $n$ a whole number, positive or negative, the spacing $\Delta f$ 100, 50, 25 or 12.5 GHz, and $c$ = 299 792 458 m/s, so the wavelengths are in vacuum. A fixed spacing in frequency is a spacing in wavelength that grows with the square of the wavelength,

\Delta\lambda \approx \frac{\lambda^2\,\Delta f}{c}

which is about 0.8 nm for 100 GHz in the C-band. The flexible grid places each channel’s central frequency on 6.25 GHz steps, $193.1\ \mathrm{THz} + n \times 6.25\ \mathrm{GHz}$ , and gives it a slot $m \times 12.5$ GHz wide; the tool packs slots edge to edge from the low-frequency end of the range, whereas a real plan assigns them one by one. The CWDM grid of G.694.2 is defined in wavelength instead: 18 channels from 1271 to 1611 nm, 20 nm apart. Band edges follow the usual ITU band names (O, E, S, C and L); the range an amplifier or a transceiver covers is set by the equipment, not by the grid.

Worked example

The anchor, 193.1 THz, is 1552.524 nm. A laser at 1550.000 nm is at 193.4145 THz, 14.49 GHz above channel $n$ = 3 of the 100 GHz grid, which is 193.4 THz or 1550.116 nm. Between 1530 and 1565 nm the 100 GHz grid holds 44 channels, from 191.6 THz (1564.679 nm) to 195.9 THz (1530.334 nm); neighbouring channels are 781.6 pm apart at the short end and 816.2 pm at the long end.

References: ITU-T Recommendation G.694.1, Spectral grids for WDM applications: DWDM frequency grid. ITU-T Recommendation G.694.2, Spectral grids for WDM applications: CWDM wavelength grid. G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed. (Wiley, 2021).