Understanding the Difference Between OM4 and OM5, Part 2

In part two of this two-part blog, we describe a multi-mode fiber metric to show when to use OM4 and when to use OM5. Part one reviewed the key characteristics of the multi-mode fibers specified by the ISO/IEC 11801 standard.

Each successive generation of multi-mode fiber specified by ISO/IEC 11801 provided performance improvements. OM1 and OM2 were designed for LEDs (Light Emitting Diodes), while OM3 was the first fiber designed for use with VCSELs (Vertical Cavity Surface Emitting Lasers). OM4, also targeting VCSELs, offered further improvement, supporting 100 Gb/s Ethernet over distances of up to 150 m. It seems reasonable to assume that OM5 fiber would provide yet another performance boost. That's not necessarily the case, however. Read on to discover why.

Effective Modal Bandwidth

Multi-mode fiber has a wider core diameter that enables it to support light propagating through the fiber over multiple paths (also described by spatial modes). The speed at which light in these modes travels varies depending upon the properties of the fiber in the optical path. DMD (Differential Mode Delay) quantifies propagation delay introduced for different optical modes excited across the diameter of the fiber.

For modern multi-mode fibers designed for use with VCSELs, the EMB (Effective Modal Bandwidth) is the metric used to define the fiber's capacity. EMB is a frequency-length product and is defined in ISO/IEC 11801 for a specific length of fiber at a specific measurement wavelength, expressed in units of MHz