step index and graded index fiber | single mode,multimode

This page describes single mode step index fiber and multimode graded index fiber. It compares both step index and graded index fibers.

There are two methods to classify fiber optic cables.
1. index of refraction variation across the cable cross section.
2. Using mode i.e. various paths light rays usually take to pass through the cable.

Figure-1 depicts fiber optic cable cross section. As shown all the fiber optic cable consists of two main layers. They are core(inner most) and cladding. Refer single mode fiber vs multimode fiber which mentions difference between both modes of fiber.

single mode step index fiber

In Single mode light takes single path through the fiber core. Step index means sharp step in the index of refraction between core and cladding interface. This indicates that in step index, core and cladding have their own constant index of refractions N1 and N2 respectively. The step index cable cross section is depicted in the figure-2.

multimode graded index fiber

In multimode light follows multiple paths through the fiber core. In graded index, index of refraction is not constant but vary smoothly across the diameter of the core. Index of refraction is increasing as one goes near the center while decreasing near outer core edges. Index of refraction is maximum at the center of the core. Index of refraction is constant for cladding part of the fiber.

Based on above, there are three types of fiber optic cable.
• multimode step index
• single mode step index
• multimode graded index

multimode step index fiber cable is widely used due to benefits of large size. It is easier to make and hence less expensive. It is used for shorter distances at very low pulse frequencies. The figure-3 above depicts graded index cable cross section.

Figure-4 depicts single mode step index cable and multimode graded index cable cross sections.