Eighteen channels on 20 nm spacing from 1271 to 1611 nm — wide enough that the lasers need no cooling.
The Grid, and Why It Is So Coarse
G.694.2 defines eighteen nominal centre wavelengths from 1271 nm to 1611 nm in exact 20 nm steps, with a passband of roughly ±6.5 nm around each one. That passband is the entire design.
An uncooled DFB laser drifts about 0.1 nm for every degree Celsius of junction temperature, so across an industrial case range it wanders five or six nanometres — comfortably inside the filter. Delete the thermoelectric cooler and you delete the largest single cost and power item in a WDM transmitter, which is why a CWDM SFP costs a fraction of its DWDM equivalent and needs no wavelength locker.
What the Filters Cost You
Every channel crosses a thin-film filter stack twice, once at each end of the span. A four-channel mux costs around 1.5 dB, an eight-channel one around 2.5 dB and a full eighteen-channel one around 4 dB.
You pay that at both ends, so an eighteen-channel pair eats roughly 8 dB before a single metre of fibre is counted. Against a long-reach 25G CWDM SFP28 with perhaps 18 dB of budget — a 10 km part has closer to 8 — that leaves very little for distance. Put the real filter numbers into the link budget calculator rather than assuming the fibre dominates — on short metro spans it usually does not.
The Water Peak
Six channels — 1351, 1371, 1391, 1411, 1431 and 1451 nm — sit on or beside the hydroxyl absorption peak near 1383 nm. On older G.652.B fibre that peak can add a decibel or two per kilometre against 0.32 dB/km at 1310 nm, which makes those lanes useless over any real distance.
Low-water-peak G.652.D flattens it to roughly the same attenuation as the rest of the band, so all eighteen become usable. This is why so much installed plant runs only the eight channels from 1471 nm upward: it was specified when nobody could rely on the fibre underneath.
Where CWDM Runs Out
Two limits. There is no amplifier: the grid spans 340 nm and an EDFA covers about 35, so once fibre loss has eaten the budget the link simply ends. Practical reach is 40 to 80 km at 1G and less at 10G or 25G, where chromatic dispersion on the 1550 nm lanes starts to bind as well.
Eighteen channels is also a hard ceiling. When mobile operators needed more lanes on installed fibre the answer was MWDM, not a nineteenth CWDM channel.
How to Specify One
Decide the channel count you will grow into rather than the one you need now, because adding channels later means replacing the mux. Fix the same channel plan at both ends and label the ports; CWDM optics are colour-coded but the codes are not universal between vendors.
Ask whether the fibre is genuinely low-water-peak before buying anything between 1351 and 1451 nm. Then choose packaging: an ABS box mux for a cabinet, a rack-mount chassis for a comms room, and an OADM wherever an intermediate site takes only one or two lanes.
CWDM reference
| Standard | ITU-T G.694.2 |
|---|---|
| Grid | 1271 to 1611 nm, exact 20 nm steps |
| Channels | 18 |
| Channel passband | typically ±6.5 nm |
| Water-peak lanes | 1351, 1371, 1391, 1411, 1431, 1451 nm |
| Mux insertion loss | about 1.5 dB (4ch) to 4 dB (18ch), each end |
| Laser | uncooled DFB, drifts about 0.1 nm/°C |
| Amplification | none — the grid spans 340 nm |
| Typical lane rate | 1G to 25G, direct detect |
Related Terms
DWDM, MWDM, LAN-WDM and DFB lasers. The four-lane CWDM4 transceiver plan is compared against parallel optics in SR4 vs PSM4 vs CWDM4, and the grid calculator lists every channel.