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Why use Graded Index Multimode Fiber?

Multimode Fiber Profiles: Step Index vs. Graded Index

There are mainly two types of multimode fibers: Step Index and Graded Index fibers

The core on a step index fiber has the same refractive index throughout.

The refractive index on a graded index fiber gradually changes from the center of the core, to where the core meets the cladding.

Step index vs. Graded index profiles compared

Step Index Fiber vs Graded Index Fiber

Step Index Fiber

Shows a clear distinction between the core and cladding.

Graded Index Fiber

Shows a gradual change from the core and cladding.

Issue with Step Index Fiber

Light rays travel somewhat uncontrolled inside a step index fiber.
The blue ray travels directly down the center of the fiber, meaning it travels a shorter distance than the red, green or black ray.
Rays that travel a longer distance will take more time to get to the end of the fiber than rays that travel a shorter distance.
The result is that some of the signal arrives before the rest of the signal. The signal becomes smeared.
This is bad for communication.

Advantage of Graded Index Fiber

Light rays are controlled well within a graded index fiber.
Just as with the step index fiber, certain rays will travel shorter distances than other rays.
However, in a graded index profile fiber the rays will reach the end of the fiber at the same time…
How does this happen?

Optics of Step and Graded Index Fiber

The speed of light depends on the refractive index of whatever the light is traveling in.

When traveling in space (air, refractive index is 1.0) light travels at 300,000km/s
When traveling in glass, where the refractive index is 1.5, it travels at 200,000km/s
Changing the refractive index changes the speed of light!!!

Since the refractive index of the core in a step index fiber is the same, the light rays will all travel the same speed. (Rays that travel a shorter distance will arrive sooner than rays that travel a longer distance.) However, the refractive index of the core in a graded index fiber changes, which means light rays inside the core will travel a different speeds.

Graded Index Fiber

In a graded index fiber, the refractive index at the center of the fiber is the highest. This means that the light traveling at the center of the fiber travels slower.
Rays that don’t travel in the center will travel faster. As the rays move farther from center, they will travel faster.
The result is that all of the rays in the core will arrive at the end of the fiber at the same time. This is ideal for communication.

Step Fiber vs Graded Fiber in the Industry

Step Fiber

Short distance applications
Lower cost
More prone to attenuation
Over shorter distances, the disadvantages of step fiber are outweighed by the lower cost of deployment

Graded Fiber

Long distance applications
Higher Cost
Less prone to attenuation
Over longer distances, the benefits of the graded fiber are worth the added cost