Understanding the Difference Between OM4 and OM5, Part 1
In part one of this two-part blog, we review the key characteristics of the multi-mode fiber specified by the ISO/IEC 11801 standard. Part two will detail when OM4 is preferable and when users can derive benefit from using OM5.
Multi-mode fiber is a mainstay of the data center and campus networking space. However, it can be a challenge to choose the specific type of multi-mode fiber for your cable infrastructure. Of the five classes of multi-mode fiber specified by the ISO/IEC 11801 standard, three are in common use: OM3, OM4 and the most recently defined version, OM5. OM5 may be difficult to understand, particularly its capabilities and when it provides advantages over OM4. To clarify the situation, we review the standard and how to quantify the fiber’s performance.
The effect of core diameter on fiber modes
Light propagates through an optical fiber as a collection of spatial modes. Each mode travels a different optical path down the fiber, with a varying transit time. The wider the core diameter of the fiber, the greater the number of modes it can support. Single-mode fiber has a core diameter 8 to 10 µm, which limits it to supporting only one mode. Multi-mode fiber has a core diameter of 50-62.5 µm, which enables it to support multiple modes.
We classify multi-mode fiber by its modal bandwidth. The ISO/IEC 11801 standard encompasses multiple generations of multi-mode fibers developed with increasing bandwidth and, hence, reach: OM1, OM2, OM3, OM4 and OM5. The standard defines OM1 and OM2 for core diameters of both 50 and 62.5 µm, while it calls out OM3, OM4 and OM5 specifications for 50-µm core diameters only.
OM1 and OM2 were created for use with LED (Light Emitting Diode) sources. The advent of VCSELs (Vertical Cavity Surface Emitting Lasers) in the late 90s, with their ability to sustain high modulation speeds, led to the development of OM3, a new class of 50-µm-core multi-mode fiber. OM3 supports 10-Gb/s speeds over links of up to 300 m.
Increasing demand for bandwidth prompted manufacturers to work toward a higher grade of fiber: LO-MMF (Laser-Optimized Multi-Mode Fiber) with tighter specifications. The result was OM4, which enables 10-Gb/s operation over distances of up to 400 m. Alternatively, OM4 supports point-to-point 40 Gb/s and 100 Gb/s Ethernet links of four or 10 pairs of fibers over distances of up to 150 m.
Most recently, the industry released OM5. OM5 is a 50/125 µm LO-MMF that is optimized for performance over the operational band of 850 nm to 953 nm. Does this mean OM5 is better for your application? Not necessarily.
In part two, we will review a key metric used to characterize multi-mode fiber performance.
For more details on this subject, see the whitepaper: