In GI type multimode fiber (GI-MMF), the refractive index is the highest at the core center. The refractive index is lower near the clad. Because of this, lower-order modes exhibit the total internal reflection in the vicinity of core center, while higher-order modes do it near the outer clad. Therefore, higher-order modes propagate a long distance in the core. However, since the light velocity is inversely proportional to the refractive index (v=c/n), light propagates faster at the outer side in the core where the refractive index is lower. By optimizing the refractive index distribution, the mode dispersion is minimized.

The stardard of measure of GI-MMF is determined by the advisory of ITU-T. The fiber loss is 0.8 [dB/km] for core/clad=50/125 µm and the length of 1310 nm. GI-MMF has a larger transmission loss than SMF, but is used for the short-distance information communication such as LAN, because the fiber connection with network devices is easy.



Fig. Light propagation and refractive index distribution in SMF and MMF.
(a) SI-SMF, (b) SI-MMF, (c) GI-MMF