In optical fibers, a step-index fiber is a fiber where a uniform refractive index exists within the core and a sharply decreased refractive index exists in the core-cladding interface because of the lower refractive index in cladding. For step-index multimode fiber, the light propagates in the shape of a zigzag along the fiber/core axis according to the principle of total reflection. Light entering the fiber at different angles of incidence will go through different paths. Although the incident lights propagate at the same speed simultaneously at the input, the time to reach the output of the fiber is different, resulting in a temporal dispersion called modal dispersion.
Since digital communications use light pulses to transmit signals down the length of the fiber, the modal dispersion causes the pulse to widen severely and spread out when they travel along with the fiber. The more modes the fiber transmits, the more pulses spread out. This significantly limits the bandwidth of step-index multimode fibers. What’s more, the modal dispersion is not good for optical fiber communication as well. For digital optical fiber systems, when the dispersion is serious, it will cause pulses to overlap with each other, causing inter-symbol interference (ISI) and increasing the bit error rate (BER). Therefore, fiber dispersion not only affects the transmission capacity of the fiber, but also limits the relay distance of optical fiber communication systems. Because of that limitation, the step-index multimode fiber is normally used in short-distance (within a few kilometers) and low-speed (8 Mb/s or less) communication systems with a relatively lower cost. However, things differ in step-index single mode fibers. In a step-index single mode fiber, it can only transmit light of one mode. Therefore, the modal dispersion is very small, causing less impact on the transmission distance. Normally, single mode fibers are all step-index fibers.