Four VCSEL wavelengths between 850 and 940 nm on one multimode fibre pair — quadrupling capacity without more fibre.
Four Short Wavelengths on One Pair
SWDM puts four VCSELs at roughly 850, 880, 910 and 940 nm onto a single multimode fibre pair, so a 100G SWDM4 module terminates in a duplex LC where 100GBASE-SR4 needs an eight-fibre MPO. Four lanes of 25G share the same glass rather than each taking its own strand.
The roughly 30 nm spacing is coarse enough that the VCSELs stay uncooled, and tight enough that the whole plan fits in the window where multimode fibre works at all. Nothing about the electrical or FEC side changes; only the optical multiplexing does.
Why This Is the One Case Where OM5 Wins
Multimode reach is set by modal bandwidth, and OM3 and OM4 are specified for effective modal bandwidth at 850 nm only. Nothing guarantees what OM4 does at 940 nm. OM5 specifies EMB across roughly 840 to 953 nm, which is exactly the range SWDM occupies.
The result is 150 m for 100G SWDM4 on OM5 against 100 m on OM4 and 75 m on OM3. Give the same fibre to an SR4 module and OM5 does nothing at all — it is an OM4-grade fibre at 850 nm. The OM3 vs OM4 vs OM5 comparison and the multimode reach calculator work the cases through.
An MSA, Not a Standard
SWDM is a multi-source agreement, not an IEEE clause, so there is no compliance test to point at and interoperability rests on vendor agreement. In practice modules from the same generation work together, but a mixed-vendor pair is something to prove on the bench rather than assume.
That status is also why SWDM never became a default. Parallel optics were cheaper per bit for anyone already pulling MPO trunk, and single-mode 100G and 400G modules kept dropping in price, so SWDM stayed a brownfield answer rather than a design choice.
How to Choose
The decision is about cabling, not optics. If a site already has duplex multimode running to every cabinet and adding MPO trunk means a civils job, SWDM4 buys 100G on the fibres already there and the arithmetic is easy. If the trunk is going in anyway, SR4 or a single-mode DR4 is normally cheaper and better supported.
Check reach honestly against the real channel, connectors included, rather than against the headline number. Where a duplex module is wanted but four wavelengths are not, 100G BiDi does the same job with two wavelengths, and the multimode fibre guide covers which grade to specify.
SWDM reference
| Wavelengths | about 850, 880, 910 and 940 nm |
|---|---|
| Fibre | duplex multimode, LC — two fibres, not eight |
| Source | one VCSEL per wavelength |
| 100G SWDM4 reach | 75 m OM3, 100 m OM4, 150 m OM5 |
| 40G SWDM4 reach | 240 m OM3, 350 m OM4, 400 m or more on OM5 |
| Standard | SWDM MSA — not an IEEE 802.3 clause |
| Fibre property that matters | OM5 specifies bandwidth from 840 to 953 nm |
| Duplex alternative | 100G BiDi, two wavelengths instead of four |
| Poor fit | greenfield builds with MPO trunk or single-mode DR4 |