Multimode Step Index

Consider the illustration in Figure 3-8. This diagram corresponds to multimode propagation with a refractive index profile that is called step index. As you can see, the diameter of the core is fairly large relative to the cladding.

There is also a sharp discontinuity in the index of refraction as you go from core to cladding. As a result, when light enters the fiber-optic cable on the left, it propagates down toward the right in multiple rays or multiple modes. This yields the designation multimode. As indicated, the lowest-order mode travels straight down the center. It travels along the cylindrical axis of the core. The higher modes, represented by rays, bounce back and forth, going down the cable to the left. The higher the mode, the more bounces per unit distance down to the right.

August 25, 2011

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