Fiber Optic Training

Section Two: What is Fiber Optics? pg.4

Step and Graded Index Multimode

The first multimode fiber was made with a core of all one type of optical glass. This is called a “step index” multimode core.

Realizing the distance limitations caused by pulse spreading, fiber engineers designed a new type of core, called “graded index”. Instead of a single type of glass, up to ninety layers of glass are put in the core in concentric circles from center to the cladding. Each layer of glass has a slightly different speed at which the light rays will travel.

The center of the core has the slowest speed of light, with the speed of light increasing with each layer until the cladding is reached. The light rays in the center of the core ("close coupled" rays) are slowed down, while the rays that bounce from cladding to cladding ("loose coupled" rays) travel faster. The net effect is to reduce the amount of pulse spreading, giving multimode fiber a usable distance of up to five miles, or eight kilometers. Virtually all multimode fiber strands manufactured today are of the graded index variety.

LESSON REVIEW

Key Points:

1. Graded index multimode fiber has many layers of glass within the core, providing a faster path for the light rays that travel the farthest from transmitter to receiver.

2. Graded index multimode fiber reduces pulse spreading.

Pop Quiz:
True or False

Step index multimode is commonly used.

Key Terms:

1. Step Index

2. Graded Index