Why is the DWDM grid defined in frequency rather than wavelength?

Because that is what the hardware controls — a laser is stabilised to a frequency and a filter passes a frequency band. Wavelength is derived, and the relationship is reciprocal rather than linear, so channels evenly spaced in frequency are unevenly spaced in wavelength.

How wide is a 50 GHz channel in nanometres?

About 0.401 nm at 1550 nm — but only 0.286 nm at 1310 nm. It is not a constant, which is why building a channel list by repeatedly adding a fixed number of nanometres produces numbers that are right at one end of the band and progressively wrong at the other.

The anchor the ITU grid is built from. Every channel is that frequency plus a whole number of spacings, in both directions. Its wavelength is 1552.524 nm, which is why that figure appears so often.

Why is CWDM spaced in wavelength instead?

Because the reason for CWDM is cost. A 20 nm channel is about a hundred times wider than a DWDM one, so an uncooled laser whose wavelength drifts with ambient temperature stays comfortably inside it — no thermoelectric cooler, no control loop. The trade is eighteen channels instead of ninety-six, and no optical amplification.

What is the water peak?

A region around 1383 nm where hydroxyl ions left from manufacturing absorb strongly. Four CWDM channels sit in it, and on older fibre they can be several times more lossy than their neighbours. Low-water-peak fibre removes the problem, but plenty of older cable is still in the ground.

Why does the checker flag channels that have different numbers?

Because a wide carrier can occupy the spectrum a neighbouring channel was meant to use. A 100 GHz carrier on channel 0 of a 50 GHz grid overlaps channel 1, and a check that only compares channel numbers passes it. Mixed line rates on one fibre make that ordinary rather than exotic.

Is anything I enter sent anywhere?

No. Grids, assignments and conversions are all calculated in your browser, and nothing is uploaded.

DWDM & CWDM Channel Planner

Wavelength to frequency, the ITU grid in both directions, and a channel plan that catches the two ways assignments clash — including the one where the channel numbers look fine.

The grid is anchored in frequency, not wavelength Every DWDM channel is 193.1 THz plus a whole number of spacings. The wavelengths follow from that and are not evenly spaced — a 50 GHz channel is 0.401 nm wide at 1550 nm and 0.286 nm at 1310 nm. Building a grid by adding a fixed number of nanometres produces channels that drift away from the real ones as you move along the band.

Convert

Value

Is a

Channel width (GHz)

The grid

Grid Spacing Band

Channel assignments

ServiceChannelWidth GHzFibre

Move the 100G service to channel 1 to see the overlap check fire — the channel numbers stay different, and the spectrum collides.

Why the anchor is a frequency

Optical channels are defined by frequency because that is what the hardware controls. A laser is stabilised to a frequency; a filter passes a frequency band. Wavelength is a derived quantity, and the relationship between them is not linear — it is a reciprocal.

The practical consequence catches people out. On a 50 GHz grid, adjacent channels differ by 0.401 nm near 1550 nm and by 0.286 nm near 1310. Anyone who builds a channel list by repeatedly adding 0.4 nm will produce numbers that look right at one end of the band and are progressively wrong at the other.

So everything here starts at 193.1 THz and counts in whole spacings. Wavelengths are shown because that is what gets printed on a patch lead, but they are the output rather than the input.

Why CWDM is the other way round

CWDM channels are spaced in wavelength — 20 nm apart, from 1271 nm — and the reason is economic rather than physical.

A DWDM laser must be held to within a few gigahertz of its channel, which needs a thermoelectric cooler and a control loop. A CWDM channel is about a hundred times wider, so an uncooled laser whose wavelength wanders with ambient temperature stays comfortably inside it. That removes the cooler, the control electronics and much of the cost.

The trade is capacity: eighteen channels instead of ninety-six, and no optical amplification, because the channels are spread far outside the band an erbium amplifier covers.

The water peak

Four CWDM channels — roughly 1371 to 1431 nm — sit in a region where hydroxyl ions left over from manufacturing absorb strongly. On older fibre those channels can be several times more lossy than their neighbours, which makes them effectively unusable.

Low-water-peak fibre removes the problem, and most fibre installed this century is. But plenty of older cable is still in the ground, and a CWDM plan that assumes all eighteen channels are available on a route of unknown vintage is a plan that will need revisiting. The grid below flags them.

Two ways a plan clashes

Two services on one channel. The obvious case, and easy to catch by sorting a list.

Overlapping spectrum on different channel numbers. The subtle one. Put a 100 GHz carrier on channel 0 of a 50 GHz grid and it occupies the space channel 1 was meant to use — but the channel numbers are different, so a check that compares numbers passes it. Mixed line rates on one fibre make this ordinary rather than exotic, and the checker here compares actual occupied spectrum rather than labels.