50/125 µm multimode at 4700 MHz·km — the default multimode grade for data centres.
What Is OM4 Fiber?
OM4 is the same 50/125 µm laser-optimised construction as OM3 held to a tighter tolerance on the graded index profile. The result is 4700 MHz·km of effective modal bandwidth at 850 nm against OM3's 2000. That is the entire technical difference between the grades.
Attenuation is specified identically — a maximum of 3.5 dB/km at 850 nm for both — which over a 100 m channel amounts to 0.35 dB and is not what decides anything. Bandwidth is.
The Reach It Buys
For the standards that matter in a data centre the gain is one step: 100GBASE-SR4, 200GBASE-SR4 and 400GBASE-SR8 all run 70 m on OM3 and 100 m on OM4. At 10G the gap is 300 m against 400 m. In a practical building that is the difference between reaching the far end of a row from one distribution point and needing two.
The channel loss allowance moves with the distance rather than against it: IEEE gives 1.8 dB for 70 m of OM3 and 1.9 dB for 100 m of OM4 on 100GBASE-SR4. Both are tight. Check yours in the link budget calculator.
How to Choose
OM4 is the sensible default for new multimode plant, and has been for a decade. The premium over OM3 is modest and the reach margin is real. The question worth asking instead is whether the site should be multimode at all — single-mode versus multimode turns on transceiver cost against fibre cost, and above 100 m the answer is usually single-mode.
OM5 is a separate decision again. It matches OM4 everywhere except on short-wavelength WDM, so it is only worth paying for if SWDM or BiDi optics are actually in the plan.
What Goes Wrong
Loss, not distance, is what usually kills an OM4 link that is well inside its reach. With a 1.9 dB channel and mated pairs at 0.25 to 0.35 dB apiece, four patch panels plus the fibre spends the whole budget. Every extra cross-connect a design adds for tidiness is a real optical cost — see insertion loss.
The other failure is a dirty MPO ferrule taking twelve fibres out at once. On parallel optics that presents as a link that trains and then flaps, which is a miserable fault to chase.
Where You Meet It
Most short-reach parallel optics in production today: SR4, SR8 and the BiDi variants, all of them landing on MPO trunks. Our MPO/MTP assemblies and multimode patch cords are built on OM4 as standard.
OM4 reference
| Core / cladding | 50 / 125 µm, graded index |
|---|---|
| Effective modal bandwidth | 4700 MHz·km at 850 nm |
| Overfilled-launch bandwidth | 3500 MHz·km at 850 nm |
| Maximum cabled attenuation | 3.5 dB/km at 850 nm, 1.5 dB/km at 1300 nm |
| 10GBASE-SR | 400 m |
| 40GBASE-SR4 | 150 m |
| 100GBASE-SR4 and 400GBASE-SR8 | 100 m, 1.9 dB channel insertion loss |
| Jacket colour | aqua, sometimes violet |
| Standards | TIA-492AAAD, ISO/IEC 11801 OM4 |
Related Terms
OM3, OM5, EMB, insertion loss, VCSEL.