Multimode fibers are optical waveguides. As the designation Multimode Fiber (MMF) indicates, several modes contribute to signal transmission inthis optical fiber. The situation is different for single- mode fibers in which only one light beam is transmitted in one mode.
As far as the modes of multimode fibers are concerned, it is about the light beams at the boundary layer between core glass and cladding glass. In multimode fibers, they are reflected frequently and differently, causing the light beams to have different propagation times. Multimode fibers are available in two versions: as step fib ers and as gradient fibers. In step fibers, the refractive indices of the core glass and cladding glass differ. The light beam coming from the core glass is always reflected by the cladding glass at the angle of incidence. The different modes are related to the angles of incidence of the light source. Step fibers have a typical core diameter of 100 µm, 120 µm or 400 µm, with a bandwidth length product of less than 100 MHz x km and an attenuation of about 6 dB/km.
Gradient fibers are different. They do not have a fixed refractive index; rather, it changes over the cross-section of the core glass. Gradient fibers have a typical core diameter of 50 µm, 62.5 µm, 85 µm or 100 µm and a cladding diameter of 125 µm or 140 µm. Attenuation values are 3 dB/km( LED 850 nm), allowing repeaterless transmission of up to 10 km. The bandwidth length product here is between 200 MHz x km at 850 nm and 500 MHz x km at 1,300 nm due to better suppression of mode dispersion.
The cable sheath of multimode fibers is orange. They support optical space division multiplexing and can transmit different information on the different modes simultaneously. As part of the new Ethernet technologies such as Gigabit Ethernet and 10 Gigabit Ethernet, new, much purer gradient fibers have been developed, the New Fiber, which have bandwidth length products of up to 10 GHz x m at 850 nm.