Laser-optimised 50/125 µm multimode fibre specified at 2000 MHz·km effective modal bandwidth — the first grade built for VCSELs.
What Is OM3 Fiber?
OM3 is 50/125 µm graded-index multimode fibre whose refractive index profile is optimised for 850 nm laser sources rather than the LEDs that earlier multimode grades were designed around. Its defining specification is an effective modal bandwidth of 2000 MHz·km at 850 nm.
That single number sets every reach figure OM3 is quoted at. It is not a loss specification and it has nothing to do with connector quality — it is a statement about how tightly the fibre's index profile was manufactured, and therefore how far a pulse can travel before it smears.
The Number That Sets the Reach
Light entering a 50-micron core populates hundreds of propagation paths. Shallow-angle paths arrive sooner than steep ones, and the graded profile exists to slow the short paths so everything arrives together. How well that compensation is executed is measured as a differential mode delay and reported as EMB.
Divide the bandwidth-distance product by the bandwidth the signal needs and you have the distance. That is why 10GBASE-SR runs 300 m on OM3 while 100GBASE-SR4, whose lanes run at 25.78 GBd, manages 70. Look the current figure up in the multimode reach calculator rather than assuming.
What Goes Wrong
The commonest error is treating the multimode reach limit as a loss budget. Cleaner end faces, lower-loss connectors and a hotter transmitter buy you nothing past the distance EMB allows, because the failure mode is pulse overlap and not a shortage of light. A link at 90 m of OM3 running 100GBASE-SR4 will test clean on insertion loss and still show errors.
The second error is mixing 50-micron and 62.5-micron plant. Launching from 62.5 into 50 loses several decibels at the joint, and the mismatch is invisible until somebody measures it.
Where You Meet It
An enormous installed base of data-centre and campus backbone, most of it laid when 10G was the ambition. For new builds OM4 is the rational default: the price difference is small and the reach difference decides whether a row can be served from one location or two. Compare the grades in OM3 vs OM4 vs OM5 or the longer multimode guide.
Where OM3 is still the right answer is short in-row work — a switch-to-switch hop of 30 m does not care that the fibre could have gone 100. Our MPO trunks are built on OM3, OM4 or OM5 to order.
OM3 reference
| Core / cladding | 50 / 125 µm, graded index |
|---|---|
| Effective modal bandwidth | 2000 MHz·km at 850 nm |
| Overfilled-launch bandwidth | 1500 MHz·km at 850 nm, 500 at 1300 nm |
| Maximum cabled attenuation | 3.5 dB/km at 850 nm, 1.5 dB/km at 1300 nm |
| 10GBASE-SR | 300 m, 2.6 dB channel insertion loss |
| 40GBASE-SR4 | 100 m |
| 100GBASE-SR4 and 400GBASE-SR8 | 70 m, 1.8 dB channel insertion loss |
| Jacket colour | aqua |
| Standards | TIA-492AAAC, ISO/IEC 11801 OM3 |