Quick answer: A multimode duplex OM4 fiber optic patch cable carries two fibers in one jacket for transmit and receive. OM4 is laser optimized with a modal bandwidth of 4700 MHz-km at 850 nm, and that fiber reaches 150 m at 40GBASE-SR4 and 100GBASE-SR4 in parallel links. Choose the jacket for the pathway, confirm polarity before install, and clean both end faces before mating.
When a 100G link has to cross a data hall without active optics, OM4 duplex cord is often the cheapest answer that still meets reach. The cord is also the easy part: bandwidth, polarity, jacket rating and end face condition all have to line up before the link stays up. This guide covers what the 4700 MHz-km figure buys you, where the 150 m reach comes from, and which jacket choices actually change the install.
What the numbers on an OM4 cord mean
OM4 is a grade of 50 micron multimode fiber optimized for 850 nm sources. The headline figure is modal bandwidth of 4700 MHz-km at 850 nm, and that is what allows higher lane counts over longer distances than older grades can manage.
Bandwidth at 850 nm
Bandwidth in MHz-km describes how much signal the fiber carries over distance before modal dispersion smears the pulses together. A higher figure means the same length keeps more margin, which is why OM4 supports 40GBASE-SR4 and 100GBASE-SR4 to 150 m while older grades fall short of that reach.
Where the 150 m reach comes from
Both of those SR4 standards use parallel optics, so they run over an eight fiber trunk or breakout rather than a duplex pair. A duplex OM4 cord serves 10G and 25G links and acts as the jumper between panels and optics. The fiber grade sets the reach figure; the cord format sets where it plugs in.
Jacket choices that change the install
The jacket decides where the cord may run and how it behaves in a tight rack. Ratings follow the pathway, not the bandwidth, so pick the rating first and the fiber grade second.
Riser, plenum and low smoke
A riser rated jacket suits vertical runs inside a shaft. A plenum rated jacket suits air handling spaces, where smoke and fuel load rules are stricter. Low smoke zero halogen jackets matter where people work in confined spaces and where corrosive smoke would damage equipment. Match the rating to the route the cord really takes.
Diameter and routing
A 2.0 mm duplex cord bends tighter and fills less tray space than a 3.0 mm cord, but it has less crush resistance. In a dense rack, thin cords improve airflow and cable management; in a tray with heavy bundles, a thicker jacket and a larger bend radius protect the fiber.
Polarity, cleaning and the connector
Two fibers mean two chances to cross a link. Confirm the A and B mapping at both ends, and clean every end face before the first mating.
Duplex polarity
A duplex cord normally crosses A to B so transmit reaches receive. The convention holds as long as both ends follow it. A cord patched straight through, or a panel that flips one side, produces a link that lights but never passes traffic.
Cleaning and inspection
A speck of dust on a 50 micron core blocks a large share of the light. Inspect and clean both ends, use one time cleaning tools rather than a cloth on a stick, and cap cords that sit in the spare bin.
| Factor | What it means | Why it matters | Check before install |
|---|
| Bandwidth | 4700 MHz-km at 850 nm | Sets reach at 40G and 100G | Grade printed on the cord |
| Polarity | A to B crossover | Transmit must reach receive | Trace both ends |
| Jacket | Riser, plenum or low smoke | Pathway and code compliance | Route the cord takes |
| Connector | LC duplex, UPC or APC | Mating and loss at the port | Polish matches the port |
Practical scenarios
- Top of rack to patch panel: Short duplex OM4 jumpers between optics and panels are the common case, and thin 2.0 mm jackets keep the vertical manager usable.
- Data hall cross connect: A 150 m run between rows carries 100GBASE-SR4 over an eight fiber trunk, with OM4 duplex cords acting as jumpers at each end.
- Plenum ceiling path: A cord that crosses an air handling space needs a plenum rated jacket whatever its length, so order the rating together with the length.
- Lab bench link: Repeated mating wears end faces, so keep a dedicated pair for testing and inspect both connectors before every campaign.
FAQ
Is OM4 the same as OM3?
Both are 50 micron laser optimized multimode, but OM4 has higher modal bandwidth, 4700 MHz-km at 850 nm, which buys more reach at the same speed. Use OM4 for new 40G and 100G work and keep OM3 for shorter legacy links.
Can a duplex OM4 cord carry 100G by itself?
Not for 100GBASE-SR4, which uses parallel optics across eight fibers. Duplex cords serve 10G and 25G links and act as jumpers between panels and optics. The 150 m figure applies to the OM4 fiber inside a parallel link.
Does jacket color matter?
Color is a convention for identifying fiber type, not a performance rating. The printed grade and the jacket fire rating are what matter. Check both before the cord goes into a pathway.
Next step
Order the right grade the first time. Our guide to OM1 to OM4 grades explains how bandwidth changes with distance, the fiber loss budget guide shows how to add up connectors and splices, and the AOC selection guide covers when an active cable beats a passive fiber run.
