Multi-mode fiber The optical fiber that is divided into multiple modes according to the operating wavelength and the possible modes of its propagation is called multi-mode fiber (MMF: MUlti ModeFiber). The fiber core diameter is 50μm, and the transmission bandwidth is dominated by the mode dispersion compared with SMF because the transmission modes can reach hundreds. Historically, it has been used for short distance transmission in cable television and communication systems. Since the emergence of SMF fiber, it seems to form a historical product. In fact, MMF has advantages over SMF in many Lans due to its larger core diameter and easier integration with leds and other light sources. Therefore, MMF is still being paid more attention in the field of short distance communication. When MMF is classified according to refractive index distribution, there are two types: gradient (GI) type and step (SI) type. The refractive index of GI type is the highest in the core, and decreases gradually along the cladding. Due to the time difference of each optical path during the reflection of SI type light wave in the fiber, the optical wave is distorted and the color excitation is large. As a result, the transmission bandwidth is narrowed and the SI MMF is rarely used.