Multimode fiber has a single core that supports multiple light modes, not multiple physical cores.

Multimode fiber is designed with a larger core diameter, typically ranging from 50 to 100 micrometers, which allows multiple light paths or modes to propagate simultaneously within the same core . The term "multimode" refers to these multiple propagation modes, not to multiple physical cores. Each mode represents a different path that light can take as it reflects off the core-cladding interface while traveling through the fiber .

Core and Light Propagation

The single core of multimode fiber is surrounded by cladding, and its larger size compared to single-mode fiber enables easier alignment with light sources such as LEDs or VCSELs . The multiple modes travel at slightly different speeds and path lengths, which can cause modal dispersion, limiting the maximum transmission distance and bandwidth . Step-index and graded-index designs are used to manage these effects, with graded-index fibers reducing modal dispersion by gradually changing the refractive index across the core .

Applications

Multimode fibers are commonly used in short- to medium-distance data transmission, such as in local area networks (LANs), data centers, and campus networks, where high data rates are required over distances typically up to 550 meters depending on the fiber type (OM3, OM4, OM5), . They are cost-effective and compatible with lower-cost light sources, making them ideal for enterprise and backbone applications . In summary, multimode fiber contains a single core that supports multiple light modes, enabling simultaneous propagation of multiple light paths, which is the source of its name and its characteristic performance features .