Step Index vs Graded Index Fiber: Single Mode and Multimode

This page delves into single mode step index fiber and multimode graded index fiber, providing a comparison between the two. Fiber optic cables can be classified using two main methods:

1. Index of refraction variation: Based on how the refractive index changes across the cable’s cross-section.
2. Mode: Determined by the various paths light rays take as they travel through the cable.

Figure 1: Fiber optic cable cross section

As shown in Figure 1, all fiber optic cables consist of two primary layers: the core (innermost layer) and the cladding.

Single Mode Step Index Fiber

Figure 2: Single mode step index fiber cable

In single mode fiber, light travels along a single path through the core. “Step index” signifies a sharp, step-like change in the refractive index at the core-cladding interface. This means that in a step index fiber, the core and cladding each have their own constant refractive indices, denoted as $N_1$ and $N_2$ respectively. Figure 2 illustrates the cross-section of a step index cable.

Multimode Graded Index Fiber

Figure 3: Multimode graded index fiber cable

In multimode fiber, light follows multiple paths through the core. In a graded index fiber, the refractive index is not constant; instead, it varies smoothly across the core’s diameter. The refractive index increases as you approach the center of the core and decreases towards the outer edges. The refractive index is at its maximum at the core’s center. The cladding, however, maintains a constant refractive index. Figure 3 depicts the cross-section of a graded index cable.

Based on these characteristics, there are three main 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 its large size, making it easier and less expensive to manufacture. It’s typically used for shorter distances at lower pulse frequencies.

Figure 4: Single mode step index cable

Figure 5: Multimode graded index cable

Figures 4 and 5 illustrate the cross-sections of single mode step index and multimode graded index cables, respectively.