Multimode fibers have a relatively large core diameter (50 μm). This means that different modes – light beams that enter the fiber at different angles – have the same phase length and run time. As a result, lower-cost light sources can be used that do not emit parallel light waves, such as light-emitting diodes (LED).

However, the attenuation is higher than for a single-mode cable. The core has a parabolic refractive index profile, with different refractive indices in the core and cladding; this profile causes light to spread more slowly in the core center than in the parts nearer the cladding, so that information can be transmitted without the need for parallel light beams.