Multimode and Multimode Fiber Parameters

Multimode fiber (MMF) is characterized by core diameter, numerical aperture, modal bandwidth, attenuation, and maximum transmission distance, with OM3, OM4, and OM5 being the most common types for modern high-speed networks.Core Diameter and Numerical ApertureMultimode fibers have a larger core diameter than single-mode fibers, typically 50–100 µm, which allows multiple light modes to propagate simultaneously . The numerical aperture (NA), often around 0.2–0.3, determines the light acceptance angle and affects the number of modes supported . Step-index fibers have a uniform refractive index, while graded-index fibers reduce modal dispersion by varying the refractive index across the core.Modal Bandwidth and AttenuationModal bandwidth is a key parameter that defines the data-carrying capacity over a given distance. It is often expressed as the bandwidth-length product (MHz·km) . Attenuation, measured in dB/km, indicates signal loss along the fiber. Both parameters are critical in determining the maximum transmission distance for a given data rate. For example, OM3 supports 10 GbE up to 300 m, while OM4 can reach 550 m at the same speed .OM Fiber TypesOM1: 62.5 µm core, LED source, suitable for lower-speed networks, short distances, orange jacket .OM2: 50 µm core, LED source, improved bandwidth over OM1, typically used for 1–10 GbE .OM3: 50 µm core, laser-optimized (LOMMF) for VCSELs, supports 10–40 GbE, aqua jacket .OM4: 50 µm core, enhanced laser-optimized, supports 10–100 GbE, longer distances than OM3, aqua jacket .OM5: 50 µm core, wideband multimode fiber, supports multiple wavelengths (850–950 nm) for shortwave WDM, lime green jacket, optimized for high-speed data center applications .Wavelength and Light SourcesMMF typically operates at 850 nm and 1300 nm wavelengths, using LEDs or VCSELs as light sources . Laser-optimized fibers (OM3/OM4/OM5) are designed for VCSELs, providing higher bandwidth and longer reach compared to LED-based fibers.ApplicationsMultimode fibers are ideal for short- to medium-distance communication, such as within buildings, campuses, or data centers . They are cost-effective due to lower-cost transceivers and simpler alignment requirements. MMF is widely used in LAN backbones, storage area networks (SANs), and high-performance computing environments .SummaryKey parameters to consider when selecting multimode fiber include core diameter, numerical aperture, modal bandwidth, attenuation, maximum distance, and compatibility with light sources. OM3, OM4, and OM5 fibers are the preferred choices for modern high-speed networks, supporting 10–100 GbE over distances up to 550 meters, while OM1 and OM2 are mostly legacy fibers for lower-speed applications .