LAN-WDM

Four Lanes, 800 GHz Apart

LAN-WDM is the four-wavelength plan carried inside LR4 and ER4 transceivers: 1295.56, 1300.05, 1304.58 and 1309.14 nm. Those numbers look arbitrary in wavelength and are not — they are exactly 800 GHz apart on the ITU frequency grid, at 231.4, 230.6, 229.8 and 229.0 THz.

Each lane is specified to roughly a nanometre either side of centre. The point of the plan is that a four-lane electrical interface becomes one duplex fibre pair rather than four, which is what distinguishes LR4 from parallel MPO-based DR4 and PSM4.

Why the O-Band

Standard G.652 fibre has its dispersion zero between 1300 and 1324 nm, and the whole LAN-WDM plan sits within a few nanometres of it. Chromatic dispersion over 10 km is therefore negligible, and even ER4 at 40 km has only a modest penalty to budget.

Move the same four lanes to 1550 nm and you would be fighting about 17 ps/nm·km, which at 25G per lane over 10 km is already a real number. The O-band placement is not a convention; it is what makes the reach achievable without compensation.

LAN-WDM Against CWDM4

CWDM4 answers the same question differently: four lanes at 1271, 1291, 1311 and 1331 nm on the 20 nm CWDM grid, uncooled, 2 km. LAN-WDM squeezes its four lanes into 13.6 nm instead of 60, which keeps every one of them close to the dispersion zero and makes 10 km straightforward.

The price is temperature control. SR4 vs PSM4 vs CWDM4 and DR4 vs FR4 vs LR4 work through which module type a given span actually wants, and it is rarely the one with the longest reach on the datasheet.

Cooling, and What It Costs

A one-nanometre passband is far tighter than an uncooled DFB's drift of about 0.1 nm per degree Celsius, so every LAN-WDM lane laser sits on a thermoelectric cooler with its own control loop.

That produces a characteristic failure mode: a module that passes traffic on the bench and starts dropping lanes once the chassis is hot and the cooler is at the end of its authority. Read the per-lane transmit power and the case temperature in DDM before blaming the fibre, because the fibre almost never fails one lane out of four.

Where You Meet It

Every 100GBASE-LR4 and ER4, every 200GBASE-LR4, and the eight-lane 400GBASE-FR8 and LR8, plus the 400G LR4 10 km modules sold outside IEEE. Note that 400GBASE-FR4 and the six-kilometre 400GBASE-LR4-6 are not LAN-WDM at all — they sit on the 20 nm CWDM grid. See our 100G LR4 and 400G LR4 transceivers.

You also meet it as a passive component: a LAN-WDM mux splits the four lanes into discrete wavelengths for test, or lets a passive design carry four separate services on one pair.

Related Terms

CWDM, MWDM, DWDM, EML and chromatic dispersion.