OM3, OM4 and OM5 are laser-optimised 50/125 micron multimode fibres. OM3 has an effective modal bandwidth of 2000 MHz·km at 850 nm and OM4 has 4700 MHz·km. OM5 matches OM4 at 850 nm and adds a specification up to 953 nm for shortwave WDM optics. Under IEEE 802.3, 100GBASE-SR4 runs 70 m on OM3 and 100 m on OM4 or OM5.
What OM3, OM4 and OM5 mean
The OM classes come from ISO/IEC 11801, and the matching TIA fibre specifications are TIA-492AAAC (OM3), TIA-492AAAD (OM4) and TIA-492AAAE (OM5). All three have a 50 micron core and a 125 micron cladding, so they use the same connectors and the same termination methods. They were designed for 850 nm VCSEL transmitters, which is why they are called laser-optimised.
The figure that separates them is effective modal bandwidth (EMB). In multimode fibre, light travels in many modes that arrive at slightly different times. Higher modal bandwidth means less pulse spreading, so a given data rate runs further.
| Grade | Core/cladding | EMB at 850 nm | Extra specification | Usual jacket colour |
|---|---|---|---|---|
| OM3 | 50/125 micron | 2000 MHz·km | None | Aqua |
| OM4 | 50/125 micron | 4700 MHz·km | None | Aqua, or heather violet |
| OM5 | 50/125 micron | 4700 MHz·km | EMB of 2470 MHz·km at 953 nm, specified across 850 to 953 nm | Lime green |
OM1 (62.5/125 micron) and OM2 (50/125 micron) are older grades with orange jackets. They are not suitable for new links above 1G and are mainly met when extending existing buildings.
Multimode fibre reach by speed under IEEE 802.3
IEEE 802.3 sets the supported distance for each Ethernet physical layer on each fibre grade. The table lists the common short-reach interfaces.
| Interface | Lanes and fibres | OM3 | OM4 | OM5 |
|---|---|---|---|---|
| 10GBASE-SR | 1 lane, 2 fibres | 300 m | 400 m | As OM4 |
| 25GBASE-SR | 1 lane, 2 fibres | 70 m | 100 m | As OM4 |
| 40GBASE-SR4 | 4 lanes, 8 fibres | 100 m | 150 m | As OM4 |
| 100GBASE-SR4 | 4 lanes, 8 fibres | 70 m | 100 m | 100 m |
| 400GBASE-SR8 | 8 lanes, 16 fibres | 70 m | 100 m | 100 m |
| 400GBASE-SR4.2 | 4 fibre pairs, 2 wavelengths each, 8 fibres | 70 m | 100 m | 150 m |
"As OM4" means OM5 meets the OM4 specification at 850 nm, so it supports at least the OM4 distance for single-wavelength 850 nm applications. These distances assume the channel also stays within the IEEE channel insertion loss limit, which our fibre loss budget guide explains. For links longer than these figures, single-mode fibre is the usual answer.
Where OM5 helps and where it does not
OM5 is wideband multimode fibre. Its extra specification covers wavelengths from 850 nm to 953 nm, which matters only for optics that send more than one wavelength down each fibre, known as shortwave wavelength division multiplexing (SWDM) or bidirectional (BiDi) optics.
- Gains from OM5: 400GBASE-SR4.2 uses two wavelengths per fibre and is specified to 150 m on OM5 against 100 m on OM4. Other multi-wavelength multimode optics follow the same pattern; check the module data sheet.
- No gain from OM5: single-wavelength 850 nm optics such as 10GBASE-SR, 100GBASE-SR4 and 400GBASE-SR8 reach the same distance on OM5 as on OM4.
OM5 is therefore worth specifying where the optics plan includes multi-wavelength modules, or where a building owner wants that option kept open. For a network built on parallel SR4 and SR8 optics, OM4 gives the same reach.
Jacket and connector colours
TIA-598 sets the industry colour code for fibre cable jackets. OM3 is aqua. OM4 is also aqua under that code, but heather violet (often called Erika violet) is widely used for OM4, especially in Europe, so that it can be told apart from OM3. OM5 is lime green. Single-mode is yellow, and the legacy OM1 and OM2 grades are orange.
Connector housings and MPO boots usually follow the jacket colour for multimode, though practice varies between manufacturers. Because colour alone is not a reliable record, the cable print legend and the label should always state the fibre grade. Our guide to fibre colour codes also covers the TIA-598 individual fibre colours.
Choosing a grade for a new installation
- OM4 is the usual default for new multimode data centre links up to 100 m at 100G and 400G, and up to 150 m at 40G.
- OM3 makes sense when extending an existing OM3 plant, where the channel reach would in any case be limited by the OM3 sections.
- OM5 fits when multi-wavelength multimode optics are part of the plan.
- Single-mode (OS2) is the better choice for runs beyond the multimode limits, or where the site expects to move to single-mode optics such as 400GBASE-DR4 and FR4.
Avoid mixing grades within one channel unless the design accounts for it. A channel that contains OM3 and OM4 sections must be designed to the OM3 reach. Mixing 50 micron fibre with 62.5 micron OM1 in the same channel causes a large loss at the joint and should not be done.
Bend-insensitive multimode and testing
Bend-insensitive multimode fibre (often shortened to BIMMF) is available in OM3, OM4 and OM5 grades. It uses a modified refractive index profile so that tight bends in crowded trays, cassettes and patch fields add less loss. It is compatible with standard multimode fibre of the same grade, so it can be used for patch cords and trunks in an existing installation.
Multimode links are tested at 850 nm for the short-reach applications in the table above, often with a second test at 1300 nm. The light source should meet the encircled flux launch condition of IEC 61280-4-1, because multimode loss results depend on how the light is launched into the fibre. A test report that states the launch condition, the reference method and the loss of every fibre gives you a record you can compare against later.
Ordering OM4 MPO cable and other multimode assemblies
When ordering OM4 MPO cable, or any multimode assembly, state:
- Fibre grade (OM3, OM4 or OM5), and whether bend-insensitive multimode fibre is required.
- Jacket colour, if the site uses violet for OM4 rather than aqua.
- Connector and fibre count (MPO-8, MPO-12, MPO-16 or MPO-24, or LC duplex).
- Polarity type and gender at each end.
- Fire rating of the jacket.
- Maximum insertion loss per mated pair and the test report you need.
Radix makes MTP OM4 patch cords and MTP OM3 48-fibre patch cords, along with multimode trunks and breakouts made to drawing. Send the details through the request a quote page.
Questions buyers ask
Can I connect OM3 and OM4 cables together?
Yes. Both are 50/125 micron fibre and use the same connectors, and the joint adds no special loss. The channel must then be designed to the OM3 reach, for example 70 m rather than 100 m for 100GBASE-SR4.
Is OM5 backward compatible with OM4?
Yes. OM5 meets the OM4 specification at 850 nm, so any optic that runs on OM4 runs on OM5 to at least the same distance. OM5 only adds reach for optics that use several wavelengths between 850 nm and 953 nm.
What colour is OM4 fibre cable?
Under the TIA-598 colour code OM4 is aqua, the same as OM3. Many manufacturers and European sites use heather violet for OM4 so the two grades can be told apart. Check the print legend on the jacket to be sure.
How far can 10G run on OM4?
IEEE 802.3 supports 10GBASE-SR to 400 m on OM4 and 300 m on OM3, provided the channel insertion loss stays within the standard's limit. Longer links need single-mode fibre and optics such as 10GBASE-LR.
Should I install multimode or single-mode for a new data centre?
Multimode with OM4 is common for links up to about 100 m, because short-reach multimode optics are generally lower in cost and power. For longer links, or where the operator plans single-mode optics such as DR4 and FR4, single-mode is the better choice.
Does OM5 give longer reach for 400G?
Only for multi-wavelength optics. 400GBASE-SR4.2 is specified to 150 m on OM5 against 100 m on OM4. 400GBASE-SR8, which uses one wavelength per fibre, has the same 100 m reach on OM4 and OM5.