In SI type multimode optical fiber (SI-MMF), the distribution of refractive index in the core is homogeneous. The refractive index steps down at the boundary from the core to the clad. Because the propagation angle is kept the same as the incident angle, the propagating distance in the core and the propagation time vary for each mode. Therefore, duration of the pulsed light incident to SI-MMF is elongated at the output edge after the light propagation in optical fiber. SI-MMF has a simple structure and is cheap, but its application is limited.


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