What is OM4 multimode fiber?
OM4 is one of the multimode fiber grades defined in cabling standards such as ISO/IEC 11801. It has a 50 micron core and a 125 micron cladding, the same size as OM2 and OM3. It is designed for use with 850 nm VCSEL light sources, and it has a higher effective modal bandwidth than OM3. The commonly published figure is 4700 MHz·km for OM4 and 2000 MHz·km for OM3.
The extra bandwidth lets OM4 carry a signal further at the same speed. For 10 Gigabit Ethernet over 850 nm multimode (10GBASE-SR), the IEEE reach is 400 m on OM4, compared with 300 m on OM3. Reach at 40G and 100G is shorter and depends on the optic type, so check the transceiver data sheet. Many OM4 cables use an aqua jacket, though some vendors use a different color for special variants, so rely on the printed cable marking and not on color alone.
OM4 is not the same as single-mode fiber. Multimode carries many light paths in a larger core and suits short runs. Single-mode (OS2) has a small core, supports much longer distances and is the usual choice between buildings and for long runs. Do not mix the two on one link: the optics, patch cords and test equipment must match the fiber type.
How it shows up in practice
- Specify the fiber grade in the scope (OM3, OM4, or OS2) and the connector type, such as LC duplex or MPO for parallel optics.
- Match patch cords and transceivers to the installed grade. A 50 micron cord must not be connected to 62.5 micron cable.
- Ask for end to end certification test results for the installed links, with the loss values recorded against the agreed limit.
- Keep fiber bend radius and cleanliness in the work method. Dirty connectors are a leading cause of fiber link problems, so clean and inspect before connecting.