3.3 OM1 Through OM5: Bandwidth, Distance, and Sources
Key Takeaways
- OM1 is 62.5/125 with modal bandwidth of about 200 MHz·km at 850 nm; OM2 is 50/125 at about 500 MHz·km at 850 nm.
- OM3 (1500 MHz·km) and OM4 (4700 MHz·km) are laser-optimized 50 µm fiber for VCSEL-based 10 Gb/s and 40/100 Gb/s links.
- OM5 (WBMMF) adds 953 nm bandwidth on top of OM4 to support short-wavelength-division multiplexing for future 400 Gb/s.
- Higher modal bandwidth directly extends distance: OM4 carries 10 Gb/s about 550 m vs OM3's 300 m, and 40/100 Gb/s about 150 m vs OM3's 100 m.
- Jacket color alone is not authoritative—verify the printed bandwidth grade (OM3/OM4/OM5) before certifying a high-speed link.
The OM Grading System
TIA-568 and ISO/IEC 11801 classify multimode fiber into performance grades OM1 through OM5. The grades are defined by modal bandwidth (MHz·km)—a measure of how much bandwidth the fiber can carry over one kilometer before modal dispersion closes the eye. Higher OM numbers mean more bandwidth and longer distances at the same data rate.
OM1 and OM2: Legacy LED Fiber
OM1 is 62.5/125 µm fiber with a modal bandwidth of about 200 MHz·km at 850 nm and 500 MHz·km at 1300 nm. It was designed for LED sources and 100 Mb/s or 1 Gb/s premises links. At 1 Gb/s, OM1 reaches roughly 300 m; at 10 Gb/s it falls to about 33 m, which makes it unsuitable for modern 10 Gb/s backbones. Jacket color is typically orange.
OM2 is 50/125 µm fiber with about 500 MHz·km at 850 nm and the same 500 MHz·km at 1300 nm. The smaller 50 µm core reduces modal dispersion, so OM2 reaches about 82 m at 10 Gb/s and about 500 m at 1 Gb/s. Jacket color is also typically orange. Because OM1 and OM2 share a jacket color, you must read the print legend to tell them apart.
OM1 and OM2 are still found in older building plants, but new installs no longer specify them. They are not engineered for VCSEL-based 10 Gb/s and faster optics.
OM3 and OM4: Laser-Optimized 50 µm
OM3 and OM4 are 50/125 µm fibers manufactured with a precise graded-index profile so that a VCSEL can launch into the center of the core with minimal pulse spreading. They are the workhorse of data center and campus backbones at 10 Gb/s and 40/100 Gb/s.
| Grade | Modal BW @850 nm | 1 Gb/s | 10 Gb/s | 40/100 Gb/s |
|---|---|---|---|---|
| OM3 | 1500 MHz·km | ~700 m | 300 m | 100 m |
| OM4 | 4700 MHz·km | ~1,000 m | 550 m | 150 m |
Jacket color for OM3 is aqua. OM4 is also commonly aqua but may be teal or magenta in some vendor schemes to distinguish it from OM3. The TIA adopted aqua as the standard for both, which is a real-world ambiguity the exam expects you to recognize: when in doubt, read the print legend.
OM5: Wideband Multimode Fiber
OM5, also called wideband multimode fiber (WBMMF), is a 50/125 µm fiber that meets OM4 bandwidth at 850 nm and adds bandwidth at 953 nm. The added 953 nm window allows short-wavelength division multiplexing (SWDM), where four wavelengths near 850–953 nm each carry a separate 10 Gb/s or 25 Gb/s lane, opening a path to 100 Gb/s and future 400 Gb/s on multimode.
- OM5 modal bandwidth: at least 4700 MHz·km at 850 nm and 1730 MHz·km at 953 nm.
- 40/100 Gb/s distance: same 150 m as OM4 for the current 4-lane standards.
- Jacket color: lime green.
OM5 is backward compatible with OM4 optics, so it can be installed as a future-proofing choice without penalty on current links. The TECH exam treats OM5 as the multimode choice when the customer explicitly wants a 400 Gb/s-ready plant.
Modal Bandwidth and Distance
The relationship between modal bandwidth (MBW) and maximum distance is intuitive: doubling the available bandwidth-distance product roughly doubles the distance at the same data rate. A 10 Gb/s link needs a fixed bandwidth; an OM3 fiber delivers 1500 MHz·km, so it can sustain 10 Gb/s for 300 m. OM4 delivers 4700 MHz·km—more than three times OM3—so it sustains 10 Gb/s for 550 m. The link is not faster; it is longer.
Source Matching and "Overfilled" vs "Restricted" Launch
Modal bandwidth numbers assume a specific launch condition. VCSEL sources produce a restricted-mode launch (RML) that concentrates power near the core center, which is what OM3/OM4/OM5 are designed for. LED sources produce an overfilled launch (OFL) that excites higher-order modes and produces more pulse spreading. The official bandwidth numbers for OM3/OM4 are specified under the effective modal bandwidth (EMB) launch used by VCSELs.
This is why a legacy OFL source on OM3 will report a much lower effective bandwidth than the fiber's rated 1500 MHz·km. Certification testers must use an encircled-flux-compliant launch (per TIA-568.3-D and IEC 61280-4-1) to get valid bandwidth-related measurements on OM3/OM4/OM5.
Reading the Jacket and Print Legend
Color coding helps but does not replace reading the print. The jacket should print the OM grade and core/cladding ratio, e.g. "OM4 50/125" or "Multimode 50/125 OM4". If the print is unreadable, the only safe option is to perform a bandwidth-related test (such as an encircled-flux-compliant loss measurement combined with length) and downgrade expectations if the test suggests legacy OM2 performance.
Selecting a Grade for a New Link
A Technician selects the fiber grade from the link length and the data rate of the electronics at each end. For example:
- 10 Gb/s switch over a 200 m backbone → OM3 is sufficient.
- 10 Gb/s over a 450 m backbone → OM4 required.
- 40/100 Gb/s over a 120 m data center run → OM4 (or OM5 for future 400 Gb/s).
- 1 Gb/s over a 280 m legacy run → existing OM1 is fine.
This kind of length-vs-grade decision shows up repeatedly on the TECH written exam and on the hands-on fiber selection task.
Which OM grade is laser-optimized 50 µm fiber with a modal bandwidth of approximately 1500 MHz·km at 850 nm and an aqua jacket?
What additional capability does OM5 (WBMMF) provide over OM4?
A customer wants 10 Gb/s over a 450 m backbone. Which multimode grade is required at minimum?